Earthquake Report: 1989 Loma Prieta!

Well, I prepared this report for the 30th anniversary of the 18 Oct 1989 Loma Prieta M 6.9 earthquake in central California, a.k.a. the World Series Earthquake (it happened during the 1989 World Series game at Candlestick Park in San Francisco). The date was 17 October in CA, but 18 Oct in England (UTC time).

Learn more about how to prepare for the next SF Bay Area quake here.

There is a treasure trove of information about this earthquake, the impacts from the earthquake, and the response of people to these impacts. The “go to” place to start looking at some of these resources is from the USGS here. Some of the information I gleaned for this report came from one of the links on that page.


I was a sophomore at the California Institute of the Arts (studying cinematography with an interest of being a DP) in October 1989. The previous year I was living at a housing coop (UCHA at 500 Landfair Ave in Westwood) while attending UCLA. One of my good friends (David Silver) from the coop was from Santa Cruz, so I called him to find out if his family was OK (they were).

That was the closest I came to experiencing the quake and this was almost a decade before I started growing my interest in geology and plate tectonics.

The earthquake had a major impact upon the entire SF Bay area. Freeway overpasses collapsed. A section of the Bay Bridge fell. Many houses were damaged. Fires started. The ground along the coast liquefied.

All of this may happen again when the next big earthquake hits.

The good thing is that, given a little bit of information, people are much more capable of experiencing an earthquake with a reduced amount of suffering. Some stuff we cannot completely prevent, but a little bit of knowledge goes a long way. If you did not participate in a shakeout this year, sign up so you can do so next year. Or, check out shakeout to see what you can learn even without the shakeout going on. If you don’t live in California or the USA, there are still lots of things that you can learn! There are shakeouts in other states and in other countries too!

Below I present several interpretive posters, as well as some figures from papers and public reports (e.g. from the USGS).

Below is my interpretive poster for this earthquake

  • I plot the seismicity from the 3 months including and after the M 6.9 earthquake, with orange circles with the symbol diameter representing magnitude (see legend). I include earthquake epicenters from 1969-2019 with magnitudes M ≥ 2.5 in one version (gray circles). I use the USGS Quaternary fault and fold database as a source for the tectonic faults on the map, with color showing their slip rates.
  • I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
  • A review of the basic base map variations and data that I use for the interpretive posters can be found on the Earthquake Reports page.
  • Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.

    I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

  • In the upper left corner there is a map that shows the major faults in the SF Bay region. The fault lines are colored (yellow to orange) that shows the chance that a given fault may slip between 2007 and 2036. The Hayward/Rodgers Creek fault system has the highest chance of having an earthquake in the next 17 years (about 31%). This is based on our knowledge of earthquakes from the past and into the prehistoric time. The region of the San Andreas fault that was involved in the Loma Prieta temblor is labeled with black arrows.
  • In the upper right corner is a map from the USGS, the Governor’s Office for Emergency Services (CalOES), and the California Geological Survey (CGS, where I work) that uses our knowledge of past earthquakes and the bedrock geology (or lack thereof) to show the potential for strong ground shaking from future earthquakes. High hazard areas are colored pink and are close to the faults (compare with the map in the upper left corner). Areas of low hazard are further away from faults. I placed a yellow circle in the general location of the M 6.9 epicenter.
  • In the lower right corner is a detailed figure from McLaughlin and Clark (2003) (labeled Wells, 2003) that shows their interpretation of the faults in the area. The mainshock is labeled by a black star.
  • Here is the map with 3 month’s seismicity plotted.

USGS Shaking Intensity

  • Here is a figure that shows a more detailed comparison between the modeled intensity and the reported intensity. Borth data use the same color scale, the Modified Mercalli Intensity Scale (MMI). More about this can be found here. The colored contours on the map are results from the USGS modeled intensity. The DYFI data are plotted as colored regions (color = MMI). I labeled some of the DYFI regions (e.g. DYFI 8.1) and MMI contours (e.g. MMI 7).
  • in the lower left-center there are two inset maps. The map on the left is the MMI shakemap from the USGS. The map on the right is shows the same DYFI regions as shown in the main map.
  • In the upper left corner is a plot showing MMI intensity (vertical axis) relative to distance from the earthquake (horizontal axis). The models are represented by the green and orange lines. The DYFI data are plotted as light blue dots. The mean and median (different types of “average”) are plotted as orand and purple dots. Note how well the reports fit the green line (the model that represents how MMI works based on quakes in California). I plot Santiago relative to distance from the earthquake with a blue arrow (compare with the poster).

Shaking Intensity and Potential for Ground Failure

  • Below are a series of maps that show the shaking intensity and potential for landslides and liquefaction. These are all USGS data products.
  • There are many different ways in which a landslide can be triggered. The first order relations behind slope failure (landslides) is that the “resisting” forces that are preventing slope failure (e.g. the strength of the bedrock or soil) are overcome by the “driving” forces that are pushing this land downwards (e.g. gravity). The ratio of resisting forces to driving forces is called the Factor of Safety (FOS). We can write this ratio like this:

    FOS = Resisting Force / Driving Force

    When FOS > 1, the slope is stable and when FOS < 1, the slope fails and we get a landslide. The illustration below shows these relations. Note how the slope angle α can take part in this ratio (the steeper the slope, the greater impact of the mass of the slope can contribute to driving forces). The real world is more complicated than the simplified illustration below.


    Landslide ground shaking can change the Factor of Safety in several ways that might increase the driving force or decrease the resisting force. Keefer (1984) studied a global data set of earthquake triggered landslides and found that larger earthquakes trigger larger and more numerous landslides across a larger area than do smaller earthquakes. Earthquakes can cause landslides because the seismic waves can cause the driving force to increase (the earthquake motions can “push” the land downwards), leading to a landslide. In addition, ground shaking can change the strength of these earth materials (a form of resisting force) with a process called liquefaction.

    Sediment or soil strength is based upon the ability for sediment particles to push against each other without moving. This is a combination of friction and the forces exerted between these particles. This is loosely what we call the “angle of internal friction.” Liquefaction is a process by which pore pressure increases cause water to push out against the sediment particles so that they are no longer touching.

    An analogy that some may be familiar with relates to a visit to the beach. When one is walking on the wet sand near the shoreline, the sand may hold the weight of our body generally pretty well. However, if we stop and vibrate our feet back and forth, this causes pore pressure to increase and we sink into the sand as the sand liquefies. Or, at least our feet sink into the sand.

    Below is a diagram showing how an increase in pore pressure can push against the sediment particles so that they are not touching any more. This allows the particles to move around and this is why our feet sink in the sand in the analogy above. This is also what changes the strength of earth materials such that a landslide can be triggered.


    Below is a diagram based upon a publication designed to educate the public about landslides and the processes that trigger them (USGS, 2004). Additional background information about landslide types can be found in Highland et al. (2008). There was a variety of landslide types that can be observed surrounding the earthquake region. So, this illustration can help people when they observing the landscape response to the earthquake whether they are using aerial imagery, photos in newspaper or website articles, or videos on social media. Will you be able to locate a landslide scarp or the toe of a landslide? This figure shows a rotational landslide, one where the land rotates along a curvilinear failure surface.


    Here is an excellent educational video from IRIS and a variety of organizations. The video helps us learn about how earthquake intensity gets smaller with distance from an earthquake. The concept of liquefaction is reviewed and we learn how different types of bedrock and underlying earth materials can affect the severity of ground shaking in a given location. The intensity map above is based on a model that relates intensity with distance to the earthquake, but does not incorporate changes in material properties as the video below mentions is an important factor that can increase intensity in places.

    Here is a map with landslide probability on the left (Jessee et al., 2017) and a map showing liquefaction susceptibility on the right (Zhu et al., 2017). Please head over to that report for more information about the USGS Ground Failure products (landslides and liquefaction). Basically, earthquakes shake the ground and this ground shaking can cause landslides. We can see that there is a moderate probability for landslides and high probability for liquefaction.


    Our primary landslide model is the empirical model of Nowicki Jessee and others (2018). The model was developed by relating 23 inventories of landslides triggered by past earthquakes with different combinations of predictor variables using logistic regression.

    Zhu and others (2017) is the preferred model for liquefaction hazard. The model was developed by relating 27 inventories of liquefaction triggered by past earthquakes to globally-available geospatial proxies (summarized below) using logistic regression. We have implemented the global version of the model and have added additional modifications.

  • Keefer (1998) presented a review of the earthquake triggered landslides from the Loma Prieta earthquake.
  • Below Keefer and Manson (1998) present a summary of observed earthquake triggered landslides, with Loma Prieta plotted as a circle. This plot shows the area affected by landslides relative to earthquake magnitude. This makes sense, that the larger the earthquake, the larger the area the landslides could be triggered by the earthquake.

  • Area of landslides generated by 1989 Loma Prieta earthquake, A, as a function of earthquake magnitude, M, in comparison with other historical earthquakes with epicenters onshore (dots) and offshore (x’s). Most data points and upper-bound curve (solid line) from Keefer (1984); additional data points and log-linear mean (dashed line) from Keefer and Wilson (1989).

Shaking Visualization & Videos

  • Below is a great visualization of the ground shaking from the ’89 shaker. This comes from the USGS here. Note how the majority of the urban areas did NOT have strong ground shaking from this earthquake, even though that lots of the damage was in those areas. Imagine what will happen when the Hayward or San Andreas faults rupture next.
  • From the USGS: The movie shows the propagation of seismic waves away from the epicenter, which lies in the Santa Cruz Mountains, about ten miles northeast of the of the city of Santa Cruz. The residual colors indicate the peak shaking intensity at locations up to the time in seconds indicated near the top center of the movie. The current intensity, at the time indicated, is indicated by shading of the colors.
  • From the USGS: One striking observation for those who experienced the 1989 Loma Prieta earthquake’s shaking is the comparison of the extent and intensity of shaking with the 1906 earthquake. The Loma Prieta rupture was about 30 times smaller in energy than the great 1906 earthquake.
  • From the USGS: he rupture in the Loma Prieta earthquake began at a depth of about 12 miles and appears to have ruptured a 25 mile long portion of the San Andreas fault. Unlike the 1906 earthquake, the rupture in the Loma Prieta earthquake did not reach the surface. As in the 1906 earthquake, the strongest shaking was concentrated along the fault. In 1989 the two areas of most intense shaking were north and south of the epicenter in the Santa Cruz mountains.

The movie’s color the landscape in each frame according to the maximum (peak) intensity of shaking (amplitude of the ground motion) up to that point in time. The color scale is the same as the one used in ShakeMap. In order to show the intensity of the current shaking, the colors darken as the shaking intensifies. At some locations, the most intense shaking lasts for several seconds, so the colors will darken as seismic waves continue to cause strong shaking. The first example shows how the colors change as the shaking at a location progresses from no shaking through weak, moderate, and strong shaking, peaking at a violent shaking level (very dark red), before the shaking dies off (red becomes brighter). The second example shows the color progression for a location that peaks at a strong level of shaking.

  • Here is a spectacular video from the California Highway Patrol.
  • Here is a documentary from NBC from 2019

Some Relevant Discussion and Figures

Loma Prieta – Geologic Setting

  • McLaughlin and Clark (2003) present two great maps that show the plate tectonic setting associated with the Loma Prieta earthquake.
  • We see maps that show the major faults associated with the Pacific-North America plate boundary. The big player is the San Andreas fault, a right-lateral strike-slip fault (see more in the geological fundamentals section to learn more about strike-slip faults).


  • Figure caption is for both maps from McLaughlin and Clark. Loma Prieta region, Calif., showing major fault blocks and fault zones. A, Regional setting. BSF, Bartlett Springs fault; CA, Calaveras fault; CSZ, Cascadia subduction zone; FF, Franklin fault; GF, Garberville fault; GLF, Garlock fault; HAY, Hayward fault; HF, Hosgri fault; MF, Maacama fault; MFZ, Mendocino Fracture Zone; NAD, Navarro discontinuity; NSAF, northern section of the San Andreas fault (north of the San Francisco peninsula); PF, Pilarcitos fault; PFZ, Pioneer Fracture Zone; PLT, Pleito thrust; PRT, Pastoria-Rand thrust zone; RCF, Rodgers Creek fault; SAF, San Andreas fault, including Peninsular segment; SGF, San Gregorio fault; SNF, Sur-Nacimiento fault; TBF, Tolay-Bloomfield fault; ZVF, Zayante-Vergeles fault. B, San Francisco Bay block, showing locations of plate 1 and figure 2A. Star, epicenter of October 18, 1989, main shock.

  • Here is the cross-section presented by McLaughlin and Clark (2003). We can see how Wells interprets the subsurface geology to be configured. First we see a deeper and more zoomed out view of the plate tectonics here. Then we see a larger scale version showing the faults in greater detail.

  • Schematic cross section across the California margin at latitude of Loma Prieta (fig. 1), showing hypothetical deep structure of the San Andreas fault system, tectonic wedging, and plate boundary relations. Depth, thickness, and compositions of crust and mantle units and location of midcrustal decollement are partly inferred from seismic reflection and refraction models of Fuis and Mooney (1990), Page and Brocher (1993), and Brocher and others (this chapter). Depth to present top of slab window (Dickinson and Snyder, 1979), configuration of lithified materials underplated in older, shallower roof area of window, and hypothetical boundary relation between the Pacific and North American plates are based on thermal and seismic models of Furlong and others (1989). CAL, Calaveras fault; SAF, San Andreas fault; SAR, Sargent fault; SGF, San Gregorio fault; TESLA–ORT, Tesla-Ortigalita fault; ZAY, Zayante fault.


    Surface deformation and crustal structure in the Summit Road-Skyland Ridge area (fig. 2B). A, Rose diagrams comparing observed and expected horizontal surface-deformation fields during 1989 Loma Prieta earthquake. B, Block diagram showing inferred crustal structure across the San Andreas fault and possible relation to primary and secondary slip during 1989 Loma Prieta earthquake. Red echelon faults at surface and shallow subsurface are fissures in the Summit Road-Skyland Ridge fault zone. Loma Prieta rupture is shown in red at depth, extending upward from main shock to base of the gabbro of Logan. Deep configuration of the San Andreas fault is partly inferred from Olson and Hill (1993). Crustal structure to about 10-km depth is partly inferred from Jachens and Griscom (this chapter), and below about 10-km depth is highly speculative and inferred from indicated seismic velocities (Fuis and Mooney, 1990; Rufus Catchings, oral commun., 1993; see Brocher and others, this chapter).

Central California – Earthquake Hazard

  • Based on our knowledge of prehistoric and historic earthquakes, the USGS and CGS have made estimates of the chance that faults may rupture in the next couple of decades (Aagaard et al., 2014). Below is a map from this report that shows the major faults and the likelihood that they may cause an earthquake in between 2014 & 2043. Note that the Hayward fault has the highest chance of slipping over this time period.

Loma Prieta – Earthquake Fault Slip Distribution

  • There are a number of slip models for the Loma Prieta Earthquake. These show the amount that the fault slipped during an earthquake. This type of modeling can be constrained by a number of factors including GPS geodetic data or seismic data.
  • Below is a figure from Jiang and Lapusta (2016). There are slip models for 3 different earthquakes. Slip is represented by color. Earthquake locations are shown as circles. B shows the depth distribution of the earthquakes.

  • (A) Spatial relations of the inferred coseismic slip during large earthquakes (in color, with hypocenters as red stars) and microseismicity before (blue circles) and after (black circles), over time periods shown in (B).The large earthquakes are: (i) 2004 Mw 6.0 Parkfield (6, 16), (ii) 1989 Mw 6.9 Loma Prieta (32), and (iii) 2002 Mw 7.9 Denali (33). Small earthquakes within 2, 4, and 5 km of the fault for the three cases, respectively, are projected onto the fault plane (except iii) and plotted using a circular crack model with the same seismic moment and 3 MPa stress drop. (B) (Left) Time evolution of the depths of seismicity (gray circles) and (right) the depth distribution of normalized total seismic moment released before (blue lines), during (red lines), and after (gray) the mainshock (MS).We considered seismicity and coseismic fault slip inside the regions of largest slip outlined by the red dashed lines in (A). Seismic moment release before the Denali event is not shown because of the small number of events.

  • These authors were investigating how faults behave. Below is another schematic illustration showing their different fault models (conventional vs. deeper-penetration).

  • (A) A strike-slip fault model with the seismogenic zone (light gray areas), creeping regions (yellow), and fault heterogeneity (dark gray circles). The initiation point and rupture fronts of a large earthquake are illustrated by the red star and contours, respectively. (B) The locked seismogenic zone and creeping regions below are typically interpreted as having VW and VS rate-and-state friction properties, respectively. In purely rate-and-state models, the VW/VS boundary and locked-creeping transition nearly coincide, and the associated concentrated shear stressing induced at the locked-creeping transition (blue line) promotes microseismicity at the bottom of the seismogenic zone in the interseismic period (blue circles). However, large earthquake rupture may extend seismic slip deeper than the VW/VS boundary, due to enhanced dynamic weakening (DW) at high slip rates, putting the locked-creeping transition and the associated concentrated stressing (red line) within the VS region and hence suppressing microseismicity nucleation.


More about the background seismotectonics

  • I place a map shows the configuration of faults in central (San Francisco) and northern (Point Delgada – Punta Gorda) CA (Wallace, 1990). Here is the caption for this map, that is on the lower left corner of my map. Below the citation is this map presented on its own.

  • Geologic sketch map of the northern Coast Ranges, central California, showing faults with Quaternary activity and basin deposits in northern section of the San Andreas fault system. Fault patterns are generalized, and only major faults are shown. Several Quaternary basins are fault bounded and aligned parallel to strike-slip faults, a relation most apparent along the Hayward-Rodgers Creek-Maacama fault trend.

  • Here is the figure showing the evolution of the SAF since its inception about 29 Ma. I include the USGS figure caption below as a blockquote.

  • EVOLUTION OF THE SAN ANDREAS FAULT.

    This series of block diagrams shows how the subduction zone along the west coast of North America transformed into the San Andreas Fault from 30 million years ago to the present. Starting at 30 million years ago, the westward- moving North American Plate began to override the spreading ridge between the Farallon Plate and the Pacific Plate. This action divided the Farallon Plate into two smaller plates, the northern Juan de Fuca Plate (JdFP) and the southern Cocos Plate (CP). By 20 million years ago, two triple junctions began to migrate north and south along the western margin of the West Coast. (Triple junctions are intersections between three tectonic plates; shown as red triangles in the diagrams.) The change in plate configuration as the North American Plate began to encounter the Pacific Plate resulted in the formation of the San Andreas Fault. The northern Mendocino Triple Junction (M) migrated through the San Francisco Bay region roughly 12 to 5 million years ago and is presently located off the coast of northern California, roughly midway between San Francisco (SF) and Seattle (S). The Mendocino Triple Junction represents the intersection of the North American, Pacific, and Juan de Fuca Plates. The southern Rivera Triple Junction (R) is presently located in the Pacific Ocean between Baja California (BC) and Manzanillo, Mexico (MZ). Evidence of the migration of the Mendocino Triple Junction northward through the San Francisco Bay region is preserved as a series of volcanic centers that grow progressively younger toward the north. Volcanic rocks in the Hollister region are roughly 12 million years old whereas the volcanic rocks in the Sonoma-Clear Lake region north of San Francisco Bay range from only few million to as little as 10,000 years old. Both of these volcanic areas and older volcanic rocks in the region are offset by the modern regional fault system. (Image modified after original illustration by Irwin, 1990 and Stoffer, 2006.)

  • Here is a map that shows the shaking potential for earthquakes in CA. This comes from the state of California here.

  • Earthquake shaking hazards are calculated by projecting earthquake rates based on earthquake history and fault slip rates, the same data used for calculating earthquake probabilities. New fault parameters have been developed for these calculations and are included in the report of the Working Group on California Earthquake Probabilities. Calculations of earthquake shaking hazard for California are part of a cooperative project between USGS and CGS, and are part of the National Seismic Hazard Maps. CGS Map Sheet 48 (revised 2008) shows potential seismic shaking based on National Seismic Hazard Map calculations plus amplification of seismic shaking due to the near surface soils.

Hayward Fault Scenarios

  • The USGS prepares earthquake shakemap scenarios for known earthquake sources in the US.
  • Below is a summary of what these scenarios are and how they can be used (from the USGS).
  • A scenario represents one realization of a potential future earthquake by assuming a particular magnitude, location, and fault-rupture geometry and estimating shaking using a variety of strategies.

    In planning and coordinating emergency response, utilities, local government, and other organizations are best served by conducting training exercises based on realistic earthquake situations—ones similar to those they are most likely to face. ShakeMap Scenario earthquakes can fill this role. They can also be used to examine exposure of structures, lifelines, utilities, and transportation corridors to specified potential earthquakes.

    A ShakeMap earthquake scenario is a predictive ShakeMap with an assumed magnitude and location, and, optionally, specified fault geometry.

  • Last year there was an effort to educate the public about earthquake hazards in the San Francisco Bay Area. This effort surrounded the 150 year anniversary of the last major earthquake on the Hayward fault. More can be found about the Haywired Project here.
  • I prepare below an interpretive poster that highlights three of the earthquake scenarios for the Hayward fault system, each with increasing magnitude (M 6.9, M 7.3, and M 7.6). Due to the uncertainty about which faults may rupture next, multiple scenarios are used to simulate earthquake effects.
  • The poster below shows the scenario earthquake fault in white (the source of the ground shaking). Earthquake intensity (using the Modified Mercalli Intensity scale) is represented by a color scale (see legend). The inset map on the right shows USGS seismicity between 1919 and 2019.

  • Look at how the same MMI extends for a larger distance across the flat areas (like Sacramento Valley). This is because the sedimentary basins in those areas amplify the seismic waves, so the ground shaking is stronger there.
  • The effect is evidenced in most valleys, such as Napa, Santa Clara, and Salinas.
  • Here is the USGS ShakeMap (Aargard et al., 2008)

  • ShakeMap for the 1906 San Francisco earthquake based on the Boatwright and Bundock (2005) intensities (processed 18 October 2005). Open circles identify the intensity sites used to construct the ShakeMap.

Geologic Fundamentals

  • For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:
  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
  • Strike Slip:

    Compressional:

    Extensional:

  • This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

  • A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

  • Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

  • Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

    Social Media

    References:

    Basic & General References

  • Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
  • Holt, W. E., C. Kreemer, A. J. Haines, L. Estey, C. Meertens, G. Blewitt, and D. Lavallee (2005), Project helps constrain continental dynamics and seismic hazards, Eos Trans. AGU, 86(41), 383–387, , https://doi.org/10.1029/2005EO410002. /li>
  • Müller, R.D., Sdrolias, M., Gaina, C. and Roest, W.R., 2008, Age spreading rates and spreading asymmetry of the world’s ocean crust in Geochemistry, Geophysics, Geosystems, 9, Q04006, https://doi.org/10.1029/2007GC001743
  • Specific References

  • Aargard, B.T. and Beroza, G.C., 2008. The 1906 San Francisco Earthquake a Century Later: Introduction to the Special Section in BSSA, v. 98, no. 2, p. 817-822, https://doi.org/10.1785/0120060401
  • Aargard, B.T. et al., 2008. Ground-Motion Modeling of the 1906 San Francisco Earthquake, Part II: Ground-Motion Estimates for the 1906 Earthquake and Scenario Events in BSSA, v. 98, no. 2, p. 1012-1046, https://doi.org/10.1785/0120060410
  • Aagaard, B.T., Blair, J.L., Boatwright, J., Garcia, S.H., Harris, R.A., Michael, A.J., Schwartz, D.P., and DiLeo, J.S., 2016, Earthquake outlook for the San Francisco Bay region 2014–2043 (ver. 1.1, August 2016): U.S. Geological Survey Fact Sheet 2016–3020, 6 p., http://dx.doi.org/10.3133/fs20163020.
  • Jessee, M.A.N., Hamburger, M. W., Allstadt, K., Wald, D. J., Robeson, S. M., Tanyas, H., et al. (2018). A global empirical model for near-real-time assessment of seismically induced landslides. Journal of Geophysical Research: Earth Surface, 123, 1835–1859. https://doi.org/10.1029/2017JF004494
  • Jiang, J. and Lapusta, N., 2016. Deeper penetration of large earthquakes on seismically quiescent faults in Science, v. 352, no. 6291, p. 1293-1297, DOI: 10.1126/science.aaf1496
  • Keefer, D.K., 1984. Landslides Caused by Earthquakes in GSA Bulletin, v. 95, p. 406-421
  • Keefer, D.K., 1998. The Loma Prieta, California, Earthquake of October 17, 1989: Strong Ground Motion and Ground Failure in Keefer, D.K., Manson, M.W., Griggs, G.B., Plant, Nathaniel, Schuster, R.L., Wieczorek, G.F., Hope, D.G., Harp, E.L., Nolan, J.M., Weber, G.E., Cole, W.F., Marcum, D.R., Shires, P.O., and Clark, B.R., Chapter C. The Loma Prieta, California, Earthquake of October 17, 1989 – Landslides, USGS Professional Paper 1551-C, https://doi.org/10.3133/pp1551C
  • Keefer, D.K. and Mason M.W., 1998. Regional Distribution and Characteristics of Landslides Generated by the Earthquake in Keefer, D.K., Manson, M.W., Griggs, G.B., Plant, Nathaniel, Schuster, R.L., Wieczorek, G.F., Hope, D.G., Harp, E.L., Nolan, J.M., Weber, G.E., Cole, W.F., Marcum, D.R., Shires, P.O., and Clark, B.R., Chapter C. The Loma Prieta, California, Earthquake of October 17, 1989 – Landslides, USGS Professional Paper 1551-C, https://doi.org/10.3133/pp1551C
  • McLaughlin, R.J. and Clark, J.C., 2003. Stratigraphy and Structure Across the San Andreas Fault Zone in the Loma Preita Region and Deformation During the Earthquake in Wells, R.E., ed., The Loma Prieta, California, Earthquake of October 17, 1989—Geologic Setting and Crustal Structure, USGS Professional Paper 11550-E, http://pubs.usgs.gov/pp/p1550e/
  • Stoffer, P.W., 2006, Where’s the San Andreas Fault? A guidebook to tracing the fault on public lands in the San Francisco Bay region: U.S. Geological Survey General Interest Publication 16, 123 p., online at http://pubs.usgs.gov/gip/2006/16/
  • USGS, 2004. Landslide Types and Processes, U.S. Geological Survey Fact Sheet 2004-3072
  • Wallace, Robert E., ed., 1990, The San Andreas fault system, California: U.S. Geological Survey Professional Paper 1515, 283 p. [http://pubs.usgs.gov/pp/1988/1434/].
  • Zhu, J., Baise, L. G., Thompson, E. M., 2017, An Updated Geospatial Liquefaction Model for Global Application, Bulletin of the Seismological Society of America, 107, p 1365-1385, doi: 0.1785/0120160198

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Posted in earthquake, education, geology, plate tectonics, San Andreas, San Francisco, strike-slip, Transform, Uncategorized

Earthquake Report: Chile

I am catching up on earthquake reports today as I was in the field the past couple of weeks…

Well, these reports are getting too long. So, I have placed the explanatory material on 2 web pages, so one does not need to read through that stuff if they have been here before. I will link those pages in all reports. You’re welcome. ;-)

This will also save me some time and make writing these reports simpler.

On 1 August 2019 there was an earthquake along the convergent plate boundary along the west coast of Chile (a subduction zone forming the Peru-Chile trench). This subduction zone megathrust fault produced the largest magnitude earthquake recorded on seismometers in 1960, the Valparaiso, Chile magnitude M9.5 earthquake that caused a trans-pacific tsunami causing damage and deaths all along the western hemispheric coastline.

This M 6.8 earthquake happened at the overlap of the southern end of the 1985 M8.0 and northern end of the 2010 M8.8 earthquakes. Does this portend that there will be another, larger, earthquake in this area soon? Only time will tell.

Below is my interpretive poster for this earthquake

  • I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1919-2019 with magnitudes M ≥ 3.0 in one version.
  • I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
  • A review of the basic base map variations and data that I use for the interpretive posters can be found on the Earthquake Reports page.
  • Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.

    I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

  • In the upper left corner is a map showing historic earthquakes along the Chile margin (Rhea et al., 2010). We may visualize the earthquake depths by checking out the color of the dots. To the below is a cross section, cutting into the Earth. Earthquakes that are along the profile D-D’ (in blue on the map) are included in this cross section. I also placed a blue line on the main map in the general location of this cross section. I placed a blue star in the general location of the M=6.8 earthquake (same for the other inset figures).
  • To the right is a map that shows a comparison between the USGS modeled intensity (using the MMI scale) with the USGS “Did You Feel It?” reports (results from real people). The model and the reported results are quite similar. See the MMI poster below for a more comprehensive comparison. In addition, I include depth contours of the subducting megathrust slab (Hayes et al., 2016; read more here).
  • In the center left bottom, I include a schematic cross section of the subduction zone. This shows where earthquakes may occur, generally. There are subduction zone megathrust earthquakes (the largest of magnitude), crustal earthquakes, slab earthquakes, and outer rise earthquakes.
  • In the lower right corner is a map that shows the relative seismic hazard for this plate boundary (Rhea et al., 2010). I plot both 2019 earthquakes.< The numbers (“80”) indicate the rate at which the Nazca Plate is subducting beneath South America. 80 mm/yr = 3 in/yr.
  • In the upper right corner is a composite figure from several figures from Metois et al., 2016. On the left is a panel that shows the latitudinal range of earthquake ruptures (I fixed it in places as the original figure did not extend the 2010 rupture sufficiently to the north). The panel on the right shows how much the subduction zone fault is “locked” (or, seismically coupled). Darker colors represent parts of the fault that are storing more energy over time and are possibly places where the fault will slip (compared to parts of the fault that are white or yellow, which may be places where the fault is currently slipping and would not generate earthquakes in the future). This is simply a model and there is not way to really know where an earthquake will happen until there is an earthquake.
  • Here is the map with a century’s seismicity plotted, for earthquakes associated with the larger earthquakes from this region (colored relative to time scale, 1960, 1985, 2010, 2015, 2019).

