Earthquake Report: Fiji

WOW

We just had a Great Earthquake in the region of the Fiji Islands, in the central-western Pacific. Great Earthquakes are earthquakes with magnitudes M ≥ 8.0.

This earthquake is one of the largest earthquakes recorded historically in this region. I include the other Large and Great Earthquakes in the posters below for some comparisons.

Today’s earthquake has a Moment Magnitude of M = 8.2. The depth is over 550 km, so is very very deep. This region has an historic record of having deep earthquakes here. Here is the USGS website for this M 8.2 earthquake. While I was writing this, there was an M 6.8 deep earthquake to the northeast of the M 8.2. The M 6.8 is much shallower (about 420 km deep) and also a compressional earthquake, in contrast to the extensional M 8.2.

This M 8.2 earthquake occurred along the Tonga subduction zone, which is a convergent plate boundary where the Pacific plate on the east subducts to the west, beneath the Australia plate. This subduction zone forms the Tonga trench.

The subduction zone megathrust fault dips downwards to the west and the location of this “slab” has been evaluated by Hayes et al. (2012). These USGS geologists have updated the global slab model and I will incorporate these new data in upcoming reports. Today’s earthquake hypocenter (the 3-dimensional location of the earthquake) is at 563 km and the slab depth is about 520 km in this location (pretty good match given the range of depths for earthquakes relative to the fault location.

Due to the large depth, this earthquake did not shake very strongly at Earth’s surface. In addition, due to the large depth, a large tsunami is not expected. I checked the UNESCO IOC Sea Level Monitoring Facility, which posts a global set of tide gage data online. Here is their online map interface.

In 1994 there was a deep Great Earthquake (M 8.0) very close to today’s M 8.2 earthquake. One interesting thing is that the 2002 earthquake was compressional (a thrust or reverse fault earthquake) and today’s M 8.2 earthquake is extensional (a normal fault earthquake).

We are still unsure what causes an earthquake at such great a depth. The majority of earthquakes happen at shallower depths, caused largely by the frictional between differently moving plates or crustal blocks (where earth materials like the crust behave with brittle behavior and not elastic behavior). Some of these shallow earthquakes are also due to internal deformation within plates or crustal blocks.

As plates dive into the Earth at subduction zones, they undergo a variety of changes (temperature, pressure, stress). However, because people cannot directly observe what is happening at these depths, we must rely on inferences, laboratory analogs, and other indirect methods to estimate what is going on.

Below is a review of possible explanations as provided by Thorne Lay (UC Santa Cruz) in an interview in response to the 2013 M 8.3 Okhtosk Earthquake.

One option could be “fluid-assisted faulting,” in which water is released from minerals as they change phases during faulting, thus lubricating the plates, Lay says.

But although this is a common mechanism for earthquakes between 70 and 400 kilometers deep, it’s unlikely to be the cause of this quake because the plate is significantly dewatered by the time it reaches 400 kilometers deep. Minerals releasing carbon dioxide as they are compacted could provide an alternative fluid to lubricate the fault, he says, much like water does at shallower depths.

And another possibility is that a transition in mineral form from low-pressure polymorphs (the form in which a mineral is stable at the surface) to high-pressure polymorphs (a denser form of a mineral that is stable at greater depths), gives the fault a start. According to this model, the plate subducts too quickly for the mineral to slowly transition to its denser form. The mineral will reach depths greater than where it is normally stable, and thus the transformation may be a catastrophic process, causing a jolt at 600 kilometers, which would allow for movement along the fault, Lay says.

There have been a number of deep earthquakes globally in the past several years. These include the 2013 M 8.3 in the Sea of Okhtosk, the 2015 M 7.8 along the Izu-Bonin Arc, and several along the central Andes. I present some interpretive posters for these earthquakes below.

In early 2017 there was an M 6.9 earthquake in this region near Fiji. Here is the report for this earthquake.

There are many interesting earthquakes on this map and I will attempt to fill in this report with discussion and figures for some of these earthquakes. For example the 2009 Samoa earthquake, the 2009 Vanuatu doublet earthquakes, and the 1995 : 1998 earthquakes at the southern New Hebrides Trench.

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 1918-2018 with magnitudes M ≥ 7.50 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 contours plotted (Hayes et al., 2012), 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.
  • Note the magnetic anomalies (alternating bands of red and blue), parallel to the spreading ridges (the green lines with diverging orange arrows in the North Fiji Basin).

    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 portion of the map from Benz et al. (2011). This map shows earthquake epicenters (2-D locations) for seismicity from the past century or so. Depth is represented by color and earthquake magnitude is represented by the size of the circle symbols. Seismicity cross sections are located along the green (H-H’) and blue (I-I’) lines. I place a blue star in the general location of today’s M 8.2 earthquake on this map, the H-H’ cross section, as well as the other inset figures.
  • Cross sections showing earthquake hypocenters along the two profiles (H-H’ and I-I’) are presented above the Benz et al. (2011) map. These seismicity cross section locations are also shown on the main map.
  • The lower right corner includes a map from de Alteriis et al. (1993) that shows some details of the plate boundaries in this region. Note the subduction zones (New Hebrides Trench and Tonga Trench). Also not some strike-slip fault systems (e.g. the Hunter fracture zone and the North Fiji fracture zone). There is a good example of a strike-slip earthquake along the Hunter fracturezone from 1990.
  • In the upper right corner is a figure that shows the tectonic development of the region surrounding Fiji (Begg and Gray, 2002). These authors worked on the volcanic and tectonic history of the Fiji Plateau.
  • Here is the map with a month’s seismicity plotted.

  • Here is the map with a centuries seismicity plotted with M ≥ 7.5.

Other Report Pages

Some Relevant Discussion and Figures

  • This is the plate tectonic map from de Alteriis et al. (1993) that shows the major fault systems in the region.

  • Location map of North Fiji Basin ridge; box indicates full multibeam covered area of Figure 2. Heavy lines denote north-south, N15°, and N160° main segments of ridge axis; dashed lines are pseudofaults indicating double propagation. F. Z.— fracture zone.

  • Here is a figure from Schellart et al. (2002) that shows their model of tectonic development of the North Fiji Basin. Schellart et al. (2002) include a long list of references for the tectonics in this region here. Below I include the text from the original figure caption in blockquote.

  • Tectonic reconstruction of the New Hebrides – Tonga region (modified and interpreted from Auzende et al. [1988], Pelletier et al. [1993], Hathway [1993] and Schellart et al.(2002a)) at (a) ~ 13 Ma, (b) ~ 9 Ma, (c) 5 Ma and (d) Present. The Indo-Australian plate is fixed. DER = d’Entrcasteaux Ridge, HFZ = Hunter Fracture Zone, NHT = New Hebrides Trench, TT = Tonga Trench, WTP = West Torres Plateau. Arrows indicate direction of arc migration. During opening of the North Fiji Basin, the New Hebrides block has rotated some 40-50° clockwise [Musgrave and Firth 1999], while the Fiji Plateau has rotated some 70-115° anticlockwise [Malahoff et al. 1982]. During opening of the Lau Basin, the Tonga Ridge has rotated ~ 20° clockwise [Sager et al. 1994]. (Click for enlargement)

  • This is the plate tectonic history map from Begg and Gray (1993) that shows how they interpret the Fiji Plateau to have formed.

  • Tectonic setting (Figures 1a–1c) and tectonic reconstructions (Figures 1d and 1e) of the Outer Melanesian region (adapted from Hathway [1993]; reprinted with permission from the Geological Society of London).

  • (a) Map of the Fiji platform and north end of the Lau Ridge showing the major islands in the Fiji area, the major early Pliocene volcanoes of Viti Levu, the major seafloor fracture zones, and part of the spreading center of the Fiji Basin (adapted from Gill and Whelan [1989]). Shoshonitic volcanoes, including the Tavua Volcano (T), are shown by squares and calc-alkaline volcanoes by circles.
  • (b) Tectonic features of the northeastern segment of the plate boundary between the Australian and Pacific plates showing the Outer Melanesian Arc of the southwest Pacific, trenches and ridge systems, and oceanic plateaus (adapted from Kroenke [1984]). Fiji, as part of the Fiji Platform, consists of a series of islands at the north end of the Lau Ridge, with the North Fiji Basin formed as part of a spreading center.
  • (c) Present plate configuration.
  • (d) Reconstruction at 5.5 Ma.
  • (e) Reconstruction at 10 Ma. In Figures 1a–1e the Australian plate is fixed and the east-west convergence rate between plates was assumed to be 9–10 cm yr-1. Shading represents submarine depths <2000 m.
  • Abbreviations are as follows: VT, Vitiaz trench; VAT, Vanuatu trench; LR, Lau Ridge; LB, Lau Basin; TR, Tonga Ridge; FFZ, Fiji Fracture Zone; LL, Lomaiviti lineament; V-BL, Vatulele-Beqa lineament. Long dashes denote southern margin of the Melanesian Border Plateau (MBP). The open square (Figures 1b and 1c) denotes the location of the Tavua Volcano.
  • Okal (1997) conducted an analysis of seismological records from a deep earthquake that happened in the region of the 2017.01.03 M 6.3 earthquake. This earthquake occurred on 26 May 1932, long before modern seismometers made it to the scene. Okal estimated the magnitude to be similar in size to earthquakes in the mid M 7 range. Here is a figure from Okal (1997) that shows some focal mechanisms for the earthquakes from 1932. Compare the mainshock (the largest focal mechanism) with the moment tensor for the 2016.01.02 M 6.3 earthquake. Below I include the text from the original figure caption in blockquote.
  • 1932.05.26 M 7.6 (USGS)

  • Focal mechanism of the 1932 earthquake, as determined in this study. We also show CMT solutions in the immediate vicinity of the event, as available from Dziewonski et al. (1983, and subsequent quarterly updates) and Huang et al. (1997). Their spatial distribution is shown in map view. The background map at the upper right sets the study area (shaded) into the familiar bathymetry of the Fiji-Tonga-Kermadec region. The separation of isobaths is 1000 m.

  • Interestingly, deep focus earthquakes take up ~66% of the deep earthquakes globally. From Yu and Wen (2012), we can see some moment tensors for deep earthquakes in this region. The 1994.07.30 earthquake is just west of the 2017 M 6.3 earthquake and also has a similar moment tensor to the 2017 M 6.3 earthquake.

  • Regional map of deep-focus similar earthquake pairs and seismicity near the Tonga–Fiji subduction zone. Deep similar earthquake pairs (black stars) and their available Global Centroid Moment Tensor (CMT) (Dziewonski et al., 1981; Ekstrom et al., 2003) are labeled with event date and doublet/cluster ID where applicable. Source parameters of the doublets/clusters are listed in Tables 1, 2. Background deep seismicity is shown as gray dots. Black lines indicate the slab contours below 300 km depth (Gudmundsson and Sambridge, 1998), with an interval of 100 km. Regional map of the Tonga–Fiji–Kermadec subduction zone is shown in the inset, with gray dotted box indicating the region blow-up in the main figure. Black lines are the slab contours below 300 km depth and the Tonga–Kermadec trench (Bird, 2003). The color version of this figure is available only in the electronic edition.

  • Green (2007) presents a great review about what may control the mechanics of deep earthquakes. I present his abstract in its entirety because it is so well written. Below are a couple supporting figures. Read the paper for more insight.
  • Abstract: Deep earthquakes have been a paradox since their discovery in the 1920s. The combined increase of pressure and temperature with depth precludes brittle failure or frictional sliding beyond a few tens of kilometers, yet earthquakes occur continually in subduction zones to ≈700 km. The expected healing effects of pressure and temperature and growing amounts of seismic and experimental data suggest that earthquakes at depth probably represent self-organized failure analogous to, but different from, brittle failure. The only high-pressure shearing instabilities identified by experiment require generation in situ of a small fraction of very weak material differing significantly in density from the parent material. This “fluid” spontaneously forms mode I microcracks or microanticracks that self-organize via the elastic strain fields at their tips, leading to shear failure. Growing evidence suggests that the great majority of subduction zone earthquakes shallower than 400 km are initiated by breakdown of hydrous phases and that deeper ones probably initiate as a shearing instability associated with breakdown of metastable olivine to its higher-pressure polymorphs. In either case, fault propagation could be enhanced by shear heating, just as is sometimes the case with frictional sliding in the crust. Extensive seismological interrogation of the region of the Tonga subduction zone in the southwest Pacific Ocean provides evidence suggesting significant metastable olivine, with implication for its presence in other regions of deep seismicity. If metastable olivine is confirmed, either current thermal models of subducting slabs are too warm or published kinetics of olivine breakdown reactions are too fast.

