Earthquake Report: Mendocino fault

I was in Humboldt County last week for the Redwood Coast Tsunami Work Group meeting. I stayed there working on my house that a previous tenant had left in quite a destroyed state (they moved in as friends of mine).
As I was grabbing a bite at Taqueria Bravo in Willits, I checked in on social media and noticed my friend Dave Bazard had posted moments earlier about an earthquake there. I had missed it by about 2 hours or so.
https://earthquake.usgs.gov/earthquakes/eventpage/nc73351710/executive
Yesterday’s earthquake was a right-lateral strike-slip earthquake on the Mendocino fault system. The Mendocino fault is a strike-slip fault formed by the eastward motion of the Gorda plate relative to the westward motion of the Pacific plate. The last major damaging earthquake on the MF was in 1994.
Interestingly, this was the 6 year commemoration of the 2014 M 6.8 Gorda plate earthquake (the last large earthquake in the region).
Also, there was a similarly sized event on the MF in 2018.

    Big “take-aways” from this:

  • This earthquake did not affect the Cascadia megathrust subduction zone fault (too small of magnitude and too far away).
  • This earthquake did not generate an observable tsunami.
  • This earthquake changed the stress in the surrounding crust, but a very very small amount (in some places it increased stress on faults and in other places it decreased stresses on faults). However, the magnitude was small and this change in stress is probably short lived. I discuss this about a previous MF earthquake here. I spend more time on this topic for a Gorda plate earthquake here.

Here is a seismic selfie from Riley, a student at Humboldt State University (taking a geology course). This photo was posted on the HSU Dept. of Geology facebook page.

Below is my interpretive poster for this earthquake

  • I plot the seismicity from the past month, with diameter representing magnitude (see legend). I include earthquake epicenters from 1920-2020 with magnitudes M ≥ 3.5 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 lower left corner is a legend, but to the right is an inset map of the Cascadia subduction zone (modified from Nelson et al., 2006). I place a blue star in the location of yesterday’s earthquake.
  • In the upper left corner is a small scale map showing the entire pacific northwest with some historic seismicity (up to central Oregon; I forgot to download the data from the entire region; there are other examples of this).
  • To the right of that is a map showing the USGS Did You Feel It observation results showing how broadly this earthquake was felt. My friend in Redding told me that they felt it. This made sense since the Mendocino fault points right at Redding, but it was also felt in southern California (probably from site amplification from sedimentary basins). The color is the same scale as in the legend for shaking intensity (MMI).
  • Here is the map with a week’s and century’s seismicity plotted. I include the USGS model for shaking intensity as a transparent overlay (with MMI intensities up to M 5 near the epicenter).

Other Report Pages

Some Relevant Discussion and Figures

  • The USGS models earthquake intensity using what we often call “Ground Motion Prediction Equations.” Some prefer to change this terminology as the word “prediction” is problematic (because one cannot predict earthquakes).
  • Basically, the further away from an earthquake, the less one feels the shaking. These GMPE “intensity-distance” relations are based on the measurements of earthquake shaking from thousands of earthquakes. There are a variety of factors that control the ground shaking in addition to the distance.
  • The USGS has a “Did You Feel It?” system where people can submit their observations using an online questionnaire. These observations are converted to an intensity value using the Modified Mercalli Intensity (MMI) scale. I explain this a little more here.
  • Here is a figure that I prepared using the USGS map of DYFI results. I also include a plot that shows how the intensity (vertical axis) decays with distance (horizontal axis) from the earthquake.

  • Here is a map of the Cascadia subduction zone, modified from Nelson et al. (2006). The Juan de Fuca and Gorda plates subduct north eastwardly 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.

  • 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 January 2010 Gorda plate earthquake. The faults are from Chaytor et al. (2004). The 1980, 1992, 1994, 2005, and 2010 earthquakes are plotted and labeled. I did not mention the 2010 earthquake, but it most likely was just like 1980 and 2005, a left-lateral strike-slip earthquake on a northeast striking fault.

  • Here is a large scale map of the 1994 earthquake swarm. The mainshock epicenter is a black star and epicenters are denoted as white circles.

  • Here is a plot of focal mechanisms from the Dengler et al. (1995) paper in California Geology.

  • 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.

  • This figure shows how a subduction zone deforms between (interseismic) and during (coseismic) earthquakes. We also can see how a subduction zone generates a tsunami. Atwater et al., 2005.

  • Here is an animation produced by the folks at Cal Tech following the 2004 Sumatra-Andaman subduction zone earthquake. I have several posts about that earthquake here and here. One may learn more about this animation, as well as download this animation here.
  • This is the map used in the animation below. Earthquake epicenters are plotted (some with USGS moment tensors) for this region from 1917-2017 with M ≥ 6.5. I labeled the plates and shaded their general location in different colors.
  • I include some inset maps.
    • In the upper right corner is a map of the Cascadia subduction zone (Chaytor et al., 2004; Nelson et al., 2004).
    • In the upper left corner is a map from Rollins and Stein (2010). They plot epicenters and fault lines involved in earthquakes between 1976 and 2010.