USGS Shaking Intensity

  • Here is a figure that shows a more detailed comparison between the modeled intensity and the reported intensity. Borth data use the same color scale, the Modified Mercalli Intensity Scale (MMI). More about this can be found here. The colors and contours on the map are results from the USGS modeled intensity. The DYFI data are plotted as colored dots (color = MMI, diameter = number of reports).
  • In the lower right corner is a plot showing MMI intensity (vertical axis) relative to distance from the earthquake (horizontal axis). The models are represented by the green and orange lines. The DYFI data are plotted as light blue dots. The mean and median (different types of “average”) are plotted as orand and purple dots. Note how well the reports fit the green line (the model that represents how MMI works based on quakes in California). I plot Santiago relative to distance from the earthquake with a blue arrow (compare with the poster).

USGS Historic Seismicity

  • Here is a poster that shows the significant earthquakes along this plate boundary. Note how there are earthquakes in the Nazca plate associated with the 2010 and 2015 megathrust subduction zone earthquakes. These are triggered earthquakes along the outer rise, not additional subduction zone earthquakes.
  • In the lower right corner is a figure from Beck (1998) that shows the spatial extent of the known earthquakes. I added the extent of the 2015 and 2010 earthquakes as green arrows.
  • In the upper right corner is an excellent figure from Horton (2018) that shows the plate tectonic setting for this area.

Other Report Pages

Some Relevant Discussion and Figures

  • Here is the overview figure from Horton, 2018.

  • Maps of (A) tectonic framework, (B) topography, and (C) sedimentary basin configuration of South America. (A) Map of plate boundaries, Andean magmatic arc (including the northern, central, and southern volcanic zones), regions of flat slab subduction, modern stress orientations from earthquake focal mechanisms, eastern front of Andean fold-thrust belt, and key segments of the retroarc foreland basin system. Plate velocities are shown relative to stable South American plate (DeMets et al., 2010). (B) DEM topographic map showing the Andes mountains and adjacent foreland region, including the Amazon, Parana, Orinoco, and Magdalena (Mag) river systems. (C) Map of Andean retroarc basins, showing isopach thicknesses (in km) of Cretaceous-Cenozoic basin fill, forebulge axis (from Chase et al., 2009), and locations of 13 sites (8 foreland basins, 5 hinterland basins) considered in this synthesis

  • Here is the seismic hazard map is from Rhea et al. (2010).

  • Here is the seismicity map and space time diagram from Métois et al. (2016). The subduction zone fault in the region of Coquimbo, Chile changes geometry, probably because of the Juan Fernandez Ridge (this structure controls the shape of the subduction zone). This figure shows a map and cross section for two parts of the subduction zone (Marot et al., 2014). Note how the subduction zone flattens out with depth here. The M=6.7 quake was shallower than this, but the shape of the downgoing slab does affect the amount of slab pull (tension in the down-dip direction) is exerted along the plate.

  • Left estimated extent of large historical or instrumental ruptures along the Chilean margin adapted from ME´ TOIS et al. (2012). Gray stars mark major intra-slab events. The recent Mw[8 earthquakes are indicated in red. Gray shaded areas correspond to LCZs defined in Fig. 3. Right seismicity recorded by the Centro Sismologico Nacional (CSN) during
    interseismic period, color-coded depending on the event’s depth. Three zones have been defined to avoid including aftershocks and preshocks associated with major events: (1) in North Chile, we plot the seismicity from 2008 to january 2014, i.e., between the Tocopilla and Iquique earthquakes; (2) in Central Chile, we plot the seismicity on the entire 2000–2014 period; (3) in South-Central Chile, we selected events that occurred between 2000 and 2010, i.e., before the Maule earthquake.

  • This figure is the 3 panel figure in the interpretive poster showing how seismicity is distributed along the margin, how historic earthquake slip was distributed, and how the fault may be locked (or slipping) along the megathrust fault.

  • a Histogram depicts the rate of Mw>3 earthquakes registered by the CSN catalog during the interseismic period defined for each zone (see Fig. 2) on the subduction interface, on 0.2° of latitude sliding windows. Stars are swarm-like sequences detected by HOLTKAMP et al. (2011) depending on their occurrence date. Swarms located in the Iquique LCZ and Camarones segment are from RUIZ et al. (2014). Empty squares are significant intraplate earthquakes. b Red curve variations of the average coupling coefficient on the first 60 km of depth calculated on 0.2° of latitude sliding windows for our best model including an Andean sliver motion. Dashed pink curves are alternative models with different smoothing options that fit the data with nRMS better than 2 (see supplementary figure 6): the pink shaded envelope around our best model stands for the variability of the coupling along strike. Green curves coseismic distribution for Maule (VIGNY et al. 2011), Iquique (LAY et al. 2014) and Illapel earthquakes (RUIZ et al. 2016). Gray shaded areas stand for the identified low coupling zones (LCZs). LCZs and high coupling segments are named on the left. The apparent decrease in the average coupling North of 30°S is considered as an artifact of the Andean sliver motion (see Sect. 5.2). c Best coupling distribution obtained inverting for Andean sliver motion and coupling amount simultaneously. The rupture zones for the three major earthquakes are indicated as green ellipses. White shaded areas are zones where we lack resolution.

  • This is a figure that shows details about the coupling compared to some slip models for the 2010, 2014, and 2015 earthquakes.

  • Left coupling maps (color coded) versus coseismic slip distributions (gray shaded contours in cm) for the last three major Chilean earthquakes (epicenters are marked by white stars). From top to bottom Iquique area, white squares are pre-seismic swarm event in the month before the main shock, green star is the 2005, Tarapaca´ intraslab earthquake epicenter, blue star is the Mw 6.7 Iquique aftershock; Illapel area, green squares show the seismicity associated with the 1997 swarm following the Punitaqui intraslab earthquake (green star); Maule area, green star is the epicenter of the 1939 Chillan intraslab earthquake. Right interseismic background seismicity in the shallow part of the subduction zone (shallower than 60 km depth) for each region (red dots) together with 80 and 90 % coupling contours. White dots are events identified as mainshock after a declustering procedure following GARDNER and KNOPOFF (1974). Yellow areas extent of swarm sequences identified by HOLTKAMP et al. (2011) for South and Central Chile, and RUIZ et al. (2014) for North Chile.

  • This is the fault locking figure from Saillard et al. (2017), showing the percent coupling (how much of the plate convergence contributes to deformation of the plate boundary, which may tell us places on the fault that might slip during an earthquake. We are still learning about why this is important and what it means.

  • Comparison between the uplift rates, interseismic coupling, major bathymetric features, and peninsulas along the Andean margin (10°S–40°S). (a) Uplift rates of marine terraces reported in the literature (we present the average rate since terrace abandonment; Table S1 in the supporting information [Jara-Muñoz et al., 2015]). Each color corresponds to a marine terrace assigned to a marine isotopic stage (MIS). Gray dots are the uplift rates of the central Andean rasa estimated from a numerical model of landscape evolution [Melnick, 2016]. (b) Major bathymetric features and peninsulas and pattern of interseismic coupling of the Andean margin from GPS data inversion (this study). Gray shaded areas correspond to the areas where the spatial resolution of inversion is low due to the poor density of GPS observations (see text and supporting information for more details). The Peru-Chile trench (thick black line), the coastline (thin black line), and the convergence direction (black arrows) are indicated. We superimposed the curve obtained by shifting the trench geometry eastward by 110 km (trench-coast distance of 110 km; blue line) with the curve reflecting the 40 km isodepth of the subducting slab (red line; Slab1.0 from Hayes and Wald [2009]), a depth which corresponds approximately with the downdip end of the locked portion of the Andean seismogenic zone (±10 km) [Ruff and Tichelaar, 1996; Khazaradze and Klotz, 2003; Chlieh et al., 2011; Ruegg et al., 2009; Moreno et al., 2011; Métois et al., 2012]. The two curves are spatially similar in the erosive part of the Chile margin (north of 34°S), whereas they diverge along the shallower slab geometry in the accretionary part of the Chile margin (south of 34°S), where the downdip end of the locked zone may be shallower (Figure 4b). Red arrows indicate the low interseismic coupling associated with peninsulas and marine terraces and evidence of aseismic afterslip (after Perfettini et al. [2010] below the Pisco-Nazca Peninsula; Pritchard and Simons [2006], Victor et al. [2011], Shirzaei et al. [2012], Bejar-Pizarro et al. [2013], and Métois et al. [2013] for the Mejillones Peninsula; Métois et al. [2012, 2014] below the Tongoy Peninsula; and Métois et al. [2012] and Lin et al. [2013] for the Arauco Peninsula). FZ: Fracture zone. Horizontal blue bands are the areas where coastline is less than 110 km (light blue) or 90 km (dark blue) from the trench (see Figure 1).

  • The following figures from Leyton et al. (2009) are great analogies, showing examples of interplate earthquakes (e.g. subduction zone megathrust events) and intraplate earthquakes (e.g. slab quakes, or events within the downgoing plate). The first figures are maps showing these earthquakes, then there are some seismicity cross sections.

  • Maps showing the location of the study and the events used ((a)–(c)). In red we present interplate earthquakes, while in blue, the intermediate depth, intraplate ones. We used beach balls to plot those events with known focal and circles for those without. White triangles mark the position of the Chilean Seismological Network used to locate the events; those with names represent stations used in the waveform analysis (either accelerometers or broadbands with known instrumental response). Labels over beach balls correspond to CMT codes.

  • Here are 2 cross sections showing the earthquakes plotted in the maps above (Leyton et al., 2009).

  • Cross-section at (a) 33.5◦S and (b) 36.5◦S showing the events used in this study. In red we present interplate earthquakes, while in blue, the intermediate depth, intraplate ones.We used beach balls (vertical projection) to plot those events with knownfocal and circles for those without. In light gray is shown the background seismicity recorded from 2000 to 2006 by the Chilean Seismological Service

  • Here is the cross section figure I prepared for the interpretive poster above.

    Social Media

    References:

    Basic & General References

  • Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
  • Hayes, G., 2018, Slab2 – A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release, https://doi.org/10.5066/F7PV6JNV.
  • Holt, W. E., C. Kreemer, A. J. Haines, L. Estey, C. Meertens, G. Blewitt, and D. Lavallee (2005), Project helps constrain continental dynamics and seismic hazards, Eos Trans. AGU, 86(41), 383–387, , https://doi.org/10.1029/2005EO410002. /li>
  • Kreemer, C., J. Haines, W. Holt, G. Blewitt, and D. Lavallee (2000), On the determination of a global strain rate model, Geophys. J. Int., 52(10), 765–770.
  • Kreemer, C., W. E. Holt, and A. J. Haines (2003), An integrated global model of present-day plate motions and plate boundary deformation, Geophys. J. Int., 154(1), 8–34, , https://doi.org/10.1046/j.1365-246X.2003.01917.x.
  • Kreemer, C., G. Blewitt, E.C. Klein, 2014. A geodetic plate motion and Global Strain Rate Model in Geochemistry, Geophysics, Geosystems, v. 15, p. 3849-3889, https://doi.org/10.1002/2014GC005407.
  • Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. https://doi.org/10.7289/V5H70CVX
  • Müller, R.D., Sdrolias, M., Gaina, C. and Roest, W.R., 2008, Age spreading rates and spreading asymmetry of the world’s ocean crust in Geochemistry, Geophysics, Geosystems, 9, Q04006, https://doi.org/10.1029/2007GC001743
  • Specific References

  • Beck, S., Barrientos, S., Kausel, E., and Reyes, M., 1998. Source Characteristics of Historic Earthquakes along the Central Chile Subduction Zone in Journal of South American Earth Sciences, v. 11, no. 2, p. 115-129, https://doi.org/10.1016/S0895-9811(98)00005-4
  • Gardi, A., A. Lemoine, R. Madariaga, and J. Campos (2006), Modeling of stress transfer in the Coquimbo region of central Chile, J. Geophys. Res., 111, B04307, https://doi.org/10.1029/2004JB003440
  • Horton, B.K., 2018. Sedimentary record of Andean mountain building< in Earth-Science Reviews, v. 178, p. 279-309, https://doi.org/10.1016/j.earscirev.2017.11.025
  • Leyton, F., Ruiz, J., Campos, J., and Kausel, E., 2009. Intraplate and interplate earthquakes in Chilean subduction zone:
    A theoretical and observational comparison in Physics of the Earth and Planetary Interiors, v. 175, p. 37-46, https://doi.org/10.1016/j.pepi.2008.03.017
  • Marot, M., Monfret, T., Gerbault, M.,. Nolet, G., Ranalli, G., and Pardo, M., 2014. Flat versus normal subduction zones: a comparison based on 3-D regional traveltime tomography and petrological modelling of central Chile and western Argentina (29◦–35◦S) in GJI, v. 199, p. 1633-164, https://doi.org/10.1093/gji/ggu355
  • Métois, M., Vigny, C., and Socquet, A., 2016. Interseismic Coupling, Megathrust Earthquakes and Seismic Swarms Along the Chilean Subduction Zone (38°–18°S) in Pure Applied Geophysics, https://doi.org/10.1007/s00024-016-1280-5
  • Rhea, S., Hayes, G., Villaseñor, A., Furlong, K.P., Tarr, A.C., and Benz, H.M., 2010. Seismicity of the earth 1900–2007, Nazca Plate and South America: U.S. Geological Survey Open-File Report 2010–1083-E, 1 sheet, scale 1:12,000,000.
  • Ruiz, S. and Madariaga, R., 2018. Historical and recent large megathrust earthquakes in Chile in Tectonophysics, v. 733, p. 37-56, https://doi.org/10.1016/j.tecto.2018.01.015
  • Saillard, M., L. Audin, B. Rousset, J.-P. Avouac, M. Chlieh, S. R. Hall, L. Husson, and D. L. Farber, 2017. From the seismic cycle to long-term deformation: linking seismic coupling and Quaternary coastal geomorphology along the Andean megathrust in Tectonics, 36, https://doi:10.1002/2016TC004156.

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Earthquake Report: Blanco fault

Well, these reports are getting too long. So, I have placed the explanatory material on 2 web pages, so one does not need to read through that stuff if they have been here before. I will link those pages in all reports. You’re welcome. ;-)

This will also save me some time and make writing these reports simpler.

The tectonics of the northeast Pacific is dominated by the Cascadia subduction zone, a convergent plate boundary, where the Explorer, Juan de Fuca, and Gorda oceanic plates dive eastward beneath the North America plate.

These oceanic plates are created (formed, though I love writing “created” in science writing) at oceanic spreading ridges/centers.

When oceanic spreading centers are offset laterally, a strike-slip fault forms called a transform fault. The Blanco transform fault is a right-lateral strike-slip fault (like the San Andreas fault). Thanks to Dr. Harold Tobin for pointing out why this is not a fracture zone.

This plate boundary fault system (BF) is quite active with ten magnitude M ≥ 6.0 earthquakes in the past 50 years (one every 5 years) and about 150 M ≥ 5 earthquakes in the same time range.

When there are quakes on the BF, people always wonder if the Cascadia megathrust is affected by this… “are we at greater risk because of those BF earthquakes?”

The main take away is that we are not at a greater risk because of these earthquakes. More on this below the interpretive poster.

Below is my interpretive poster for this earthquake

  • I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1919-2019 with magnitudes M ≥ 3.0 in one version.
  • I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
  • A review of the basic base map variations and data that I use for the interpretive posters can be found on the Earthquake Reports page.
  • Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.

    I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

  • In the upper left corner is a map of the Cascadia subduction zone (CSZ) and regional tectonic plate boundary faults. This is modified from several sources (Chaytor et al., 2004; Nelson et al., 2004). I placed a blue stars in the general location of today’s earthquake (as in other inset figures in this poster). As for all insets in this poster, I place a cyan star in the general location of this M 6.3 earthquake.
  • In the lower left corner is an illustration modified from Plafker (1972). This figure shows how a subduction zone deforms between (interseismic) and during (coseismic) earthquakes. Today’s earthquake did not occur along the CSZ, so did not produce crustal deformation like this. However, it is useful to know this when studying the CSZ. Today’s earthquakes happened in the lower Gorda plate
  • In the upper right corner is a map that shows 21st century earthquakes along the Blanco transform fault system.
  • In the lower right corner is a map from Dziak et al. (2000) that shows the topography (in the upper panel) and the faulting (in the lower panel) along the BFZ. I outline the location of this figure in the main part of the poster. Blue = lower elevation, deeper oceanic depths; Red = shallower oceanic depth, higher elevation. I placed orange arrows to help one locate the normal faults (perpendicular to the strike-slip faults) in this map. Compare this inset map with the Google Earth bathymetry in the main map. Can you see the BFZ perpendicular ridges?
  • Here is the map with a century’s seismicity plotted, for earthquakes of magnitude M ≥ 6.0.

Stress Triggering

When earthquake faults slip, the surrounding crust and faults change shape and this causes areas of the faults to get imparted increased or decreased amounts of stress. If these faults are almost ready to slip and the change of stress is increased sufficiently, those source earthquakes may trigger earthquakes on the receiver fault (the one with increased stress). This is termed “static coulomb stress triggering.”

Typically the maximum distance from an earthquake that these stress changes can trigger an earthquake is about twice the length of the source earthquake.

If we use data from historic earthquakes to correlate earthquake fault slip length to magnitude, we can estimate the length of the BF that slipped during the M 6.3 temblor (Wells and Coppersmith, 1994).

Below is a figure from Wells and Coppersmith (1994) that shows the empirical relations between surface rupture length (SRL, the length of the fault that ruptures to the ground surface) and magnitude. If one knows the SRL (horizontal axis), they can estimate the magnitude (vertical axis). The left plot shows the earthquake data. The right plot shows how their formulas “predict” these data.


* note, i corrected this caption by changing the word “relationships” to “relations.”

(a) Regression of surface rupture length on magnitude (M). Regression line shown for all-slip-type relations. Short dashed line indicates 95% confidence interval. (b) Regression lines for strike-slip, reverse, and normal-slip relations. See Table 2 for regression coefficients. Length of regression lines shows the range of data for each relation.

We don’t really know what the SRL for the M 6.3, but using these empirical relations, the length of the M 6.3 fault is probably between 11-14 km. So, the distance that the M 6.3 could probably trigger another quake is limited to 30 km or so. The westward tip of North America is about 230 km from the M 6.3 epicenter, with the locked zone (the part of the megathrust that might slip during an earthquake) is tens of km even further away (maybe more than 300 km).

To give us an idea about this stress triggering stuff, below is a figure from Rollins and Stein (2010). This figure shows the results from their model. This model shows the change in stress imparted upon the megathrust from a strike-slip fault in the Gorda plate (a 1980 M 7.3 earthquake, which was very close to the megathrust).

The red areas show areas of increased stress, blue areas show decreased stress. This is based on a left-lateral strike-slip fault (so a right-lateral quake would produce changes in stress the opposite as this, red regions would be blue and blue regions would be red, generally).

The M 7.3 SRL may have been between 86-104 km. Compare this with the 12-14 km SRL for a M 6.3. The changes in coulomb stress for the M 6.3 is much much less than for the > 7.3.


Coulomb stress changes imparted by the 1980 Mw = 7.3 earthquake (B) to a matrix of faults representing the Mendocino Fault Zone, the Cascadia subduction zone, and NE striking left‐lateral faults in the Gorda zone. (con’t)

So, now you may have more insight about whether or not a BF earthquake could affect the CSZ megathrust. (If a M 7.8 BF earthquake happened, it would be at the outer limits of beginning to influence the megathrust, but this affect would be quite small)

2018.08.22 M 6.2 Blanco transform fault

About a year ago, there was a magnitude M 6.2 temblor on the same plate boundary fault system. Here is the earthquake report for that M 6.2 event. Below I include the 2 posters from that Earthquake Report.

  • I include two main interpretive posters for this earthquake. One includes information from this earthquake, including the MMI contours and USGS “Did You Feel It?” colored polygons. This way we can compare the modeled estimate of intensity (MMI contours) and the reports from real people (DYFI data). There are some good matches and some mismatches (in western Oregon). Check this out and try to think about why there may be mismatches.

  • The second poster includes earthquake information for earthquakes with M ≥ 6.0. I place fault mechanisms for all existing USGS mechanisms from the Blanco fracture zone and I include some examples from the rest of the region. These other mechanisms show how different areas have different tectonic regimes. Earthquakes within the Gorda plate are largely responding to being deformed in a tectonic die between the surrounding stronger plates (northeast striking (oriented) left-lateral strike-slip earthquakes). I include one earthquake along the Mendocino fracture zone, a right-lateral (dextral) strike-slip earthquake from 1994. I include one of the more memorable thrust earthquakes, the 1992 Cape Mendocino earthquake. I also include an extensional earthquake from central Oregon that may represent extension (basin and range?) in the northwestern region of the basin and range.

Other Report Pages

Some Relevant Discussion and Figures

Cascadia subduction zone

  • Here is a map of the Cascadia subduction zone, modified from Nelson et al. (2006). The Juan de Fuca and Gorda plates subduct norteastwardly beneath the North America plate at rates ranging from 29- to 45-mm/yr. Sites where evidence of past earthquakes (paleoseismology) are denoted by white dots. Where there is also evidence for past CSZ tsunami, there are black dots. These paleoseismology sites are labeled (e.g. Humboldt Bay). Some submarine paleoseismology core sites are also shown as grey dots. The two main spreading ridges are not labeled, but the northern one is the Juan de Fuca ridge (where oceanic crust is formed for the Juan de Fuca plate) and the southern one is the Gorda rise (where the oceanic crust is formed for the Gorda plate).

  • Here is a version of the CSZ cross section alone (Plafker, 1972). This shows two parts of the earthquake cycle: the interseismic part (between earthquakes) and the coseismic part (during earthquakes). Regions that experience uplift during the interseismic period tend to experience subsidence during the coseismic period.

  • I was inspired today to prepare a new plate tectonic setting map for the Cascadia subduction zone. More about the materials on this poster can be found on this page.
  • This poster includes seismicity from the past 5 decades, for temblors M > 3.0. I also include the map and cross section as explained above. On the left is a map that shows the possible shaking intensity from a future CSZ earthquake.

Blanco transform fault

  • This is the figure from Dziak et al. (2000) for us to evaluate. I include their long figure caption below.

  • (Top) Sea Beam bathymetric map of the Cascadia Depression, Blanco Ridge, and Gorda Depression, eastern Blanco Transform Fault Zone (BTFZ).Multibeam bathymetry was collected by the NOAA R/V’s Surveyor and Discoverer and the R/V Laney Chouest during 12 cruises in the 1980’s and 90’s. Bathymetry displayed using a 500 m grid interval. Numbers with arrows show look directions of three-dimensional diagrams in Figures 2 and 3. (Bottom) Structure map, interpreted from bathymetry, showing active faults and major geologic features of the region. Solid lines represent faults, dashed lines are fracture zones, and dotted lines show course of turbidite channels. When possible to estimate sense of motion on a fault, a filled circle shows the down-thrown side. Inset maps show location and generalized geologic structure of the BTFZ. Location of seismic reflection and gravity/magnetics profiles indicated by opposing brackets. D-D’ and E-E’ are the seismic reflection profiles shown in Figures 8a and 8b, and G-G’ is the gravity and magnetics profile shown in Figure 13. Submersible dive tracklines from sites 1 through 4 are highlighted in red. L1 and L2 are two lineations seen in three-dimensional bathymetry shown in Figures 2 and 3. Location of two Blanco Ridge slump scars indicated by half-rectangles, inferred direction of slump shown by arrow, and debris location (when identified) designated by an ‘S’. CD stands for Cascadia Depression, BR is Blanco Ridge, GD is Gorda Depression, and GR is Gorda Ridge. Numbers on north and south side of transform represent Juan de Fuca and Pacific plate crustal ages inferred from magnetic anomalies. Long-term plate motion rate between the Pacific and southern Juan de Fuca plates from Wilson (1989).

BF Historic Seismicity

  • There were two Mw 4.2 earthquakes associated with this plate boundary fault system in mid 2015. I plot the moment tensors for these earthquakes (USGS pages: 4/7/15 and 4/11/15) in this map below. I also have placed the relative plate motions as arrows, labeled the plates, and placed a transparent focal mechanism plot above the BFZ showing the general sense of motion across this plate boundary. There have been several earthquakes along the Mendocino fault recently and I write about them 1/2015 here and 4/2015 here.

  • There was also seismic activity along the BFZ later in 2015. Here are my report and report update.
  • Here is a map showing these earthquakes, with moment tensors plotted for the M 5.8 and M 5.5 earthquakes. I include an inset map showing the plate configuration based upon the Nelson et al. (2004) and Chaytor et al. (2004) papers (I modified it). I also include a cross section of the subduction zone, as it is configured in-between earthquakes (interseismic) and during earthquakes (coseismic), modified from Plafker (1972).

  • I put together an animation that includes the seismicity from 1/1/2000 until 6/1/2015 for the region near the Blanco fracture zone, with earthquake magnitudes greater than or equal to M = 5.0. The map here shows all these epicenters, with the moment tensors for earthquakes of M = 6 or more (plus the two largest earthquakes from today’s swarm). Here is the page that I posted regarding the beginning of this swarm. Here is a post from some earthquakes earlier this year along the BFZ.
  • Earthquake epicenters are plotted with the depth designated by color and the magnitude depicted by the size of the circle. These are all fairly shallow earthquakes at depths suitable for oceanic lithosphere.

    Here is the list of the earthquakes with moment tensors plotted in the above maps (with links to the USGS websites for those earthquakes):

  • 2000/06/02 M 6.0
  • 2003/01/16 M 6.3
  • 2008/01/10 M 6.3
  • 2012/04/12 M 6.0
  • 2015/06/01 M 5.8
  • 2015/06/01 M 5.9
    Here are some files that are outputs from that USGS search above.

  • csv file
  • kml file (not animated)
  • kml file (animated)

VIDEOS

    Here are links to the video files (it might be easier to download them and view them remotely as the files are large).

  • First Animation (20 mb mp4 file)
  • Second Animation (10 mb mp4 file)

Here is the first animation that first adds the epicenters through time (beginning with the oldest earthquakes), then removes them through time (beginning with the oldest earthquakes).

Here is the second animation that uses a one-year moving window. This way, one year after an earthquake is plotted, it is removed from the plot. This animation is good to see the spatiotemporal variation of seismicity along the BFZ.

Here is a map with all the fore- and after-shocks plotted to date.

Gorda Plate Seismicity

  • Here is a map from Chaytor et al. (2004) that shows some details of the faulting in the region. The moment tensor (at the moment i write this) shows a north-south striking fault with a reverse or thrust faulting mechanism. While this region of faulting is dominated by strike slip faults (and most all prior earthquake moment tensors showed strike slip earthquakes), when strike slip faults bend, they can create compression (transpression) and extension (transtension). This transpressive or transtentional deformation may produce thrust/reverse earthquakes or normal fault earthquakes, respectively. The transverse ranges north of Los Angeles are an example of uplift/transpression due to the bend in the San Andreas fault in that region.

  • A: Mapped faults and fault-related ridges within Gorda plate based on basement structure and surface morphology, overlain on bathymetric contours (gray lines—250 m interval). Approximate boundaries of three structural segments are also shown. Black arrows indicated approximate location of possible northwest- trending large-scale folds. B, C: uninterpreted and interpreted enlargements of center of plate showing location of interpreted second-generation strike-slip faults and features that they appear to offset. OSC—overlapping spreading center.

  • These are the models for tectonic deformation within the Gorda plate as presented by Jason Chaytor in 2004.
  • Mw = 5 Trinidad Chaytor

    Models of brittle deformation for Gorda plate overlain on magnetic anomalies modified from Raff and Mason (1961). Models A–F were proposed prior to collection and analysis of full-plate multibeam data. Deformation model of Gulick et al. (2001) is included in model A. Model G represents modification of Stoddard’s (1987) flexural-slip model proposed in this paper.

  • Here is a map from Rollins and Stein, showing their interpretations of different historic earthquakes in the region. This was published in response to the Januray 2010 Gorda plate earthquake. The faults are from Chaytor et al. (2004).

  • Tectonic configuration of the Gorda deformation zone and locations and source models for 1976–2010 M ≥ 5.9 earthquakes. Letters designate chronological order of earthquakes (Table 1 and Appendix A). Plate motion vectors relative to the Pacific Plate (gray arrows in main diagram) are from Wilson [1989], with Cande and Kent’s [1995] timescale correction.

  • In this map below, I label a number of other significant earthquakes in this Mendocino triple junction region. Another historic right-lateral earthquake on the Mendocino fault system was in 1994. There was a series of earthquakes possibly along the easternmost section of the Mendocino fault system in late January 2015, here is my post about that earthquake series.

The Gorda and Juan de Fuca plates subduct beneath the North America plate to form the Cascadia subduction zone fault system. In 1992 there was a swarm of earthquakes with the magnitude Mw 7.2 Mainshock on 4/25. Initially this earthquake was interpreted to have been on the Cascadia subduction zone (CSZ). The moment tensor shows a compressional mechanism. However the two largest aftershocks on 4/26/1992 (Mw 6.5 and Mw 6.7), had strike-slip moment tensors. These two aftershocks align on what may be the eastern extension of the Mendocino fault.

There have been several series of intra-plate earthquakes in the Gorda plate. Two main shocks that I plot of this type of earthquake are the 1980 (Mw 7.2) and 2005 (Mw 7.2) earthquakes. I place orange lines approximately where the faults are that ruptured in 1980 and 2005. These are also plotted in the Rollins and Stein (2010) figure above. The Gorda plate is being deformed due to compression between the Pacific plate to the south and the Juan de Fuca plate to the north. Due to this north-south compression, the plate is deforming internally so that normal faults that formed at the spreading center (the Gorda Rise) are reactivated as left-lateral strike-slip faults. In 2014, there was another swarm of left-lateral earthquakes in the Gorda plate. I posted some material about the Gorda plate setting on this page.