  • Here is a profile into the Earth that shows depths for various chemical – mechanical process that are thought to control seismicity in various ways (Green, 2007).

  • Earthquake depth distribution. (a) Semilog plot of global earthquake frequency per 10-km-thick depth interval, showing a bimodal distribution. All earthquakes below 50 km are in subduction zones, the coldest parts of the mantle. The boundary between the mantle transition zone and lower mantle in subduction zones is at 700 km. No earthquake has ever been detected in the lower mantle. Modified from ref. 35. (b) Cartoon of subduction zone and earthquake distribution. Lithosphere (speckled) at right, with uppermost layers altered to antigorite (serpentine), is subducting beneath lithosphere at left. Earthquakes in olivine-dominated upper mantle are shown as red dots in serpentine and white diamonds. In the mantle transition zone, olivine is hypothesized to remain present despite being no longer thermodynamically stable and to slowly react away to spinel (wadsleyite or ringwoodite) during descent, occasionally generating earthquakes (black dots) by the process discussed in the text. Note volume reductions accompanying phase transformations at 410 and 660 km. Modified from ref. 36.

  • Here is an illustration showing a visualization of the slab associated with the Tonga subduction zone (Green, 2007).

  • Cartoon showing active Tonga subduction zone and fossil slab floating above it. Original figure is modified after ref. 26. Yellow and orange stars and circles were added in ref. 28.

  • The Goes et al. (2017) paper presents an excellent review of the various forces and earthquake types along subduction zones globally. This paper is open source and free to download. Below are some summary figures.
  • This shows the general relations between various forces exerted on a subducting slab.

  • Schematic diagram showing the main forces that affect how slabs interact with the transition zone. The slab sinks driven by its negative thermal buoyancy (white filled arrows). Sinking is resisted by viscous drag in the mantle (black arrows) and the frictional/viscous coupling between the subducting and upper plate (pink arrows). To be able to sink, the slab must bend at the trench. This bending is resisted by slab strength (curved green arrow). The amount the slab needs to bend depends on whether the trench is able to retreat, a process driven by the downward force of the slab and resisted (double green arrow) by upper-plate strength and mantle drag (black arrows) below the upper plate. At the transition from ringwoodite to the postspinel phases of bridgmanite and magnesiowüstite (rg – bm + mw), which marks the interface between the upper and lower mantle, the slab’s further sinking is hampered by increased viscous resistance (thick black arrows) as well as the deepening of the endothermic phase transition in the cold slab, which adds positive buoyancy (open white arrow) to the slab.

    By contrast, the shallowing of exothermic phase transition from olivine to wadsleyite (ol-wd) adds an additional driving force (downward open white arrow), unless it is kinetically delayed in the cold core of the slab (dashed green line), in which case it diminishes the driving force. Phase transitions in the crustal part of the slab (not shown) will additionally affect slab buoyancy. Buckling of the slab in response to the increased sinking resistance at the upper-lower mantle boundary is again resisted by slab strength.

  • Here is a plot showing their summary of observations for various subduction zones globally.

  • Summary of morphologies of transition-zone slabs as imaged by tomographic studies and their Benioff stress state. Arrows on the map indicate the approximate locations of the cross sections shown around the map, with their points in downdip direction. Blue shapes are schematic representations of slab morphologies (based on the extent of fast seismic anomalies that were tomographically resolvable from the references listed). Horizontal black lines indicate the base of the transition zone (~660 km depth). For flattened slabs, the approximate length of the flat section is given in white text inside the shapes. For penetrating slabs, the approximate depth to which the slabs are continuous is given in black text next to the slabs. Circles inside the slabs indicate whether the mechanisms of earthquakes at intermediate (100–350 km) and deep (350–700 km) are predominantly downdip extensional (black) or compressional (white). Stress states are from the compilations of Isacks and Molnar (1971), Alpert et al. (2010), Bailey et al. (2012), complemented by Gorbatov et al. (1997) for Kamchatka, Stein et al. (1982) for the Antilles, McCrory et al. (2012) for Cascadia, Papazachos et al. (2000) for the Hellenic zone, and Forsyth (1975) for Scotia. The subduction zones considered are (from left to right and top to bottom): RYU—Ryukyu, IZU—Izu, HON—Honshu, KUR—Kuriles, KAM—Kamchatka, ALE—Aleutians, ALA—Alaska, CAL—Calabria, HEL—Hellenic, IND—India, MAR—Marianas, CAS—Cascadia, FAR—Farallon, SUM—Sumatra, JAV—Java, COC—Cocos, ANT—Antilles, TON—Tonga, KER—Kermadec, CHI—Chile, PER—Peru, SCO—Scotia. Numbers next to the red subduction zone codes refer to the tomographic studies used to define the slab shapes

    I include some inset figures in this interpretive poster for the 2017.01.03 M 6.9 Fiji Earthquake.

  • In the lower left corner I include map that shows the historic seismicity for this region (Martin, 2014). The color shows well how the earthquakes that happen along the Tonga Trench get deeper along with the subducting slab. Shallow earthquakes are generally subduction zone earthquakes and deeper earthquakes are related (generally) to processes happening withing the downgoing slab. The 2017.01.02 M 6.3 earthquake is one of these deep earthquakes. I will briefly compare this M 6.3 earthquake with an earthquake from the region that occurred in 1932 (Okal, 1997).
  • In the center top I include a figure that shows a small scale map of the southwestern Pacific (a) and a large scale map of the North Fiji Basin (b) from Martin, 2013. The various spreading ridges are indicated as double lines. I present this figure below.
  • In the upper right corner I include a figure from Schellart et al. (2002) that shows a conceptual model for the development of the North Fiji Basin formed by extension in the plate as the Basin rotated clockwise towards the New Hebrides Trench. I present this below.
  • In the lower right corner I include a figure from Richards et al. (2011) that shows their model of how the subducting slabs have interacted through time. These authors think that there is a stalled out and torn slab at depth below the North Fiji Basin. The M 7.2 earthquake occurred near the cross section c-c’.


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).

  • 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.

    References:

  • Auzende, J-M., Pelletier, B., Lafoy, Y., 1994. Twin active spreading ridges in the North Fiji Basin (southwest Pacific) in Geology, v. 22, p. 63-66.
  • Begg, G. and Gray, D.R., 2002. Arc dynamics and tectonic history of Fiji based on stress and kinematic analysis of dikes and faults of the Tavua Volcano, Viti Levu Island, Fiji in Tectonics, v. 21, no. 4, DOI: 10.1029/2000TC001259
  • Benz, H.M., Herman, Matthew, Tarr, A.C., Furlong, K.P., Hayes, G.P., Villaseñor, Antonio, Dart, R.L., and Rhea, Susan, 2011. Seismicity of the Earth 1900–2010 eastern margin of the Australia plate: U.S. Geological Survey Open-File Report 2010–1083-I, scale 1:8,000,000.
  • de Alterris, G. et al., 1993. Propagating rifts in the North Fiji Basin southwest Pacific in Geology, v. 21, p. 583-586.
  • Goes, S., Agrusta, R., van Hunen, J., and Garel, F., 2017. Subduction-transition zone interaction: A review: Geosphere, v. 13, no. 3, p. 1–21, doi:10.1130/GES01476.1.
  • Green, H.W., 2007. Shearing instabilities accompanying high-pressure phase transformations and the mechanics of deep earthquakes in PNAS, v. 104, no. 22, DOI: https://doi.org/10.1073/pnas.0608045104
  • 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
  • Martin, A.K., 2013. Double-saloon-door tectonics in the North Fiji Basin in EPSL, v. 374, p. 191-203.
  • Martin, A.K., 2014. Concave slab out board of the Tonga subduction zone caused by opposite toroidal flows under the North Fiji Basin in Tectonophysics, v. 622, p. 56-61.
  • 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. doi:10.7289/V5H70CVX
  • Okal, 1997. A reassessment of the deep Fiji earthquake of 26 May 1932 in Tectonophysics v., 275, p. 313-329.
  • Richards, S., Holm., R., Barber, G., 2011. When slabs collide: A tectonic assessment of deep earthquakes in the Tonga-Vanuatu region, Geology, v. 39, pp. 787-790.
  • Schellart, W., Lister, G. and Jessell, M. 2002. Analogue modelling of asymmetrical back-arc extension. In: (Ed.) Wouter Schellart, and Cees W. Passchier, Analogue modelling of large-scale tectonic processes, Journal of the Virtual Explorer, Electronic Edition, ISSN 1441-8142, volume 7, paper 3, doi:10.3809/jvirtex.2002.00046
  • Yu, W. and Wen, L., 2012. Deep-Focus Repeating Earthquakes in the Tonga–Fiji Subduction Zone, BSSA, v. 102, no. 4, pp. 1829-1849

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Posted in College Redwoods, earthquake, education, geology, pacific, plate tectonics, subduction

Earthquake Report: Andreanof Islands, Aleutians

Well, yesterday while I was installing the final window in a reconstruction project, there was an earthquake along the Aleutian Island Arc (a subduction zone) in the region of the Andreanof Islands. Here is the USGS website for the M 6.6 earthquake. This earthquake is close to the depth of the megathrust fault, but maybe not close enough. So, this may be on the subduction zone, but may also be on an upper plate fault (I interpret this due to the compressive earthquake fault mechanism). The earthquake has a hypocentral depth of 20 km and the slab model (see Hayes et al., 2013 below and in the poster) is at 40 km at this location. There is uncertainty in both the slab model and the hypocentral depth.

The Andreanof Islands is one of the most active parts of the Aleutian Arc. There have been many historic earthquakes here, some of which have been tsunamigenic (in fact, the email that notified me of this earthquake was from the ITIC Tsunami Bulletin Board).

Possibly the most significant earthquake was the 1957 Andreanof Islands M 8.6 Great (M ≥ 8.0) earthquake, though the 1986 M 8.0 Great earthquake is also quite significant. As was the 1996 M 7.9 and 2003 M 7.8 earthquakes. Lest we forget smaller earthquakes, like the 2007 M 7.2. So many earthquakes, so little time.

I include some earthquakes along this plate boundary system that are also interesting as they reveal how the plate boundary changes along strike, and how the margins of the plate boundary (e.g. the western and eastern termini) behave.

The M 6.6 earthquake is the result of north-northwest compression from the subduction of the Pacific plate underneath the North America plate to the north.

The majority of the Aleutian Islands are volcanic arc islands formed as a result of the subduction of the Pacific plate beneath the North America plate. As the oceanic crust subducts, the water in the rock tends is released into the overlying mantle, leading to magma formation. This magma is less dense and rises to form volcanoes that comprise this magmatic arc.

This and other earthquakes have occurred in the region of the subduction zone west of where the Adak fracture zone is aligned. Further to the east is the Amlia fracture zone. The Amlia fracture zone is a left lateral strike slip oriented fracture zone, which displaces crust of unequal age, beneath the megathrust. The difference in age results in a variety of factors that may contribute to differences in fault stress across the fracture zone (buoyancy, thermal properties, etc). For example, older crust is colder and denser, so it sinks lower into the mantle and exerts a different tectonic force upon the overriding plate.

To the west, there is another subduction zone along the Kuril and Kamchatka volcanic arcs. These subduction zones form deep sea trenches (the deepest parts of the ocean are in subduction zone trenches). Between these 2 subduction zones is another linear trough, but this does not denote the location of a subduction zone. The plate boundary between the Kamchatka and Aleutian trenches is the Bering Kresla shear zone (BKSZ). Below I present some earthquake reports that help explain the western terminus of the Aleutian subduction zone.