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    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>
  • 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
  • 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
  • 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
  • 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, https://doi.org/0.1785/0120160198
  • 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.
  • 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/
  • 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/
  • 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: 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

Return to the Earthquake Reports page.


Earthquake Report: Berkeley, CA (Hayward fault)

There was an earthquake last night (local time) in Berkeley, aligned with the Hayward fault. The Hayward fault is one of the synthetic sister faults to the San Andreas fault, the major player in the dextral (right-lateral, strike-slip) plate boundary between the Pacific plate and the North America plate to the east.
Over 35,000 people have reported their observations on the USGS “Did You Feel It?” website for this earthquake. If you live in this region, please visit this website and register your observations!
The San Andreas fault is a right-lateral strike-slip transform plate boundary between the Pacific and North America plates. The plate boundary is composed of faults that are parallel to sub-parallel to the SAF and extend from the west coast of CA to the Wasatch fault (WF) system in central Utah (the WF runs through Salt Lake City and is expressed by the mountain range on the east side of the basin that Salt Lake City is built within).
About 75% of the relative plate motion is accommodated along the SAF and its synthetic sister faults in the northern CA region. The rest of the plate boundary motion is accommodated along the Eastern CA shear zone and Walker Lane, along with the Central Nevada Seismic Belt, and the Wasatch fault systems. In Northern CA, there is about 33-37 mm/yr strain accumulated on the SAF plate boundary system. About 18-25 mm/yr is on the SAF, 8-11 mm/yr on the MF, and 5-7 mm/yr on the Bartlett Springs fault system (Geist and Andrews, 2000).
The three main faults in the region north of San Francisco are the SAF, the Hayward fault (HF), and the Calaveras fault (CF). However, there are several others that pose a risk to the inhabitants here. Most of the faults in the region are right-lateral strike-slip faults, just like the SAF.

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 1917-2017 with magnitudes M > 4.0.
I use the USGS Quaternary fault and fold database for the faults.
I plot the USGS fault plane solutions (moment tensors in blue,focal mechanisms in orange) for 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. Based upon the tectonics associated with the San Andreas and Hayward faults, I interpret this M 4.4 earthquake to be a right-lateral strike-slip fault.
  • 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 some inset figures.

  • On the right, I include generalized fault map of northern California from Wallace (1990). I place a blue star in the general location of today’s M 4.4 earthquake.
  • In the upper left corner is a map from Aagaard et al. (2016) that shows the probability (chance of) an earthquake along various faults for the next 30 years or so. Note that the HF has the highest likelihood of generating an earthquake with magnitude M ≥ 6.7.
  • In the lower left corner I include a larger scale map showing the details of the mapped faults.



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 a map from McLaughlin et al. (2012) that shows the regional faulting. I include the figure caption as a blockquote below.

  • Maps showing the regional setting of the Rodgers Creek–Maacama fault system and the San Andreas fault in northern California. (A) The Maacama (MAFZ) and Rodgers Creek (RCFZ) fault zones and related faults (dark red) are compared to the San Andreas fault, former and present positions of the Mendocino Fracture Zone (MFZ; light red, offshore), and other structural features of northern California. Other faults east of the San Andreas fault that are part of the wide transform margin are collectively referred to as the East Bay fault system and include the Hayward and proto-Hayward fault zones (green) and the Calaveras (CF), Bartlett Springs, and several other faults (teal). Fold axes (dark blue) delineate features associated with compression along the northern and eastern sides of the Coast Ranges. Dashed brown line marks inferred location of the buried tip of an east-directed tectonic wedge system along the boundary between the Coast Ranges and Great Valley (Wentworth et al., 1984; Wentworth and Zoback, 1990). Dotted purple line shows the underthrust south edge of the Gorda–Juan de Fuca plate, based on gravity and aeromagnetic data (Jachens and Griscom, 1983). Late Cenozoic volcanic rocks are shown in pink; structural basins associated with strike-slip faulting and Sacramento Valley are shown in yellow. Motions of major fault blocks and plates relative to fi xed North America, from global positioning system and paleomagnetic studies (Argus and Gordon, 2001; Wells and Simpson, 2001; U.S. Geological Survey, 2010), shown with thick black arrows; circled numbers denote rate (in mm/yr). Restraining bend segment of the northern San Andreas fault is shown in orange; releasing bend segment is in light blue. Additional abbreviations: BMV—Burdell Mountain Volcanics; QSV—Quien Sabe Volcanics. (B) Simplifi ed map of color-coded faults in A, delineating the principal fault systems and zones referred to in this paper.

  • 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 Mendicino 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 Mendicino 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 Mendicino 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.

  • Here is the earthquake probability map for the SF Bay area (Aagard et al., 2016).