    References:

    Basic & General References

  • Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
  • Hayes, G., 2018, Slab2 – A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release, https://doi.org/10.5066/F7PV6JNV.
  • Holt, W. E., C. Kreemer, A. J. Haines, L. Estey, C. Meertens, G. Blewitt, and D. Lavallee (2005), Project helps constrain continental dynamics and seismic hazards, Eos Trans. AGU, 86(41), 383–387, , https://doi.org/10.1029/2005EO410002. /li>
  • Kreemer, C., J. Haines, W. Holt, G. Blewitt, and D. Lavallee (2000), On the determination of a global strain rate model, Geophys. J. Int., 52(10), 765–770.
  • Kreemer, C., W. E. Holt, and A. J. Haines (2003), An integrated global model of present-day plate motions and plate boundary deformation, Geophys. J. Int., 154(1), 8–34, , https://doi.org/10.1046/j.1365-246X.2003.01917.x.
  • Kreemer, C., G. Blewitt, E.C. Klein, 2014. A geodetic plate motion and Global Strain Rate Model in Geochemistry, Geophysics, Geosystems, v. 15, p. 3849-3889, https://doi.org/10.1002/2014GC005407.
  • Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. https://doi.org/10.7289/V5H70CVX
  • Müller, R.D., Sdrolias, M., Gaina, C. and Roest, W.R., 2008, Age spreading rates and spreading asymmetry of the world’s ocean crust in Geochemistry, Geophysics, Geosystems, 9, Q04006, https://doi.org/10.1029/2007GC001743
  • Specific References

  • Atwater, B.F., Musumi-Rokkaku, S., Satake, K., Tsuju, Y., Eueda, K., and Yamaguchi, D.K., 2005. The Orphan Tsunami of 1700—Japanese Clues to a Parent Earthquake in North America, USGS Professional Paper 1707, USGS, Reston, VA, 144 pp.
  • Chaytor, J.D., Goldfinger, C., Dziak, R.P., and Fox, C.G., 2004. Active deformation of the Gorda plate: Constraining deformation models with new geophysical data: Geology v. 32, p. 353-356.
  • Dengler, L.A., Moley, K.M., McPherson, R.C., Pasyanos, M., Dewey, J.W., and Murray, M., 1995. The September 1, 1994 Mendocino Fault Earthquake, California Geology, Marc/April 1995, p. 43-53.
  • Dziak, R.P., Fox, C.G., Embleey, R.W., Nabelek, J.L., Braunmiller, J., and Koski, R.A., 2000. Recent tectonics of the Blanco Ridge, eastern blanco transform fault zone in Marine Geophysical Researches, vol. 21, p. 423-450
  • Geist, E.L. and Andrews D.J., 2000. Slip rates on San Francisco Bay area faults from anelastic deformation of the continental lithosphere, Journal of Geophysical Research, v. 105, no. B11, p. 25,543-25,552.
  • Irwin, W.P., 1990. Quaternary deformation, in Wallace, R.E. (ed.), 1990, The San Andreas Fault system, California: U.S. Geological Survey Professional Paper 1515, online at: http://pubs.usgs.gov/pp/1990/1515/
  • Lin, J., R. S. Stein, M. Meghraoui, S. Toda, A. Ayadi, C. Dorbath, and S. Belabbes (2011), Stress transfer among en echelon and opposing thrusts and tear faults: Triggering caused by the 2003 Mw = 6.9 Zemmouri, Algeria, earthquake, J. Geophys. Res., 116, B03305, doi:10.1029/2010JB007654.
  • McCrory, P.A.,. Blair, J.L., Waldhauser, F., kand Oppenheimer, D.H., 2012. Juan de Fuca slab geometry and its relation to Wadati-Benioff zone seismicity in JGR, v. 117, B09306, doi:10.1029/2012JB009407.
  • McLaughlin, R.J., Sarna-Wojcicki, A.M., Wagner, D.L., Fleck, R.J., Langenheim, V.E., Jachens, R.C., Clahan, K., and Allen, J.R., 2012. Evolution of the Rodgers Creek–Maacama right-lateral fault system and associated basins east of the northward-migrating Mendocino Triple Junction, northern California in Geosphere, v. 8, no. 2., p. 342-373.
  • Nelson, A.R., Asquith, A.C., and Grant, W.C., 2004. Great Earthquakes and Tsunamis of the Past 2000 Years at the Salmon River Estuary, Central Oregon Coast, USA: Bulletin of the Seismological Society of America, Vol. 94, No. 4, pp. 1276–1292
  • Rollins, J.C. and Stein, R.S., 2010. Coulomb stress interactions among M ≥ 5.9 earthquakes in the Gorda deformation zone and on the Mendocino Fault Zone, Cascadia subduction zone, and northern San Andreas Fault: Journal of Geophysical Research, v. 115, B12306, doi:10.1029/2009JB007117, 2010.
  • Stoffer, P.W., 2006, Where’s the San Andreas Fault? A guidebook to tracing the fault on public lands in the San Francisco Bay region: U.S. Geological Survey General Interest Publication 16, 123 p., online at http://pubs.usgs.gov/gip/2006/16/
  • Yue, H., Zhang, Z., Chen, Y.J., 2008. Interaction between adjacent left-lateral strike-slip faults and thrust faults: the 1976 Songpan earthquake sequence in Chinese Science Bulletin, v. 53, no. 16, p. 2520-2526
  • Wallace, Robert E., ed., 1990, The San Andreas fault system, California: U.S. Geological Survey Professional Paper 1515, 283 p. [http://pubs.usgs.gov/pp/1988/1434/].

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Earthquake Report: Sunda Strait, Indonesia

Around the beginning of the month, I was helping get a fundraiser prepared for a weekend concert series (audio crew for load in and strike; stage manager during the show). So, I was away from the computers when there was a magnitude M6.9 earthquake offshore of Sumatra and Java, Indonesia.

There was also an interesting earthquake in Chile, but I can’t do it all. (If I get a chance, I will write that one up too.)

The tectonics are both simple and complicated in this part of the world. The islands of Sumatra and Java (and more) are rows of volcanoes (called an island arc) formed by the partial melt of mantle material associated with the subduction of the oceanic India-Australia plate beneath the Sunda plate (part of Eurasia).

The downgoing plate has lots of water embedded in the rocks and sediments, when this plate is subducted, those fluids make their way into the overlying mantle. This changes the conditions so that the mantle partially melts, which results in the material being less dense, so it rises and erupts as volcanoes.

We can see some of these volcanoes in the interpretive poster below (look at the eastern part of the Island of Java).

So, the subduction zone is the main player on the scene. But the orientation (strike and changes in strike) of the subduction zone megathrust fault, in comparison to the relative motion between these plates, and in comparison to pre-existing structures in the India-Australia plate, leads to a number of additional faults.

The major fault system that accommodates the different relative plate motions is the Great Sumatra fault. The relative plate motion is oblique (not perpendicular to) the orientation of the subduction zone fault. Therefore, while the megathrust accommodates the fault perpendicular motion, the Sumatra accommodates the fault parallel motion (as a strike slip-fault). There are other strike slip faults too. These faults are called “forearc sliver faults.”

Some of the historic faults in the interpretive poster below are subduction zone earthquakes. The 2007 M 8.4 quake is a great example of this.

There are a couple good examples of “outer rise” earthquakes, temblors that occur in the downgoing plate, where there is flexure of the plate, causing the plate to bend and cause earthquakes along these bends. These are extensional earthquakes (the 2011 & 2013 quakes near Christmas Island).

There are two quakes that appear related to the Sumatra fault (19994 and 1995 quakes).

The 2 Aug 2019 M 6.9 quake is interesting because it does not appear to be a megathrust quake, an outer rise quake, or a Sumatra fault quake. The M 6.9 is (1) too deep for those types of quakes and (2) has an orientation that is not consistent with those types of quakes. This quake is in the India-Australia plate and could be along a reactivated fracture zone. The inset maps shows several of these north-south trending fracture zones (e.g. the Investigator fracture zone).

Thus, I interpret this as a north-south oriented left-lateral strike-slip earthquake. It is pretty deep, and could also be related to some other processes going on within the slab or uppermost mantle. The slab depth at this location is 20 km, so the quake is possibly about 35 km beneath the top of the India Australia plate. Oceanic crust is, on average, 7km. So, this M 6.9 is probably within the mantle beneath the slab.

There is an analogous M 7.0 earthquake on 2009.09.02 to the east, just south of the label “Java” on the interpretive poster. This earthquake shows trench parallel compression (perpendicular to the compression from the subsudction zone). This quake is almost 40 km deep, so is also probably beneath the slab, within the uppermost “lithospheric” mantle.

So, these 2019 M 6.9 and 2009 M 7.0 earthquakes are really cool.

Below is my interpretive poster for this earthquake


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1919-2019 with magnitudes M ≥ 6.5 in one version.

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), in addition to some relevant historic earthquakes.

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours on the map (in transparent shaded colors, see legend). These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • The inset map that shows a comparison of MMI and DYFI results includes the slab 2.0 contours plotted (Hayes, 2018), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.

    Magnetic Anomalies

  • The inset map that shows a comparison of MMI and DYFI results includes a transparent overlay of the magnetic anomaly data from EMAG2 (Meyer et al., 2017). As oceanic crust is formed, it inherits the magnetic field at the time. At different points through time, the magnetic polarity (north vs. south) flips, the North Pole becomes the South Pole. These changes in polarity can be seen when measuring the magnetic field above oceanic plates. This is one of the fundamental evidences for plate spreading at oceanic spreading ridges (like the Gorda rise).
  • Regions with magnetic fields aligned like today’s magnetic polarity are colored red in the EMAG2 data, while reversed polarity regions are colored blue. Regions of intermediate magnetic field are colored light purple.

    I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

  • In the upper left corner is a map showing the major plate boundary faults offshore and onshore of Sumatra, Indonesia (Patton et al., 2015). Historic earthquake slip patches are shown (Bilham, 2005; Malik et al., 2011), including slip contours for the 2004 and 2005 subduction zone earthquakes (Chileh et al., 2007, 2008). I include moment tensors for the 2004 and 2005 subduction zone earthquakes, as well as the 2 Aug 2019 M 6.9 quake for reference.
  • In the lower right corner is a low angle oblique view of a cut away of the Earth along the subduction zone in Java, Indonesia. The Sunda plate is part of Eurasia. The subduction zone offshore of Sumatra is similar in some ways to Java.
  • In the upper right corner is a map that shows a comparison between the USGS computer modeled estimates of ground shaking (shown as MMI contours) and the USGS “Did You Feel It?” (DYFI) results. DYFI data are compiled when real people submit their observations to the USGS DYFI part of the webpage for the earthquake. These levels of intensity are quite close. I spot checked several and the DYFI spot values are generally between the MMI contours for that range of values.
  • Here is the map with a year’s (orange) and a century’s (gray) seismicity plotted.

    Landslide, Liquefaction, and Shaking Intensity

  • Keefer (1984) studied a global data set of earthquake triggered landslides and found that larger earthquakes trigger larger and more numerous landslides across a larger area than do smaller earthquakes. Earthquakes can cause landslides because the seismic waves can cause the driving force to increase (the earthquake motions can “push” the land downwards), leading to a landslide. In addition, ground shaking can change the strength of these earth materials (a form of resisting force) with a process called liquefaction.
  • Sediment or soil strength is based upon the ability for sediment particles to push against each other without moving. This is a combination of friction and the forces exerted between these particles. This is loosely what we call the “angle of internal friction.” Liquefaction is a process by which pore pressure increases cause water to push out against the sediment particles so that they are no longer touching.
  • An analogy that some may be familiar with relates to a visit to the beach. When one is walking on the wet sand near the shoreline, the sand may hold the weight of our body generally pretty well. However, if we stop and vibrate our feet back and forth, this causes pore pressure to increase and we sink into the sand as the sand liquefies. Or, at least our feet sink into the sand.
  • Below is the liquefaction susceptibility map. I discuss liquefaction more in my earthquake report on the 28 September 20018 Sulawesi, Indonesia earthquake, landslide, and tsunami here.
  • I use the same color scheme that the USGS uses on their website. Note how the areas that are more likely to have experienced earthquake induced liquefaction are in the valleys. The fact that this earthquake happened in the summer time suggests that there may not have been any liquefaction from this earthquake.

Other Report Pages

Some Relevant Discussion and Figures

  • Here is the plate tectonic map from Zahirovic et al (2014).

  • Regional tectonic setting with plate boundaries (MORs/transforms = black, subduction zones = teethed red) from Bird (2003) and ophiolite belts representing sutures modified from Hutchison (1975) and Baldwin et al. (2012). West Sulawesi basalts are from Polvé et al. (1997), fracture zones are from Matthews et al. (2011) and basin outlines are from Hearn et al. (2003).

  • In addition to the orientation of relative plate motion (that controls seismogenic zone and strain partitioning), the Indo Australia plate varies in crustal age (Lasitha et al., 2006). I include their figure caption below as a blockquote.

  • Tectonic sketch map of the Sumatra–Java trench-arc region in eastern Indian Ocean Benioff Zone configuration. Hatched line with numbers indicates depth to the top of the Benioff Zone (after Newcomb and McCann13). Magnetic anomaly identifications have been considered from Liu et al.14 and Krishna et al.15. Magnitude and direction of the plate motion is obtained from Sieh and Natawidjaja. O indicates the location of the recent major earthquakes of 26 December 2004, i.e. the devastating tsunamigenic earthquake (Mw = 9.3) and the 28 March 2005 earthquake (Mw = 8.6).

  • Here is a figure showing the regional geodetic motions (Bock et al., 2003). I include their figure caption below as a blockquote.

  • Topographic and tectonic map of the Indonesian archipelago and surrounding region. Labeled, shaded arrows show motion (NUVEL-1A model) of the first-named tectonic plate relative to the second. Solid arrows are velocity vectors derived from GPS surveys from 1991 through 2001, in ITRF2000. For clarity, only a few of the vectors for Sumatra are included. The detailed velocity field for Sumatra is shown in Figure 5. Velocity vector ellipses indicate 2-D 95% confidence levels based on the formal (white noise only) uncertainty estimates. NGT, New Guinea Trench; NST, North Sulawesi Trench; SF, Sumatran Fault; TAF, Tarera-Aiduna Fault. Bathymetry [Smith and Sandwell, 1997] in this and all subsequent figures contoured at 2 km intervals.

  • Here is a map that shows the subduction zone offshore of Sumatra. Note the fracture zones in the India Australia plate (Krabbenhoeft et al., 2010). These authors looked at deep seismic profiles and seafloor bathymetry to interpret the structures of the accretionary prisms forming above the megathrust faults.
  • Here is a cogent summary of their findings:
  • We find the differences along the Sunda margin, especially the wider extent of the seismogenic zone off Sumatra, producing larger earthquakes, to result from the interaction of different age and subduction direction of the oceanic plate. We attribute a major role to the sediment income and continental/oceanic upper plate nature of Sumatra/Java influencing the composition and deformation style along the forearc and subduction fault.


    Bathymetry off Sumatra (multibeam bathymetry, where available underlain by satellite derived bathymetry; Smith and Sandwell, 1997). Tectonic setting is after Newcomb and McCann, 1987. Fracture zones (after Kopp et al., 2008) on the incoming plate as well as subduction direction and velocity (after Simons et al., 2007) are indicated by annotated black arrows on Indo-Australian plate. Major Mentawai islands as well as major faults are annotated along the forearc. Dashed lines sub-parallel to the trench mark the updip and downdip limit of the SZ. The seaward trench-parallel dashed line marking the updip limit of the SZ coincides with the slope break. Profiles and regions are marked and annotated, where additional investigations were available to constrain or refute their limits of the SZ.

  • Here is a map that shows the subduction zone offshore of Java. Note the fracture zones in the India Australia plate.

  • Bathymetry off Java and the Lesser Sunda islands (multibeam bathymetry (for YK0207 see Soh et al., 2002), where available underlain by satellite derived bathymetry; Smith and Sandwell, 1997). Tectonic setting (after Newcomb and McCann, 1987) on the incoming plate as well as subduction direction and velocity (after Simons et al., 2007) are indicated by annotated black arrows on Indo-Australian plate. Lesser Sunda islands as well as major tectonic features are annotated along the forearc. Dashed lines sub-parallel to the trench mark the updip and downdip limit of the SZ. The seaward trench-parallel dashed line marking the updip limit of the SZ coincides with the slope break. Profiles and regions are marked and annotated, where additional investigations were available to constrain or refute their limits of the SZ.

  • This is the main figure from Hayes et al. (2013) from the Seismicity of the Earth series. There is a map with the slab contours and seismicity both colored vs. depth. There are also some cross sections of seismicity plotted, with locations shown on the map.

  • Here is a cutaway figure showing the subduction zone beneath the island of Sumatra (from Earth Observatory of Singapore).

  • Here is a great figure from Philobosian et al. (2014) that shows the slip patches from the subduction zone earthquakes in this region.

  • Map of Southeast Asia showing recent and selected historical ruptures of the Sunda megathrust. Black lines with sense of motion are major plate-bounding faults, and gray lines are seafloor fracture zones. Motions of Australian and Indian plates relative to Sunda plate are from the MORVEL-1 global model [DeMets et al., 2010]. The fore-arc sliver between the Sunda megathrust and the strike-slip Sumatran Fault becomes the Burma microplate farther north, but this long, thin strip of crust does not necessarily all behave as a rigid block. Sim = Simeulue, Ni = Nias, Bt = Batu Islands, and Eng = Enggano. Brown rectangle centered at 2°S, 99°E delineates the area of Figure 3, highlighting the Mentawai Islands. Figure adapted from Meltzner et al. [2012] with rupture areas and magnitudes from Briggs et al. [2006], Konca et al. [2008], Meltzner et al. [2010], Hill et al. [2012], and references therein.

    • For a review of the 2004 and 2005 Sumatra Andaman subduction zone (SASZ) earthquakes, please check out my Earthquake Report here. Below is the poster from that report. On that report page, I also include some information about the 2012 M 8.6 and M 8.2 Wharton Basin earthquakes.
      • I include some inset figures in the poster.
      • In the upper left corner, I include a map that shows the extent of historic earthquakes along the SASZ offshore of Sumatra. This map is a culmination of a variety of papers (summarized and presented in Patton et al., 2015).
      • In the upper right corner I include a figure that is presented by Chlieh et al. (2007). These figures show model results from several models. Each model is represented by a map showing the amount that the fault slipped in particular regions. I present this figure below.
      • In the lower right corner I present a figure from Prawirodirdjo et al. (2010). This figure shows the coseismic vertical and horizontal motions from the 2004 and 2005 earthquakes as measured at GPS sites.
      • In the lower left corner are the MMI intensity maps for the two SASZ earthquakes. Note these are at different map scales. I also include the MMI attenuation curves for these earthquakes below the maps. These plots show the reported MMI intensity data as they relate to two plots of modeled estimates (the orange and green lines). These green dots are from the USGS “Did You Feel It?” reports compared to the estimates of ground shaking from Ground Motion Prediction Equation (GMPE) estimates. GMPE are empirical relations between earthquakes and recorded seismologic observations from those earthquakes, largely controlled by distance to the fault, ray path (direction and material properties), and site effects (the local geology). When seismic waves propagate through sediment, the magnitude of the ground motions increases in comparison to when seismic waves propagate through bedrock. The orange line is a regression of data for the central and eastern US and the green line is a regression through data from the western US.


    • The 2004/2005 SASZ earthquakes also tended to load strain in the crust in different locations. On 2012.04.11 there was a series of strike-slip earthquakes in the India plate crust to the west of the 2004/2005 earthquakes. The two largest magnitudes for these earthquakes were M 8.6 and M 8.2. The M 8.6 is the largest strike-slip earthquake ever recorded.
    • On 2016.03.22 there was another large strike-slip earthquake in the India-Australia plate. This is probably related to this entire suite of subduction zone and intraplate earthquakes. I presented an interpretive poster about this M 7.8 earthquake here. Below is my interpretive poster for the M 7.8 earthquake. Here is the USGS website for this earthquake.
    • I include a map in the upper right corner that shows the historic earthquake rupture areas.

    • Here is a poster that shows some earthquakes in the Andaman Sea. This is from my earthquake report from 2015.11.08.

    • This map shows the fracture zones in the India-Australia plate.

    • Here is a map showing seismicity and cross section locations along the Java trench (Jones et al., 2010). Below the map is the seismicity cross section A-A.’ shown on the map.


    • Here is a cutaway figure showing the subduction zone beneath the island of Java (from Earth Observatory of Singapore).

    • Kopp (2011) presents an analysis of the deep structure of the subduction zone beneath and offshore of Java. Below is their figure that shows how the structure changes along strike (from east to west, a series of cross sections.
    • First is a map showing where these sections are located. This figure also shows nicely where this convergent plate boundary changes from a subduction zone on the west (Java trench) to a collision zone on the east (e.g. Timor trough). The India-Australia plate is oceanic on the west and continental crust on the east.

    • Morphology of the Java margin based on satellite altimetry data (Smith & Sandwell 1997). A large bivergent accretionary wedge is expressed as a continuous bathymetric ridge fronting the Java fore-arc basin offshore western Java. This ridge structure is broken and highly deformed offshore central Java, where the oceanic Roo Rise is colliding with the margin. The eastern Java trench offshore Bali to Sumba is characterized by the subduction of smooth oceanic crust of the Argo Abyssal Plain. The transition from oceanic subduction to continent–island arc collision occurs south of Sumba where the Scott plateau enters the trench. Black lines show wide-angle refraction profiles.

    • Here are the structural profiles.

    • Tomographic images and velocity–depth distribution along seven refraction seismic dip lines crossing the fore-arc between western Java and east of Sumba island. The profiles document the variation from the accretionary domain (a and b) to the erosional seamount/plateau subduction regime off central to eastern Java (c and d). To the east, the transition from oceanic subduction offshore Lombok (e) to continent–island arc collision (f and g) occurs. All profiles west of Sumba show a shallow hydrated upper plate mantle, which limits the downdip extent of the seismogenic zone. Profiles are approximately aligned along the vertical stippled line. Vertical exaggeration in all profiles is 2.5.

    • In 2018, there was a series of earthquakes along the island of Lombok, Indonesia. My third report on that sequence is here. Below is an interpretive poster from the early part of the Lombok sequence, but includes some historic earthquakes associated with the subduction zone. The 2018 Lombok earthquakes were related to a fault that opposes the subduction zone, not on the subduction zone itself. Learn more about these structures in that report series.
    • Here is the interpretive posted from the M 6.4 7/28 earthquake, with historic seismicity and earthquake mechanisms.

    • Below are the maps and cross sections from Darman et al., 2012.
    • Here is the map in the interpretive poster above.

    • Tectonic map of the Lesser Sunda Islands, showing the main tectonic units, main faults, bathymetry and location of seismic sections discussed in this paper.

    • Here is the seismicity cross section in the interpretive poster above.

    • This plot shows the earthquake localizations on a South-North cross section for the lat -14°/-4° long 114°/124° quadrant corresponding to the Lesser Sunda Islands region. The localizations are extracted from the USGS database and corresponds to magnitude greater than 4.5 in the 1973-2004 time period (shallow earthquakes with undetermined depth have been omitted.

    • Here is their interpretations of seismic data used to interpret the tectonics of the subduction zone and Flores thrust.

    • Six 15 km deep seismic sections acquired by BGR from west to east traversing oceanic crust, deep sea trench, accretionary prism, outer arc high and fore-arc basin, derived from Kirchoff prestack depth migration (PreSDM) with a frequency range of 4-60 Hz. Profile BGR06-313 shows exemplarily a velocity-depth model according to refraction/wide-angle
      seismic tomography on coincident profile P31 (modified after Lüschen et al, 2011).

    • Here is the tectonic map from Hangesh and Whitney (2016). At the eastern part of the plate boundary, things get more complicated. Check out my earthquake report for a recent intermediate depth earthquake to learn more about this part of the world.

    • Illustration of major tectonic elements in triple junction geometry: tectonic features labeled per Figure 1; seismicity from ISC-GEM catalog [Storchak et al., 2013]; faults in Savu basin from Rigg and Hall [2011] and Harris et al. [2009]. Purple line is edge of Australian continental basement and fore arc [Rigg and Hall, 2011]. Abbreviations: AR = Ashmore Reef; SR = Scott Reef; RS = Rowley Shoals; TCZ = Timor Collision Zone; ST = Savu thrust; SB = Savu Basin; TT = Timor thrust; WT =Wetar thrust; WASZ = Western Australia Shear Zone. Open arrows indicate relative direction of motion; solid arrows direction of vergence.

Geologic Fundamentals

  • For more on the graphical representation of moment tensors and focal mechanisms, check this IRIS video out:
  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

  • Here is another way to look at these beach balls.
  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
  • Strike Slip:

    Compressional:

    Extensional:

  • This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

  • A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

  • Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

  • Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

  • Here is a great tweet that discusses the different parts of a seismogram and how the internal structures of the Earth help control seismic waves as they propagate in the Earth.

    References:

    Basic & General References

  • Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
  • Hayes, G., 2018, Slab2 – A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release, https://doi.org/10.5066/F7PV6JNV.
  • Holt, W. E., C. Kreemer, A. J. Haines, L. Estey, C. Meertens, G. Blewitt, and D. Lavallee (2005), Project helps constrain continental dynamics and seismic hazards, Eos Trans. AGU, 86(41), 383–387, , https://doi.org/10.1029/2005EO410002. /li>
  • Kreemer, C., J. Haines, W. Holt, G. Blewitt, and D. Lavallee (2000), On the determination of a global strain rate model, Geophys. J. Int., 52(10), 765–770.
  • Kreemer, C., W. E. Holt, and A. J. Haines (2003), An integrated global model of present-day plate motions and plate boundary deformation, Geophys. J. Int., 154(1), 8–34, , https://doi.org/10.1046/j.1365-246X.2003.01917.x.
  • Kreemer, C., G. Blewitt, E.C. Klein, 2014. A geodetic plate motion and Global Strain Rate Model in Geochemistry, Geophysics, Geosystems, v. 15, p. 3849-3889, https://doi.org/10.1002/2014GC005407.
  • Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. https://doi.org/10.7289/V5H70CVX
  • Müller, R.D., Sdrolias, M., Gaina, C. and Roest, W.R., 2008, Age spreading rates and spreading asymmetry of the world’s ocean crust in Geochemistry, Geophysics, Geosystems, 9, Q04006, https://doi.org/10.1029/2007GC001743
  • Specific References