This earthquake sequence is unrelated to the earthquakes in northern Alaska earlier this week. Here is my report for that sequence.

There was also a sequence (that is still experiencing aftershocks) in the Gulf of Alaska. Here is my main report (there were updates) for this Gulf of Alaska earthquake.

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 1918-2018 with magnitudes M ≥ 3.0 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.

Mechanisms for historic earthquakes that come from publications other than the USGS fault plane solutions include the 1957 M 8.7 (Brown et al., 2013), the 1965 M 8.7 (Stauyder, 1968), and the 1965 M 7.6 earthquakes (Abe, 1972).

  • 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 contours plotted (Hayes et al., 2012), 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. 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.

    I include some inset figures.

  • In the upper center is a map from IRIS that shows seismicity plotted relative to depth using color. One may observe that the earthquakes get deeper to the north, relative to the subduction zone fault (labeled Aleutain Trench in the posters below). I place a yellow star in the general location of this earthquake sequence (same for other figures here).
  • In the center right is a companion figure from IRIS that shows a low angle oblique view of this Pacific – North America plate boundary. Note how the downgoing Pacific plate subducts beneath the North America plate as a megathrust fault.
  • In the lower left corner is a figure from Torsvik et al. (2017) which shows the age progression for the seamounts along the Emperor and Hawai’i seamount chains. This age progression is a key evidence for plate tectonic theory and a foundation for our knowledge of plate motion rates globally.
  • In the lower right corner is a figure from Sykes et al. (1980) that includes a map and a space-time diagram (shows spatial extent and timing for historic earthquakes along various fault systems.
  • In the upper right corner is a figure that shows the historic earthquake ruptures along the Aleutian Megathrust (Peter Haeussler, USGS).
  • Here is the map with a month’s seismicity plotted.

  • Here is the map with a centuries seismicity plotted for earthquakes M ≥ 6.6.

Other Report Pages

Some Background about the North America – Pacific plate boundary

  • Here is a map that shows historic earthquake slip regions as pink polygons (Peter Haeussler, USGS). Dr. Haeussler also plotted the magnetic anomalies (grey regions), the arc volcanoes (black diamonds), and the plate motion vectors (mm/yr, NAP vs PP).

  • Speaking of the 1964 earthquake, here is a map that shows the regions of coseismic uplift and subsidence observed following that earthquake. The 27 March, 1964 M 9.2 earthquake is the second largest earthquake ever recorded on modern seismometers. This figure can be compared to the cross section below.

  • Here is the Plafker (1972)cross-section graphic on its own.

  • Here is a figure recently published in the 5th International Conference of IGCP 588 by the Division of Geological and Geophysical Surveys, Dept. of Natural Resources, State of Alaska (State of Alaska, 2015). This is derived from a figure published originally by Plafker (1969). There is a cross section included that shows how the slip was distributed along upper plate faults (e.g. the Patton Bay and Middleton Island faults).

  • This figure shows a summary of the measured horizontal and vertical displacements from the Good Friday Earthquake. I include a figure caption from here below as a blockquote.

  • Profile and section of coseismic deformation associated with the 1964 Alaska earthquake across the Aleutian arc (oriented NW-SE through Middleton and Montague Islands). Profile of horizontal and vertical components of coseismic slip (above) and inferred slip partitioning between the megathrust and intraplate faults (below). From Plafker (1965, 1967; 1972)

  • Here is a graphic showing the sediment-stratigraphic evidence of earthquakes in Cascadia, but the analogy works for Alaska also. Atwater et al., 2005. There are 3 panels on the left, showing times of (1) prior to earthquake, (2) several years following the earthquake, and (3) centuries after the earthquake. Before the earthquake, the ground is sufficiently above sea level that trees can grow without fear of being inundated with salt water. During the earthquake, the ground subsides (lowers) so that the area is now inundated during high tides. The salt water kills the trees and other plants. Tidal sediment (like mud) starts to be deposited above the pre-earthquake ground surface. This sediment has organisms within it that reflect the tidal environment. Eventually, the sediment builds up and the crust deforms interseismically until the ground surface is again above sea level. Now plants that can survive in this environment start growing again. There are stumps and tree snags that were rooted in the pre-earthquake soil that can be used to estimate the age of the earthquake using radiocarbon age determinations. The tree snags form “ghost forests.

  • This is a photo that I took along the Seward HWY 1, that runs east of Anchorage along the Turnagain Arm. I attended the 2014 Seismological Society of America Meeting that was located in Anchorage to commemorate the anniversary of the Good Friday Earthquake. This is a ghost forest of trees that perished as a result of coseismic subsidence during the earthquake. Copyright Jason R. Patton (2014). (Please contact me for a higher resolution version of this image: quakejay at gmail.com)

  • Below is an educational video from the USGS that presents material about subduction zones and the 1964 earthquake and tsunami in particular.
    Youtube Source IRIS
  • WMV file for downloading.
    mp4 file for downloading.

      Credits:

    • Animation & graphics by Jenda Johnson, geologist
    • Directed by Robert F. Butler, University of Portland
    • U.S. Geological Survey consultants: Robert C. Witter, Alaska Science Center Peter J. Haeussler, Alaska Science Center
    • Narrated by Roger Groom, Mount Tabor Middle School
  • Here is a map for the earthquakes of magnitude greater than or equal to M 7.0 between 1900 and 2016. This is the USGS query that I used to make this map. One may locate the USGS web pages for all the earthquakes on this map by following that link.

Some Relevant Discussion and Figures

  • In june 2017, there was an M 6.8 earthquake that happened in a region where the Pacific-North America plate boundary transitions from a subduction zone to a shear zone. To the east of this region, the Pacific plate subducts beneath the North America plate to form the Alaska-Aleutian subduction zone. As a result of this subduction, a deep oceanic trench is formed. To the west of this earthquake, the plate boundary is in the form of a shear zone composed of several strike-slip faults. The main fault that is positioned in the trench is the Bering-Kresla shear zone (BKSZ), a right-lateral strike-slip fault. In the oceanic basin to the north of the BKSZ there are a series of parallel fracture zones, also right-lateral strike-slip faults. Below are my thoughts, some from my Earthquake Report here.
  • My initial thought is that the entire Aleutian trench was a subduction zone prior to about 47 million years ago (Wilson, 1963; Torsvik et al., 2017). Prior to 47 Ma, the relative plate motion in the region of the BKSZ would have been more orthogonal (possibly leading to subduction there). After 47 Ma, the relative plate motion in the region of the BKSZ has been parallel to the plate boundary, owing to the strike-slip motion here. However, Konstantinovskaia (2001) used paleomagnetic data for a plate motion reconstruction through the Cenozoic and they have concluded that there is a much more complicated tectonic history here (with strike-slip faults in the region prior to 47 Ma and other faults extending much farther east into the plate boundary). When considering this, I was reminded that the relative plate motion in the central Aleutian subduction zone is oblique. This results in strain partitioning where the oblique motion is partitioned into fault-normal fault movement (subduction) and fault-parallel fault movement (strike-slip, along forearc sliver faults). The magmatic arc in the central Aleutian subduction zone has a forearc sliver fault, but also appears to have blocks that rotate in response to this shear (Krutikov, 2008).
  • There have been several other M ~6 earthquakes to the west that are good examples of this strike-slip faulting in this area. On 2003.12.05 there was a M 6.7 earthquake along the Bering fracture zone (the first major strike-slip fault northeast of the BKSZ). On 2016.09.05 there was a M 6.3 earthquake also on the Bering fracture zone. Here is my earthquake report for the 2016 M 6.3 earthquake. The next major strike-slip fault, moving away from the BKSZ, is the right-lateral Alpha fracture zone. The M 6.8 earthquake may be related to this northwest striking fracture zone. However, aftershocks instead suggest that this M 6.8 earthquake is on a fault oriented in the northeast direction. There is no northeast striking strike-slip fault mapped in this area and the Shirshov Ridge is mapped as a thrust fault (albeit inactive). There is a left-lateral strike-slip fault that splays off the northern boundary of Bowers Ridge. If this fault strikes a little more counter-slockwise than is currently mapped at, the orientation would match the fault plane solution for this M 6.8 earthquake (and also satisfies the left-lateral motion for this orientation). The bathymetry used in Google Earth does not reveal the orientation of this fault, but the aftershocks sure align nicely with this hypothesis.
  • I include some inset figures in the poster
    • In the upper right corner is a figure that shows the historic earthquake ruptures along the Aleutian Megathrust (Peter Haeussler, USGS). I place a yellow star in the general location of this earthquake sequence (same for other figures here).
    • In the upper left corner is a figure from Gaedicke et al. (2000) which shows some of the major tectonic faults in this region.
    • In the lower right corner is a figure from Konstantnovskaia et al. (2001) that shows a very detailed view of all the faults in this complicated region.


  • Here is the interpretive poster from the 2016.09.05 M 6.3 #EarthquakeReport.

  • Here are several figures from Gaedicke et al. (2000) showing their tectonic reconstructions. I include their figure captions below in blockquote. The first map shows the general tectonic setting as in the poster above.

  • Map of the Aleutian–Bering region and location of the study area (rectangle). Lines with barbs indicate subduction zones: (1) Kamchatka Trench and (2) Aleutian Trench; lines with sense of displacement mark fracture zones (FZs): (3) Steller, (4) Pikezh and (5) Bering FZs. Single arrows show relative direction of convergence of the Pacific (P) and North American (NA) plates. Bathymetric contours are in meters.

  • This figure shows the complicated intersection of the BKSZ and the Kuril-Kamchatka Trench (a subduction zone).

  • The main tectonic features of the Kamchatka–Aleutian junction area modified from Seliverstov (1983), Seliverstov et al. (1988) and Baranov et al. (1991). The eastern side of the Central Kamchatka depression is bounded by normal faults. Contour interval is 1000 m. Lines A and B indicate the locations of profiles shown in Fig. 3; the rectangle marks the location of the area shown in Fig. 4.

  • This figure shows a medium scale view of the faults here, along with the major historic earthquakes. In this figure the BKSZ is labeled the Aleutian fracture zone (AFZ).

  • Rupture zones of the major earthquakes in the Kamchatka–Aleutian junction area [according to Vikulin (1997)]. Earthquakes with a magnitude of Mw>7 are shown.

  • Here is a great illustration that shows how forearc sliver faults form due to oblique convergence at a subduction zone (Lange et al., 2008). Strain is partitioned into fault normal faults (the subduction zone) and fault parallel faults (the forearc sliver faults, which are strike-slip). This figure is for southern Chile, but is applicable globally.

  • Proposed tectonic model for southern Chile. Partitioning of the oblique convergence vector between the Nazca plate and South American plate results in a dextral strike-slip fault zone in the magmatic arc and a northward moving forearc sliver. Modified after Lavenu and Cembrano (1999).

  • Here is a figure from Krutikov (2008) showing the block rotation and forearc sliver faults associated with the oblique subduction in the central Aleutian subduction zone. Note that there are blocks that are rotating to accommodate the oblique convergence. There are also margin parallel strike slip faults that bound these blocks. These faults are in the upper plate, but may impart localized strain to the lower plate, resulting in strike slip motion on the lower plate (my arm waving part of this). Note how the upper plate strike-slip faults have the same sense of motion as these deeper earthquakes.

  • Here are several figures from Konstantnovskaia et al. (2001) showing their tectonic reconstructions. I include their figure captions below in blockquote. The first figure is the one included in the poster above.

  • Geodynamic setting of Kamchatka in framework of the Northwest Pacific. Modified after Nokleberg et al. (1994) and Kharakhinov (1996)). Simplified cross-section line I-I’ is shown in Fig. 2. The inset shows location of Sredinny and Eastern Ranges. [More figure caption text in the publication].

  • Here are 4 panels that show the details of their reconstructions. Panels shown are for 65 Ma, 55 Ma, 37 Ma, and Present.