  • This shows a timeline for historic earthquakes in this region.

  • 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. Many of the earthquakes people are familiar with in the Mendocino triple junction region are either compressional or strike slip. The following three animations are from IRIS.
  • Strike Slip:
  • Compressional:
  • Extensional:
  • This figure shows what a transform plate boundary fault is. Looking down from outer space, the crust on either side of the fault moves side-by-side. When one is standing on the ground, on one side of the fault, looking across the fault as it moves… If the crust on the other side of the fault moves to the right, the fault is a “right lateral” strike slip fault. The Mendocino and San Andreas faults are right-lateral (dextral) strike-slip faults. I believe this is from Pearson Higher Ed.

  • 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).


    References:

  • Aagaard, B.T., Blair, J.L., Boatwright, J., Garcia, S.H., Harris, R.A., Michael, A.J., Schwartz, D.P., DiLeo, J.S., Jacques, K., and Donlin, C., 2016. Earthquake Outlook for the San Francisco Bay Region 2014–2043 in USGS Fact Sheet 2016–3020 Revised August 2016 (ver. 1.1) ISSN 2327-6916 (print) ISSN 2327-6932 (online) http://dx.doi.org/10.3133/fs20163020
  • 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/
  • 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/
  • Wallace, Robert E., ed., 1990, The San Andreas fault system, California: U.S. Geological Survey Professional Paper 1515, 283 p. [https://pubs.er.usgs.gov/publication/pp1515].

San Francisco Bay: San Ramon Earthquake Swarm!

Those of you in the region of the San Francisco Bay are probably wondering what the likelihood that this recent and ongoing swarm of earthquakes in the San Ramon area may lead to a larger earthquake. I do not know, but I will lay out some things for you to chew on.

    Two possibilities exist. The prior has a slightly higher likelihood in my opinion. Others may have a stronger opinion and when they chime in, I will add more commentary here.

  1. These are foreshocks to an earthquake on the Pleasanton fault zone.
  2. These earthquakes are loading the Calaveras fault and we can expect an earthquake on that fault system.

Here is a map that shows the regional fault lines and the focal mechanisms for the two largest (M = 3.4 & 3.4) earthquakes from this swarm. Given the presence of the dominantly right-lateral strike-slip faulting in the region, I interpret these two earthquakes to be right-lateral (dextral) strike-slip earthquakes. It appears that these earthquakes are shallow and are associated with deformation along the Pleasanton fault or nearby faults. It appears that these earthquakes are near the northward termination of this fault zone.
I placed a moment tensor / focal mechanism legend in the upper left corner of the map. 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.

    Here are the USGS web pages for these two earthquakes:

  • 2015.10.15 M 3.4
  • 2015.10.19 M 3.6

Here is a locally zoomed map showing these earthquakes as they relate to the cities of San Ramon and Danville. Notice how the Pleasanton fault ends in the region of the swarm. This swarm may have loaded the fault segments to the south, along the Pleasanton fault. However, it probably did not load significantly the Calaveras fault system to the west. The faults at depth may be complicated, so this 2-D plan view interpretation of the stresses is probably an oversimplification.


The USGS, have made estimates of the probabilities of earthquake ruptures on the major fault zones in the San Francisco Bay area. Here is a link to their web site. Below is a map showing the probabilities that they have assigned to each of these major fault zones. Note the low probability assigned to the Calaveras fault zone. Click on the map for a higher resolution (10 MB) pdf map.


Keith I. Kelson and Sean T. Sundermann published a report based on their USGS National Earthquake Hazards Reduction Program funded study of the Calaveras fault. They report a couple swarms possibly associated with the Pleasanton fault zone. In 1976 there was a swarm with a M 4.0 largest earthquake and in 1970 there was a swarm with a M 4.3 as a largest earthquake. Then in 2002 there was a swarm with a M 3.9 as a largest earthquake.


Based on all these different sources of information, it would appear that this swarm is similar to swarms in 1970, 1976, and 2002. I do not expect either of the two possibilities, that I list at the top of this page, to occur. We cannot predict the future, but can only look for patterns based upon the past. This is sort of inverse uniformitarianism.

Small Earthquake in San Francisco Region

There was a M 3.6 earthquake near Concord, CA. This earthquake was felt widely across the S.F. Bay area. The two earthquakes plotted in the map below are both just southeast of the 2014 August Napa earthquake. The Napa earthquake, a magnitude M = 6.0 earthquake that generated surface rupture. Here was my original post about the Napa earthquake and here is an update. This is the USGS web site for the 2015.05.03 Concord earthquake.
Here is a map that shows the epicenter of the Concord earthquake, as well as some explanation diagrams for the moment tensor plots. The region of the 2014 Napa Earthquake is depicted by the orange and red polygon.


This is a cool map that shows the “Did You Feel It?” survey results in a grid. The colors are light blue, matching the lower intensities of the Modified Mercalli Intensity Scale.