  • Abercrombie, R.E., Antolik, M., Ekstrom, G., 2003. The June 2000 Mw 7.9 earthquakes south of Sumatra: Deformation in the India–Australia Plate. Journal of Geophysical Research 108, 16.
  • Bassin, C., Laske, G. and Masters, G., The Current Limits of Resolution for Surface Wave Tomography in North America, EOS Trans AGU, 81, F897, 2000.
  • Bock, Y., Prawirodirdjo, L., Genrich, J.F., Stevens, C.W., McCaffrey, R., Subarya, C., Puntodewo, S.S.O., Calais, E., 2003. Crustal motion in Indonesia from Global Positioning System measurements: Journal of Geophysical Research, v. 108, no. B8, 2367, doi: 10.1029/2001JB000324.
  • Bothara, J., Beetham, R.D., Brunston, D., Stannard, M., Brown, R., Hyland, C., Lewis, W., Miller, S., Sanders, R., Sulistio, Y., 2010. General observations of effects of the 30th September 2009 Padang earthquake, Indonesia. Bulletin of the New Zealand Society for Earthquake Engineering 43, 143-173.
  • Chlieh, M., Avouac, J.-P., Hjorleifsdottir, V., Song, T.-R.A., Ji, C., Sieh, K., Sladen, A., Hebert, H., Prawirodirdjo, L., Bock, Y., Galetzka, J., 2007. Coseismic Slip and Afterslip of the Great (Mw 9.15) Sumatra-Andaman Earthquake of 2004. Bulletin of the Seismological Society of America 97, S152-S173.
  • Chlieh, M., Avouac, J.P., Sieh, K., Natawidjaja, D.H., Galetzka, J., 2008. Heterogeneous coupling of the Sumatran megathrust constrained by geodetic and paleogeodetic measurements: Journal of Geophysical Research, v. 113, B05305, doi: 10.1029/2007JB004981.
  • DEPLUS, C. et al., 1998 – Direct evidence of active deformation in the eastern Indian oceanic plate, Geology.
  • DYMENT, J., CANDE, S.C. & SINGH, S., 2007 – Oceanic lithosphere subducting beneath the Sunda Trench: the Wharton Basin revisited. European Geosciences Union General Assembly, Vienna, 15-20/05.
  • Hayes, G. P., Wald, D. J., and Johnson, R. L., 2012. Slab1.0: A three-dimensional model of global subduction zone geometries in J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524.
  • Hayes, G.P., Bernardino, Melissa, Dannemann, Fransiska, Smoczyk, Gregory, Briggs, Richard, Benz, H.M., Furlong, K.P., and Villaseñor, Antonio, 2013. Seismicity of the Earth 1900–2012 Sumatra and vicinity: U.S. Geological Survey Open-File Report 2010–1083-L, scale 1:6,000,000, https://pubs.usgs.gov/of/2010/1083/l/.
  • Ishii, M., Shearer, P.M., Houston, H., Vidale, J.E., 2005. Extent, duration and speed of the 2004 Sumatra-Andaman earthquake imaged by the Hi-Net array. Nature 435, 933.
  • JACOB, J., DYMENT, J., YATHEESH, V. & BHATTACHARYA, G.C., 2009 – Marine magnetic anomalies in the NE Indian Ocean: the Wharton and Central Indian basins revisited. European Geosciences Union General Assembly, Vienna, 19-24/04.
  • Ji, C., D.J. Wald, and D.V. Helmberger, Source description of the 1999 Hector Mine, California earthquake; Part I: Wavelet domain inversion theory and resolution analysis, Bull. Seism. Soc. Am., Vol 92, No. 4. pp. 1192-1207, 2002.
  • Kanamori, H., Rivera, L., Lee, W.H.K., 2010. Historical seismograms for unravelling a mysterious earthquake: The 1907 Sumatra Earthquake. Geophysical Journal International 183, 358-374.
  • Konca, A.O., Avouac, J., Sladen, A., Meltzner, A.J., Sieh, K., Fang, P., Li, Z., Galetzka, J., Genrich, J., Chlieh, M., Natawidjaja, D.H., Bock, Y., Fielding, E.J., Ji, C., Helmberger, D., 2008. Partial Rupture of a Locked Patch of the Sumatra Megathrust During the 2007 Earthquake Sequence. Nature 456, 631-635.
  • Kopp, H., 2011. The Java convergent margin: structure, seismogenesis and subduction processes in Geological Society, London, Special Publications 2011; v. 355; p. 111-137, doi: 10.1144/SP355.6
  • Krabbenhoeft, A., Weinrebe, R.W., Kopp, H., Flueh, E.R., Ladage, S., Papenberg, C., Planert, L., and Djajadihardja, Y., 2010. Bathymetry of the Indonesian Sunda margin-relating morphological features of the upper plate slopes to the location and extent of the seismogenic zone in NHESS, v. 10, p. 1899-1911, doi:10.5194/nhess-10-1899-2010
  • Lasitha, S., Radhakrishna, M., Sanu, T.D., 2006. Seismically active deformation in the Sumatra–Java trench-arc region: geodynamic implications in Current Science, v. 90, p. 690-696.
  • Maus, S., et al., 2009. EMAG2: A 2–arc min resolution Earth Magnetic Anomaly Grid compiled from satellite, airborne, and marine magnetic measurements, Geochem. Geophys. Geosyst., 10, Q08005, doi:10.1029/2009GC002471.
  • Malik, J.N., Shishikura, M., Echigo, T., Ikeda, Y., Satake, K., Kayanne, H., Sawai, Y., Murty, C.V.R., Dikshit, D., 2011. Geologic evidence for two pre-2004 earthquakes during recent centuries near Port Blair, South Andaman Island, India: Geology, v. 39, p. 559-562.
  • Meltzner, A.J., Sieh, K., Chiang, H., Shen, C., Suwargadi, B.W., Natawidjaja, D.H., Philobosian, B., Briggs, R.W., Galetzka, J., 2010. Coral evidence for earthquake recurrence and an A.D. 1390–1455 cluster at the south end of the 2004 Aceh–Andaman rupture. Journal of Geophysical Research 115, 1-46.
  • Meng, L., Ampuero, J.-P., Stock, J., Duputel, Z., Luo, Y., and Tsai, V.C., 2012. Earthquake in a Maze: Compressional Rupture Branching During the 2012 Mw 8.6 Sumatra Earthquake in Science, v. 337, p. 724-726.
  • Natawidjaja, D.H., Sieh, K., Chlieh, M., Galetzka, J., Suwargadi, B., Cheng, H., Edwards, R.L., Avouac, J., Ward, S.N., 2006. Source parameters of the great Sumatran megathrust earthquakes of 1797 and 1833 inferred from coral microatolls. Journal of Geophysical Research 111, 37.
  • Newcomb, K.R., McCann, W.R., 1987. Seismic History and Seismotectonics of the Sunda Arc. Journal of Geophysical Research 92, 421-439.
  • Philibosian, B., Sieh, K., Natawidjaja, D.H., Chiang, H., Shen, C., Suwargadi, B., Hill, E.M., Edwards, R.L., 2012. An ancient shallow slip event on the Mentawai segment of the Sunda megathrust, Sumatra. Journal of Geophysical Research 117, 12.
  • Prawirodirdjo, P., McCaffrey,R., Chadwell, D., Bock, Y, and Subarya, C., 2010. Geodetic observations of an earthquake cycle at the Sumatra subduction zone: Role of interseismic strain segmentation, JOURNAL OF GEOPHYSICAL RESEARCH, v. 115, B03414, doi:10.1029/2008JB006139
  • Rivera, L., Sieh, K., Helmberger, D., Natawidjaja, D.H., 2002. A Comparative Study of the Sumatran Subduction-Zone Earthquakes of 1935 and 1984. BSSA 92, 1721-1736.
  • Shearer, P., and Burgmann, R., 2010. Lessons Learned from the 2004 Sumatra-Andaman Megathrust Rupture, Annu. Rev. Earth Planet. Sci. v. 38, pp. 103–31
  • SATISH C. S, CARTON H, CHAUHAN A.S., et al., 2011 – Extremely thin crust in the Indian Ocean possibly resulting from Plume-Ridge Interaction, Geophysical Journal International.
  • Sieh, K., Natawidjaja, D.H., Meltzner, A.J., Shen, C., Cheng, H., Li, K., Suwargadi, B.W., Galetzka, J., Philobosian, B., Edwards, R.L., 2008. Earthquake Supercycles Inferred from Sea-Level Changes Recorded in the Corals of West Sumatra. Science 322, 1674-1678.
  • Singh, S.C., Carton, H.L., Tapponnier, P, Hananto, N.D., Chauhan, A.P.S., Hartoyo, D., Bayly, M., Moeljopranoto, S., Bunting, T., Christie, P., Lubis, H., and Martin, J., 2008. Seismic evidence for broken oceanic crust in the 2004 Sumatra earthquake epicentral region, Nature Geoscience, v. 1, pp. 5.
  • Smith, W.H.F., Sandwell, D.T., 1997. Global seafloor topography from satellite altimetry and ship depth soundings: Science, v. 277, p. 1,957-1,962.
  • Sorensen, M.B., Atakan, K., Pulido, N., 2007. Simulated Strong Ground Motions for the Great M 9.3 Sumatra–Andaman Earthquake of 26 December 2004. BSSA 97, S139-S151.
  • Subarya, C., Chlieh, M., Prawirodirdjo, L., Avouac, J., Bock, Y., Sieh, K., Meltzner, A.J., Natawidjaja, D.H., McCaffrey, R., 2006. Plate-boundary deformation associated with the great Sumatra–Andaman earthquake: Nature, v. 440, p. 46-51.
  • Tolstoy, M., Bohnenstiehl, D.R., 2006. Hydroacoustic contributions to understanding the December 26th 2004 great Sumatra–Andaman Earthquake. Survey of Geophysics 27, 633-646.
  • Zhu, Lupei, and Donald V. Helmberger. “Advancement in source estimation techniques using broadband regional seismograms.” Bulletin of the Seismological Society of America 86.5 (1996): 1634-1641.

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Posted in earthquake, education, geology, plate tectonics, strike-slip, subduction

Earthquake Report: Ridgecrest Update #3 Literature Review

I have reviewed a small portion of the literature for the tectonics of the northern Eastern California shear zone, Owens Valley fault, Garlock fault, etc. I have a basic knowledge of this region and have attended several Pacific Cell Friends of the Pleistocene field trips in this area, but have not done extensive literature review for this area (though I did help Steve Bacon (DRI PhD. student defending soon) for his work on the Owens Valley fault for his M.S. thesis at Humboldt State University, Dept. of Geology, while I was a graduate student there with “early morning” Steve).

Below I present some key overview figures from some of the papers I reviewed today. See the reference list for additional papers. However, first I present a new map.

Global Strain Rate Map

  • Strain is basically the change in shape or volume of a material through time. The Earth deforms with space and time in relation to geospatial variations in plate tectonic motions.
  • Tectonic strain can be measured in a variety of methods. Most people are familiar with geodetic methods. Geodesy is the study of the motion of the Earth as measured at discrete locations (e.g. with GPS observations). One may use changes in position at GPS sites to measure how the Earth moves, so we can directly measure changes in shape this way.
  • Geodetic data can be combined with geologic and seismicity data to evaluate tectonic strain at global, regional, and local scales.
  • In 1998 the International Lithosphere Program started compiling a global dataset to support the construction of a Global Strain Rate Map (GSRM; Kreemer et al., 2000, 2002, 2003, 2014).
  • The GSRM has been incorporated into the Global Earthquake Model of Seismic Hazard, v 2.1 presented online here.
  • I present a map for the Ridgecrest Earthquake Sequence that uses an older version of the GSRM (v 1.2). The color ramp is based on the “second invariant” of strain. Warmer colors show regions of greater tectonic strain. Units are in 10 per year. I acquired these data here.

Geologic Map

  • There are some larger scale geology maps for this region, but they cost money (Dibblee Foundation/AAPG). Needless to say, I don’t have the $50 to buy them right now. They are geotiffs, so would overlay nicely.
  • The map below shows seismicity for the past month overlain upon the 1962 California Division of Mines and Geology 1:250,000 scale geologic map (Jennings et al., 1962). I prepared this on 21 July 2019 after georeferencing the map from the CGS website..


UNAVCO Response Page

  • UNAVCO has event response pages where people post visualizations of data. Here is the Ridgecrest Earhquake Response Page.

  • OTA-measured GNSS static displacements from the real-time GNSS system (blue) compared to the seismically derived static displacements (pink).


    Preliminary coseismic horizontal vector displacements for the July 4, 2019 M 6.4 earthquake. The 5-minute sample rate time series were obtained using rapid orbits from the Jet Propulsion Laboratory.


    Ultra rapid analysis coseismic offsets calculated by the Nevada Geodetic Laboratory (NGL) for a subset of continuous GPS stations in the region of the July 6, 2019 M 7.1 earthquake.


    Rapid analysis coseismic offset pattern for the July 6, 2019 M 7.1 Ridgecrest earthquake, from the Nevada Geodetic Laboratory (NGL)


    GPS derived coseismic displacements of Mw6.4 foreshock. Five days of GPS data spanning the foreshock and prior to the mainshock were processed to obtain the solution.


    GPS derived coseismic displacements of Mw7.1 mainshock. Four days of GPS data spanning the mainshock and after the foreshock were processed to obtain the solution.


    Preliminary slip results derived from geodetic and seismic data for the July 6, 2019 M 7.1 Ridgecrest earthquake, from the Pacfic Northwest Seismic Network. The slip model was run through G-FAST.

  • Here is a video showing real time GPS displacement from 1Hz GPS/GNSS NOTA data analyzed by Christine Puskas.

Background Literature – Tectonics

  • Here is a great overview map of the faults in the region from Oskin et al. (2008). Their paper is about their research to quantify the tectonic loading of faults in the Eastern California shear zone. Note that they use about 12 mm per year of Pacific-North America relative plate motion across this region.

  • A: Index map of southwest North America showing geodetic provinces from Bennett et al. (2003) and location of Mojave block. Velocities of geodetically stable regions are shown relative to Colorado Plateau. ECSZ—eastern California shear zone in Mojave block. Shear zone continues northward into western Great Basin province. GF—Garlock fault. B: Index map of the Mojave block with active faults and locations of recent earthquake ruptures. Circles show localities of slip-rate measurements that sum to ≤6.2 ± 1.9 mm/yr across the ECSZ. GPS—global positioning system.

  • Here is another good overview map, showing the faults for which Petersen and Wesnousky (1994) reviewed slip rates in that publication. They present an excellent review of all slip rate and paleoseismic investigations at the time that paper was published.

  • Map showing sites of slip rate studies in southern California for the San Andreas (SAI-14), San Jacinto (SJI-13), Elsinore-Whittier (El-8), Newport- Inglewood (N1-3), Palos Verdes (N4-6), Rose Canyon (N7), Transverse Ranges (T1-50), Mojave (MI-6), and Garlock (G1-9) faults.

  • Oskin and Iriondo studied the Blackwater fault, the right-lateral strike-slip fault system that extends from the south into the region of the Ridgecrest Earthquake Sequence. The Blackwater fault is connected to the south with the Calico fault (a fault between the 1992 and 1999 earthquakes). This appears to be the major Eastern California Shear zone fault that extends towards the Airport Valley and Little Lake faults (which ruptured during the Ridgecrest Earthquake Sequence).

  • A: Index map of Pacific–North America plate boundary through southwest North America. Principal faults are shown as thick black lines. Tectonically stable areas are outlined by dotted lines. Walker Lane and Eastern California shear zone, shown as dark gray band encompassing network of active faults, together absorb 9%–23% of total plate boundary shear (Dixon et al., 2000; Dokka and Travis, 1990a). JDF—Juan de Fuca; MTJ— Mendocino triple junction. B: Index map of Eastern California shear zone showing fault slip rates (in parentheses, mm/yr) determined by paleoseismic studies (Klinger and Piety, 2000; Lee et al., 2001; McGill and Sieh, 1993; Rockwell et al., 2000; Zhang et al., 1990). Heavy dark gray lines outline historic earthquake ruptures (Beanland and Clark, 1994; Sieh et al., 1993; Treiman et al., 2002). Heavy, medium gray band highlights Blackwater–Calico fault system. Light gray band surrounding Blackwater fault and passing north of Garlock fault is zone of localized 1.2 6 0.5 mm/yr strain accumulation documented by radar interferometry (Peltzer et al., 2001). C: Neotectonic map of Blackwater fault, showing type and orientation of fault line scarps with ticks on downthrown side. Dark patterned areas are lava flows cut by Blackwater fault (Dibblee, 1968, 1967; Smith, 1964)

  • Peltzer et al. (2001) evaluate the amount of tectonic strain that has accumulated over time (see geodesy section to learn more about strain). First I present their tectonic map.

  • Tectonic map of southern California. Solid lines are active faults (Jennings, 1975). Yellow dots are relocated earthquakes between 1981 and 2000 (Hauksson, 2000). Dashed-line box is area covered by Earth Resource Satellite (ERS) data used in this study. White dashed line shows location of concentrated shear observed in synthetic aperture radar (SAR) data. Black stars indicate epicenters of recent earthquakes: OV—1872 Owens Valley, JT—1992 Joshua Tree, L—1992 Landers, BB—1992 Big Bear, N—1994 Northridge, RC—1994 and 1995 Ridgecrest, HM—1999 Hector Mine. Heavy solid lines depict surface ruptures of Landers (Sieh et al., 1993), Hector Mine (U.S. Geological Survey and California Division of Mines and
    Geology, 2000; Peltzer et al., 2001), and Owens Valley (Beanland and Clark, 1994; only southern half of rupture is shown) earthquakes. Black dots and arrows show locations and observed velocities of 11 stations of Yucca GPS array (Gan et al., 2000).

    * Faults are listed in the paper

  • Guest et al. (2003) used geologic mapping and geochronologic data (ages of geologic units) to constrain a tectonic model. They suggest that some of the faults in the region developed as a result of tectonic blocks rotating about a vertical axis. First we see their geologic map.

  • Segment of Trona sheet geologic map showing Owlshead block, southern Death Valley, and Northeast Mojave block. WWFZ— Wingate Wash fault zone, BMF—Brown Mountain fault, OLF—Owl Lake fault, DVFZ—Death Valley fault zone, MSS—Mule Springs strand, LLS—Leach Lake strand, DWLF—Drink Water Lake fault, FIF—Fort Irwin fault, CCF—Coyote Canyon fault, TMF—Tiefort
    Mountain fault.

  • Here is the Guest et al. (2003) map showing their interpretation of how these faults developed over time.

  • In this model the Owlshead and southern Panamint blocks are hypothesized to have undergone sinistral transtension in response to a clockwise rotation of their southern confining boundary (Garlock fault zone).

    RTR—Radio Tower Range, SOM—Southern Owlshead Mountains, WWFZ—WingateWash fault zone, BMF—Brown Mountain fault, OLF—Owl Lake fault, GF—Garlock fault, MSS—Mule Springs strand, LLZ—Leach Lake fault zone, SDVFZ—Southern Death Valley fault zone.

Background Literature – Geodesy

  • Gan et al. (2003) present a summary of geodetic data where they show that the Owens Valley, Little Lake, and Helendale faults form the generalized western boundary of the Eastern California shear zone (there are additional right-lateral faults to the west however).

  • Map showing the location of the ECSZ, the GPS arrays, the station velocities (relative to the fixed North America), and the principal faults in southern California (from Jennings [1992]). The thick dashed lines directed N23°W show the boundaries of the assumed parallel-sided ECSZ. The thin dashed lines extended from the segments of the Garlock fault show the trends of the segments.

  • One of the challenges with interpreting geodetic data is comparing earthquake fault slip rates inferred from geodetic methods with rates calculated using geologic data (either from long term offsets of bedrock, or from more recent rates using fault trenches).
  • Chuang and Johnson (2011) present their comparisons of GPS slip rates with geologic rates.
    • Blue = geologic rate
    • Black = geodetic rate
    • Magenta = block model rate from their analyses


    Comparison of geologic fault slip rates (blue, mm/yr) used in model, range of estimates from elastic block models (black) of Becker et al. (2005) and Meade and Hager (2005), and estimates from our block model (magenta) along major faults. Negative is left lateral. Light red lines are surface fault traces, and white thick lines are model blocks. Blue arrows are Southern California Earthquake Center (SCEC) crustal motion map 3 (Shen et al., 2003) velocities with respect to stable North America.

    *See their paper for fault abbreviations.

  • Here is an interesting figure showing their (Chuang and Johnson, 2011) estimate of the relative position in the earthquake cycle for these faults. This is based on published recurrence intervals for these faults (the average time between earthquakes given paleoseismic investigation data).

  • Summary of assumed geologic rates, recurrence interval (T), and time since last earthquake (teq) in Southern California. (For further discussion of sources of T and teq, see footnote 1). Blue numbers are expert opinion slip rates from Working Group on California Earthquake Probabilities (2008) and red numbers are rates from other paleoseismology data.

    Color of rupture segment represents ratio of time since last earthquake and recurrence interval. Hot (red) colors show segments are in early earthquake cycle, and cold (blue) colors show late earthquake cycle.

  • This is the summary of the Chuang and Johnson (2011) slip rate comparison.

  • A: Geologic fault slip rates versus slip rates inferred from geodetic data. Geologic rates are summarized in Table DR1 (see footnote 1). Blue bars are slip rate comparisons from Meade and Hager (2005) and red bars are from this study. B: Normalized velocity across Garlock fault (blue), Mojave segment of San Andreas fault (red), and eastern California shear zone (ECSZ, green) from our cycle model. Black line is normalized velocity derived from elastic model.

  • Here is an earlier analysis comparing geodetic rates with geologic rates (Dixon et al., 2003). First we see a map showing the faults from which the fault comparisons are shown.

  • Sketch map of study area, modified from Dixon et al. (1995). Bar marks approximate location of Global Positioning System transect (Gan et al., 2000). GF— Garlock fault. Labeled faults of Eastern California shear zone: ALF— Airport Lake fault zone; OVF—Owens Valley fault zone; HMF—Hunter Mountain–Panamint Valley fault zone; DVF— Death Valley–Furnace
    Creek fault zone; FLV— Fish Lake Valley fault zone.

  • Here is the east west profile from Dixon et al. (2003). The horizontal axis is distance and the vertical axis is the rate that each site moves in mm per year. Their fault modeling is represented by the dark black line.

  • Global Positioning System velocity (triangles) and one standard error (bars) from Gan et al. (2000) compared to prediction of viscoelastic coupling model (heavy solid line), representing summed velocity contributions from four parallel faults (light dashed lines). SAF—San Andreas fault; DVF—Death Valley–Furnace Creek fault zone; HMF—Hunter Mountain–Panamint Valley fault zone; OVF—Owens Valley fault zone. Inset shows model rheology for Eastern California shear zone. SNB—Sierra Nevada block;B&R— Basin and Range Province; h is fault depth (depth of elastic layer) for three faults (a, b, or c), m is rigidity, h is viscosity. Arrows mark location of major shear-zone faults.

  • Peltzer et al. (2001) use synthetic aperture radar interferometry (see my second update report for more on InSAR anslysis) to measure tectonic deformation that accumulated between 1992-2000.
  • The Coso Geothermal Field is the rainbow area in the northernmost part of the map. Indian Wells Valley is the green area to the south of the Coso Field. This is an area of elevated strain. The Garlock fault is the ~east-west black line in the center of the white inset box.

  • Surface velocity map obtained by averaging 25 interferograms of Los Angeles–Mojave region. One color cycle depicts 10 mm/yr of surface displacement along radar line of sight (at lat N348; ERS [Earth Resource Satellite] descending track trends S13.68W, radar looking westward at 238 off vertical incidence angle in middle of imaged swath). Gray areas are zones of low phase coherence that have been masked in processing. Black lines are active faults (Jennings, 1975). White box indicates subset of synthetic aperture radar (SAR) data that was used for profile in Figure 4. Note conspicuous shear strain along San Andreas fault and shear zone parallel to Blackwater–Little Lake fault system. Large deformation signal in northwest corner of frame is ground subsidence related to Coso volcanic and geothermal field (Fig. 1). Surface displacement associated with 1994 and 1995 Ridgecrest earthquakes is visible south of Coso area. Other patterns of surface deformation include ground subsidence due to groundwater withdrawal in Los Angeles and Lancaster areas (Fig. 1) and to seasonal change of water table level around dry lakes.

  • Peltzer et al. (2001) plot observations from their radar data showing relative plate motion associated with dislocation along the Blackwater-Little Lake fault system.

  • Profiles of observed and modeled line-of-sight displacement projected on vertical plane perpendicular to shear zone. Gray dots are individual data points for all radar-image pixels included in box shown in Figure 3. Solid line shows 2 km running mean of observed displacement along profile length. Note that apparent standard deviation of projected data relative to average profile reflects in part displacement gradient parallel to fault strike and not only error in data. Groups of dots that deviate from dense part of profile are due to ground subsidence near lake shores and to surface displacement associated with Ridgecrest earthquakes (Figs. 1, 3). Short-dash line is profile predicted by long-term velocity model used to estimate interferometric baseline (Shen et al., 1996). Long-dash line is profile predicted by velocity model, including additional buried dislocation along Blackwater–Little Lake fault system. Parameters of added fault are given in text. Black dots and error bars (2s) are line-of-sight projections of horizontal velocities observed by GPS at stations of Yucca transect (Gan et al., 2000).

Background Literature – Little Lake fault

  • Amos et al. (2013) presented an analysis of “tectonic, geomorphic, and volcanic” features to derive a slip rate for the Little Lake fault near Little Lake, California. This is just northwest of the 2019 Ridgecrest Earthquake Sequence. Here is their tectonic map.

  • Overview of active faults and regional topography of the Eastern California shear zone (ECSZ) and southern Walker Lane belt. Labeled faults are abbreviated as follows: ALF—Airport Lake fault, BF—Blackwater fault, GF—Garlock fault, KCF—Kern Canyon fault, LLF—Little Lake fault, OVF—Owens Valley fault, SNFF—Sierra Nevada frontal fault. OL—Owens Lake, IWV—Indian Wells Valley. Major historical earthquake surface ruptures in the Eastern California shear zone and Walker Lane belt are outlined in white, with stars denoting epicentral locations: OV—1872 Owens Valley, L—Landers 1992, HM—1999 Hector Mine. Active fault traces are taken from the U.S. Geological Survey Quaternary fault and fold database, with the exception of the Kern Canyon fault, taken from Brossy et al. (2012).

  • Here is a geologic map from Amos et al. (2013) that shows the mapped faults and topographic controls of river drainage for the area.

  • Simplified geologic map of the Little Lake fault, highlighting Quaternary volcanic and alluvial deposits bearing on the Pleistocene drainage of Owens River through the Little Lake area. Map units are named and modified from Duffield and Bacon (1981). The 30 m elevation contours are taken from the National Elevation Database (NED). The 40Ar/39Ar dates are labeled as in Table 1. SNFF—Sierra Nevada frontal fault.

  • Here is a figure that shows the topography at the Amos et al. (2003) slip rate site along the Owens River. They measured topographic profiles of the ground surface across topographic landforms. These profiles were taken along the thin white lines on the map on the left.
  • On the right are the profiles from the western (B) and the eastern (C) profiles are shown on the right. They use these offset features, and the distance that they are offset, to calculate the slip rate here.

  • (A) 50 cm digital elevation model derived from terrestrial laser scanning (TLS) of displaced terrace risers in Little Lake narrows. (B–C) Stacked topographic profiles along the western and eastern edges of the Qt1 surface, respectively, used to reconstruct the total dextral offset of the Qt1-Qt2 terrace riser. Individual profiles were extracted perpendicular to the average riser orientation and were then projected onto a plane parallel to the local fault strike. Profile locations for each margin are shown in A. VE—vertical exaggeration.

  • This is one of the coolest figures I found during my literature review. Amos et al. (2013) back calculate what the ground surface would look like if back in time, before the fault started to offset the topography here.

  • Geometric reconstruction of (A) the modern geomorphic configuration of the upper Little Lake narrows indicates between ~140 and 250 m of dextral offset for the base (B) and upper edge (C) of the eastern canyon wall. Geologic units are labeled as in Figure 3. The base image includes a hillshade image from our terrestrial laser scanning (TLS) survey, as
    well as 10 m contours overlain on a National Elevation Database (NED) hillshade map. The map location is shown by the boxed area in Figure 3. Geologic units are labeled as in Figures 2 and 3.

  • This is a cool figure, but not as cool as the above map. Amos et al. (2013) plot their slip rate estimates compared to published rates. First they show their observations of displacement relative to the age of the offset topographic landform. Then they plot slip rate estimates in the same manner.

  • (A) Compiled dextral displacements and (B) corresponding fault-slip rates as a function of age for the Little Lake, Blackwater, and Garlock faults. Linear regressions in A indicate constant slip rates through time. Geologic slip rate estimates in B are for time intervals since the respective age measurements. Geodetic measurements represent
    interseismic deformation measured from interferometric synthetic aperture radar (InSAR) and global positioning system (GPS). […]

Background Literature – Garlock fault

  • Astiz and Allen (1983) studied the seismicity of southern California and looked specifically at earthquake mechanisms associated with the Garlock fault. First we see their seismicity map for the region, then we zoom into the Garlock fault.

  • Southern California seismiclty during 1981 from Caltech-USGS catalog. The outer border corresponds to the limits of the southern California array. The inner frame is the limit of Figures 2 and 6. Notice the cluster of earthquakes along the Garlock fault trace and the smaller activity w~th respect to many other faults in southern California.

  • Astiz and Allen (1983) plot the earthquake locations that they relocated for their analyses. This map shows a detailed map of the faults in the area..

  • Earthquake relocations from 1932 to 1981 in the Garlock fault zone. The light line corresponds to the 25-km-wide zone around the fault from which the earthquakes were taken from the catalog. The numbers m the figure corresponds to kilometers along the fault northeast from Gorman quarry (vertical axes in Figure 3). Sohd circles are quarries, and solid triangles are alignment array locations (from Keller et al., 1978). Faults are taken from Jennings and Strand (1969), Smith (1964), and Jennings et al. (1962).

  • This figure shows the earthquake mechanisms for some events that Astiz and Allen (1983) worked on to show how many faults have strike slip mechanisms, but that there are changes in earthquake type (some thrust (compression) and normal (extension) events).

  • Focal mechanisms for selected events that occurred m the Garlock fault zone between 1977 and 1981 Numbers correspond to those m Table 3 Event 5 is a composite mechanism of six nearby events.

  • McGill et al. (2009) late Pleistocene sediments (alluvial fan) and alluvial channels (with radiocarbon ages) to constrain an earthquake fault slip rate for the Garlock fault. First we see a tectonic map for the region.

  • Location of the Clark Wash site (large white circle) as well as other slip-rate and paleoseismic sites (small white circles) along the Garlock fault. AM—Avawatz Mountains; EPM—El Paso Mountains; GF—Garlock fault; PM—Providence Mountains; SAF—San Andreas fault; SLB—Soda Lake Basin; SM—Soda Mountains; SR—Slate Range; SSH—Salt Spring Hills; SV—Searles
    Valley.

  • These authors used a variety of observations to derive a statistical estimate (using probabilistic model) for a slip rate based on an estimate of offset and radiocarbon age (which both had a range of probabilities, plotted as a probability density function). This is really cool.

  • Probability density functions for left-lateral offset (A) and age (B) of Clark Wash that were assigned on the basis of quantitative constraints and subjective judgment (see text), and the resulting probability density function for the slip rate of the Garlock fault (C).

  • Here is a compilation of their slip rate estimates (McGill et al., 2009).

  • Comparison of slip-rate estimates for the Garlock fault. The three values in italics, associated with boxes that outline sections of the fault, are the slip rates and formal uncertainties from Meade and Hager’s (2005) best-fitting elastic block model of available geodetic data. They report, however, that experience with a range of models suggests that true uncertainties are ~3 mm/yr. White-filled circles mark the locations of Holocene and Late Quaternary geologic slip-rate estimates. The Holocene rates that are constrained by radiocarbon dates and are thus considered most reliable are shown in bold […].

    * more abbreviations and explanation in the paper

Background Literature – Owens Valley fault

  • Kylander-Clark et al. (2005) use the lateral offset of plutonic dikes (igneous rocks) to constrain a long term slip rate across the Owens Valley fault. This map shows one of the dike pairs used in their analysis. By knowing the age of these dieks, and the distance that they have been offset, we can obtain a slip rate.

  • Locations of the Golden Bear and Coso dikes, adjacent to Owens Valley. Main figure shows the Golden Bear and Coso dikes striking into the valley, where they intrude 102 Ma plutons. Both the dikes and the plutons provide distinctive markers that can be matched across the valley and are consistent with 65 km of dextral displacement since 84 Ma. Inset shows other markers across Owens Valley that earlier workers suggested indicate from 0 to 65 km of dextral offset across the valley. Also shown are the traces of the Tinemaha fault (Stevens et al., 1997; Stevens and Stone, 2002) and intrabatholithic break 3 (IBB3; Kistler, 1993), which are hypothesized to accommodate offset of these markers. Note that the section of IBB3 between 38°N and 36.5°N is correlative with the eastern intrabatholithic break (EIB) of Saleeby and Busby (1993). Not all known locations of Independence dikes are indicated. Instead, patterned areas show only the densest parts of the dike swarm as defi ned by Glazner et al. (2003). AR—Argus Range; CR—Coso Range; IR—Inyo Range; WM—White Mountains

  • Bacon and Pezzopane used trench excavations across earthquake faults to construct a prehistoric earthquake history for the Owens Valley fault. Below is their tectonic map for the region.