  • The Cenozoic evolution in the Northwest Pacific. Plate kinematics is shown in hotspot reference frame after (Engebretson et al., 1985). Keys distinguish zones of active volcanism (thick black lines), inactive volcanic belts (thick gray lines), deformed arc terranes (hatched pattern), subduction zones: active (black triangles), inactive *(empty triangles). In letters: sa = Sikhote-aline, bs = Bering shelf belts; SH = Shirshov Ridge; V = Vitus arch; KA = Kuril; RA = Ryukyu’ LA = Luzon; IBMA = Izu-Bonin-Mariana arcs; WPB = Western Philippine, BB = Bowers basins.

  • On 2017.05.08 there was an earthquake further to the east, with a magnitude M 6.2. Here is my interpretive poster for this earthquake, which includes fault plane solutions for several historic earthquakes in the region. These fault plane solutions reveal the complicated intersection of these two different types of faulting along this plate boundary. Here is my earthquake report for this earthquake sequence.

  • Here is the figure from Bassett and Watts (2015) for the Aleutians. They use gravity profile data to characterize subduction zones globally.

  • Aleutian subduction zone. Symbols as in Figure 3. (a) Residual free-air gravity anomaly and seismicity. The outer-arc high, trench-parallel fore-arc ridge and block-bounding faults are dashed in blue, black, and red, respectively. Annotations are AP = Amchitka Pass; BHR = Black-Hills Ridge; SS = Sunday Sumit Basin; PD = Pratt Depression. (b) Published asperities and slip-distributions/aftershock areas for large magnitude earthquakes. (c) Cross sections showing residual bathymetry (green), residual free-air gravity anomaly (black), and the geometry of the seismogenic zone [Hayes et al., 2012].

  • Here is the schematic figure from Bassett and Watts (2015).

  • Schematic diagram summarizing the key spatial associations interpreted between the morphology of the fore-arc and variations in the seismogenic behavior of subduction megathrusts.

  • Here is a beautiful illustration for the Aleutian Trench from Alpha (1973) as posted on the David Rumsey Collection online.

  • Here is the figure from Sykes et al. (1980) that shows the space time relations for historic earthquakes in relation to the map.

  • Above: Rupture zones of earthquakes of magnitude M > 7.4 from 1925-1971 as delineated by their aftershocks along plate boundary in Aleutians, southern Alaska and offshore British Columbia [after Sykes, 1971]. Contours in fathoms. Various symbols denote individual aftershock sequences as follows: crosses, 1949, 1957 and 1964; squares, 1938, 1958 and 1965; open triangles, 1946; solid triangles, 1948; solid circles, 1929, 1972. Larger symbols denote more precise locations. C = Chirikof Island. Below: Space-time diagram showing lengths of rupture zones, magnitudes [Richter, 1958; Kanamori, 1977 b; Kondorskay and Shebalin, 1977; Kanamori and Abe, 1979; Perez and Jacob, 1980] and locations of mainshocks for known events of M > 7.4 from 1784 to 1980. Dashes denote uncertainties in size of rupture zones. Magnitudes pertain to surface wave scale, M unless otherwise indicated. M is ultra-long period magnitude of Kanamori 1977 b; Mt is tsunami magnitude of Abe[ 1979]. Large shocks 1929 and 1965 that involve normal faulting in trench and were not located along plate interface are omitted. Absence of shocks before 1898 along several portions of plate boundary reflects lack of an historic record of earthquakes for those areas.

  • This is a map from Sykes et al. (1980) that shows the regions of slip inferred for these historic earthquakes.

  • Aftershock areas of earthquakes of magnitude M > 7.4 in the Aleutians, southern Alaska and offshore British Columbia from 1938 to 1979, after Sykess [1971] and McCann et al. [1979]. Heavy arrows denote motion of Pacific plate with respect to North American plate as calculated by Chase [1978]. Two thousand fathom contour is shown for Aleutian trench. Ms and Mw denote magnitude scales described by Kanamori [1977b].

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).

  • 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.

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Posted in alaska, College Redwoods, earthquake, education, geology, pacific, plate tectonics, subduction, tsunami

Earthquake Report: northern Alaska

Well, I awakened shortly after this M 6.4 earthquake hit the northern part of Alaska, along the north Slope, north of the Brooks Range.

My inbox has had a lower frequency of USGS ENS notifications since Kilauea has settled down somewhat. However, today, the aftershocks just keep rolling in. Those who are on the north slope are getting rattled for sure. I have had to reproduce my seismicity maps several times as the epicenters keep getting updated (thanks USGS). The two largest earthquakes are now actually aligned with the west northwest strike of the earthquake.

There are no active faults mapped in the region of today’s earthquakes. There is a series of thrust faults that form the mountains in this area (e.g. the Sadlerochit Mountains). To the north is a Quaternary active fold (the Marsh Creek anticline), however, this structure is too far away to be related to today’s activity.

The interesting thing is that today’s series of earthquakes are strike-slip earthquakes. It is possible that one of these thrust faults has been reactivated as a strike-slip fault (but they are probably dipping too shallow to do this). So, i suspect that these earthquakes are either on an un-mapped active fault or are distributed throughout the region on a variety of different faults (seems more likely, but I would defer to those who are studying the tectonics on the North Slope to be more informed about this). These earthquakes remind me of the 2002 dextral (right-lateral) strike-slip Denali fault earthquake. More on the Denali Earthquake can be found here too.

I include a second poster below that has more details about the regional geology. On this map I include faults and folds from the Alaska Quaternary Active Faults and Folds database (Keohler et al., 2013).

Based upon the seismicity, I interpret these earthquakes (at least the ones with mechanisms) as east striking right-lateral strike-slip earthquakes. The historic earthquakes are not as easy to interpret, so I include both nodal plane solutions as being possible. However, if they are related in some way to today’s seismicity, they are probably also right-lateral strike-slip earthquakes.

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 1918-2018 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.

  • 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 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 map from IRIS that shows historic seismicity in the Alaska region. Color represents depth. One may visualize the subduction zone as shallower earthquakes are green in color and the deeper earthquakes are red in color.
  • In the lower right corner is a USGS map showing the major historic earthquakes in Alaska. Most of these are subduction zone earthquakes, however, the 2002 Denali Earthquake also shows up. This was a right-lateral strike-slip earthquake on the Denali fault. I place a blue star in the general location of today’s first M 6.4 earthquake.
  • In the upper left corner is a large scale map showing the tectonics on the eastern North Slope (O’Sullivan et al., 2012). This map shows the anticlines and thrust faults. Anticlines are folds in the crust that are formed by compression, with the fold being pushed upwards (viewed from the side, it would look like a frown). The thrust fautls are symbolized with triangles pointed in the direction down dip (into the earth). There is a thrust fault on the north flank of the southern of the two anticlines in the Sadlerochit Mountains.
  • In the lower left corner is a cross section showing how these thrust faults and anticlines are possibly configured (O’Sullivan et al., 2012).
  • Here is the map with a month’s seismicity plotted.

  • Here is the map with a centuries’ seismicity plotted.

  • Here is the larger scale map showing more detail. This includes faults from the Alaska QFF (Koehler et al., 2013). I include a shaded relief map as a base map. I also include the state geological map (Wheeler et al., 1997), colored relative to the age of the geologic unit.

  • UPDATE This is the same map with ESRI imagery as a basemap.

Other Report Pages

Some Relevant Discussion and Figures

  • Here is an informational video from IRIS explaining the tectonics in Alaska. There is a paucity of information about the geology of the north slope in this video, but it is still very educational.

  • Here is the USGS mpa showing historic earthquakes in Alaska.

  • Here is the IRIS map showing seismicity relative to depth (color).

  • This is the low angle oblique view of the Alaska-Aleutian subduction zone. Note how the downgoing Pacific plate subducts beneath the North America plate.

  • Below are 3 figures from O’Sullivan et al. (2012) that present their interpretations for the tectonic structures along the eastern portion of the North Slope in Alaska.
  • Here is their intro overview map. The second map below is outlined here.

  • Generalized geologic map of northeastern Alaska, showing the location of the Arctic National Wildlife Refuge (ANWR), the northeastern Brooks Range, and other features specifically mentioned in the text.

  • This is a larger scale map showing the details for the structures in the area. The cross section locations are labeled here.

  • Tectonic map of the northeastern Brooks Range, showing the location of the Sadlerochit and Shublik mountain ranges, Ignek Valley, the Beli Unit #1 well, seismic line 84-6, and other features mentioned in the text. Map modified from Wallace and Hanks (1990).

  • This is a cross section showing their interpretation of how these thrust faults relate to each other. Note the lack of a strike slip fault in this area.

  • Presentation of known structures recognized within the Sadlerochit Mountains region. (A) Balanced cross section through the northern part of northeastern Brooks Range (modified from Wallace, 1993). Each basement-cored anticlinorium is interpreted to mark a horse in a duplex formed above a detachment at depth in basement (dark shading). The roof thrust in Kayak Shale terminates to north in the Sadlerochit Mountains owing to depositional discontinuity. All structures shown are interpreted to be Cenozoic in age. (B) Reproduced interpretation of seismic line 84-6 by Potter et al. (1999, plate BD2), indicating that basement rocks were involved in deformation beneath the coastal plain to the north of the Sadlerochit Mountains (at same scale as A).

  • Here is a map from Cox et al. (2015) that shows some detailed geologic mapping in the region.

  • Simplified geologic map of the Shublik and Sadlerochit Mountains, northeastern Brooks Range, Alaska. The Kikiktat volcanics are shown in green and outcrop in the hanging wall of large N-directed Cretaceous–Tertiary Brookian thrust sheets. Geologic is mapping by Strauss and Macdonald, with modifications from Robinson et al. (1989) and Bader and Bird (1986).

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).

  • 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.

    Arctic


    General Overview

    Earthquake Reports

  • 2017.01.08 M 5.8 Arctic

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Posted in alaska, College Redwoods, earthquake, education, geology, plate tectonics, strike-slip

Earthquake Report: Lombok, Indonesia: Update #1

Yesterday morning, as I was recovering from working on stage crew for the 34th Reggae on the River (fundraiser for the non profit, the Mateel Community Center), I noticed on social media that there was an M 6.9 earthquake in Lombok, Indonesia. This is sad because of the likelihood for casualties and economic damage in this region.

However, it is interesting because the earthquake sequence from last week (with a largest earthquake with a magnitude of M 6.4) were all foreshocks to this M 6.9. Now, technically, these were not really foreshocks. The M 6.4 has an hypocentral (3-D location) depth of ~6 km and the M 6.9 has an hypocentral depth of ~31 km. These earthquakes are not on the same fault, so I would interpret that the M 6.9 was triggered by the sequence from last week due to static coulomb changes in stress on the fault that ruptured. Given the large difference in depths, the uncertainty for these depths is probably not sufficient to state that they may be on the same fault (i.e. these depths are sufficiently different that this difference is larger than the uncertainty of their locations).

I present a more comprehensive analysis of the tectonics of this region in my earthquake report for the M 6.4 earthquake here. I especially address the historic seismicity of the region there. This M 6.9 may have been on the Flores thrust system, while the earthquakes from last week were on the imbricate thrust faults overlying the Flores Thrust. See the map from Silver et al. (1986) below. I include the same maps as in my original report, but after those, I include the figures from Koulani et al. (2016) (the paper is available on researchgate).

UPDATE 2018.08.08

Based on Eric Fielding and JD Dianala’s interpretation of the InSAR data, the M 6.4 and M 6.9 earthquakes could possibly have a similar hypocentral depth. See Social Media update below.

Find out more about InSAR (Interferometric Synthetic Aperture Radar) here.

In addition, as Dr. Anthony Lomax pointed out, the USGS depth uncertainty is large enough for these earthquakes that they may be along the same fault.

Dr. Fielding uses the InSAR data (see update below) to interpret the fault geometry.

UPDATE 2018.08.12

People have been asking me if we might expect another large or larger earthquake in this region. So, here is what I have told them:

  • It is difficult to say if there will be a larger or another large earthquake or not.
  • Based upon historic seismicity, the M 6.9 is probably the mainshock in this sequence. But the historic record is short (100 yrs +-), so may not be a perfect sample of what could happen.
  • The M 6.9 probably ruptured the Flores thrust fault, a back thrust to the subduction zone.
  • There is probably a small chance that the Flores thrust fault (east west fault dipping to the south) to the east and west of the M 6.9 has an increased amount of stress imparted upon it from the M 6.9 (small amount, so if the fault was almost ready to go, this change might make it go). but this is a small possibility (but still possible). (i.e. Bali).
  • There is also a small chance that the subduction zone (south of the islands, dipping to the north) also has an increased amount of stress from this M 6.9 earthquake. but this is probably less likely than the other example (due to the distance between the M .6.9 and the subduction zone fault.
  • Though there will probably be earthquakes up to M 5 or mid M 5 as aftershocks… and as time passes, the chance of a larger earthquake diminish to the background risk of such an earthquake. by the time it is Sept through Dec, we will probably have passed the increased risk due to the M 6.9 sequence.
  • But we must always remember, we cannot absolutely know what will happen. our observational history is only a few centuries and seismometers are only a century old (and modern ones, with a global network, maybe 50 years). so it is challenging to think that we know about how this region (or any region) behaves tectonically.

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 1918-2018 with magnitudes M ≥ 6.0 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 contours plotted (Hayes et al., 2012), 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. 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.

    I include some inset figures.

  • In the upper right corner is a low angle oblique view of the Sunda subduction zone beneath Java, Bali, Lombok, and Sumbawa (from Earth Observatory Singapore). I place a blue star in the general location of today’s earthquake’s epicenter (as for all figures here). The India-Australia plate is subducting northwards beneath the Sunda plate (part of the Eurasia plate).
  • In the upper left corner is a map from Koulali et al. (2016) that presents a plate tectonic map for this region. They present earthquake mechanisms for some historic earthquakes. These authors favor the interpretation that the Flores thrust fault system extends to eastern Java.
  • To the right of the Koulali et al. (2016) map is a cross section of seismicity presented by Hengresh and Whitney (2016). These authors argue for a north vergent Flores thrust in this region, though most of their work was on the subduction/collision zone.
  • In the lower right corner is another Koulali et al. (2016) map that shows the relative amount of motion across these plate boundary fault systems as modeled in their analysis. Based on their modeling, there is about 10-20 mm/yr of strain accumulating on the Flores thrust system north of Lombok, Indonesia.
  • In the lower left corner is a Koulali et al. (2016) that shows their estimate of this strain accumulation (via fault slip deficit) for the Flores thrust fault.
  • Here is the map with a month’s seismicity plotted.

  • Here is the map with a centuries’ seismicity plotted.

  • Here is the interpretive posted from last week, with historic seismicity and earthquake mechanisms.

Other Report Pages

Some Relevant Discussion and Figures

  • Below is a map showing historic seismicity (Jones et al., 2014). Cross sections B-B’ and C-C’ are shown. The seismicity for the cross sections below are sourced from within each respective rectangle.

  • Here are the seismcity cross sections.

  • Here is the map from McCaffrey and Nabelek (1987). They used seismic reflection profiles, gravity modeling along these profiles, seismicity, and earthquake source mechanism analyses to support their interpretations of the structures in this region.

  • Tectonic and geographic map of the eastern Sunda arc and vicinity. Active volcanoes are represented by triangles, and bathymetric contours are in kilometers. Thrust faults are shown with teeth on the upper plate. The dashed box encloses the study area.

  • 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.

  • This are the seismicity cross sections from Hangesh and Whitney (2016). These are shown to compare the subduction zone offshore of Java and the collision zone in the Timor region.

  • Comparison of hypocentral profiles across the (a) Java subduction zone and (b) Timor collision zone (paleo-Banda trench). Catalog compiled from multiple reporting agencies listed in Table 1. Events of Mw>4.0 are shown for period 1815 to 2015.

  • Here is a map of the same general area from Silver et al. (1986), used here to locate the following large scale map.

  • Location of SeaMARC II survey (Plate 1 and Figures 2) and geographic features discussed in text. Triangles on upper plates of thrust zones.

  • This is the large scale map showing the detailed thrust fault mapping (Silver et al., 1986).


  • Bathymetry, faults, and mud diapirs of the central Flores thrust zone, based on interpretation of SeaMARC II data and seismic reflection profiles. Shown also are locations (circled numbers) of all seismic profiles. Mud diapirs are solid black. Triangles on upper plates of thrust faults.

  • Here is the tectonic map from Hangesh 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 are some focal mechanisms from earthquakes in the region from Hangesh and Whitney (2016). Symbol color represents depth.

  • (a) Focal mechanism solutions for the study region. The focal mechanisms are classified based on depth intervals to illustrate the style of faulting within the different structural domains. Note (b) sinistral reverse motion along Timor trough, (c) subduction related pattern along Java trench, and dextral solutions along the western Australia extended margin (Figure 4a) north of 20°S. Centroid moment tensor (CMT) solutions [Dziewonski et al., 1981] are from the CMT project [Ekström et al., 2012; http://www.globalcmt.org/CMTcite.html] for events of Mw>5.0 for the period 1976 onward.

  • 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.

  • This map from Hangesh and Whitney (2016) shows the GPS velocities in this region. Note the termination of the Flores thrust and the north-northeast striking (oriented) cross fault between Lombok and Sumbawa.

  • GPS velocities of Sunda and Banda arc region. Large black and grey arrow shows motion of Australia relative to Eurasia [DeMets et al., 1994]. Thin black arrows show GPS velocities of Sunda and Banda arc regions relative to Australia [Nugroho et al., 2009]. Seismicity from ISC-GEM catalog [Storchak et al., 2013]. Note reduction of station velocities from west to east indicating progressive coupling of the Banda arc to the Australian plate compared to the area along the Sunda arc.

  • Below are the 4 figures from Koulani et al., 2016. First is the plate tectonic map. I include their figure captions in block quote.

  • Seismotectonic setting of the Sunda-Banda arc-continent collision, East Indonesia. Major faults (thick black lines) [Hamilton, 1979]. Topography and bathymetry are from Shuttle Radar Topography Mission (http://topex.ucsd.edu/www_html/srtm30_plus.html). Focal mechanisms are from the Global Centroid Moment Tensor. Blue mechanisms correspond to earthquakes with Mw>7 (brown transparent ellipses are the corresponding rupture areas for Flores 1992 and Alor 2004 earthquakes), while the green focal mechanism shows the highest magnitude recorded in Sumbawa. Red dots indicate the locations of major historical earthquakes [Musson, 2012].

  • This figure shows their estimates for plate motion relative velocities as derived from GPS data, constrained by the fault geometry in their block modeling.

  • GPS velocities determined in this study with respect to Sunda Block. Uncertainty ellipses represent 95% confidence level. The inset figure corresponds to the area of the dashed rectangle in the map. Light blue arrows show the velocities for East and West Makassar Blocks.

  • This figure shows their estimates of slip rate deficit along all the plate boundary faults in this region.

  • Relative slip vectors across block boundaries, derived from our best fit model. Arrows show motion of the hanging wall (moving block) relative to the footwall (fixed block) with 95% confidence ellipses. The tails of arrows is located within the “moving” block. Black thick lines show well-defined boundaries we use as active faults in our model and dashed lines show less well-defined boundaries (green : free-slipping boundaries and black: fixed locked faults) . Principal axes of the horizontal strain tensor estimated for the SUMB, EMAK, and EJAV are shown in pink. The thick pink arrow shows the relative motion of Australia with respect to Sunda (AUST/SUND). Abbreviations are Sumba Block (SUMB), West Makassar Block (WMAK), East Makassar Block (EMAK), East Java Block (EJAV), and Timor Block (TIMO). The background seismicity is from the International Seismological Centre catalog with magnitudes ≥5.5 and depths <40 km.

  • Here is their figure that shows the slip deficit along the plate boundary faults.

  • Fault slip rate components: (a) fault normal (extension positive) and (b) fault parallel (right-lateral positive).

UPDATE 2018.08.08

NASA InSAR

  • Here is the InSAR result from Eric Fielding at NASA, the files are available here.
  • These data are from a change in position between 2018.07.30 and 2018.08.05, so they compare the ground motion of only the M 6.9 earthquake (generally speaking).

  • From Dr. Fielding
  • Deformation of Lombok Island, Indonesia due to 5 August 2018 earthquake shows uplift of northwest corner due to fault slip at depth, measured with #InSAR of Copernicus Sentinel-1 radar images processed by Caltech-JPL ARIA project. Data at https://go.nasa.gov/2OlbxY6

    Black contours are 5 cm (2 inches). Copernicus Sentinel-1 data acquired on 30 July and 5 August 2018. White areas where measurement not possible, largely due to dense forests.

    Measurements with #InSAR are in direction towards satellite, so not purely vertical or horizontal. Mostly vertical in this case.

    My preliminary interpretation is that uplift is due to a north-dipping blind thrust fault that would project to the surface near the “zero” level of the interferogram, but a south-dipping thrust fault is also possible with down-dip end of rupture beneath the “zero” line

Rusi P InSAR

  • These two InSAR images allow us to compare ground deformation from these two earthquakes. Rusi P presents these results on twitter here. This tweet is also posted below in the Social Media section.
  • This is the analysis for the M 6.4 earthquake. This interferogram is made from SAR data collected on 7/18 and 7/30.

  • This is the analysis for the M 6.9 earthquake. This interferogram is made from SAR data collected on 7/30 and 8/05.

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.

    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.
  • Darman, H., 2012. Seismic Expression of Tectonic Features in the Lesser Sunda Islands, Indonesia in Berita Sedimentologi, Indonesian Journal of Sedimentary Geology, no. 25, po. 16-25.
  • 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.
  • 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
  • Jones, E.S., Hayes, G.P., Bernardino, Melissa, Dannemann, F.K., Furlong, K.P., Benz, H.M., and Villaseñor, Antonio, 2014. Seismicity of the Earth 1900–2012 Java and vicinity: U.S. Geological Survey Open-File Report 2010–1083-N, 1 sheet, scale 1:5,000,000, https://dx.doi.org/10.3133/ofr20101083N.
  • Koulali, A., S. Susilo, S. McClusky, I. Meilano, P. Cummins, P. Tregoning, G. Lister, J. Efendi, and M. A. Syafi’i, 2016. Crustal strain partitioning and the associated earthquake hazard in the eastern Sunda-Banda Arc in Geophys. Res. Lett., 43, 1943–1949, doi:10.1002/2016GL067941
  • McCaffrey, R., and Nabelek, J.L., 1984. The geometry of back arc thrusting along the Eastern Sunda Arc, Indonesia: Constraints from earthquake and gravity data in JGR, Atm., vol., 925, no. B1, p. 441-4620, DOI: 10.1029/JB089iB07p06171
  • 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.
  • Silver, E.A., Breen, N.A., and Prastyo, H., 1986. Multibeam Study of the Flores Backarc Thrust Belt, Indonesia, in JGR., vol. 91, no. B3, p. 3489-3500
  • 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

Posted in College Redwoods, collision, earthquake, education, geology, Indonesia, pacific, plate tectonics, subduction

Earthquake Report: San Pablo Bay, CA

Well well.

There was a small earthquake in the San Francisco Bay area today, with an epicenter in San Pablo Bay northwest of Richmond and San Pablo, CA. This earthquake is cool, at least in part, because of its location.

There are several fault systems that bisect the SF Bay area, which are all part of the San Andreas fault system, the right-lateral strike-slip plate boundary between the North America and Pacific plates. A large proportion of the relative motion along this plate boundary is localized on the San Andreas fault proper. The faults in the SF Bay area are thought to accommodate 85% of the plate boundary relative motion. The rest of this boundary relative motion can be observed along the east side of the Sierra Nevada, with smaller proportions extending through central Nevada and as far east as the Wasatch fault system in Utah.

The San Andreas and sister faults initiated this right-lateral relative motion at the plate boundary approximately 29 million years ago, in a location near where Los Angeles currently is. Over time, various subparallel strike-slip faults have formed. The main strands in the SF Bay area are the San Gregorio, San Andreas, Hayward – Rogers Creek, Maacama, Calaveras – Paicines, and Hunting Creek – Berryessa – Green Valley – Concord – Greenville fault systems. Geologists have been debating about how the Hayward and Rogers Creek faults interact.