  • (A) Map of major Quaternary faults in the northern Eastern California shear zone and southern and central Walker Lane, as well as the locations of the Owens Valley fault. Faults are modified from Reheis and Dixon (1996) and Wesnousky (2005)

    (B) Generalized fault and geology map of south-central Owens Valley, showing the A.D. 1872 Owens Valley fault rupture and major fault zones in the valley (modified from Hollett et al. [1991] and Beanland and Clark [1994]).

    (For fault abbreviations, see their paper.)

  • This map shows a more detailed view of the Owens Valley fault and the Owens Lake topography (Bacon and Pezzopane, 2007).

  • Shaded relief map of southern Owens Valley showing fault zones and the ages of the most recent prominent highstands and recessional shorelines of Owens Lake during the latest Quaternary (modified from Bacon et al., 2006).

  • This map shows the Bacon and Pezzopane (2007) field sites.

  • Map of the field area and locations of paleoseismic study sites in relation to the A.D. 1872 Owens Valley earthquake fault trace near Lone Pine. Study sites are located on the Alabama Hills (AHS), Diaz Lake (DLS), and Manzanar (MZS) sections of the Owens Valley fault zone mapped by Bryant (1988) and Beanland and Clark (1994) from 1:12,000 aerial photographs.

  • An essential part of any earthquake fault investigation is knowledge about the geologic units that are offset by the fault. Bacon and Pezzopane (2007) also described and interpreted the sediment stratigraphy in southern Owens Valley as part of their research.

  • Schematic composite stratigraphic column. The generalized stratigraphic and geochronologic relations, developed from exposures at the Alabama Gates and Quaker paleoseismic sites and Owens River bluffs near Lone Pine (Bacon et al., 2006), show the positions of radiocarbon dates, sequence boundaries, and event chronologies as discussed in the text.

  • The geologic method (McCalpin, 1996) is based on the offset of geologic materials like sedimentary deposits or bedrock lithologic units. Below are trench logs showing the geologic units that Bacon and Pezzopane (2007) use to infer an earthquake history. Geologic evidence is “primary” evidence for earthquakes.

  • Here is a time series showing the sedimentary and earthquake history as interpreted by Bacon and Pezzopane (2007).

  • Schematic depiction of stratigraphy and structural relations at the Quaker paleoseismic site prior to the penultimate event and after the A.D. 1872 earthquake (depictions A–H). The stratigraphy and structure exposed in trench T5 (Fig. 7) was retrodeformed and reconstructed one event at a time (while also accounting for other stratigraphic and
    paleoseismic relations exposed in adjacent fault trenches and stratigraphic pits). The locations of sequence boundaries (SB0–SB4) are shown and can be referenced on Figure 5.

Background Literature – Earthquake History

  • Here are the results of the paleoseismic (prehistoric earthquake history) investigation for the Owens Valley fault (Bacon and Pezzopane, 2007).

  • Fault segmentation and section map of central and southern Owens Valley showing overlap and possible distributive faulting and linkage between the northern segment of the Owens Valley fault (OVF) and southern White Mountains fault (WMF) near Big Pine. The trace of the A.D. Owens Valley fault rupture and section boundaries of Beanland and Clark (1994) and segment boundaries of dePolo et al. (1991) are shown in relation to the central and southern White Mountains fault and the location of the Black Mountain rupture of dePolo (1989). RRF—Red Ridge fault; LP—Lone Pine; I—Independence; BP—Big Pine; OSL—optically stimulated luminescence; PE—Penultimate event; APE—antepenultimate event; MRE—most recent event.

  • McGill and Rockwell (1998) and Dawson et al. (2003) used fault trenching near El Paso Peaks, California to conduct a paleoseismic investigation along the Garlock fault. Below is a map that shows their trench site relative to tectonic features in the region.

  • Map showing the location of the trench site along the Garlock fault. Mountains are shaded, and valleys are shown open. Stippled areas are dry lake beds. SAF is San Andreas fault, DV is Death Valley, QM is Quail Mountains, LTC is Lone Tree Canyon, and SL is Searles (dry) Lake. Modified from McGill and Sieh [1993].

  • McGill and Rockwell (1998) present this figure that shows an aerial image and a geologic map showing topographic features labeled in the aerial image. Note how there is a stream channel that is left-laterally offset.

  • Geomorphic and geologic expression ofthe Garlock fault at the trench site. (top) An annotated aerial photograph (courtesy U.S. Geological Survey) showing the trench site and selected geomorphic features. Unlabeled arrows mark the locations of fault scarps and benches. (bottom) A geologic map of the same area. Scale and orientation of the air photo are the same as shown on the map.

  • Here is an annotated aerial image that was acquired when the light from the sun was at an angle that highlights the topographic features. This low-angle sun aerial photography method was pioneered by Bert Slemmons, one of the fathers of paleoseismology (who advised my HSU professor, Gary Carver when Gary was a student).
  • Note how some features on the north side of the fault are to the left of features on the south side of the fault. This is why we call these left-lateral strike-slip faults. If one turns the image upside down, they will notice that the stuff on the other side of the fault still moves to the left. So, it does not matter what side of the fault one is standing on. I rotated the image below so we can see this first hand (see how features on the top of the image are offset to the left compared to the bottom of the image.


  • Annotated aerial photograph showing local tectonic geomorphology of the trench site. Scale is approximate. Solid lines are mappable fault traces, and dashed lines are inferred fault traces.

  • This photo shows how huge and impressive the fault trenches were that Dawson et al. (2003) excavated for this study. Note the heavy equipment for scale. Read their paper to see the impressive amount of details that they used to unravel the earthquake history.

  • Annotated photograph illustrating some of the additional exposures that were created and documented. Note the location of trench 2, which had been backfilled at the time this photograph was taken. Trench 2 was later reexcavated to create the final and deepest exposure.

  • This is but one example of the complicated sediment stratigraphy and faulting evidence that Dawson et al. (2003) used as a basis for their observations and interpretations. I show both the trench log (artwork) and the annotated panchromatic photo mosaic.


  • Event Y logs and three-dimensional excavation. Figure 7a is a log of a portion of trench 1 with evidence for event Y taken from McGill and Rockwell [1998]. Units shown shaded were interpreted to have been deposited in a collapse pit and then subsequently faulted by event Y. Figure 7b shows the three-dimensional excavation of this feature that shows units 90 and 92 actually being tubular in shape and units 78–42 correlative with units outside of the interpreted collapse feature. Scale varies in this mosaic due to three-dimensionality of the exposure, but the total width of the area shown is about 2.5 m. Dashed lines represent corners of 3-D exposure.

  • These are complicated figures, yet elegant (McGill and Rockwell, 1998; Dawson et al., 2003). My favorite type of figure. The horizontal axis is time in calendar years (now is on the right and the past is on the left). The vertical axis is the thickness of the sedimentary deposits, with the ground surface at the top.
  • Each earthquake is named an event (e.g. Event W). The dots represent radiocarbon ages (and the horizontal lines are the uncertainty associated with these ages). In the Dawson figure, the gray region represents the envelope of possible ages for the sediments between the radiocarbon ages. They assume a linear sedimentation rate between ages. Often people call these radiocarbon dates, but they are ages (it is not possible to obtain a date from radiocarbon age determinations because a date is a single day and these analyses are not that precise).

  • Variation of calibrated radiocarbon dates with stratigraphic depth. Errors shown are 2-sigma. The thick, diagonal line connecting the best estimates of most of the radiocarbon ages illustrates the simplest sedimentation rate history. Thinner, diagonal lines on either side represent the 2-sigma error envelope on the sedimentation rate, assuming that the date of each sample closely approximates its time of deposition. The faulting events visible within the trench are labelled along the right side of the graph, according to their stratigraphic depth; implied, preferred ages are plotted explicitly. Uncertain events are shown in parentheses. Stratigraphic depths to the earthquake horizons and to each depositional unit containing a radiocarbon sample were taken from the composite stratigraphic section shown in Figure 4.


    Variation of calibrated radiocarbon dates with stratigraphic depth. Errors on the calibrated radiocarbon dates are 2-sigma. The curve connecting the solid circles connects the best estimates of the radiocarbon ages, providing the sedimentation rate. The dashed lines give the 2-sigma error envelope on the sedimentation rate.

  • Here is a table showing McGill and Rockwell (1998) earthquake event times and return interval for each prehistoric earthquake.

  • Here is the summary of prehistoric earthquake event times for this part of the Garlock fault (Dawson et al., 2003).

Social Media (UPDATE 2019.07.21

This is not about Ridgecrest, but about the time, 20 June.

    References:

  • Amos, C.B., Bwonlee, S.J., Hood, D.H., Fisher, G.B., Bürgmann, R., Renne, P.R., and Jayko, A.S., 2013. Chronology of tectonic, geomorphic, and volcanic interactions and the tempo of fault slip near Little Lake, California in GSA Bulletin, v. 125, no. 7-8, https://doi.org/10.1130/B30803.1
  • Astiz, L. and Allen, C.R., 1983. Seismicity of the Garlock Fault, California in BSSA v. 73, no. 6, p. 1721-1734
  • Bacon, S.N. and Pezzopane, S.K., 2007. A 25,000-year record of earthquakes on the Owens Valley fault near Lone Pine, California: Implications for recurrence intervals, slip rates, and segmentation models in GSA Bulletin, v. 119, no. 7/8, p. 823-847, https://doi.org/10.1130/B25879.1
  • Bakun, W.H., Ralph A. Haugerud, Margaret G. Hopper, Ruth S. Ludwin, 2002. The December 1872 Washington State Earthquake in BSSA, v. 92, no. 8., https://doi.org/10.1785/0120010274
  • Brocher, T., Margaret G. Hopper, S.T. Ted Algermissen, David M. Perkins, Stanley R. Brockman, and Edouard P. Arnold, 2048. Aftershocks, Earthquake Effects, and the Location of the Large 14 December 1872 Earthquake near Entiat, Central Washington in BSSA, v. 108, no. 1., https://doi.org/10.1785/0120170224
  • Chuang, R.Y. and Johnson, K.M., 2011. Reconciling geologic and geodetic model fault slip-rate discrepancies in Southern California: Consideration of nonsteady mantle flow and lower crustal fault creep in Geology, v. 39, no. 7, p. 627630, https://doi.org/10.1130/G32120.1
  • Dawson, T. E., S. F. McGill, and T. K. Rockwell, Irregular recurrence of paleoearthquakes along the central Garlock fault near El Paso Peaks, California, J. Geophys. Res., 108(B7), 2356, https://doi.org/10.1029/2001JB001744, 2003.
  • Dixon, T.H., Norabuena, E., and Hotaling, L., 2003. Paleoseismology and Global Positioning System: Earthquake-cycle effects and geodetic versus geologic fault slip rates in the Eastern California shear zone in Geology, v. 31, no. 1., p. 55-58,
  • Frankel, K.L., Glazner, A.F., Kirby, E., Monastero, F.C., Strane, M.D., Oskin, M.E., Unruh, J.R., Walker, J.D., Anandakrishnan, S., Bartley, J.M., Coleman, D.S., Dolan, J.F., Finkel, R.C., Greene, D., Kylander-Clark, A., Morrero, S., Owen, L.A., and Phillips, F., 2008, Active tectonics of the eastern California shear zone, in Duebendorfer, E.M., and Smith, E.I., eds., Field Guide to Plutons, Volcanoes, Faults, Reefs, Dinosaurs, and Possible Glaciation in Selected Areas of Arizona, California, and Nevada: Geological Society of America Field Guide 11, p. 43–81, doi: 10.1130/2008.fl d011(03).
  • Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
  • Gan, W., Zhang, P., Shen, Z-K., Prescott, W.H., and Svarc, J.L., 2003. Initiation of deformation of the Eastern California Shear Zone: Constraints from Garlock fault geometry and GPS observations in GRL, v. 30, no. 10, https://doi.org/10.1029/2003GL017090
  • Guest, B., Pavlis, T.L., Goldberg, H., and Serpa, L., 2003. Chasing the Garlock: A study of tectonic response to vertical axis rotation in Geology, v. 31, no. 6, p. 553-556
  • Hayes, G., 2018, Slab2 – A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release, https://doi.org/10.5066/F7PV6JNV.
  • Holt, W. E., C. Kreemer, A. J. Haines, L. Estey, C. Meertens, G. Blewitt, and D. Lavallee (2005), Project helps constrain continental dynamics and seismic hazards, Eos Trans. AGU, 86(41), 383–387, , https://doi.org/10.1029/2005EO410002. /li>
  • Kreemer, C., J. Haines, W. Holt, G. Blewitt, and D. Lavallee, 2000. On the determination of a global strain rate model, Geophys. J. Int., 52(10), 765–770.
  • Kreemer, C. , W.E. Holt, and A.J. Haines, 2002. The global moment rate distribution within plate boundary zones. In S. Stein and J.T. Freymueller (eds.): Plate Boundary Zones, Geodynamics Series, Vol. 30, https://doi.org/10/1029/030GD10
  • Kreemer, C., W. E. Holt, and A. J. Haines, 2003. An integrated global model of present-day plate motions and plate boundary deformation, Geophys. J. Int., 154(1), 8–34, , https://doi.org/10.1046/j.1365-246X.2003.01917.x.
  • Kreemer, C., G. Blewitt, E.C. Klein, 2014. A geodetic plate motion and Global Strain Rate Model in Geochemistry, Geophysics, Geosystems, v. 15, p. 3849-3889, https://doi.org/10.1002/2014GC005407.
  • Kylander-Clark, A.R.C., Coleman, D.S., Glazner, A.F., and Bartley, J.M., 2005. Evidence for 65 km of dextral slip across Owens Valley, California, since 83 Ma in GSA Bulletin, v. 117, no. 7/8, https://doi.org/10.1130/B25624.1
  • McAuliffe, L. J., Dolan, J. F., Kirby, E., Rollins, C., Haravitch, B., Alm, S., & Rittenour, T. M., 2013. Paleoseismology of the southern Panamint Valley fault: Implications for regional earthquake occurrence and seismic hazard in southern California. Journal of Geophysical Research: Solid Earth, 118, 5126-5146, https://doi.org/10.1029/jgrb.50359.
  • McGill, S.F. and Rockwell, T., 1998. Ages of late Holocene earthquakes on the central Garlock fault near El Paso Peaks, California in JGR, v. 103, no. B4, p. 7265-7279
  • McGill, S.F., Wells, S.G., Fortner, S.K., Kuzma, H.A., and McGill, J.D., 2009. Slip rate of the western Garlock fault, at Clark Wash, near Lone Tree Canyon, Mojave Desert, California in GSA Bulletin, v. 121, no. 3/4, https://doi.org/10.1130/B26123.1
  • Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. https://doi.org/10.7289/V5H70CVX
  • Müller, R.D., Sdrolias, M., Gaina, C. and Roest, W.R., 2008, Age spreading rates and spreading asymmetry of the world’s ocean crust in Geochemistry, Geophysics, Geosystems, 9, Q04006, https://doi.org/10.1029/2007GC001743
  • Oskin, M. and Iriondo, A., 2004. Large-magnitude transient strain accumulation on the Blackwater fault, Eastern California shear zone in Geology, v. 32, no. 4, https://doi.org/10.1130/G20223.1
  • Oskin, M., L. Perg, D. Blumentritt, S. Mukhopadhyay, and A. Iriondo, 2007. Slip rate of the Calico fault: Implications for geologic versus geodetic rate discrepancy in the Eastern California Shear Zone, J. Geophys. Res., v. 112, B03402, https://doi.org/10.1029/2006JB004451
  • Oskin, M., Perg, L., Shelef, E., Strane, M., Gurney, E., Singer, B., and Zhang, X., 2008. Elevated shear zone loading rate during an earthquake cluster in eastern California in Geology, v. 36, no. 6, https://doi.org/10.1130/G24814A.1
  • Peltzer, G., Crampe, F., Hensely, S., and Rosen, P., 2001. Transient strain accumulation and fault interaction in the Eastern California shear zone in geology, v. 29, no. 11
  • Petersen, M.D. and Wesnousky, S.G., 1994. Review Fault Slip Rates and Earthquake Histories for Active Faults in Southern California in BSSA, v. 84, no. 5, p. 1608-1649
  • Stein, R.S., Earthquake Conversations, Scientific American, vol. 288, 72-79, January issue, 2003. Republished in: Our Ever Changing Earth, Scientific American, Special Edition, v. 15 (2), 82-89, 2005.
  • Toda, S., Stein, R. S., Richards-Dinger, K. & Bozkurt, S. Forecasting the evolution of seismicity in southern California: Animations built on earthquake stress transfer. J. Geophys. Res. 110, B05S16 (2005) https://doi.org/10.1029/2004JB003415

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Earthquake Report: Ridgecrest Update #2

Well Well Well

Here is a commercial from Sony for Sony Discman following the 1995-96 Ridgecrest Earthquake (from which we have usurped this name for this July 2019 sequence).

The story continues to unfold.

  • Here is a graphic from the USGS that summarizes our observations as of 16 July.

Field Work Narrative

Last week I was lucky enough to spend a week in the field with my coworkers (California Geological Survey) and colleagues (U.S. Geological Survey) making observations of surface rupture from the Ridgecrest Earthquake Sequence (RES). It was initially termed the Searles Valley Earthquake Sequence, but we have since changed the name. Just check out #RidgecrestEarthquake on social media. Our work will be presented in several publications in the coming future. Stay tuned.

Many of us were granted rare access to the Naval Air Weapons Station China Lake. This emergency earthquake response effort was an unprecedented collaborative effort between the Navy, the CGS, and the USGS. We worked together as a team and accomplished our mission goals with due diligence. The CGS/USGS team is out in the field again this week, working off base. We plan to continue doing additional field work for weeks to come. (Though I need to get back to my tsunami stuff as we have deadlines to prepare new tsunami hazard products in the next few weeks to months.)

These collaborative efforts were based on a mutual respect between team agencies and team members. The field team members all appreciated the very special access we were granted. The commanding officer, Captain Paul Dale, is very supportive of scientific research and his support of our mission was evidence of this.

We were granted permission to take photos of the geologic evidence of the earthquake and ground shaking. We reviewed our images with the Public Affairs Officer to ensure that we did not take photos of any facilities or equipment that was on the base. This was important and we were very careful about this. We even double checked the images after we got back from the field.

I will add some photos to this page tomorrow.

Remote Sensing Narrative

There has also been a large number of Earth scientists using remote sensing data to evaluate the RES. These data are primarily from satellite images of different types (spectral imagery (another word for what we used to call air photos), RADAR, Global Positioning Systems (GPS), seismometer observations, etc.).

For most of these methods, pre-earthquake data are compared with post-earthquake data for a comparison. The methods used for these comparisons is advancing at a lightning pace. Every year, these models get better and better.

These remote sensing methods allow us to infer how the ground moved and slipped during and after the earthquake. We can get estimates of the slip on the fault from this type of analysis.

Combining different sources of remote sensing data also allows us to make estimates of the faults, where they moved, and how much they moved (in the subsurface).

I will present some of these observations below.

USGS Data Products

I prepared some interpretive posters for the M 7.1 earthquake shortly after it happened. The USGS earthquake pages are a source of great information as evidenced by how hard they are hit by web visitors following events as significant as the M 7.1. The website was unusable for periods of time. This demonstrates that the USGS is doing something right.

Last weekend, I spent Saturday preparing the same types of interpretive posters that I presented here, but as comparisons between the M 6.4 and M 7.1 temblors.

  • Here is an updated seismicity map. There are two main types of earthquakes on this map. I present this map both with aerial imagery and with a topographic (“hillshade”) basemap. I outline the general area of Ridgecrest in purple.
    1. First, there are an abundance of aftershocks aligned with the two main faults that ruptured during this sequence (the northwest trending M 7.1 fault and the northeast trending M 6.4 fault). Part of the northwest striking fault ruptured during the M 6.4 event.
    2. Second, there are several areas that show earthquakes that were triggered by this sequence. There are some triggered earthquakes along the Coso Range (where the Coso Geothermal Field is located), some events along the Garlock fault, and some temblors along the Ash Hill fault (in Panamint Valley, to the north of Searles Valley).



  • This is a seismicity comparison for the two earthquakes. on the left are earthquakes (USGS) from prior to the M 7.1 earthquake and on the right are quakes after and including the M 7.1 temblor. I plot the USGS Quaternary fault and fold database on the left as black lines.

  • Here is a map with landslide probability on it. Please head over to that report for more information about the USGS Ground Failure products (landslides and liquefaction). Basically, earthquakes shake the ground and this ground shaking can cause landslides. We can see that there is a low probability for landslides. However, we have already seen photographic evidence for landslides and the lower limit for earthquake triggered landslides is magnitude M 5.5 (from Keefer 1984-ish).

  • Here is a map showing liquefaction susceptibility. I explain more about this type of map in my original report for the M 6.4 earthquake. Scroll down a bit to find the landslide and liquefaction maps for that event.

  • Finally, here is a map that shows the shaking intensity for the M 6.4 and M 7.1 earthquakes. As I mention in my original report, this is based on a model that relates earthquake shaking intensity with earthquake magnitude and distance from the earthquake. Note that there was violent shaking from the M 7.1 event (MMI IX).

NASA JPL ARIA Data Products

  • NASA Jet Propulsion Laboratory (JPL) prepares Advanced Rapid Imaging and Analysis (ARIA) data products for major events worldwide. Their data are presented online here. I used the data from this event in a GIS computer program, but the data are prepared in Google Earth files too (so everyone can use them if they have a modern computer with an internet connection). This is a valuable government service.
  • This first map shows the results of modeling Synthetic Aperature Radar Interferometry data. Basically, Radar satellite imagery data from before and from after the earthquake are compared to model the amount of ground deformation that occurred between the satellite acquisitions. Each color band represents a certain amount of motion. This is referred to as the wrapped image.
  • Here are a series of sources of background information about InSAR analysis.

  • This map is made using the same basic data, though it has been processed in a way to show the overall ground motion with just two colors, instead of color bands. This is called the unwrapped image.

  • Below is the first in a series of videos that explains more about SAR and InSAR analyses.

Dr. Sotiris Valkaniotis

  • Dr. Valkaniotis is a Greek geologist who has a great set of remote sensing skills who studies earthquake geology and paleoseismology. I include lots of social media posts below where people share their analyses. However, I select two images from Dr. Valkaniotis for this earthquake. Contact him for more information about his processing. As embedded below in the social media section, here is the tweet that is the source of these two maps.
  • These images are similar to the NASA JPL ARIA unwrapped maps above. I include his description below in blockquote.

  • Gradient render from unwrapped LOS displacement map (higher quality 20m from SNAP). Surface ruptures (major & minor) are easily visible as dark linear features (high displacement gradient). Processing in @esa_gep. Descending pair from #Sentinel1, #Ridgecrestearthquake


    And the ascending pair from #Sentinel1, #Ridgecrestearthquake. Gradient render from unwrapped LOS displacement map (higher quality 20m from SNAP). Processing in @esa_gep.

  • Here is a map that Dr. Valkaniotis prepared showing fault lines he has interpreted from his model results.

  • Complex and detailed pattern of co-seismic ruptures for the #RidgecrestEarthquake sequence. Red lines are primary & secondary surface ruptures, together with small triggered ruptures away from main faults. Previously mapped Quaternary Faults with yellow, for comparison.

PBS News Hour: 2019.07.08

Death Valley at Devil’s Hole

The clip shows water violently sloshing around, rising and falling 10 to 15 feet, according to a park estimate. The video captures two angles, one looking into the cave and the other underwater inside it.

Devils Hole is a part of the desert uplands and spring-fed oases that make up the Ash Meadows complex, a national wildlife refuge.

Temblor Articles

Ross Stein (Ph.D.), Volkan Sevilgan (M.Sc.), Tiegan Hobbs (Ph.D.), Chris Rollins (Ph.D.), Geoffrey Ely, (Ph.D.), and Shinji Toda (Ph.D.) are coauthors to a suite of 5 articles presented on Temblor.net. Temblor is a National Science Foundation funded organization that promotes earthquake insurance and seismic retrofits for people in earthquake country. I wrote several articles for Temblor prior to starting work at the California Geological Survey. (My efforts at earthjay.com are purely volunteer and do not reflect endorsement nor review from or by CGS.)

These reports are excellent sources of interpretive information at the detail for non experts (sometimes my reports are at a detail more aimed towards undergraduate geology students, though I attempt to make them available to a broad audience as well). I include a few figures from their reports that I find most interesting, but please check out their articles for more information!

  • Dr. Stein begins by presenting an hypothesis that these earthquakes are in a region of increased tectonic stress following the 1872 Owens Valley Earthquake, estimated to have a magnitude of M 7.6 (though it happened prior to modern seismometer instrumentation, so magnitude estimates have considerable uncertainty).
  • When earthquake faults slip, the surrounding crust is squished and squashed. This deformation changes the tectonic stresses in the crust. In some places this change causes an increase in the amount of stress on earthquake faults and in some places it decreases the tectonic stress. In places where the stress increases, the fault is brought closer to having an earthquake, and vice versa for places where the stress is diminished.
  • These stress changes are very small, so for a fault to be triggered by these changes in “static coulomb stress,” the fault had to be almost ready to slip before these changes happened. More can be found in Stein (2003) and Toda et al. (2005) linked below in the references.
  • In the map below, warm colors represent areas with an increase in (static coulomb stress) and cool colors represent a decrease in stress. I include their figure caption in blockquote below the figure (as for all their figures).

  • he site of the July 4th shock was likely brought closer to failure in the 1872 M~7.6 shock. Notice that the (red) stress trigger zones of the this 148-year-old quake are all seismically active today, whereas the (blue) stress shadows are generally devoid of shocks.

  • The Owens Valley fault triggering is speculative of course, since that earthquake was so long ago. However, there are other cases where aftershocks or triggered earthquakes are happening a long time after the main event. For example, there are ongoing aftershocks following an 1872 earthquake near Lake Chelan (Bakun et al., 2002; Brocher et al., 2018).
  • Stein and his colleagues calculated “static coulomb” stress changes imparted by the Ridgecrest Earthquake Sequence onto a series of other faults in the area. Read more about their analyses here.

  • Here we calculate stress transferred to the principal mapped faults, using the USGS slip model for the 7.1 and a model based on University of Nevada Reno GPS displacements for the 6.4 (not shown here for simplicity, but included). Most of the stress change is from the 7.1: it was several times larger than the 6.4 and torqued the surrounding crust far more. This fault inventory might be woefully incomplete, of course: the 7.1 itself struck on an unmapped fault. Nevertheless, the most striking result is the >2-bar stress increase on a 30-km (20-mile) section of the Garlock Fault. An end-to-end rupture on the Garlock, if (still) possible, would be in the magnitude 7.6-7.8 range.

  • In my interpretive posters above, I mention the areas where there have been triggered earthquakes (e.g. the Coso Geothermal Field, the Garlock fault, the Ash Hill fault). Turns out, Stein and his colleagues were thinking the same thing.
  • They prepared a figure in their report here where they show changes in “static coulomb” stress. They label the same areas I mention (except the Ash Hill fault in Panamint Valley). Take a look at the areas of increased stress compared to these three regions (even the Ash Hill fault is in an area of increased stress).

  • Faults in the red lobes are calculated to be brought closer to failure; those in the blue ‘stress shadows’ are inhibited from failure. The calculation estimates what the dominant fault orientations are around the earthquakes by interpolating between major mapped faults (shown in red lines). So, we would expect strong stressing in the Coso Volcanic Field to the north (where the aftershocks lie), and along the Garlock Fault to the south (but not where most of them lie).

  • Hobbs and Rollins speculate that the San Andreas fault may also have changes in (static coulomb) stress imparted by the Garlock fault if that were to slip. Read more in their article here.

  • If the western and central Garlock were to rupture, it would load the section of the San Andreas just north of Los Angeles. The jog in the San Andreas under the S in “Source” is at Palmdale. Figure from McAuliffe et al. [2013].

Below are all the Temblor articles to read


2019.07.04 Southern California M 6.4 earthquake stressed by two large historic ruptures
2019.07.05 Earthquake early warning system challenged by the largest SoCal shock in 20 years
2019.07.06 Magnitude 7.1 earthquake rips northwest from the M6.4 just 34 hours later
2019.07.06 M 7.1 SoCal earthquake triggers aftershocks up to 100 mi away: What’s next?
2019.07.09 The Ridgecrest earthquakes: Torn ground, nested foreshocks, Garlock shocks, and Temblor’s forecast
  • Here are the references for these Temblor articles.
    • Stein, R. S., and Sevilgen, V., (2019), Southern California M 6.4 earthquake stressed by two large historic ruptures, Temblor, http://doi.org/10.32858/temblor.034
    • Hobbs, T.E. and Rollins, C., (2019), Earthquake early warning system challenged by the largest SoCal shock in 20 years, Temblor, http://doi.org/10.32858/temblor.035
    • Ross S. Stein, Tiegan Hobbs, Chris Rollins, Geoffrey Ely, Volkan Sevilgen, and Shinji Toda, (2019), Magnitude 7.1 earthquake rips northwest from the M6.4 just 34 hours later, Temblor, http://doi.org/10.32858/temblor.037
    • Ross S. Stein, Chris Rollins, Volkan Sevilgen, and Tiegan Hobbs, (2019), M 7.1 SoCal earthquake triggers aftershocks up to 100 mi away: What’s next?, Temblor, http://doi.org/10.32858/temblor.038
    • Chris Rollins, Ross S. Stein, Guoqing Lin, and Deborah Kilb (2019), The Ridgecrest earthquakes: Torn ground, nested foreshocks, Garlock shocks, and Temblor’s forecast, Temblor, http://doi.org/10.32858/temblor.039

Field Photos

  • Below are some field photos I took. I cannot tell anyone where they were taken (at least not yet) as we don’t have clearance. I may post more later, but wanted to post some to show people the type of observations we were making.
  • This is Dr. Chris DuRoss (USGS) as we walked across the scarp at our first site working together.