The cool part about the location of this earthquake is that it happened in a place that holds great interest to those in the seismic hazard community. In the SF Bay area, the Hayward – Rogers Creek fault system is the fault that has the highest probability of having an earthquake with a magnitude of M 6.7. There is a 33% chance that there will be a M ≥ 6.7 earthquake between 2014 and 2043.

Some have proposed that there is a step over, where these two faults overlap and there is some proportion of extension in this transfer zone. This hypothesis includes the idea that ruptures here may cause subsidence between the fault strands, causing a local tsunami in the area.

Others hypothesize that these faults are more directly linked. USGS geologists like Janet Watt have been conducting seismic reflection and sediment coring studies to evaluate this likelihood. Here is a review of their work in San Pablo Bay.

    From the USGS: Why does it matter?

  • The longer the stretch of fault that breaks during an earthquake, the stronger the quake. When two faults are close to one another, the earthquake can jump from one to the other, making the rupture longer and the shaking stronger. When two faults are directly connected, it’s even easier for earthquake rupture to continue from one fault to the next.
  • A break along the combined length of the Hayward and Rodgers Creek faults could produce a major earthquake of magnitude 7.4. That earthquake would release more than five times the energy released by the 1989 magnitude 6.9 Loma Prieta earthquake, which caused about $6 billion in damage and killed 63 people.
  • To estimate the earthquake hazard posed by the Hayward and Rodgers Creek faults, scientists need to understand whether and how the faults connect. Previous work showed that the two faults approach each other closely beneath San Pablo Bay, but their exact relationship remained a mystery.

The USGS researchers found that the Hayward fault appears to connect directly to the Rodgers Creek fault.

    From the USGS: Unexpected trajectory

  • “Where the faults enter San Pablo Bay, the Hayward from the south and the Rodgers Creek from the north, their orientations suggest that they’ll run parallel to one another, separated by a space, or ‘stepover,’ of about 5 kilometers [3 miles],” said Janet Watt, USGS research geophysicist and lead author of the study. “So when we went out to map, we thought we were going to map details of the stepover—like minor fractures that might enable an earthquake rupture to cross from one fault to the other.”
  • As the data accumulated, however, the researchers saw that the Hayward fault strand they were mapping bends slightly to the right as it traverses San Pablo Bay, heading toward the Rodgers Creek fault.
  • They realized that they might be looking at a fault bend. The distinction is important because fault bends affect earthquake hazards differently than stepovers. These differences include how likely a break on one fault will continue on to the next, how much slip (movement of rock on either side of the fault) will occur, and where ground shaking will be strongest.

Another cool thing about today’s earthquake is that this year marks the anniversary of the last major earthquake on the Hayward fault in 1868. The USGS, CGS, and other organizations and agencies are rolling out Haywired, an earthquake scenario designed to help people to learn to become more prepared and more resilient to earthquake hazards in the SF Bay area. More can be found out about Haywired here. There is more layperson speak material on the Hayward fault here.

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 also include earthquake epicenters from 1918-2018 with magnitudes M ≥ 3.0 in one version of the poster.

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes. Most earthquakes are strike-slip and aligned with the orientation of the plate boundary fault system. Some are normal (extensional) earthquakes which are probably places where faults are bending or stepping over. The 1989 event is the compressional Loma Prieta Earthquake. There is a good example of the earthquake types in the Geysers area (2016.12.14 M 5.0). These earthquakes often share this type of moment tensor, showing a poorly fit double couple (note how the corners of the beach ball are rounded, not sharp like the 1989 Loma Prieta focal mechanism. Most tectonic earthquakes are slip along a fault, which would ideally produce a double couple mechanism. When other types of seismic events occur (like volcanic explosions, nuclear test explosions, steam explosions, etc.) they have mechanisms that are not double couples.

  • 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 Did You Feel It?” felt reports data as a transparent overlay. The colors are based upon reports that people submit when they feel an earthquake. This is shown for a comparison between the modeled data (the MMI contours) and the DYFI data.
  • 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 that shows the potential for shaking from earthquakes in central California, the San Francisco Bay area. This was published in 2003.
  • In the lower left corner is a figure from Watt et al. (2016). In the upper right is a map showing the blue San Pablo Bay. These authors collected subsurface data (seismic reflection, CHIRP) along lines shown in gray. In the main part of the map is a magnetic anomaly map, which shows how the magnetic field can be used to interrogate the structures and material types within the earth. This figure shows that the Hayward fault location can be inferred by the magnetic anomaly data. The gray bands labeled A, B, C, and D show the location of the seismic profiles shown on this poster.
  • To the right of the magnetic anomaly map is a series of 4 seismic profiles. The color changes (white to black) represent locations in the subsurface that have changes in material properties. These are most likely layers of sediment. Where layers are flat, the sediment may still be in place where it was deposited. Where the layers are folded, this may be evidence for tectonic deformation. Where these layers terminate along a line, this may show that there is an earthquake fault along that line. Note the observations you can make along the proposed location of the northern Hayward fault. I include this figure below so one may zoom in further.
  • Here is the interpretive poster with seismicity from the past month.

  • Here is the interpretive poster with seismicity from 1918-2018 for earthquakes M ≥ 3.0.

USGS Earthquake Pages

Some Relevant Discussion and Figures

  • Here is a fault map from Parsons et al., 2003. Note the configuration of the Hayward relative to the Rodgers Creek fault. These authors use seismic reflection and seismic tomography to interpret the subsurface in San Pablo Bay. The seismic profile below is located where the red line is shown on this map.

  • Fault map of San Pablo Bay based on the cross sections of Wright and Smith (1992) (cross section locations shown with black lines) and our new section in south San Pablo Bay (cross-section location shown with red line). We carry the Pinole fault offshore at least as far north as our cross section, and we connect the Rodgers Creek fault as far south as our section.

  • Here is the seismic profile (Parsons et al., 2003). Color represents material properties (seismic velocity). The black and white layers represent geologic materials with varying material properties (where there are layers of alternating material properties). These authors interpret faulting along this profile.

  • A structural cross section (upper 2 km) across south San Pablo Bay and the Hayward–Rodgers Creek step-over (see Fig. 1 for location). Seismic reflection data are overlain on a tomographic seismic velocity section. A significant lateral contrast is observed about 1 km east of the presently active trace of the Hayward fault and may represent an older fault trace. Another fault ~4 km east of the active Hayward fault separates east-dipping from west-dipping bedding within a basin; this structure is located on the Rodgers Creek fault trend projected from two structural sections in central and north San Pablo Bay developed by Wright and Smith (1992). Sparse relocated earthquake hypocenters (Waldhauser and Ellsworth, 2002) are shown in the lower panels with and without the Wright and Smith (1992) interpretation.

  • Here is their gravity map. Compare these results with the map from Watts et al. (2016; below and on the interpretive poster).

  • Gravity map of the San Pablo Bay region. An upward continued (500 m) signal was subtracted from the data, which filters out the broader wavelength features and allows us to focus on the shallowest part of the crust. A simplified fault map is shown by the black lines, and gravity gradients are identified by the white dots. A broad gravity low characterizes San Pablo Bay, and a subtly lower anomaly appears to coincide with the region between the Hayward and Rodgers Creek faults. This low is truncated near the north edge of San Pablo Bay by a strong gravity gradient that might be a normal fault boundary of a pull-apart basin.

  • This is the figure from Watts et al. (2016). I include their figure caption below.

  • Marine magnetic map of San Pablo Bay. Warm colors show magnetic highs, and cool colors show magnetic or dipole lows. Plus signs show locations of the offshore Hayward fault along chirp seismic profiles. Thick red lines show Late Pleistocene and younger traces of Hayward and Rodgers Creek faults (23). Black lines are older Quaternary faults (23). Thick gray lines show locations of seismic profiles in Fig. 3. Capital letters E, F, G, H, and J are discussed in the text. Black circles show exploratory well locations (5). Base map, 2010 (1m) National Oceanic and Atmospheric Administration Lidar. Inset: New Hayward fault strand (yellow) connecting directly to the Rodgers Creek fault. Gray lines show locations of chirp seismic track lines. Small black circles show relocated earthquakes (22).

  • Here are the 4 seismic profiles from Watt et al. (2016), their locations shown as gray bands in the above map. I include their figure caption below.

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    Chirp seismic profiles along the offshore Hayward fault. (A) to (D) are discussed in the text. Note vertical exaggeration of ~195:1. NW, northwest; SE, southeast.

  • Here is the isostatic gravity map from Watt et al. (2016). Gravity measurements have been modeled to account for the geometry of the earth (e.g. thickness of the crust or sections of the crust, sediment bodies, etc.), topography, etc. The result of this modeling can let us learn about structures like faults. I include their figure caption below.

  • Isostatic gravity map of San Pablo Bay. Map shows isostatic gravity anomalies in San Pablo Bay (45–47). Thick dashed white line shows the location of the horizontal gravity gradient maxima relative to the location of the Hayward fault (black plus signs). Orange star shows the location of a steep tomographic gradient along a seismic velocity profile (dotted black line) (6).


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 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.)

Tectonic History of Western North America and Southern California

  • Here is an animation from Tanya Atwater that shows how the Pacific-North America plate margin evolved over the past 40 million years (Ma).

  • 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 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 (Aagaard et al., 2016).

  • Here is a map from the CGS that shows some of the detailed fault mapping done in this region. One can view this map on the CGS website here.

HayWired

  • There is much more about the Haywired scenario here.
  • However I include here a couple graphics to help us key into this knowledge of the past so we can prepare for the future.
  • Here is an estimate (from ground motion modeling) of the amount of shaking that might happen when the Hayward fault ruptures next (USGS, 2018). Remember that the Hayward fault is the fault in the SF Bay area that has the highest chance of going off in the next few decades. Red = severe shaking, green = moderate shaking.

  • This map of the San Francisco Bay region, California, shows simulated ground shaking caused by the hypothetical magnitude-7.0 mainshock of the HayWired earthquake scenario on the Hayward Fault. Red shows the most extreme ground shaking and where damage is the worst. The mainshock begins beneath the City of Oakland (star) and causes the Hayward Fault to rupture along about 52 miles of its length (thick black line). White lines are other major faults in the region.

  • Here is a map that shows what aftershocks and triggered earthquakes may happen as part of the result of the Hayward fault rupturing (USGS, 2018).

  • This map of California’s San Francisco Bay region shows the hypothetical mainshock and aftershock sequence of the HayWired earthquake scenario on the Hayward Fault. In the scenario, aftershocks are modeled for 2 years after the mainshock—an innovation unique to HayWired. Early in the sequence, most aftershocks are concentrated near the Hayward Fault.

Informational Video: HayWired

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.

    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
  • Detweiler, S.T., and Wein, A.M., eds., 2017, The HayWired earthquake scenario—Earthquake hazards (ver. 1.1, March 2018): U.S. Geological Survey Scientific Investigations Report 2017–5013–A–H, 126 p., https://doi.org/10.3133/sir20175013v1.
  • Detweiler, S.T., and Wein, A.M., eds., 2018, The HayWired earthquake scenario—Engineering implications: U.S. Geological Survey Scientific Investigations Report 2017–5013–I–Q, 429 p., https://doi.org/10.3133/sir20175013v2.
  • 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.
  • Hudnut, K.W., Wein, A.M., Cox, D.A., Porter, K.A., Johnson, L.A., Perry, S.C., Bruce, J.L., and LaPointe, D., 2018, The HayWired earthquake scenario—We can outsmart disaster: U.S. Geological Survey Fact Sheet 2018–3016, 6 p., https://doi.org/10.3133/fs20183016.
  • 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/
  • Parsons, T., Sliter, R., Geist, E.L., Jachens, R.C., Jaffe, B.E., Foxgrover, A., Hart, P.E., and McCarthy, J., 2003. Structure and Mechanics of the Hayward–Rodgers Creek Fault Step-Over, San Francisco Bay, California in BSSA, vol. 93, no. 5, pp. 2187–2200
  • 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, 2018. The HayWired Earthquake Scenario – We Can Outsmart Disaster, USGS Fact Sheet 2018-3016, April, 2018.
  • 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].
  • Watt, J., Ponce, D., Parsons, T., and Hart, P., 2016. Missing link between the Hayward and Rodgers Creek faults in Science Advances, v. 2, no. 10, e1601441 DOI: 10.1126/sciadv.1601441

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Earthquake Report: Blanco fracture zone

Well, so exciting to have more earthquakes to write about! This summer has been a low seismic summer. The entire year actually.