  • Here is a great one of Dr. Jessie T. Jobe (USGS, soon to be USBR) taking notes at that same scarp (DuRoss’ boots for scale).

  • This is a portion of a road where the fault crossed. There were several dm of lateral offset on either side of the road, but the road itself had an imperceptible amount of lateral offset (i.e. 1 ± 1 cm offset). There was some amount of compression here.

  • Here we were projecting the ground surface across the fault to estimate the amount of vertical displacement. Dr. Ryan Gold (USGS) is measuring while a Navy Base geologist is holding the profile stick along the ground surface.

  • Here is a photo very similar to Mr. Brian Olson’s tweeted photo, but I took this one instead. Dr. Belle Philibosian (USGS) is on the left and Kelly (NAWCL geologist) is on the right. This shows right-lateral strike-slip displacement of 420 cm. We thought nobody would believe us, so we made another measurement nearby to confirm.

  • I located some beautiful slickenlines (grooves in the fault surface created when the fault slips) and this is Dr. Beth Haddon (USGS) collecting strike, dip and rake data for these lines. We collected many photos of this site so that we can create a 3-D model (using structure from motion).

  • Here is Dr. Belle Philibosian looking spectacular as usual, providing scale to help us understand the amount of vertical separation across the fault in this location.

  • We located some evidence for liquefaction too. Here is a sand volcano, where lots of the sediment got washed away by the fluid that possibly shot up through this hole.

  • This was a great opportunity to show the compass orientation of these conjugate fault offsets in the road. The road material properties probably controlled the location of the faults here (there were pre-existing planes of weakness as evidenced by the tar patches, but some of the pavement faulting was new).

    References:

  • Amos, C.B., Bwonlee, S.J., Hood, D.H., Fisher, G.B., Bürgmann, R., Renne, P.R., and Jayko, A.S., 2013. Chronology of tectonic, geomorphic, and volcanic interactions and the tempo of fault slip near Little Lake, California in GSA Bulletin, v. 125, no. 7-8, https://doi.org/10.1130/B30803.1
  • Bakun, W.H., Ralph A. Haugerud, Margaret G. Hopper, Ruth S. Ludwin, 2002. The December 1872 Washington State Earthquake in BSSA, v. 92, no. 8., https://doi.org/10.1785/0120010274
  • Brocher, T., Margaret G. Hopper, S.T. Ted Algermissen, David M. Perkins, Stanley R. Brockman, and Edouard P. Arnold, 2048. Aftershocks, Earthquake Effects, and the Location of the Large 14 December 1872 Earthquake near Entiat, Central Washington in BSSA, v. 108, no. 1., https://doi.org/10.1785/0120170224
  • Frankel, K.L., Glazner, A.F., Kirby, E., Monastero, F.C., Strane, M.D., Oskin, M.E., Unruh, J.R., Walker, J.D., Anandakrishnan, S., Bartley, J.M., Coleman, D.S., Dolan, J.F., Finkel, R.C., Greene, D., Kylander-Clark, A., Morrero, S., Owen, L.A., and Phillips, F., 2008, Active tectonics of the eastern California shear zone, in Duebendorfer, E.M., and Smith, E.I., eds., Field Guide to Plutons, Volcanoes, Faults, Reefs, Dinosaurs, and Possible Glaciation in Selected Areas of Arizona, California, and Nevada: Geological Society of America Field Guide 11, p. 43–81, doi: 10.1130/2008.fl d011(03).
  • Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
  • Hayes, G., 2018, Slab2 – A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release, https://doi.org/10.5066/F7PV6JNV.
  • Holt, W. E., C. Kreemer, A. J. Haines, L. Estey, C. Meertens, G. Blewitt, and D. Lavallee (2005), Project helps constrain continental dynamics and seismic hazards, Eos Trans. AGU, 86(41), 383–387, , https://doi.org/10.1029/2005EO410002. /li>
  • Kreemer, C., J. Haines, W. Holt, G. Blewitt, and D. Lavallee (2000), On the determination of a global strain rate model, Geophys. J. Int., 52(10), 765–770.
  • Kreemer, C., W. E. Holt, and A. J. Haines (2003), An integrated global model of present-day plate motions and plate boundary deformation, Geophys. J. Int., 154(1), 8–34, , https://doi.org/10.1046/j.1365-246X.2003.01917.x.
  • Kreemer, C., G. Blewitt, E.C. Klein, 2014. A geodetic plate motion and Global Strain Rate Model in Geochemistry, Geophysics, Geosystems, v. 15, p. 3849-3889, https://doi.org/10.1002/2014GC005407.
  • McAuliffe, L. J., Dolan, J. F., Kirby, E., Rollins, C., Haravitch, B., Alm, S., & Rittenour, T. M., 2013. Paleoseismology of the southern Panamint Valley fault: Implications for regional earthquake occurrence and seismic hazard in southern California. Journal of Geophysical Research: Solid Earth, 118, 5126-5146, https://doi.org/10.1029/jgrb.50359.
  • Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. https://doi.org/10.7289/V5H70CVX
  • Müller, R.D., Sdrolias, M., Gaina, C. and Roest, W.R., 2008, Age spreading rates and spreading asymmetry of the world’s ocean crust in Geochemistry, Geophysics, Geosystems, 9, Q04006, https://doi.org/10.1029/2007GC001743
  • Stein, R.S., Earthquake Conversations, Scientific American, vol. 288, 72-79, January issue, 2003. Republished in: Our Ever Changing Earth, Scientific American, Special Edition, v. 15 (2), 82-89, 2005.
  • Toda, S., Stein, R. S., Richards-Dinger, K. & Bozkurt, S. Forecasting the evolution of seismicity in southern California: Animations built on earthquake stress transfer. J. Geophys. Res. 110, B05S16 (2005) https://doi.org/10.1029/2004JB003415

Return to the Earthquake Reports page.

Posted in earthquake, education, geology, plate tectonics, San Andreas, strike-slip

Earthquake Report: Halmahera, Indonesia

Well, yesterday I was preparing some updates to the Ridgecrest Earthquake following my field work with my colleagues at the California Geological Survey (where I work) and the U.S. Geological Survey. We spent the week documenting surface ruptures associated with the M 6.4 and M 7.1 Ridgecrest Earthquake Sequence. (it is currently named the Searles Valley Earthquake Sequence, but I am calling it the Ridgecrest Earthquake)

I was just about done with these new maps and getting ready to start writing them up in an updated earthquake report when I noticed that there was an interesting earthquake, with few historic analogues, along the Western Australia Shear Zone offshore of northwestern Australia. I probably won’t get to that earthquake, but I started downloading some material and reviewing my literature for the region. I considered doing both of these tasks on Sunday (today). That was not to be as I awakened to an email about this magnitude M 7.3 earthquake in Halmahera, Indonesia. I have several earthquake reports for the Molucca Strait, west of Halmahera. So, I have some background literature and knowledge about this region already.

There was an earthquake along Molucca Strait that I could not work on due to my field work. So I will briefly mention that quake here. There was also a recent earthquake to the south, in the Banda Sea (here is my earthquake report for that event). The June earthquake had the same magnitude as today’s shaker, M = 7.3. However, the earlier quake was too deep to cause a tsunami (unlike today’s temblor). Earthquakes along the Molucca Strait have generated tsunami with wave heights of over 9 meters (30 feet) according toe Harris and Major, 2016.

The Molucca Strait is a north-south oriented seaway formed by opposing subduction zone / thrust faults (convergent plate boundaries). See the “Geologic Fundamentals” section below for an explanation of different fault types. On the west of the Molucca Strait is a thrust fault that dips downwards to the west. On the east, there is a thrust fault that dips down to the east (beneath the island of Halmahera).

There is a major east-west trending (striking) strike-slip fault that comes into the region from the east, called the Sorong fault. There are multiple strands of this system. A splay of this Sorong fault splays northwards through the island of Halmahera. There may be additional details about how this splay relates to the Sorong fault, but I was unable to locate any references (or read the details) today. According to BMKG, the fault that is associated with this earthquake is the Sorong-Bacan fault.

Today’s M 7.3 Halmahera earthquake is a strike-slip earthquake (the plates move side-by-side, like the San Andreas or North Anatolia faults). Often people don’t think of tsunami when a strike-slip earthquake happens because there is often little vertical ground motion. Many people are otherwise familiar with thrust or subduction zone earthquakes, which can produce significant uplift and subsidence (vertical land motion), that can lead to significant tsunami.

However, there is abundant evidence that strike-slip earthquakes do cause tsunami, though often of much smaller size than their thrust/subduction siblings. The main difference is that these strike-slip generated tsunami are much smaller in size.

For example, the 1999 Izmit and 2012 Wharton Basin earthquakes provided empirical evidence of strike-slip earthquake triggered tsunami. More recently, the 28 September 2018 magnitude M 7.5 Dongalla-Palu earthquake caused a tsunami in Palu Bay, Sulawesi, Indonesia that exceeded 10 meters (33 feet) in wave height (wave run up elevation)!!! I just got an email from Dr. Lori Dengler who is an a conference where people claim that the earthquake is possibly singlehandedly responsible for this large wave. Previously people thought that there may have been submarine landslides that contributed to the size.

Here is the tide gage record from a gage near today’s M 7.3 earthquake. The earthquake epicenter appears to be on land, so the tsunami is possibly smaller because of this. Indonesia operates a network of tide gages throughout the region here. The gage data below are from the island of Gebe, about 50 miles to the east of the M 7.3 epicenter.


Here is a quote from the Meteorology, Climatology and Geophysics Agency (BMKG) website:

Impact of Earthquake

Based on community reports, it was shown that shocks were felt in Bitung and Manado with the intensity of IV-V MMI (felt by almost all residents, many people built), and in Ternate III-IV MMI (felt by many people in the house). Until now there have been no reports of damage due to a strong earthquake shock in northern Maluku last night. The impact of the North Maluku earthquake only caused a tremendous panic among the people. In the city of Manado, some of the houses of the walls had cracks in the building walls of the building with very light categories.

Now I can get back to working on a Ridgecrest update… stay tuned. (the maps are already made)

Below is my interpretive poster for this earthquake


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend).

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange). Due to the high rate of seismicity in this region, I do not have an historic seismicity poster for this event.

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours transparently on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include the slab 2.0 contours plotted transparently (Hayes, 2018), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.

    Magnetic Anomalies

  • In the map below, I include a transparent overlay of the magnetic anomaly data from EMAG2 (Meyer et al., 2017). As oceanic crust is formed, it inherits the magnetic field at the time. At different points through time, the magnetic polarity (north vs. south) flips, the North Pole becomes the South Pole. These changes in polarity can be seen when measuring the magnetic field above oceanic plates. This is one of the fundamental evidences for plate spreading at oceanic spreading ridges (like the Gorda rise).
  • Regions with magnetic fields aligned like today’s magnetic polarity are colored red in the EMAG2 data, while reversed polarity regions are colored blue. Regions of intermediate magnetic field are colored light purple.

    I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

  • In the upper left corner is a plate tectonic map showing major fault lines for the Molucca Strait and Halmahera region (Waltham et al., 2008). I place a blue star in the general location of today’s M 7.3 earthquake.
  • In the lower left corner is a low angle oblique view of the tectonic plates in this region (Hall, 2011). The view is from the southeast looking into the Earth towards the northwest.
  • In the lower right corner are the tide gage data from the tide gage at Pulau Gebe. These data were provided by the Indonesian Government here. These appear to be tsunami waves, they lasted over 5 hours and had a small wave height of 12 centimeters..
  • In the upper right corner is a part of the Global Earthquake Model (GEM) seismic hazard map that uses cool colors to represent a lower level of shaking intensity than warm colors (Silva et al., 2018). The units are in g (gravitational acceleration). 1 g = Earth’s gravity, so hypothetically, “rocks can get thrown in the air at 1g.” This map is prepared based on the chance an area will have earthquakes of a given size based on a combination of many different seismic hazard models. The region where today’s earthquake happened is colored yellow and has a 10% chance of shaking that 0.2g to 0.35 g (or stronger) over the next 50 years.
  • Below the hazard map is the GEM seismic risk map presents the geographic distribution of average annual loss (USD) due to ground shaking in the residential, commercial and industrial building stock, considering contents, structural and non-structural components. Warmer colors represent larger loss over time. Risk is the overlap of hazard and population. If there are no people, but there is seismic hazard, there is no seismic risk.
  • To the left of the GEM maps is a map of Halmahera and some surrounding islands. The color shows the level of seismic hazard for these islands (Zulkifli et al.,l 2017). The color shows the estimated Peak level of ground shaking for a period of 500 years (i.e. 10% probability of exceedance in 50 years). The units are the same (g). The M 7.3 earthquake generated up to ~.25 g, which is higher than the model would suggest (between 0.03 and 0.06 g).
  • Here is the map with a month’s seismicity plotted.

  • Here is the map with a century’s seismicity plotted. In the future I hope to get around to plotting earthquake mechanisms on this map. Yellow fault lines are from the Coordinating Committee Geoscience East-Southeast Asia consortium (CCOF). Red fault lines are from the Global Earthquake Model (GEM) Foundation.

Other Report Pages

Shaking Intensity and Potential for Ground Failure

  • Below are a series of maps that show the shaking intensity and potential for landslides and liquefaction. These are all USGS data products.
  • There are many different ways in which a landslide can be triggered. The first order relations behind slope failure (landslides) is that the “resisting” forces that are preventing slope failure (e.g. the strength of the bedrock or soil) are overcome by the “driving” forces that are pushing this land downwards (e.g. gravity). The ratio of resisting forces to driving forces is called the Factor of Safety (FOS). We can write this ratio like this:

    FOS = Resisting Force / Driving Force

    When FOS > 1, the slope is stable and when FOS < 1, the slope fails and we get a landslide. The illustration below shows these relations. Note how the slope angle α can take part in this ratio (the steeper the slope, the greater impact of the mass of the slope can contribute to driving forces). The real world is more complicated than the simplified illustration below.


    Landslide ground shaking can change the Factor of Safety in several ways that might increase the driving force or decrease the resisting force. Keefer (1984) studied a global data set of earthquake triggered landslides and found that larger earthquakes trigger larger and more numerous landslides across a larger area than do smaller earthquakes. Earthquakes can cause landslides because the seismic waves can cause the driving force to increase (the earthquake motions can “push” the land downwards), leading to a landslide. In addition, ground shaking can change the strength of these earth materials (a form of resisting force) with a process called liquefaction.

    Sediment or soil strength is based upon the ability for sediment particles to push against each other without moving. This is a combination of friction and the forces exerted between these particles. This is loosely what we call the “angle of internal friction.” Liquefaction is a process by which pore pressure increases cause water to push out against the sediment particles so that they are no longer touching.

    An analogy that some may be familiar with relates to a visit to the beach. When one is walking on the wet sand near the shoreline, the sand may hold the weight of our body generally pretty well. However, if we stop and vibrate our feet back and forth, this causes pore pressure to increase and we sink into the sand as the sand liquefies. Or, at least our feet sink into the sand.

    Below is a diagram showing how an increase in pore pressure can push against the sediment particles so that they are not touching any more. This allows the particles to move around and this is why our feet sink in the sand in the analogy above. This is also what changes the strength of earth materials such that a landslide can be triggered.


    Below is a diagram based upon a publication designed to educate the public about landslides and the processes that trigger them (USGS, 2004). Additional background information about landslide types can be found in Highland et al. (2008). There was a variety of landslide types that can be observed surrounding the earthquake region. So, this illustration can help people when they observing the landscape response to the earthquake whether they are using aerial imagery, photos in newspaper or website articles, or videos on social media. Will you be able to locate a landslide scarp or the toe of a landslide? This figure shows a rotational landslide, one where the land rotates along a curvilinear failure surface.


    Here is a map with landslide probability on it (Jessee et al., 2017). Please head over to that report for more information about the USGS Ground Failure products (landslides and liquefaction). Basically, earthquakes shake the ground and this ground shaking can cause landslides. We can see that there is a low probability for landslides. However, we have already seen photographic evidence for landslides and the lower limit for earthquake triggered landslides is magnitude M 5.5 (from Keefer 1984)


    Nowicki Jessee and others (2018) is the preferred model for earthquake-triggered landslide hazard. Our primary landslide model is the empirical model of Nowicki Jessee and others (2018). The model was developed by relating 23 inventories of landslides triggered by past earthquakes with different combinations of predictor variables using logistic regression. The output resolution is ~250 m. The model inputs are described below. More details about the model can be found in the original publication. We modify the published model by excluding areas with slopes <5° and changing the coefficient for the lithology layer "unconsolidated sediments" from -3.22 to -1.36, the coefficient for "mixed sedimentary rocks" to better reflect that this unit is expected to be weak (more negative coefficient indicates stronger rock).To exclude areas of insignificantly small probabilities in the computation of aggregate statistics for this model, we use a probability threshold of 0.002.

    Here is an excellent educational video from IRIS and a variety of organizations. The video helps us learn about how earthquake intensity gets smaller with distance from an earthquake. The concept of liquefaction is reviewed and we learn how different types of bedrock and underlying earth materials can affect the severity of ground shaking in a given location. The intensity map above is based on a model that relates intensity with distance to the earthquake, but does not incorporate changes in material properties as the video below mentions is an important factor that can increase intensity in places.

    Here is a map showing liquefaction susceptibility (Zhu et al., 2017).


    Zhu and others (2017) is the preferred model for liquefaction hazard. The model was developed by relating 27 inventories of liquefaction triggered by past earthquakes to globally-available geospatial proxies (summarized below) using logistic regression. We have implemented the global version of the model and have added additional modifications proposed by Baise and Rashidian (2017), including a peak ground acceleration (PGA) threshold of 0.1 g and linear interpolation of the input layers. We also exclude areas with slopes >5°. We linearly interpolate the original input layers of ~1 km resolution to 500 m resolution. The model inputs are described below. More details about the model can be found in the original publication.

Here is a map that shows a comparison of modeled shaking intensity for both the M 6.9 Molucca Strait (the left panel) and M 7.3 Halmahera (the right panel) earthquakes. The legend shows the MMI scale, which I discuss above.


Seismic Hazard and Seismic Risk

  • These are the two maps shown in the map above, the GEM Seismic Hazard and the GEM Seismic Risk maps from Pagani et al. (2018) and Silva et al. (2018).
    • The GEM Seismic Hazard Map:


    • The Global Earthquake Model (GEM) Global Seismic Hazard Map (version 2018.1) depicts the geographic distribution of the Peak Ground Acceleration (PGA) with a 10% probability of being exceeded in 50 years, computed for reference rock conditions (shear wave velocity, VS30, of 760-800 m/s). The map was created by collating maps computed using national and regional probabilistic seismic hazard models developed by various institutions and projects, and by GEM Foundation scientists. The OpenQuake engine, an open-source seismic hazard and risk calculation software developed principally by the GEM Foundation, was used to calculate the hazard values. A smoothing methodology was applied to homogenise hazard values along the model borders. The map is based on a database of hazard models described using the OpenQuake engine data format (NRML); those models originally implemented in other software formats were converted into NRML. While translating these models, various checks were performed to test the compatibility between the original results and the new results computed using the OpenQuake engine. Overall the differences between the original and translated model results are small, notwithstanding some diversity in modelling methodologies implemented in different hazard modelling software. The hashed areas in the map (e.g. Greenland) are currently not covered by a hazard model. The map and the underlying database of models are a dynamic framework, capable to incorporate newly released open models. Due to possible model limitations, regions portrayed with low hazard may still experience potentially damaging earthquakes.

    • The GEM Seismic Risk Map:


    • The Global Seismic Risk Map (v2018.1) presents the geographic distribution of average annual loss (USD) normalised by the average construction costs of the respective country (USD/m2) due to ground shaking in the residential, commercial and industrial building stock, considering contents, structural and non-structural components. The normalised metric allows a direct comparison of the risk between countries with widely different construction costs. It does not consider the effects of tsunamis, liquefaction, landslides, and fires following earthquakes. The loss estimates are from direct physical damage to buildings due to shaking, and thus damage to infrastructure or indirect losses due to business interruption are not included. The average annual losses are presented on a hexagonal grid, with a spacing of 0.30 x 0.34 decimal degrees (approximately 1,000 km2 at the equator). The average annual losses were computed using the event-based calculator of the OpenQuake engine, an open-source software for seismic hazard and risk analysis developed by the GEM Foundation. The seismic hazard, exposure and vulnerability models employed in these calculations were provided by national institutions, or developed within the scope of regional programs or bilateral collaborations. This global map and the underlying databases are based on best available and publicly accessible datasets and models. Due to possible model limitations, regions portrayed with low risk may still experience potentially damaging earthquakes.

Tsunami Hazard

  • Here are two maps that show the results of probabilistic tsunami modeling for the nation of Indonesia (Horspool et al., 2014). These results are similar to results from seismic hazards analysis and maps. The color represents the chance that a given area will experience a certain size tsunami (or larger).
  • The first map shows the annual chance of a tsunami with a height of at least 0.5 m (1.5 feet). The second map shows the chance that there will be a tsunami at least 3 meters (10 feet) high at the coast.

  • Annual probability of experiencing a tsunami with a height at the coast of (a) 0.5m (a tsunami warning) and (b) 3m (a major tsunami warning).

Some Relevant Discussion and Figures

  • Here is a tectonic map for this part of the world from Zahirovic et al., 2014. They show a fracture zone where the M 7.3 earthquake happened. I left out all the acronym definitions (you’re welcome), but they are listed in the paper.

  • Regional tectonic setting with plate boundaries (MORs/transforms = black, subduction zones = teethed red) from Bird (2003) and ophiolite belts representing sutures modified from Hutchison (1975) and Baldwin et al. (2012). West Sulawesi basalts are from Polvé et al. (1997), fracture zones are from Matthews et al. (2011) and basin outlines are from Hearn et al. (2003).

  • Here are maps showing the regional tectonics (Smoczyk et al., 2013).

  • Along its western margin, the Philippine Sea plate is associated with a zone of oblique convergence with the Sunda plate. This highly active convergent plate boundary extends along both sides the Philippine Islands, from Luzon in the north to Sulawesi in the south. The tectonic setting of the Philippines is unusual in several respects: it is characterized by opposite-facing subduction systems on its east and west sides; the archipelago is cut by a major transform fault, the Philippine Fault; and the arc complex itself is marked by volcanism, faulting, and high seismic activity. Subduction of the Philippine Sea plate occurs at the eastern margin of the archipelago along the Philippine Trench and its northern extension, the East Luzon Trough. The East Luzon Trough is thought to be an unusual example of a subduction zone in the process of formation, as the Philippine Trench system gradually extends northward (Hamburger and others, 1983).

  • This shows Global Positioning System (GPS) velocities at various locations. These plate motions are represented as vectors in mm/yr. (see legend) Here note how the vector labeled phil/eura (for the motion of the PSP relative to the Eurasia plate) is oblique to the plate margin along the Philippine trench (i.e. the PSP is not subducting perpendicular to the megathrust fault). The oblique relative motion seems to lead to strain partitioning, leading to a forearc sliver fault (the Philippine fault, shown in maps above). Below I include the text from the original figure caption in blockquote.

  • Topographic and tectonic map of the Indonesian archipelago and surrounding region. Labeled, shaded arrows show motion (NUVEL-1A model) of the first-named tectonic plate relative to the second. Solid arrows are velocity vectors derived from GPS surveys from 1991 through 2001, in ITRF2000. For clarity, only a few of the vectors for Sumatra are included. The detailed velocity field for Sumatra is shown in Figure 5. Velocity vector ellipses indicate 2-D 95% confidence levels based on the formal (white noise only) uncertainty estimates. NGT, ew Guinea Trench; NST, North Sulawesi Trench; SF, Sumatran Fault; TAF, Tarera-Aiduna Fault. Bathymetry [Smith and Sandwell, 1997] in this and all subsequent figures contoured at 2 km intervals

  • This is one of my favorite figures of all time (Hall, 2011). Read below for more details.

  • 3D cartoon of plate boundaries in the Molucca Sea region modified from Hall et al. (1995). Although seismicity identifies a number of plates there are no continuous boundaries, and the Cotobato, North Sulawesi and Philippine Trenches are all intraplate features. The apparent distinction between different crust types, such as Australian continental crust and oceanic crust of the Philippine and Molucca Sea, is partly a boundary inactive since the Early Miocene (east Sulawesi) and partly a younger but now probably inactive boundary of the Sorong Fault. The upper crust of this entire region is deforming in a much more continuous way than suggested by this cartoon.

  • Here is a map and cross section presented by Waltham et al. (2008). They use a variety of data sources as a basis for their interpretations (seismic reflection data, gravity data). Note how the Molucca Sea plate subducts both to the west and to the east. Below I include the text from the original figure caption in blockquote.

  • (A) Location and major tectonic features of the Molucca Sea region. Small, black-fi lled triangles are modern volcanoes. Bathymetric contours are at 200, 2000, 4000, and 6000 m. Large barbed lines are subduction zones, and small barbed lines are thrusts. (B) Cross section across the Halmahera and Sangihe Arcs on section line B. Thrusts on each side of the Molucca Sea are directed outward toward the adjacent arcs, although the subducting Molucca Sea plate dips east beneath Halmahera and west below the Sangihe Arc. (C) Inset is the restored cross section of the Miocene–Pliocene Weda Bay Basin of SW Halmahera on section line C, fl attened to the Pliocene unconformity, showing estimated thickness of the section

  • Early work done in the region was presented by McCaffrey et al. (1980). Here is a map showing seismic refraction lines that they used to constrain the structures in this region. Below I include the text from the original figure caption in blockquote.

  • Map of the Molucca Sea, eastern Indonesia, showing I~tions of seismic refraction lines (solid straight lines) and gravity traverses (duhed-dotted lines). Thrust faults are shown with teeth on hanging wall. Triangles represent active volcanoes defining the Sangihe and Halmahera magmatic arcs. Isobath interval is 1 km from Mammericks et al. [1976].

  • Here is a cross section that shows the gravity model they used to interpret this region.

  • Gravity model for the central Molucca Sea. (II) Crustal model with layers designated by their density contrasts and refraction control points by open circles and vertical bars. (b) Mantle structure used in modeling the gravity profiles in the central Molucca Sea. Figure 124 fits into the small box at the apex of the inverted-V-ehaped lithosphere. Slab dimensions are controlled by earthquake foci (dots) from Hlltherton 11M Dickinaon [1969J, and mantle densities are taken from Grow 11M Rowin [1975J. The column at the left shows assumed densities for the range of depths between the tick marks. The small v pattern represents oceanic crust, and island arc crust is designated by a short parallel line pattern. East is to the right of the figure.

  • Here is another tectonic map showing the Sorong fault and some splay faults (dashed lines running along Halmahera), one of which may be involved in today’s earthquake.

  • Location map and active faults of the Molucca Sea region. Fault colours: blue, convergence; red, transvergence; yellow, divergence; grey, uncertain motion. Fault abbreviations: CF, Catabato Fault; GF, Gorontalo Fault; NST, North Sulawesi Trench; PKF, Palu-Koro Fault; SF, Sorong Fault.

  • This is a geologic map for the islands in the region (Hall et al., 1988).

  • Sketch geological map of Halmahera based on Apandi & Sudana (1980), Silitonga et al. (1981), Supriatna (1980) & Yasin (1980) and modified after our own observations. Note in particular the absence of thrusting in the NE arm and the major NE-SW fault (the Subaim Fault) running parallel to the south side of Kau Bay.

Geologic Fundamentals

  • For more on the graphical representation of moment tensors and focal mechanisms, check this IRIS video out:
  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

  • Here is another way to look at these beach balls.
  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
  • Strike Slip:

    Compressional:

    Extensional:

  • This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

  • A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

  • Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

  • Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

  • Here is a great tweet that discusses the different parts of a seismogram and how the internal structures of the Earth help control seismic waves as they propagate in the Earth.