There was an earthquake within the Gorda plate a few days ago, but these M 5.3 and M 5.6 earthquakes are unlikely to be related, at least in a physical reality sort of way. Here is my Earthquake Report for the Gorda plate earthquake sequence.

This morning (my time) there was an earthquake along the Blanco fracture zone system (BFZ). Today’s earthquake(s) are too small and too far away to directly affect or impact the Cascadia subduction zone megathrust fault. However, I prepare this report because it is a great way to explore the complexities along the BFZ.

The BFZ is a transform plate boundary that connects the Juan de Fuca ridge with the Gorda rise spreading centers. This active fault zone consists of numerous right-lateral (dextral) faults. There is some debate as to how far east the BFZ extends beyond the Gorda rise (some pose it extends far past the trench and ambient noise tomographic data supports this interpretation; Porritt et al., 2011). I remember a colleague of mine who once adamantly stated that there is no evidence for the extension of the BFZ eastwards past the megathrust fault tip. However, this colleague made this statement a decade before the Porritt et al. (2011) data were to be published. My colleague is may still be correct as other experts agree with them.

The interesting thing about today’s M 5.3 earthquake is that it is extensional (normal faulting). This is not altogether unexpected, but interesting nonetheless. Most people might expect the BFZ to have dominantly strike-slip earthquakes. This is largely true, but there are “pull-apart basins” along the BFZ. As strike-slip faults may not be oriented perfectly to the strain field (the tectonic forces driving plate motion and deformation of the lithosphere or crust), other structures may form to accommodate this imperfection. One example of this is a pull apart basin. There are various other causes for pull apart basins too. For example, as faults may bend or change orientation (also in response to the strain field), pull apart basins (or compressional pop up structures) may form.

However, it is possible (probable, given the bathymetric data) that this M 5.3 is not associated with a pull apart basin, but simply the reactivation of a spreading ridge normal fault in response to the complicated tectonics along the BFZ.

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.
  • Note that along the Gorda rise, the magnetic anomaly is red, showing that the spreading ridge has a normal polarity, like that of today. Prior to about 780,000 years ago, the polarity was reversed. During the Bruhnes-Matuyama magnetic polarity reversal, the polarity flipped to the way it is today. Note how as one goes away from the Gorda rise (east or west), the magnetic anomaly changes color to blue. At the boundary between red and blue is the Bruhnes-Matuyama magnetic polarity reversal.
  • The structures in the Gorda, Juad de Fuca, and Pacific plates in this region are largely inherited from the extensional tectonic and volcanic processes at the Gorda rise and Juan de Fuca Ridge. However, the Gorda plate is being pulverized by the surrounding tectonic plates. There are several interpretations about how the plate is deforming and some debate about whether the Gorda plate is even behaving like a plate.
  • Note how some of the magnetic anomalies appear to be offset along lines that are sub-parallel to the BFZ. This is because they are.

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 one version, I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 6.0.

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 include some inset figures.

  • In the upper right 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).
  • In the lower right 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 left corner is a map showing the details for the faulting along the BFZ (Braunmiller and Nabelek (2008). Note that this zone is quite complicated and includes several nornal fault bounded pull-apart basins.
  • In the lower left 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. 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?

USGS Earthquake Pages

    These are from this current sequence

  • 2018-07-29 M 5.3
  • Here is the map with the seismicity from the past 30 days.

  • Here is the same map with the seismicity from 1918-2018.

Some Relevant Discussion and Figures

  • 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.

  • This is a diagram that shows how a pull apart basin might form (Wu et al., 2009).

  • General characteristics of a pull-apart basin in a dextral side-stepping fault system. The pull-apart basin is defined to develop in pure strike-slip when alpha = 0 degrees and in transtension when 0 degrees < alpha 45 degrees.

  • This figure shows the results of modeling in clay, showing a pull apart basin form (Wu et al., 2009).

  • Plan view evolution of transtensional pull-apart basin model illustrated with: (a) time-lapse overhead photography; and (b) fault interpretation and incremental basin subsidence calculated from differential laser scans. Initial and final baseplate geometry shown with dashed lines; (c) basin topography at end of experiment.

  • With these analog models in mind, consider the map below from Braunmiller and Nabelek (2008). This map shows bathymetry (depth in the ocean) in color (units in meters below sea level). They also plot earthquake mechanisms to show how there is extension at the boundary of these basins and strike-slip motion along the strike slip faults. The uncertainty in their locaions are represented by the crosses. I include their figure caption below

  • Close-up of the BTFZ. Plotted are fault plane solutions (gray scheme as in Figure 10) and well-relocated earthquake epicenters. SeaBeam data are from the RIDGE Multibeam Synthesis Project (http://imager.ldeo.columbia.edu) at the Lamont-Doherty Earth Observatory. Solid and dashed lines mark inferred [Embley and Wilson, 1992] locations of active
    and inactive faults, respectively.

  • Here is a detailed map showing a pull apart basin just to the southwest of today’s M 5.3 (Braunmiller and Nabelek, 2008). I include their figure caption below.

  • Close-up of the BTFZ-Juan de Fuca ridge-transform intersection. The deep basins are East Blanco Depression (EBD) and West Blanco Depression (WBD); the bathymetric high south of WBD is the Parks Plateau. White arrows are slip vector azimuths of strike-slip events (Figure 16) with tails at their epicenters. Possible active fault strands are shown schematically as solid and dashed lines and are marked (WBDN, WBDC, WBDS, and PPF); solid northerly trending lines illustrate right stepping of (some) transform motion at the EBD.

  • 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).

BFZ 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.

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:

  • 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
  • Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
  • 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.
  • 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. doi:10.7289/V5H70CVX
  • 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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Posted in cascadia, earthquake, education, Extension, geology, pacific, plate tectonics, strike-slip

Earthquake Report: Lombok, Indonesia

Earlier today there was a shallow M 6.4 earthquake with an epicenter on the island of Lombok, Indonesia. With a hypocentral depth of about 7.5 km, this size of an earthquake can be quite damaging. The USGS PAGER estimate of impact suggests that there is about a 10% chance that there are more than 10 fatalities. Hopefully there are none. There have been several aftershocks, two M > 5.

This earthquake is probably along a thrust fault associated with the Flores thrust fault, a north vergent (dipping into the earth in a southerly direction) back thrust fault to the Sunda subduction zone fault. The Flores thrust possibly extends from east of Timor on the east to the northern shore of Java (McCaffrey and Nabelek, 1987). Others suggest that the Flores thrust ends at a cross fault just east of Lombok (Hengresh and Whitney, 2016). However, the seismic profiles from Silver et al. (1986) are convincing that there are east-west compressional structures extending between the northern shore of Java to where the Flores thrust is mapped.

Detailed mapping of the seafloor to the east of Lombok, north of the island of Sumbawa, reveals that there are imbricate (overlapping) thrust faults (Silver et al., 1986). I think that it is reasonable to presume that there are similar structures on the northern flank of Lombok.

Lombok is also a volcano complex as part of the Sunda magmatic arc. There may be fault systems associated with the volcanic activity. I include tectonic faults that are included in the global scale fault data set from the Coordinating Committee for Geoscience Programme in East and Southeast Asia. The most active volcano on Lombok is the Rinjani volcano. Here is a great place to learn about this volcano (the Volcano Discovery website).

If the M 6.4 earthquake was on the Flores fault, it would need to dip at about 10°. The Flores thrust fault proposed by Hengesh and Whitney (2016) has a much steeper dip. So this sequence is probably in the upper plate somewhere.

There was a M 6.0 earthquake to the east of the M 6.4, but it was much deeper (almost 600 km), so is unlikely to be genetically related to the M 6.4 sequence.

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. 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.

Historic Seismicity

  • Below I discuss analogues to today’s M 6.4 earthquake.
  • To the west, between Lombok and Bali, there was a series of earthquakes all in 1979. They happened several months apart, but had a similar magnitude and orientation. The hypocentral depths were in the 25-40 km depth range, so some of these may have been on the Flores thrust system. These alone suggest that the Flores thrust extends at least this far west.
  • To the east, along the eastern part of Sumbawa, there was a series of earthquakes in the first decade of the 21st century, from 2002-2009. These also all share a similar magnitude range and orientation. These earthquakes all happened within a narrow range of depths (18-20 km; though the 2002 earthquake has a default depth on 10 km).
  • Based on earthquakes in the regions to the east and to the west, it is possible that this M 6.4 is the first of a series of mid M 6 earthquakes (either within a year like in Bali or over several years like Sumbawa).

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 1918-2018 with magnitudes M ≥ 6.0.

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 contours plotted (Hayes et al., 2012), 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.
  • I include some inset figures.

  • In the upper right corner is a low angle oblique view of the Sunda subduction zone beneath Java, Bali, Lombok, and Sumbawa (from Earth Observatory Singapore). I place a blue star in the general location of today’s earthquake’s epicenter (as for all figures here). The India-Australia plate is subducting northwards beneath the Sunda plate (part of the Eurasia plate).
  • In the upper left corner is a plate tectonic map showing the major fault systems, volcanic arc islands, and oceanic plateaus and basins of the region (Darman, 2012). The map shows the Flores thrust extending as far west as Lombok. Compare the complicated tectonics in the eastern portion of this region compared to the western portion of this region.
  • To the right of the Darman (2012) map is a cross section of seismicity presented by Hengresh and Whitney (2016). These authors argue for a north vergent Flores thrust in this region, though most of their work was on the subduction/collision zone.
  • In the lower right corner is another, earlier, tectonic map from Silver et al. (1986). These authors use seismic reflection and multibeam bathymetry data to map the Flores thrust as far as Java, west of Bali. The location for the map in the lower left corner of this interpretive poster is outlined here as a dashed line rectangle.
  • In the lower left corner is a map from Silver et al. (1986) that shows the detailed mapping of imbricate north (and some south) vergent thrust faults.
  • Here is the same map but with seismicity from the past month.


  • Here is the same map but with historic seismicity.


USGS Earthquake Pages

    These are from this current sequence

  • 2018-07-28 17:07:23 UTC M 6.0
  • 2018-07-28 22:47:37 UTC M 6.4
  • 2018-07-28 23:06:49 UTC M 5.4
  • 2018-07-29 01:50:32 UTC M 5.3

Other Report Pages

Some Relevant Discussion and Figures

  • Below is a map showing historic seismicity (Jones et al., 2014). Cross sections B-B’ and C-C’ are shown. The seismicity for the cross sections below are sourced from within each respective rectangle.

  • Here are the seismcity cross sections.

  • Here is the map from McCaffrey and Nabelek (1987). They used seismic reflection profiles, gravity modeling along these profiles, seismicity, and earthquake source mechanism analyses to support their interpretations of the structures in this region.

  • Tectonic and geographic map of the eastern Sunda arc and vicinity. Active volcanoes are represented by triangles, and bathymetric contours are in kilometers. Thrust faults are shown with teeth on the upper plate. The dashed box encloses the study area.

  • 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.

  • This are the seismicity cross sections from Hangesh and Whitney (2016). These are shown to compare the subduction zone offshore of Java and the collision zone in the Timor region.

  • Comparison of hypocentral profiles across the (a) Java subduction zone and (b) Timor collision zone (paleo-Banda trench). Catalog compiled from multiple reporting agencies listed in Table 1. Events of Mw>4.0 are shown for period 1815 to 2015.

  • Here is a map of the same general area from Silver et al. (1986), used here to locate the following large scale map.

  • Location of SeaMARC II survey (Plate 1 and Figures 2) and geographic features discussed in text. Triangles on upper plates of thrust zones.

  • This is the large scale map showing the detailed thrust fault mapping (Silver et al., 1986).