    References:

  • Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
  • Hall, R., 2011. Australia-SE Asia collision: plate tectonics and crustal flow in Geological Society, London, Special Publications 2011; v. 355; p. 75-109 doi: 10.1144/SP355.5
  • Hall., R., Audley-Charles, M.G., Banner, F.T., Hidayat, S., Tobing, S.L., 1988. Basement rocks of the Halmahera region, eastern Indonesia: a Late Cretaceous-early Tertiary arc and fore-arc in Journal of the Geological Society, v. 145, p. 65-84
  • Harris, R. and Major, J., 2016. Waves of destruction in the East Indies: the Wichmann catalogue of earthquakes and tsunami in the Indonesian region from 1538 to 1877 in Cummins, P. R. & Meilano, I. (eds) Geohazards in Indonesia: Earth Science for Disaster Risk Reduction. Geological Society, London, Special Publications, 441, http://doi.org/10.1144/SP441.2
  • Hayes, G., 2018, Slab2 – A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release, https://doi.org/10.5066/F7PV6JNV.
  • Highland, L.M., and Bobrowsky, P., 2008. The landslide handbook—A guide to understanding landslides, Reston, Virginia, U.S. Geological Survey Circular 1325, 129 p.
  • Holt, W. E., C. Kreemer, A. J. Haines, L. Estey, C. Meertens, G. Blewitt, and D. Lavallee (2005), Project helps constrain continental dynamics and seismic hazards, Eos Trans. AGU, 86(41), 383–387, , https://doi.org/10.1029/2005EO410002. /li>
  • Horspool, N., Pranantyo, I., Griffin, J., Latief, H., Natawidjaja, D. H., Kongko, W., Cipta, A., Bustaman, B., Anugrah, S. D., and Thio, H. K., 2014. A probabilistic tsunami hazard assessment for Indonesia, Nat. Hazards Earth Syst. Sci., 14, 3105-3122, https://doi.org/10.5194/nhess-14-3105-2014, 2014.
  • Jessee, M.A.N., Hamburger, M. W., Allstadt, K., Wald, D. J., Robeson, S. M., Tanyas, H., et al. (2018). A global empirical model for near-real-time assessment of seismically induced landslides. Journal of Geophysical Research: Earth Surface, 123, 1835–1859. https://doi.org/10.1029/2017JF004494
  • Keefer, D.K., 1984. Landslides Caused by Earthquakes in GSA Bulletin, v. 95, p. 406-421
  • Kreemer, C., J. Haines, W. Holt, G. Blewitt, and D. Lavallee (2000), On the determination of a global strain rate model, Geophys. J. Int., 52(10), 765–770.
  • Kreemer, C., W. E. Holt, and A. J. Haines (2003), An integrated global model of present-day plate motions and plate boundary deformation, Geophys. J. Int., 154(1), 8–34, , https://doi.org/10.1046/j.1365-246X.2003.01917.x.
  • Kreemer, C., G. Blewitt, E.C. Klein, 2014. A geodetic plate motion and Global Strain Rate Model in Geochemistry, Geophysics, Geosystems, v. 15, p. 3849-3889, https://doi.org/10.1002/2014GC005407.
  • McCaffrey, R., Silver, E.A., and Raitt, R.W., 1980. Crustal Structure of the Molucca Sea Collision Zone, Indonesia in The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands-Geophysical Monograph 23, p. 161-177.
  • Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. https://doi.org/10.7289/V5H70CVX
  • Müller, R.D., Sdrolias, M., Gaina, C. and Roest, W.R., 2008, Age spreading rates and spreading asymmetry of the world’s ocean crust in Geochemistry, Geophysics, Geosystems, 9, Q04006, https://doi.org/10.1029/2007GC001743
  • Pagani,M. , J. Garcia-Pelaez, R. Gee, K. Johnson, V. Poggi, R. Styron, G. Weatherill, M. Simionato, D. Viganò, L. Danciu, D. Monelli (2018). Global Earthquake Model (GEM) Seismic Hazard Map (version 2018.1 – December 2018), DOI: 10.13117/GEM-GLOBAL-SEISMIC-HAZARD-MAP-2018.1
  • Silva, V ., D Amo-Oduro, A Calderon, J Dabbeek, V Despotaki, L Martins, A Rao, M Simionato, D Viganò, C Yepes, A Acevedo, N Horspool, H Crowley, K Jaiswal, M Journeay, M Pittore, 2018. Global Earthquake Model (GEM) Seismic Risk Map (version 2018.1). https://doi.org/10.13117/GEM-GLOBAL-SEISMIC-RISK-MAP-2018.1
  • Smoczyk, G.M., Hayes, G.P., Hamburger, M.W., Benz, H.M., Villaseñor, Antonio, and Furlong, K.P., 2013. Seismicity of the Earth 1900–2012 Philippine Sea plate and vicinity: U.S. Geological Survey Open-File Report 2010–1083-M, 1 sheet, scale 1:10,000,000.
  • Waltham et al., 2008. Basin formation by volcanic arc loading in GSA Special Papers 2008, v. 436, p. 11-26.
  • Zahirovic et al., 2014. The Cretaceous and Cenozoic tectonic evolution of Southeast Asia in Solid Earth, v. 5, p. 227-273, doi:10.5194/se-5-227-2014.
  • Zulkifli, M., Rudyanto, A., and Sakti, A.P., 2016. The View of Seismic Hazard in The Halmahera Region in proceedings from International Symposium on Earth Hazard and Disaster Mitigation (ISEDM) 2016 AIP Conf. Proc. 1857, 050004-1–050004-7; doi:10.1063/1.4987082

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Posted in collision, earthquake, education, geology, Indonesia, plate tectonics, tsunami

Earthquake Report: Ridgecrest Update #1

There have been well over 1000 aftershocks with magnitudes M ≥ 0.5.

Last night there was the largest aftershock (so far) a magnitude M 5.4 earthquake.

It is clear that this sequence has involved at least 2 main faults. I interpret the mainshock (the M 6.4) to be on a northeast trending (striking) left-lateral strike-slip fault. This is largely because (1) the longer of the 2 aftershock trends is has this orientation and (2) the majority of field observations of surface rupture are along this orientation. The M 5.4 aftershock is located along the right-lateral northwest trending fault. The M 6.4 could be on the nw striking fault.

Lots of information about the regional tectonics is in my original report, so I won’t rehash that here.

  • I present two summaries below:
    1. A video showing seismicity for the past day or two.
    2. An updated seismicity map.

Seismicity Visualization

  • I use the USGS earthquake website to query for earthquakes for a given time range, spatial extent, and minimum magnitude. Using the query results, I export these data as a text file (for the GIS based maps) and as Google Earth kmz files.
  • I use the animated version of the kmz, use computer software to capture the animation, and then do some video editing with this software. The music is copyright free.

Updated Seismicity Map

  • I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1919-2019 with magnitudes M ≥ 5.0 in one version.
  • I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the earthquakes for which the USGS has prepared earthquake mechanism plots. Read more about these plots in my original report under “Geologic Fundamentals” at the bottom of the report.
  • I label these earthquakes relative to the date and time of their ocurrence. I also label them in red in order of appearance. There was a M 4.0 foreshock to the M 6.4 mainshock.
  • It is not clear, for some quakes, which fault they are on, so i include both nodal plane solutions (purple and green arrows). Obviously, these are just my interpretations based on a simple overlay with the faults. Upon more rigorous analysis, we will learn more about how these earthquakes relate to each other.
  • I place red dashed lines in the general location of the proposed faults involved in this sequence. They are not well constrained, though the northeast striking fault is somewhat located where the surface rupture is located crossing Highway 178 (the major road that traverses Ridgecrest).
  • Note that there are 3 normal faulting events (#5, 6, 7) and they happen at about the same time. As I mention in my original report, there are lots of normal faults mapped in this region.
  • The northeast striking fault is about 22 km long and the northwest striking fault is about 17 km long (if the most westerly eqs are not included as they appear to be on a separate fault based on the gap in seismicity). Using Wells and Coppersmith (1995), I calculate that a 22 km long fault could produce a M 6.6 earthquake. This is pretty close to M 6.4 given the uncertainty in those fault magnitude : fault lenth relations.


LATE BREAKING NEWS

USGS


Berkeley Seismology Lab


UPDATE

  • Here is a photo taken Dr. Mark Hemphill-Haley. This shows the Humboldt State University Baby Benioff seismograph for the M 7.1 earthquake. As Hemphill-Haley states, the gain is turned up so people can see smaller quakes. This is why the record maxes out. The beginning of the earthquake is on the bottom.

  • Here is an updated map. Please see previous maps and reports for more information about what is on this map.
  • I have plotted epicenters from the past few days for magnitudes M > 0.5 in green and earthquakes for the past century for magnitudes M > 5.0.
  • As I have discussed earlier and here, there are 2 major faults that are participating in this sequence. One fault is a northeast striking (oriented) left-lateral strike-slip fault (analog = Garlock fault). The other fault is a northwest striking (oriented) right-lateral strike-slip fault (analog = San Andreas, Owens Valley faults). There are also many other smaller faults too.
  • Earlier I interpreted the M 6.4 to have been on the northeast striking left-lateral strike-slip fault. This is still my favored interpretation as (1) using the empirical fault scaling relations from Wells and Coppersmith (1995), the 23 km length could produce a magnitude M 6.6 earthquake (close enough to > 6.4), (2) this is where the aftershocks were following the M 6.4, and (3) the surface rupture was identified along the northeast striking region. However, it may be on the northwest striking fault.
  • The M 7.1 earthquake is clearly on the nw striking right-lateral strike-slip fault. The aftershocks are filling in, showing us the spatial extent (the length) of this fault rupture. At first, there were several M 4+ aftershocks at the northwestern end of the aftershocks. As I was preparing the map below, some starting ripping off to the southeast of the ne striking fault. The nw fault appears to be about 60 km long. Using Wells and Coppersmith, a 56 km long rupture would produce a M 7.0 magnitude earthquake. Imagine that!

  • In the above map, there is an inset map showing the eastern California Shear Zone, the San Andreas fault, and the Garlock fault. I highlight several key historic earthquakes. The 1872 M 7.6+- Owens Valley, the 1992 M 7.4 Landers, and the 1999 M 7.1 Hector Mine earthquakes (the faults that ruptured are shown as red lines in the inset map).
  • In Dr. Ross Stein’s article on Temblor from late yesterday, Stein suggested that these Ridgecrest earthquakes are in a region of increased static coulomb stress from the 1872, 1992, and 1999 earthquakes. Below is one of his figures that shows regions that have an increased (in red) and decreased stress following the 1872 earthquake. Of particular interest is that there is a region of faults that lie between this ongoing Ridgecrest sequence and the 1872 rupture.

  • Here is their analysis that shows an expected increase in rates of seismicity follwing earthquakes from 1992-2005 (Toda et al., 2005). This is also from the Temblor report.

  • The reason I bring all this up is that there is a possibility that other faults in the region may rupture as a large earthquake. Of course, this could happen tomorrow or months or years from now. Recall that the last earthquake this size was in 1999 and the one prior to that was in 1992. Regardless, there is a stretch of the plate boundary faults (e.g. Owens Valley) that are between the 1872 and this 2019 slip.
  • My cousin (a famous blues guitarist, Barry Levenson) just asked me on social media about the “Big One.” (I am paraphrasing.). I wrote to him this: as far as the San Andreas fault (SAF), this Ridgecrest sequence probably does not affect the chance that the SAF might rupture. The SAF is getting ready to go every day, but this Ridgecrest sequence is probably not affecting that… the Ridgecrest sequence is just too far away from the SAF to affect it.
  • Here is the intensity map from the CISN/California Geological Survey. The color represents the shaking intensity from the M 7.1 earthquake.

  • The map above is based on an empirical relation between earthquake shaking intensity, earthquake magnitude, and distance to the earthquake. These relations depend also on other factors, like the type of earthquake and the type of Earth materials.
  • Here is a plot showing the empirical plot (blue lines) based on the attenuation relations of Boore and Atkinson (2008). The black dots represent observations from seismometers operated by the California Geological Survey. Note the limitation that there are few observations less than 100 km from the earthquake.

UPDATE: 2019.07.06 afternoon

  • Here are some updated maps. I am heading to the field tomorrow, so probably won’t be providing more updates, but we will see.
  • Here is an updated seismicity map. The aftershock zone is now extending all the way to the Garlock fault. Also, there are some triggered events far to the northwest of the aftershock zone. These are probably not part of the main northwest trending fault, which appears to end near where the aftershocks are. The pdf version of this map is 167 MB.
  • Stay Tuned

  • Here is a map with landslide probability on it. I prepared one like this for the M 6.4 earthquake. Please head over to that report for more information about the USGS Ground Failure products (landslides and liquefaction). Basically, earthquakes shake the ground and this ground shaking can cause landslides. We can see that there is a low probability for landslides. However, we have already seen photographic evidence for landslides and the lower limit for earthquake triggered landslides is magnitude M 5.5 (from Keefer 1984-ish).

  • Here is a map showing liquefaction susceptibility. I explain more about this type of map in my original report for the M 6.4 earthquake. Scroll down a bit to find the landslide and liquefaction maps for that event.

  • Finally, here is a map that shows the shaking intensity for the M 7.1 earthquake. As I mention in my original report, this is based on a model that relates earthquake shaking intensity with earthquake magnitude and distance from the earthquake. Note that there was violent shaking from the M 7.1 event (MMI IX).

    USGS Earthquake Forecast (UPDATED 5 July 2019)

  • The USGS has been increasing the list of products that are produced in association with their earthquake pages. One of these products is an earthquake forecast (not a prediction as nobody can predict earthquakes yet) that lists the chance of an earthquake with a given magnitude over a certain period of time. The forecast for the M 6.4 earthquake is found here. These forecasts are updated periodically, so the information will change with time. Below is a table where I present the forecast as it was when I checked the page this morning (would be nice if the USGS would produce an easy to read table).
  • Thanks to Dr. Harold Tobin for reviewing these tables (I reformat them) as he noticed a mistake. They are now fixed.
  • From the USGS:

    Be ready for more earthquakes

    • More earthquakes than usual (called aftershocks) will continue to occur near the mainshock.
    • When there are more earthquakes, the chance of a large earthquake is greater which means that the chance of damage is greater.
    • The USGS advises everyone to be aware of the possibility of aftershocks, especially when in or around vulnerable structures such as unreinforced masonry buildings.
    • This earthquake could be part of a sequence. An earthquake sequence may have larger and potentially damaging earthquakes in the future, so remember to: Drop, Cover, and Hold on.

    About this earthquake and related aftershocks

    • So far in this sequence there have been 97 magnitude 3 or higher earthquakes, which are strong enough to be felt, and 1 magnitude 5 or higher earthquakes, which are large enough to do damage.

    What we think will happen next

    • According to our forecast, over the next 1 Week there is a 3 % chance of one or more aftershocks that are larger than magnitude 6.4. It is likely that there will be smaller earthquakes over the next 1 Week, with 47 to 88 magnitude 3 or higher aftershocks. Magnitude 3 and above are large enough to be felt near the epicenter. The number of aftershocks will drop off over time, but a large aftershock can increase the numbers again, temporarily.

    About our earthquake forecasts

    • No one can predict the exact time or place of any earthquake, including aftershocks. Our earthquake forecasts give us an understanding of the chances of having more earthquakes within a given time period in the affected area. We calculate this earthquake forecast using a statistical analysis based on past earthquakes.
    • Our forecast changes as time passes due to decline in the frequency of aftershocks, larger aftershocks that may trigger further earthquakes, and changes in forecast modeling based on the data collected for this earthquake sequence.
    • The first table presents this forecast in terms of percent chance and the second table presents the forecast in terms of number of earthquakes.



    References:

  • Amos, C.B., Bwonlee, S.J., Hood, D.H., Fisher, G.B., Bürgmann, R., Renne, P.R., and Jayko, A.S., 2013. Chronology of tectonic, geomorphic, and volcanic interactions and the tempo of fault slip near Little Lake, California in GSA Bulletin, v. 125, no. 7-8, https://doi.org/10.1130/B30803.1
  • Frankel, K.L., Glazner, A.F., Kirby, E., Monastero, F.C., Strane, M.D., Oskin, M.E., Unruh, J.R., Walker, J.D., Anandakrishnan, S., Bartley, J.M., Coleman, D.S., Dolan, J.F., Finkel, R.C., Greene, D., Kylander-Clark, A., Morrero, S., Owen, L.A., and Phillips, F., 2008, Active tectonics of the eastern California shear zone, in Duebendorfer, E.M., and Smith, E.I., eds., Field Guide to Plutons, Volcanoes, Faults, Reefs, Dinosaurs, and Possible Glaciation in Selected Areas of Arizona, California, and Nevada: Geological Society of America Field Guide 11, p. 43–81, doi: 10.1130/2008.fl d011(03).
  • Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
  • Hayes, G., 2018, Slab2 – A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release, https://doi.org/10.5066/F7PV6JNV.
  • Holt, W. E., C. Kreemer, A. J. Haines, L. Estey, C. Meertens, G. Blewitt, and D. Lavallee (2005), Project helps constrain continental dynamics and seismic hazards, Eos Trans. AGU, 86(41), 383–387, , https://doi.org/10.1029/2005EO410002. /li>
  • Kreemer, C., J. Haines, W. Holt, G. Blewitt, and D. Lavallee (2000), On the determination of a global strain rate model, Geophys. J. Int., 52(10), 765–770.
  • Kreemer, C., W. E. Holt, and A. J. Haines (2003), An integrated global model of present-day plate motions and plate boundary deformation, Geophys. J. Int., 154(1), 8–34, , https://doi.org/10.1046/j.1365-246X.2003.01917.x.
  • Kreemer, C., G. Blewitt, E.C. Klein, 2014. A geodetic plate motion and Global Strain Rate Model in Geochemistry, Geophysics, Geosystems, v. 15, p. 3849-3889, https://doi.org/10.1002/2014GC005407.
  • Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. https://doi.org/10.7289/V5H70CVX
  • Müller, R.D., Sdrolias, M., Gaina, C. and Roest, W.R., 2008, Age spreading rates and spreading asymmetry of the world’s ocean crust in Geochemistry, Geophysics, Geosystems, 9, Q04006, https://doi.org/10.1029/2007GC001743

Return to the Earthquake Reports page.

Posted in earthquake, education, geology, San Andreas, strike-slip

Earthquake Report: Ridgecrest, CA

Well, happy fourth of July!

There was a good sized earthquake in southern California today. The largest earthquake since the 1999 M 7.1 Hector Mine earthquake. (The 2003 San Simeon earthquake was larger, but much farther to the west, at about the same latitude.)

Today’s earthquake sequence has a mainshock (so far) with a magnitude M = 6.4. If you live in southern California or southern Nevada, please visit this website to describe your observations.

This region is at the intersection of several different fault systems. The Pacific-North America plate boundary, which most people associate with the San Andreas fault, includes the South Sierra Nevada fault zone and other right-lateral strike-slip faults that trend along the eastern side of the Sierra Nevada Mountains (including the Eastern California Shear Zone). There is also an interesting conjugate fault, the Garlock fault, which is a left-lateral strike-slip fault.

If we zoom into the area where this earthquake sequence is happening, we can locate some mapped faults. Some are parallel to the S. Sierra Nevada system and some are parallel to the Garlock fault. The faults parallel to the Sierra Nevada system are right-lateral and the faults parallel to the Garlock are left-lateral.

The sequence today appears to involve faults with both orientations. Looking at the aftershocks, it looks like the main shock is left-lateral (more aftershocks along the northeast trend).

These strike-slip faults also have normal motion on them (so they are both strike-slip and normal, i.e. “oblique”).

There are photographic reports of surface rupture (where the earthquake fault breaks the ground surface) across Highway 178.

This earthquake will be studied over the coming weeks, so I will be preparing updates in the near and far future.

The USGS earthquake products I review below include (1) the probability (“chance for”) landslides and liquefaction and (2) an earthquake forecast (the chance of future earthquakes for given time ranges).

Below, check out the social media links. There are field observations and a link to a Temblor report where they suggest this earthquake was possibly triggered by earthquakes in the 20th century.

Here is the Baby Benioff Seismograph from Humboldt State University, Department of Geology (photo credit Dr. Mark Hemphill-Haley).


This is in a tweet below, but the figure is so telling, I am placing it up here. Some may need to read more background material (below) to understand this figure.

This figure shows earthquake mechanisms (focal mechanisms) for seismicity associated with this ongoing sequence in Ridgecrest.


There are lots of great field photos in tweets below. Here is one of them. The reason I show this here is to mention one of the principles of geologic time. Relative time is based on several principles (e.g. law of superposition, principle of original horizontality). The principle demonstrated here is cross cutting relations.

The spectacular example spans different time scales. First the road was built, then the paint stripes were painted (superposed above the road, so are younger than the road). Then the driver of the Jeep felt the earthquake (inferred by the black rubber skid marks). The skid marks were then offset by the earthquake (the skid marks are cross-cut by the earthquake fault).

This objective information tells us several things about the earthquake. I already mentioned that the driver may have felt the earthquake, leading them to skid to a stop. The cool thing is that we can tell that the fault slipped in this area after the person skid across the fault. This is really cool… at this location, the shaking started prior to the fault slip.

UPDATE: Ian Pierce tells us that the black mark is not a skid mark, but road tar. So, I was incorrect.


UPDATE (2019.07.05): Here is my first Earthquake Report Update

Below is my interpretive poster for this earthquake


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1919-2019 with magnitudes M ≥ 5.0 in one version.

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include the slab 2.0 contours plotted (Hayes, 2018), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.

    Global Strain

  • In a map below, I include a transparent overlay of the Global Strain Rate Map (Kreemer et al., 2014).
  • The mission of the Global Strain Rate Map (GSRM) project is to determine a globally self-consistent strain rate and velocity field model, consistent with geodetic and geologic field observations. The overall mission also includes:
    1. contributions of global, regional, and local models by individual researchers
    2. archive existing data sets of geologic, geodetic, and seismic information that can contribute toward a greater understanding of strain phenomena
    3. archive existing methods for modeling strain rates and strain transients
  • The completed global strain rate map will provide a large amount of information that is vital for our understanding of continental dynamics and for the quantification of seismic hazards.
  • The version used in the poster(s) below is an update to the original 2004 map (Kreemer et al., 2000, 2003; Holt et al., 2005).

    I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

  • In the upper right corner is a regional plate tectonic map (Amos et al., 2013). Earthquake faults are shown and labeled. I place a blue star in the location of today’s sequence. I label the Eastern California Shear zone. This map shows the locations of the historic major surface rupturing earthquakes (1872 Owen’s Valley, 1992 Landers, and 1999 Hector Mine).
  • In the lower left corner is a screen shot of the USGS website showing earthquakes from this sequence for M > 0.5.
  • In the upper left corner are two maps.
    • The one on the left shows the thickness of sedimentary deposits (Stevens et al., 2013). As these faults move, they create space for sediments to deposit. The faster the faults move (the slip rate) and the more time that passes, the thicker the sedimentary deposits can be. The thickest deposits are warmer in color. There is a cross section labeled (A-A’).
    • The one on the right shows how these authors interpret how the North America plate is broken up into “blocks.” These blocks are bounded by the different fault systems. The Indian Wells Valley is bisected by the Airport Lane/Little Lake fault.
  • In the lower right corner is cross section A-A’ through the Indian Wells Valley (Stevens et al., 2013). The gray areas represent the sedimentary deposits. The faults curring across and bounding this valley are shown (with arrows showing relative motion). The Little Lake fault is shown as a right-lateral strike-slip fault.
  • Here is the map with a month’s seismicity plotted. I include transparent colors that are based on the USGS “Did You Feel It?” (DYFI) felt reports database. This way we can compare the computer model based intensity data (the MMI contours) with the reports provided by real people. The comparison is decent.

  • This is a plot that allows us to take a closer look at the comparison between the modeled data relative to the reported data.
  • The vertical axis is shaking intensity (MMI). The horizontal axis is distance from the fault that slipped.
  • The orange line shows the result from the USGS application of a model from Atkinson and Wald (2007) called an “Attenuation Relation” model. This is an empirical model that relates shaking intensity with earthquake magnitude and distance.
  • These attenuation relations also take into account earthquake type, material properties, and other parameters that affect shaking intensity.
  • The blue dots are the actual reported values of intensity from people who used the USGS website to report their direct observations. As I write this, over 42,800 people reported what they experienced and observed. The bigger dots represent the meand and median intensity from the DYFI reports.

  • Here is the map with a century’s seismicity plotted, for quakes M ≥ 5.0.

  • Here is the map with a century’s seismicity plotted, for quakes M ≥ 5.0 with the Global Strain Map as an overlay.

    Landslide, Liquefaction, and Shaking Intensity

  • Here is a suite of maps that use USGS earthquake products to help us learn about how earthquakes may affect the landscape: landslide probability and liquefaction susceptibility (a.k.a. the Ground Failure data products)..
  • First I present the landslide probability model. This is a GIS data product that relates a variety of factors to the probability (the chance of) landslides as triggered by this earthquake. There are a number of assumptions that are made in order to be able to produce this model across such a large region, though this is still of great value (like other aspects from the USGS, e.g. the PAGER alert). Learn more about all of these Ground Failure products here.
  • There are many different ways in which a landslide can be triggered. The first order relations behind slope failure (landslides) is that the “resisting” forces that are preventing slope failure (e.g. the strength of the bedrock or soil) are overcome by the “driving” forces that are pushing this land downwards (e.g. gravity). I spend more time discussing landslides and liquefaction in this recent earthquake report.
  • This model, like all landslide computer models, uses similar inputs. I review these here:
    1. Some information about ground shaking. Often, people use Peak Ground Acceleration, though in the past decade+, it has been recognized that the parameter “Arias Intensity” is a better measure of the energy imparted by the earthquake across the land and seascape. Instead of simply accounting for the peak accelerations, AI integrates the entire energy (duration) during the earthquake. That being said, PGA is a more common parameter that is available for people to use. For example, when I was modeling slope stability for the 2004 Sumatra-Andaman subduction zone earthquake, the only model that was calibrated to observational data were in units of PGA. The first order control to shaking intensity (energy observed at any particular location) is distance to the earthquake fault that slipped.
    2. Some information about the strength of the materials (e.g. angle of internal friction (the strength) and cohesion (the resistance).
    3. Information about the slope. Steeper slopes, with all other things being equal, are more likely to fail than are shallower slopes. Think about skiing. Beginners (like me) often choose shallower slopes to ski because they will go down the slope slower, while experts choose steeper slopes.
  • I use the same color scheme that is presented by the USGS on their website. Note that the majority of areas that may have experienced earthquake triggered landslides are cream in color (0.3-1%). There are a few places with a slightly higher chance that there were triggered landslides. It is possible that there were no significant landslides from this earthquake. The lower bounds for earthquake triggered landslides on land is about M 5.5 and a M 6.4 releases much more energy than that.

  • Landslide ground shaking can change the Factor of Safety in several ways that might increase the driving force or decrease the resisting force. Keefer (1984) studied a global data set of earthquake triggered landslides and found that larger earthquakes trigger larger and more numerous landslides across a larger area than do smaller earthquakes. Earthquakes can cause landslides because the seismic waves can cause the driving force to increase (the earthquake motions can “push” the land downwards), leading to a landslide. In addition, ground shaking can change the strength of these earth materials (a form of resisting force) with a process called liquefaction.
  • Sediment or soil strength is based upon the ability for sediment particles to push against each other without moving. This is a combination of friction and the forces exerted between these particles. This is loosely what we call the “angle of internal friction.” Liquefaction is a process by which pore pressure increases cause water to push out against the sediment particles so that they are no longer touching.
  • An analogy that some may be familiar with relates to a visit to the beach. When one is walking on the wet sand near the shoreline, the sand may hold the weight of our body generally pretty well. However, if we stop and vibrate our feet back and forth, this causes pore pressure to increase and we sink into the sand as the sand liquefies. Or, at least our feet sink into the sand.
  • Below is the liquefaction susceptibility map. I discuss liquefaction more in my earthquake report on the 28 September 20018 Sulawesi, Indonesia earthquake, landslide, and tsunami here.
  • I use the same color scheme that the USGS uses on their website. Note how the areas that are more likely to have experienced earthquake induced liquefaction are in the valleys. The fact that this earthquake happened in the summer time suggests that there may not have been any liquefaction from this earthquake.

  • Finally, here is a map showing the earthquake shaking intensity. The scale is the Modified Mercalli Intensity scale (explained above).
  • I also include two inset maps (also in the landslide and liquefaction maps). These are seismic hazard and seismic risk maps. Read more about these maps here.
    • On the right is the Global Earthquake Model Seismic Hazard map. Color represents the amount of shaking that an area may experience over the next 50 years. The units are “g” (which stands for gravity, where g= 1 is the gravity at the Earth’s surface). If g > 1, objects can be thrown into the air.
    • On the left is the GEM Seismic Risk map. Risk is the combination of hazard and people. If there is seismic hazard where there are no people, then there is no seismic risk. If there are people where there is no seismic hazard, there is no seismic risk. Seismic risk happens when there are people exposed to seismic hazard. The color represents the financial expense due to seismic hazards.


    USGS Earthquake Forecast (UPDATED 5 July 2019)

  • The USGS has been increasing the list of products that are produced in association with their earthquake pages. One of these products is an earthquake forecast (not a prediction as nobody can predict earthquakes yet) that lists the chance of an earthquake with a given magnitude over a certain period of time. The forecast for the M 6.4 earthquake is found here. These forecasts are updated periodically, so the information will change with time. Below is a table where I present the forecast as it was when I checked the page this morning (would be nice if the USGS would produce an easy to read table).
  • Thanks to Dr. Harold Tobin for reviewing these tables (I reformat them) as he noticed a mistake. They are now fixed.
  • From the USGS:

    Be ready for more earthquakes

    • More earthquakes than usual (called aftershocks) will continue to occur near the mainshock.
    • When there are more earthquakes, the chance of a large earthquake is greater which means that the chance of damage is greater.
    • The USGS advises everyone to be aware of the possibility of aftershocks, especially when in or around vulnerable structures such as unreinforced masonry buildings.
    • This earthquake could be part of a sequence. An earthquake sequence may have larger and potentially damaging earthquakes in the future, so remember to: Drop, Cover, and Hold on.

    About this earthquake and related aftershocks

    • So far in this sequence there have been 97 magnitude 3 or higher earthquakes, which are strong enough to be felt, and 1 magnitude 5 or higher earthquakes, which are large enough to do damage.

    What we think will happen next

    • According to our forecast, over the next 1 Week there is a 3 % chance of one or more aftershocks that are larger than magnitude 6.4. It is likely that there will be smaller earthquakes over the next 1 Week, with 47 to 88 magnitude 3 or higher aftershocks. Magnitude 3 and above are large enough to be felt near the epicenter. The number of aftershocks will drop off over time, but a large aftershock can increase the numbers again, temporarily.

    About our earthquake forecasts

    • No one can predict the exact time or place of any earthquake, including aftershocks. Our earthquake forecasts give us an understanding of the chances of having more earthquakes within a given time period in the affected area. We calculate this earthquake forecast using a statistical analysis based on past earthquakes.
    • Our forecast changes as time passes due to decline in the frequency of aftershocks, larger aftershocks that may trigger further earthquakes, and changes in forecast modeling based on the data collected for this earthquake sequence.
    • The first table presents this forecast in terms of percent chance and the second table presents the forecast in terms of number of earthquakes.



Other Reports for this Earthquake

  • Temblor: Southern California M 6.4 earthquake stressed by two large historic ruptures
  • Some Relevant Discussion and Figures

    • Here is the Amos et al. (2013) plate tectonic map. Check out the location of the historic surface rupturing earthquakes. Their figure caption is below (as for other figures here).

    • Overview of active faults and regional topography of the Eastern California shear zone (ECSZ) and southern Walker Lane belt. Labeled faults are abbreviated as follows: ALF—Airport Lake fault, BF—Blackwater fault, GF—Garlock fault, KCF—Kern Canyon fault, LLF—Little Lake fault, OVF—Owens Valley fault, SNFF—Sierra Nevada frontal fault. OL—Owens Lake, IWV—Indian Wells Valley. Major historical earthquake surface ruptures in the Eastern California shear zone and Walker Lane belt are outlined in white, with stars denoting epicentral locations: OV—1872 Owens Valley, L—Landers 1992, HM—1999 Hector Mine. Active fault traces are taken from the U.S. Geological Survey Quaternary fault and fold database, with the exception of the Kern Canyon fault, taken from Brossy et al. (2012).

    • This map extends a little farther to the east (Frankel et al., 2008). This map shows nicely how the Sierra Nevada and Owens Valley faults (the Pacific-North America plate boundary) and Eastern California Shear Zone, aka ECSZ (the maps south of the Garlock fault, 35.5N°) interact with east-west trending left-lateral strike-slip faults like the Garlock fault. The ’92 Landers and ’99 Hector Mine Earthquakes are on faults in the ECSZ.

    • Shaded relief index map of Quaternary faults, roads, towns, and fi eld trip stops in the eastern California shear zone. Most faults are from the U.S. Geological Survey Quaternary fault and fold database (http://earthquake.usgs.gov/regional/qfaults). Arrows indicate relative fault motion for strike slip faults. Bar and circle indicates the hanging wall of normal faults. Field trip stop location numbers are tied to site descriptions in the fi eld guide section. AHF—Ash Hill fault; ALF—Airport Lake fault; B—Bishop; BF—Blackwater fault; BLF—Bicycle Lake fault; BM—Black Mountains; BP—Big Pine; Br—Baker; Bw—Barstow; By—Beatty; CA—California; CF—Cady fault; CLF—Coyote Lake fault; CoF—Calico fault; CRF—Camp Rock fault; DSF—Deep Springs fault; DV-FLVF—Death Valley–Fish Lake Valley fault; EPF—Emigrant Peak fault; EV— Eureka Valley; FIF—Fort Irwin fault; FM—Funeral Mountains; GF—Garlock fault; GFL—Goldstone Lake fault; GM—Grapevine Mountains; HF—Helendale fault; HLF—Harper Lake fault; HMSVF—Hunter Mountain–Saline Valley fault; I—Independence; LF—Lenwood fault; LLF— Lavic Lake fault; LoF—Lockhart fault; LP—Lone Pine; LuF—Ludlow fault; LV—Las Vegas; M—Mojave; MF—Manix fault; NV—Nevada; O—Olancha; OL—Owens Lake; OVF—Owens Valley fault; P—Pahrump; PF—Pisgah fault; PV—Panamint Valley; PVF—Panamint Valley fault; R—Ridgecrest; S—Shoshone; SAF—San Andreas fault; SDVF—southern Death Valley fault; SLF—Stateline fault; SPLM—Silver Peak–Lone Mountain extensional complex; SNF—Sierra Nevada frontal fault; SP—Silver Peak Range; T—Tonopah; TF—Tiefort Mountain fault; TMF—Tin Mountain fault; TPF—Towne Pass fault; WMF—White Mountains fault; YM—Yucca Mountain.

    • This is also from Amos et al. (2013) that shows how some northeast striking normal faults are related to the Little Lake fault, in the northern part of Indian Wells Valley. The Little Lake fault connects to the Sierra Nevada frontal fault.

    • Simplified geologic map of the Little Lake fault, highlighting Quaternary volcanic and alluvial deposits bearing on the Pleistocene drainage of Owens River through the Little Lake area. Map units are named and modified from Duffield and Bacon (1981). The 30 m elevation contours are taken from the National Elevation Database (NED). The 40Ar/39Ar dates are labeled as in Table 1. SNFF—Sierra Nevada frontal fault.

    • Here is the Frankel et al. (2008) larger scale fault map, also focusing on the northern Indian Wells Valley.

    • Northward branching of the Holocene-active Airport Lake fault zone in northern Indian Wells Valley, Rose Valley, the Coso Range, and Wild Horse Mesa. AL—Airport Lake playa; BR— basement ridge; CB—Central branch; CWF—Coso Wash fault; EB—Eastern branch; GF—geothermal field; HS— Haiwee Spring; LCF—Lower Cactus Flat; MF—McCloud Flat; UCF—Upper Centennial Flat; WB—Western branch; WHA—White Hills anticline; WHM—Wild Horse Mesa; WHMFZ— Wild Horse Mesa fault zone. Faults with especially prominent scarps in Wild Horse Mesa are highlighted in bold. Late Quaternary faults modified from Duffield and Bacon (1981) and Whitmarsh (1998), with additional original mapping. A and B indicate two faults that display evidence for late Quaternary dextral offset.

    • In 1995-1996 there was a sequence in Ridgecrest that had a mainshock of M 5.8. This sequence is also in the same forecast area suggested by Toda et al. (2005) to have a higher chance of earthquakes following the ECSZ temblors.
    • This figure from Dreger et al. (2008) show some earthquake mechanisms from the Ridgecrest sequence. Note that most of the quakes are strike slip, but there are some normal (extensional) earthquakes too. This matches the fault configuration, which represents longer term strain.

    • Map showing the locations of events from the SCSN Earthquake Catalog and seismic moment tensors obtained by inverting low-frequency waveforms recorded at BDSN stations CMB, PKD1, and SAO.

    • Speaking of earthquake triggering and aftershocks, this figure shows some triggered earthquakes following the 1992 Landers earthquake Rouqemore and Simila (1994). They extend to and beyond the Indian Wells Valley. One aftershock near the Little Lake fault zone has a strike-slip mechanism and is located nearby today’s M 6.4.

    • Seismicity (M 4 or greater) for 28 June 1992 to 1 June 1993. See Figure 1 legend for definitions of abbreviations. The 28 June 1992 Landers rupture is shown as shaded fault lines. Faults are from Jennings (1992).

    • Here is a figure that Dr. Ross Stein prepared in the Temblor article linked and tweeted below.
    • When earthquake faults slip, the surrounding crust deforms like jello. This deformation and the fault slip lead to changes in the forces within the Earth. These changes can increase or decrease the stress on faults in these areas.
    • The map below shows regions that have an increase in fault stress as red and areas that have a decrease in stress as blue.
    • Note that there are sections of faults that experience both increases and decreases in stress. Take note that these changes in stress are tiny compared to the amount of stress that it takes for a fault to create an earthquake. So, for this type of stress change to lead to an earthquake, the fault would need to already be highly stressed. If the fault is just about ready to slip before this M 6.4, it probably would not be triggered.
    • Read more in Dr. Stein’s article here.

    • Coulomb 3.3 calculation of stress transferred by the 4th July shock to the surrounding region and major faults. Here we use a simple source based on the moment tensor (geometry, sense of slip, and size) of the earthquake, as determined by the USGS.

    • Here is a low-angle oblique image from Roquemore (1980) that shows some normal faults (the Airport Lake fault). North is up in this case.

    • Aerial view of a 2 km wide tension graben located along the south end of the right slip Airport Lake fault.

    • Here is a map I put together showing the faulting in the area where the above aerial image was taken Guess which faults are more strike-slip in nature, compared to extensional (normal). North isn’t always up.

    • This is the Stevens et al. (2013) map that shows the sedimentary basins in the region.

    • Map showing interpreted thickness of Cenozoic deposits and major faults outlining the deep basins, based on inversion of gravity data [56]. Connection between West Inyo and Southwest Argus faults from Pluhar et al. [58]. ALFZ = Airport Lake Fault Zone; CWF = Coso Wash Fault; EIF = East Inyo Fault; LLF = Little Lake Fault. A-A’ to H-H’ indicate lines of cross sections and gravity profiles shown in Figure 10.

    • Here are the fault blocks presented by Stevens et al. (2013).

    • Map showing deep basins, relatively shallow down-dropped blocks, extended mountain blocks, and structural zones in the ESVS, which is bounded by largely unextended mountain blocks. CB = Chalfant Basin; NBB = North Bishop Block; RVB = Round Valley Basin

    • This is cross section A-A’ showing the normal and normal oblique faults that cross the Indian Wells Valley (Stevens et al., 2013).

    • Structural cross sections across the East Sierra Valley System (ESVS), with corresponding gravity profiles. Locations of sections are shown in Figure 5. No vertical exaggeration. Shading represents Quaternary sedimentary and volcanic deposits, with thicknesses based on inversion of gravity data [53].

    Geologic Fundamentals

    • For more on the graphical representation of moment tensors and focal mechanisms, check this IRIS video out:
    • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

    • Here is another way to look at these beach balls.
    • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
    • Strike Slip:

      Compressional:

      Extensional:

    • This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

    • A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

    • Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

    • Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

    • Here is a great tweet that discusses the different parts of a seismogram and how the internal structures of the Earth help control seismic waves as they propagate in the Earth.

      References:

    • Amos, C.B., Bwonlee, S.J., Hood, D.H., Fisher, G.B., Bürgmann, R., Renne, P.R., and Jayko, A.S., 2013. Chronology of tectonic, geomorphic, and volcanic interactions and the tempo of fault slip near Little Lake, California in GSA Bulletin, v. 125, no. 7-8, https://doi.org/10.1130/B30803.1
    • Frankel, K.L., Glazner, A.F., Kirby, E., Monastero, F.C., Strane, M.D., Oskin, M.E., Unruh, J.R., Walker, J.D., Anandakrishnan, S., Bartley, J.M., Coleman, D.S., Dolan, J.F., Finkel, R.C., Greene, D., Kylander-Clark, A., Morrero, S., Owen, L.A., and Phillips, F., 2008, Active tectonics of the eastern California shear zone, in Duebendorfer, E.M., and Smith, E.I., eds., Field Guide to Plutons, Volcanoes, Faults, Reefs, Dinosaurs, and Possible Glaciation in Selected Areas of Arizona, California, and Nevada: Geological Society of America Field Guide 11, p. 43–81, doi: 10.1130/2008.fl d011(03).
    • Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
    • Hayes, G., 2018, Slab2 – A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release, https://doi.org/10.5066/F7PV6JNV.
    • Holt, W. E., C. Kreemer, A. J. Haines, L. Estey, C. Meertens, G. Blewitt, and D. Lavallee (2005), Project helps constrain continental dynamics and seismic hazards, Eos Trans. AGU, 86(41), 383–387, , https://doi.org/10.1029/2005EO410002. /li>
    • Kreemer, C., J. Haines, W. Holt, G. Blewitt, and D. Lavallee (2000), On the determination of a global strain rate model, Geophys. J. Int., 52(10), 765–770.
    • Kreemer, C., W. E. Holt, and A. J. Haines (2003), An integrated global model of present-day plate motions and plate boundary deformation, Geophys. J. Int., 154(1), 8–34, , https://doi.org/10.1046/j.1365-246X.2003.01917.x.
    • Kreemer, C., G. Blewitt, E.C. Klein, 2014. A geodetic plate motion and Global Strain Rate Model in Geochemistry, Geophysics, Geosystems, v. 15, p. 3849-3889, https://doi.org/10.1002/2014GC005407.
    • Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. https://doi.org/10.7289/V5H70CVX
    • Müller, R.D., Sdrolias, M., Gaina, C. and Roest, W.R., 2008, Age spreading rates and spreading asymmetry of the world’s ocean crust in Geochemistry, Geophysics, Geosystems, 9, Q04006, https://doi.org/10.1029/2007GC001743

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    Posted in earthquake, education, geology, plate tectonics, strike-slip

    Earthquake Report: Indonesia

    I had been making an update to an earthquake report on a regionally experienced M 5.6 earthquake from coastal northern California when I noticed that there was a M 7.3 earthquake in eastern Indonesia.

    This earthquake is in a region of strike-slip faulting (if in downgoing plate for example) or subduction thrusting, so I thought it may or may not produce a tsunami. There are also intermediate depth quakes here (deeper than subduction zone megathrust events), like this earthquake (which reduces the chance of a tsunami). While we often don’t think of strike-slip earthquakes as those that could cause a tsunami, they can trigger tsunami, albeit smaller in size than those from subduction zone earthquakes or locally for landslides. But, I checked tsunami.gov just in case (result = no tsunami locally nor regionally). I also took a look at the tide gages in the region here and here (result = no observations).

    South of this earthquake is a convergent plate boundary, where the Australia plate dives northwards beneath a part of the Sunda plate (Eurasia) forming the Java and Timor trenches (subduction zones). Far to the west, on 2 June 1994 there was a subduction zone megathrust earthquake along the Java Trench. Earlier, on 19 August 1977 there was an M 8.3 earthquake, but it was not a subduction zone thrust event, but an extensional earthquake in the downgoing Australia plate (Given and Kanamori, 19080). Both 1977 and 1994 events are shown on one of the maps below. The 1977 earthquake was tsunamigenic, creating a wave observed on tide gages at Damier, Hampton, and Port Hedland in Australia (Gusman et al., 2009).

    To the north of the subduction zone, there is a parallel fault system that dips in the opposite direction as the subduction zone. This is referred to as a backthrust fault (it is a thrust fault and “backwards” to the main fault). The Wetar and Flores faults are both part of this backthrust system. In JUly and August of 2018 there was a series of earthquakes near the Island of Flores associated with this backthrust. Here is my final of 3 reports on those earthquakes.

    The Timor trough wraps around to the north on its eastern end and eventually forms the Seram Trench, which dips to the south. The shape of these linked trenches forms a “U” shape with the open part of the U pointing to the west. Recently it has been published that the basin formed by these fault systems is the deepest forearc basin on Earth (Pownall et al., 2016). There was a subduction zone earthquake in 1938, called the Great Banda Sea Earthquake. Okal and Reymond (2003) prepared an earthquake mechanism for this M 8.5 earthquake.

    To complicate matters, there is a large strike-slip system that comes into the area from the east (Papua New Guinea) and bisects the crest of the “U” shape. This strike slip system feeds into the backthrust so that the backthrust is both a thrust fault and a strike-slip fault. There are probably separate faults that accommodate these different senses of motion. There have been a series of strike-slip earthquakes in the 20th century associated with the strike-slip motion along this boundary. For example, Osada and Abe (1981) uses seismologic records (e.g. from seismometers) to prepare an earthquake mechanism for this M 8.1 earthquake. They found that it was an oblique strike-slip earthquake. The depth was pretty shallow compared to the M 7.3 earthquake I am reporting about today.

    On 17 June 1987 there was another relatively shallow M 7.1 strike-slip earthquake on this strike-slip fault system.

    However, there is also a deeper strike-slip fault within the Australia plate. This fault is probably what ruptured on 2 March 2005 (M 7.1) and 10 December 2012 (M 7.1). The M 7.3 earthquake from a day ago had a similar magnitude, depth, mechanism, and location as these earlier quakes. These may have all ruptured the same fault (or not).

    Below is my interpretive poster for this earthquake


    I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1919-2019 with magnitudes M ≥ 7.0 in one version.

    I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes. Some earthquakes have older focal mechanisms plotted in black and white.

    • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
    • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
    • I include the slab 2.0 contours plotted (Hayes, 2018), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.

      Magnetic Anomalies

    • In the map below, I include a transparent overlay of the magnetic anomaly data from EMAG2 (Meyer et al., 2017). As oceanic crust is formed, it inherits the magnetic field at the time. At different points through time, the magnetic polarity (north vs. south) flips, the North Pole becomes the South Pole. These changes in polarity can be seen when measuring the magnetic field above oceanic plates. This is one of the fundamental evidences for plate spreading at oceanic spreading ridges (like the Gorda rise).
    • Regions with magnetic fields aligned like today’s magnetic polarity are colored red in the EMAG2 data, while reversed polarity regions are colored blue. Regions of intermediate magnetic field are colored light purple.
    • We can see the roughly east-west trends of these red and blue stripes in the Caroline and Australia plates. These lines are parallel to the ocean spreading ridges from where they were formed. The stripes disappear at the subduction zone because the oceanic crust with these anomalies is diving deep beneath the Sunda plate (part of Eurasia), so the magnetic anomalies from the overlying Sunda plate mask the evidence for the Australia plate.

      Global Strain

    • In a map below, I include a transparent overlay of the Global Strain Rate Map (Kreemer et al., 2014).
    • The mission of the Global Strain Rate Map (GSRM) project is to determine a globally self-consistent strain rate and velocity field model, consistent with geodetic and geologic field observations. The overall mission also includes:
      1. contributions of global, regional, and local models by individual researchers
      2. archive existing data sets of geologic, geodetic, and seismic information that can contribute toward a greater understanding of strain phenomena
      3. archive existing methods for modeling strain rates and strain transients
    • The completed global strain rate map will provide a large amount of information that is vital for our understanding of continental dynamics and for the quantification of seismic hazards.
    • The version used in the poster(s) below is an update to the original 2004 map (Kreemer et al., 2000, 2003; Holt et al., 2005).

      I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

    • In the upper left corner, I include a map from Benz et al. (2011) that shows historic earthquake locations (epicenters) along with some of the plate boundary faults. Note the strike slip fault (with the opposing black arrows) that cross the location of the 1938 earthquake (labeled in yellow on that map). I placed a blue star in the location of the M 7.3 quake. There is a cross section to the right of the map that shows how earthquakes dive down with a westward trend (following the plate down the subduction zone). The cross section location is shown on the map (B-B’).
    • In the upper right corner is a larger scale tectonic map from Audley (2011) showing the major thrust faults and the large forearc basin is labeled “Weber Deep.”
    • Hangesh and Whitney (2016) did lots of work on the faulting in the region to the south of the M 7.3. They show block boundaries and relative plate motion arrows in white. Note how they extend strike-slip motion along the Timor trough. This may be in addition to the strike-slip along the backthrust.
    • Here is the map with a month’s seismicity plotted. I included MMI contours from a recent M 6.3 earthquake in PNG, which led to a sequence of additional M~6 quakes to the southeast of that main shock. I won’t be writing a report for those quakes, even though it is interesting (check it out!). Sorry to have misspelled Hengesh as Hangesh.

    • Here is the map with a century’s seismicity (M ≥ 7.0) plotted.

    • Here is the map with a month’s seismicity (M ≥ 0.5) plotted with the Global Strain data plotted. We can see the 2018 Flores swarm show up here.

    Other Report Pages

    Some Relevant Discussion and Figures

    • Here is a tectonic map for this part of the world from Zahirovic et al., 2014. They show a fracture zone where the M 7.3 earthquake happened. I left out all the acronym definitions (you’re welcome), but they are listed in the paper.

    • Regional tectonic setting with plate boundaries (MORs/transforms = black, subduction zones = teethed red) from Bird (2003) and ophiolite belts representing sutures modified from Hutchison (1975) and Baldwin et al. (2012). West Sulawesi basalts are from Polvé et al. (1997), fracture zones are from Matthews et al. (2011) and basin outlines are from Hearn et al. (2003).

    • This is a great visualization showing the Australia plate and how it formed the largest forearc basin on Earth (Pownall et al., 2014).
    • The maps on the left show a time history of the tectonics. The low angle oblique view on the right shows the dipping crust (north is not always up, as in this figure).
    • In the lower right, they show how there is strike-slip faulting along the Seram trough also (I left out the figure caption for E).

    • Reconstructions of eastern Indonesia, adapted from Hall (2012), depict collision of Australia with Southeast Asia and slab rollback into Banda Embayment. Yellow star indicates Seram. Oceanic crust is shown in purple (older than 120 Ma) and blue (younger than 120 Ma); submarine arcs and oceanic plateaus are shown in cyan; volcanic island arcs, ophiolites, and material accreted along plate margins are shown in green. A: Reconstruction at 15 Ma. B: Reconstruction at 7 Ma. C: Reconstruction at 2 Ma. D: Visualization of present-day slab morphology of proto–Banda Sea based on earthquake hypocenter distribution and tomographic models

    • Here is a map and some cross sections showing seismic tomography (like C-T scans into the Earth using seismic waves instead of X-Rays). The map shows the location of the cross sections (Spakman et al., 2010).

    • The Banda arc and surrounding region. 200 m and 4,000 m bathymetric contours are indicated. The numbered black lines are Benioff zone contours in kilometres. The red triangles are Holocene volcanoes (http://www.volcano.si.edu/world/). Ar=Aru, Ar Tr=Aru trough, Ba=Banggai Islands, Bu=Buru, SBS=South Banda Sea, Se=Seram, Sm=Sumba, Su=Sula Islands, Ta=Tanimbar, Ta Tr=Tanimbar trough, Ti=Timor, W=Weber Deep.


      Tomographic images of the Banda slab. Vertical sections through the tomography model along the lines shown in Fig. 1. Colours: P-wave anomalies with reference to velocity model ak135 (ref. 30). Dots: earthquake hypocentres within 12 km of the section. The dashed lines are phase changes at ~410 km and ~660 km. The sections are plotted without vertical exaggeration; the horizontal axis is in degrees. The labelled positive anomalies are the Sunda (Su) and Banda (Ba) slabs: BuDdetached slab under Buru, FlDslab under Flores, SDslab under Seram, TDslab under Timor. a, The Sunda slab enters the lower mantle whereas the Banda embayment slab is entirely in the upper mantle with the change under Sulawesi. b–e, Banda slab morphology in sections parallel to Australia plate motion shows a transition from a steep slab with a flat section (fs) (b) to a spoon shape shallowing eastward (c–e).

    • Here is the tectonic map from Hengesh and Whitney (2016)

    • Illustration of major tectonic elements in triple junction geometry: tectonic features labeled per Figure 1; seismicity from ISC-GEM catalog [Storchak et al., 2013]; faults in Savu basin from Rigg and Hall [2011] and Harris et al. [2009]. Purple line is edge of Australian continental basement and fore arc [Rigg and Hall, 2011]. Abbreviations: AR = Ashmore Reef; SR = Scott Reef; RS = Rowley Shoals; TCZ = Timor Collision Zone; ST = Savu thrust; SB = Savu Basin; TT = Timor thrust; WT =Wetar thrust; WASZ = Western Australia Shear Zone. Open arrows indicate relative direction of motion; solid arrows direction of vergence.

    • Here is the Audley (2011) cross section showing how the backthrust relates to the subduction zone beneath Timor. I include their figure caption in blockquote below.

    • Cartoon cross section of Timor today, (cf. Richardson & Blundell 1996, their BIRPS figs 3b, 4b & 7; and their fig. 6 gravity model 2 after Woodside et al. 1989; and Snyder et al. 1996 their fig. 6a). Dimensions of the filled 40 km deep present-day Timor Tectonic Collision Zone are based on BIRPS seismic, earthquake seismicity and gravity data all re-interpreted here from Richardson & Blundell (1996) and from Snyder et al. (1996). NB. The Bobonaro Melange, its broken formation and other facies are not indicated, but they are included with the Gondwana mega-sequence. Note defunct Banda Trench, now the Timor TCZ, filled with Australian continental crust and Asian nappes that occupy all space between Wetar Suture and the 2–3 km deep deformation front north of the axis of the Timor Trough. Note the much younger decollement D5 used exactly the same part of the Jurassic lithology of the Gondwana mega-sequence in the older D1 decollement that produced what appears to be much stronger deformation.

    • Here is a figure showing the regional geodetic motions (Bock et al., 2003). I include their figure caption below as a blockquote.

    • Topographic and tectonic map of the Indonesian archipelago and surrounding region. Labeled, shaded arrows show motion (NUVEL-1A model) of the first-named tectonic plate relative to the second. Solid arrows are velocity vectors derived from GPS surveys from 1991 through 2001, in ITRF2000. For clarity, only a few of the vectors for Sumatra are included. The detailed velocity field for Sumatra is shown in Figure 5. Velocity vector ellipses indicate 2-D 95% confidence levels based on the formal (white noise only) uncertainty estimates. NGT, New Guinea Trench; NST, North Sulawesi Trench; SF, Sumatran Fault; TAF, Tarera-Aiduna Fault. Bathymetry [Smith and Sandwell, 1997] in this and all subsequent figures contoured at 2 km intervals.

    • Whitney and Hengesh (2015) used GPS modeling to suggest a model of plate blocks. Below are their model results.

    • Plate boundary segments in the Banda Arc region from Nugroho et al (2009). Numbers inside rectangles show possible micro-plate blocks near the Sumba Triple Junction (colored) based on GPS velocities (black arrows) with in a stable Eurasian reference frame.

    • Here is the conceptual model from Whitney and Hengesh (2015) that shows how left-lateral strike-slip faulting can come into the region.

    • Schematic map views of kinematic relations between major crustal elements in the Sumba Triple Junction region. CTZ= collisional tectonic zone. Red arrow size designates schematic plate motion relations based on geological data relative to a fixed Sunda shelf reference frame (pin).

    Geologic Fundamentals

    • For more on the graphical representation of moment tensors and focal mechanisms, check this IRIS video out:
    • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

    • Here is another way to look at these beach balls.
    • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
    • Strike Slip:

      Compressional:

      Extensional:

    • This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

    • A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

    • Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

    • Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

    • Here is a great tweet that discusses the different parts of a seismogram and how the internal structures of the Earth help control seismic waves as they propagate in the Earth.

      Social Media

      References:

    • Audley-Charles, M.G., 1986. Rates of Neogene and Quaternary tectonic movements in the Southern Banda Arc based on micropalaeontology in: Journal of fhe Geological Society, London, Vol. 143, 1986, pp. 161-175.
    • Audley-Charles, M.G., 2011. Tectonic post-collision processes in Timor, Hall, R., Cottam, M. A. &Wilson, M. E. J. (eds) The SE Asian Gateway: History and Tectonics of the Australia–Asia Collision. Geological Society, London, Special Publications, 355, 241–266.
    • Baldwin, S.L., Fitzgerald, P.G., and Webb, L.E., 2012. Tectonics of the New Guinea Region in Annu. Rev. Earth Planet. Sci., v. 41, p. 485-520.
    • Benz, H.M., Herman, Matthew, Tarr, A.C., Hayes, G.P., Furlong, K.P., Villaseñor, Antonio, Dart, R.L., and Rhea, Susan, 2011. Seismicity of the Earth 1900–2010 New Guinea and vicinity: U.S. Geological Survey Open-File Report 2010–1083-H, scale 1:8,000,000.
    • Given, J. W., and H. Kanamori (1980). The depth extent of the 1977 Sumbawa, Indonesia, earthquake, in EOS Trans. AGU., v. 61, p. 1044.
    • Gusnman, A.R., Tanioka, Y., Matsumoto, H., and Iwasakai, S.-I., 2009. Analysis of the Tsunami Generated by the Great 1977 Sumba Earthquake that Occurred in Indonesia in BSSA, v. 99, no. 4, p. 2169-2179, https://doi.org/10.1785/0120080324
    • Hall, R., 2011. Australia-SE Asia collision: plate tectonics and crustal flow in Geological Society, London, Special Publications 2011; v. 355; p. 75-109 doi: 10.1144/SP355.5
    • Hangesh, J. and Whitney, B., 2014. Quaternary Reactivation of Australia’s Western Passive Margin: Inception of a New Plate Boundary? in: 5th International INQUA Meeting on Paleoseismology, Active Tectonics and Archeoseismology (PATA), 21-27 September 2014, Busan, Korea, 4 pp.
    • Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
    • Hayes, G., 2018, Slab2 – A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release, https://doi.org/10.5066/F7PV6JNV.
    • Holt, W. E., C. Kreemer, A. J. Haines, L. Estey, C. Meertens, G. Blewitt, and D. Lavallee (2005), Project helps constrain continental dynamics and seismic hazards, Eos Trans. AGU, 86(41), 383–387, , https://doi.org/10.1029/2005EO410002. /li>
    • Kreemer, C., J. Haines, W. Holt, G. Blewitt, and D. Lavallee (2000), On the determination of a global strain rate model, Geophys. J. Int., 52(10), 765–770.
    • Kreemer, C., W. E. Holt, and A. J. Haines (2003), An integrated global model of present-day plate motions and plate boundary deformation, Geophys. J. Int., 154(1), 8–34, , https://doi.org/10.1046/j.1365-246X.2003.01917.x.
    • Kreemer, C., G. Blewitt, E.C. Klein, 2014. A geodetic plate motion and Global Strain Rate Model in Geochemistry, Geophysics, Geosystems, v. 15, p. 3849-3889, https://doi.org/10.1002/2014GC005407.
    • Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. https://doi.org/10.7289/V5H70CVX
    • Müller, R.D., Sdrolias, M., Gaina, C. and Roest, W.R., 2008, Age spreading rates and spreading asymmetry of the world’s ocean crust in Geochemistry, Geophysics, Geosystems, 9, Q04006, https://doi.org/10.1029/2007GC001743
    • Okal, E. A., & Reymond, D., 2003. The mechanism of great Banda Sea earthquake of 1 February 1938: applying the method of preliminary determination of focal mechanism to a historical event in EPSL, v. 216, p. 1-15.
    • Osada, M. and Abe, K., 1981. Mechanism and tectonic implications of the great Banda Sea earthquake of November 4, 1963 in Physics of the Earth and Plentary Interiors, v. 25, p. 129-139
    • Pownall, J.M., Hall, R., Armstrong,, R.A., and Forster, M.A., 2014. Earth’s youngest known ultrahigh-temperature granulites discovered on Seram, eastern Indonesia in Geology, v. 42, no. 4, p. 379-282, https://doi.org/10.1130/G35230.1
    • Spakman, W. and Hall, R., 2010. Surface deformation and slab–mantle interaction during Banda arc subduction rollback in Nature Geosceince, v. 3, p. 562-566, https://doi.org/10.1038/NGEO917
    • Whitney, B.B. and Hengesh, J.V., 2015. A new model for active intraplate tectonics in western Australia in Proceedings of the Tenth Pacific Conference on Earthquake Engineering Building an Earthquake-Resilient Pacific 6-8 November 2015, Sydney, Australia, paper number 82
    • Zahirovic, S., Seton, M., and Müller, R.D., 2014. The Cretaceous and Cenozoic tectonic evolution of Southeast Asia in Solid Earth, v. 5, p. 227-273, doi:10.5194/se-5-227-2014

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    Posted in earthquake, education, Indonesia, New Zealand, pacific, plate tectonics, strike-slip, Transform