  • Bathymetry, faults, and mud diapirs of the central Flores thrust zone, based on interpretation of SeaMARC II data and seismic reflection profiles. Shown also are locations (circled numbers) of all seismic profiles. Mud diapirs are solid black. Triangles on upper plates of thrust faults.

  • Here is the tectonic map from Hangesh 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 are some focal mechanisms from earthquakes in the region from Hangesh and Whitney (2016). Symbol color represents depth.

  • (a) Focal mechanism solutions for the study region. The focal mechanisms are classified based on depth intervals to illustrate the style of faulting within the different structural domains. Note (b) sinistral reverse motion along Timor trough, (c) subduction related pattern along Java trench, and dextral solutions along the western Australia extended margin (Figure 4a) north of 20°S. Centroid moment tensor (CMT) solutions [Dziewonski et al., 1981] are from the CMT project [Ekström et al., 2012; http://www.globalcmt.org/CMTcite.html] for events of Mw>5.0 for the period 1976 onward.

  • 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.

  • This map from Hangesh and Whitney (2016) shows the GPS velocities in this region. Note the termination of the Flores thrust and the north-northeast striking (oriented) cross fault between Lombok and Sumbawa.

  • GPS velocities of Sunda and Banda arc region. Large black and grey arrow shows motion of Australia relative to Eurasia [DeMets et al., 1994]. Thin black arrows show GPS velocities of Sunda and Banda arc regions relative to Australia [Nugroho et al., 2009]. Seismicity from ISC-GEM catalog [Storchak et al., 2013]. Note reduction of station velocities from west to east indicating progressive coupling of the Banda arc to the Australian plate compared to the area along the Sunda arc.

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.

    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.
  • Darman, H., 2012. Seismic Expression of Tectonic Features in the Lesser Sunda Islands, Indonesia in Berita Sedimentologi, Indonesian Journal of Sedimentary Geology, no. 25, po. 16-25.
  • 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.
  • 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
  • Jones, E.S., Hayes, G.P., Bernardino, Melissa, Dannemann, F.K., Furlong, K.P., Benz, H.M., and Villaseñor, Antonio, 2014. Seismicity of the Earth 1900–2012 Java and vicinity: U.S. Geological Survey Open-File Report 2010–1083-N, 1 sheet, scale 1:5,000,000, https://dx.doi.org/10.3133/ofr20101083N.
  • McCaffrey, R., and Nabelek, J.L., 1984. The geometry of back arc thrusting along the Eastern Sunda Arc, Indonesia: Constraints from earthquake and gravity data in JGR, Atm., vol., 925, no. B1, p. 441-4620, DOI: 10.1029/JB089iB07p06171
  • 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.
  • Silver, E.A., Breen, N.A., and Prastyo, H., 1986. Multibeam Study of the Flores Backarc Thrust Belt, Indonesia, in JGR., vol. 91, no. B3, p. 3489-3500
  • 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

Posted in australia, collision, earthquake, education, geology, Indian Ocean, Indonesia, pacific, plate tectonics, subduction

#Earthquake Report: Gorda plate

Over the past night and morning, there was a sequence of earthquakes within the Gorda plate due west of Crescent City. Some people even felt these earthquakes, culminating (so far) with a M 5.6. There was a Gorda plate earthquake in March of this year, but it was in a different location.

These earthquakes did not occur along the Gorda Rise as some have reported, but within a region of oceanic crust over a million years old.

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.

Note that along the Gorda rise, the magnetic anomaly is red, showing that the spreading ridge has a normal polarity, like that of today. Prior to about 780,000 years ago, the polarity was reversed. During the Bruhnes-Matuyama magnetic polarity reversal, the polarity flipped to the way it is today. Note how as one goes away from the Gorda rise (east or west), the magnetic anomaly changes color to blue. At the boundary between red and blue is the Bruhnes-Matuyama magnetic polarity reversal. The earthquakes from today occurred within this blue region, so the oceanic crust is older than about 780,000 years old, probably older than a million years old.

The structures in the Gorda plate in this region are largely inherited from the extensional tectonic and volcanic processes at the Gorda rise. However, the Gorda plate is being pulverized by the surrounding tectonic plates. There are several interpretations about how the plate is deforming and some debate about whether the Gorda plate is even behaving like a plate. These normal fault (extensional) structures have been reactivating as left-lateral strike-slip faults as a result of this deformation. This region is called the Mendocino deformation zone (a.k.a. the Triangle of Doom).

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 1918-2018 with magnitudes M ≥ 5.0 in a second poster).

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 include some inset figures.

  • In the upper right 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 earthquakes.
  • In the upper left corner is a map from Chaytor et al. (2004) that shows some details of the faulting in the region. This figure shows the predominant tectonic fabric in the GP (northeast striking left-lateral faults). More about this figure can be found below.
  • In the lower right corner is a figure from Rollins and Stein (2010). In their paper they discuss how static coulomb stress changes from earthquakes may impart (or remove) stress from adjacent crust/faults. I place a blues star in the general location of today’s earthquakes.
  • In the lower left corner is a figure from Chaytor et al. (2004) that shows the different models for the internal deformation within the Gorda plate.


  • This version includes earthquakes M ≥ 5.0 from the USGS. Note how the region where today’s earthquakes happened is a region of higher levels of seismicity. Perhaps this is because this region is the locus of the deformation within the Mendocino deformation zone?


USGS Earthquake Pages

  • However, this region is typified by these normal (extensional earthquakes. Below are some of these.
  • 1985.07.23 M 5.3
  • 1990.01.05 M 5.4
  • 2013.12.01 M 5.5

Some Relevant Discussion and Figures

  • 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.

  • 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.

Cascadia subduction zone Earthquake Reports


General Overview

  • 1700.09.26 M 9.0 Cascadia’s 315th Anniversary 2015.01.26
  • 1700.09.26 M 9.0 Cascadia’s 316th Anniversary 2016.01.26 updated in 2017 and 2018
  • 1992.04.25 M 7.1 Cape Mendocino 25 year remembrance
  • 1992.04.25 M 7.1 Cape Mendocino 25 Year Remembrance Event Page
  • Earthquake Information about the CSZ 2015.10.08
  • Earthquake Reports

    Gorda plate

    Blanco fracture zone

    Mendocino fault

    Mendocino triple junction

    North America plate

    Explorer plate

    Uncertain

    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:

    • 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.
    • Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
    • 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.
    • 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. doi:10.7289/V5H70CVX
    • 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/].

    Posted in cascadia, earthquake, education, Extension, geology, gorda, pacific, plate tectonics

    Earthquake Report: Madagascar!

    Today there was an earthquake in a region that we don’t hear about that often. Madagascar is off the coast of southeastern Africa and the oceanic basin to the west is likely formed as part of the East Africa Rift system, but also due to the post Gondwana plate tectonics. Madagascar was once part of India, back when India was part of Gondwana.

    Today’s (and of the last few days) earthquakes are located along the Comoros Archipelago, volcanic islands formed from hotspot volcanism.

    There exist fracture zones in this region. Below, we see that these fracture zones have been interpreted as right-lateral strike-slip faults. However, the relative offsets of magnetic anomalies (and spreading ridges) show that these faults are instead left-lateral. So, that is my interpretation for this M 5.8 earthquake, a left-lateral strike-slip earthquake. I placed white dashed lines in the poster below to show where some of these fracture zones may be located, based upon the magnetic anomaly data (EMAG2).

    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 1918-2018 with magnitudes M ≥ 4.5.

    I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the M 5.8 earthquake.

    • 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 some inset figures.

    • In the upper center right is a figure from James Wood and Alex Guth, showing the rift systems in eastern Africa.
    • In the upper left corner is a figure showing the estimated (reconstructed) location of Madagascar within Gondwana (Reeves, 2014).
    • In the lower left corner is a map that shows the East Africa Rift system, along with the offshore branch (associated with the Davie fracture zone). The M 5.8 earthquake is just to the east of the larger scale map (Franke et al., 2015).
    • In the upper right corner is a figure that shows a reconstruction of the position of Madagascar (Phethean et al., 2017). The blue lines are pathway lines showing how Madagascar moved away from Africa. Spreading ridges are shown in red. The offsets of the spreading ridges show left lateral strike slip offsets of these ridges.
    • In the lower right corner is a map that shows the free-air gravity data that Phethean et al. (2017) used to make their reconstruction. Note that they interpret these faults as right lateral strike-slip. This interpretation is in contrast to the relative offsets of the oceanic spreading ridges. I placed a blue star in the general location of today’s M 5.8 earthquake.


    USGS Earthquake Pages

    Some Relevant Discussion and Figures

    • Here is the plate reconstruction figure from Phethean et al. (2017).

    • Plate tectonic reconstruction of Madagascar’s escape from Africa from the Early Jurassic to the cessation of spreading in the Cretaceous. Madagascar is shown without the remainder of East Gondwana (India, Antarctica, and Australia) attached. (a) Present-day sediment thickness in the Western Somali Basin taken from the CRUST1.0 model. (b–e) The key stages of Madagascar’s motion out of Africa. Modeled flow lines are shown as blue-arrowed lines where the center of symmetry is marked by orange circles. (f) Madagascar’s present-day position, which is reached at around 125 Ma. Flow lines closely match the fracture zone pattern of the basin (additional black lines), and the basin’s predicted final symmetry (orange circles) lies in good agreement with the interpreted extinct mid-ocean ridge system (red lines). Locations of magnetic anomalies used to temporally constrain plate motions shown with symbols as interpreted by Davis et al. [2016].

    • Here is the summary figure, showing their interpretation of the different fracture zones (Phethean et al., 2017).

    • (a) Commonly interpreted basin configuration, where the continent-ocean transition is assumed to follow the DFZ [e.g., Bunce and Molnar, 1977; Coffin and Rabinowitz, 1987; Gaina et al., 2013]. (b) Schematic of the basin configuration suggested in this study, with strike-slip tectonics dominating along the edge of the Rovuma Basin, while much of the Tanzania Coastal Basin should be considered as an obliquely rifted margin. The Davie Fracture Zone is a major ocean-ocean fracture zone, not the continent-ocean transform margin. DFZ, Davie Fracture Zone; DHOW, Dhow Fracture Zone; VLCC, Very Large Crude Carrier Fracture Zone; ARS, Auxiliary Rescue and Salvage Fracture Zone. (c) Free-air gravity overlain with interpretation as for Figure 9b

    • This is the map from Franke et al. (2015) showing the EAR system, including the offshore branch, the Davie Ridge. These authors work offshore and use seismic reflection and bathymetric data to show the extension in the offshore basins, as they respond to EAR extension.

    • General geological overview of the study area. Dark grey lines indicate the position of geophysical profiles acquired during R/V Sonne cruise SO-231 in 2014. Earthquake locations and magnitudes (1973–2014; mb>4.0) are shown as magenta circles according to the National Earthquake Information Center catalog and earthquake moment tensors from the Global Centroid Moment Tensor catalog [Ekström et al., 2012]. The Lurio Belt separates the northern from the southern high-grade metamorphic basement of northern Mozambique [Emmel et al., 2011]. The inlay shows the main faults of the western and eastern branches of the East African Rift System (from Chorowicz [2005] and Macgregor [2015]).

    • Emerick and Duncan (1983) demonstrated the age progression for the volcanic islands in this region. Below is a map showing the paths for the Cororos and Seychelles hotspots.

    • Hotspot paths predicted by African absolute motions [3], which are shown as solid lines connected by circles of 20 m.y. increments, are systematically offset from the observed paths for the Comores (A) and Reunion (B) hotspots, outlined by the 2000 m bathymetric contour. The difference between predicted and observed paths can be used to determine Somali-African relative motion between 0 and 10 m.y.B.P. For the period 10-60 m.y. the predicted paths parallel the observed paths, indicating no significant relative motion prior to about 10 m.y. ago. The reported ages for the Comores trend are from this paper and reference 7; for the Reunion trend, from references 24-26, 37, 38.

    • Here is a plot showing the ages for the rocks studied by Emerick and Duncan (1982).

    • A. Distance from present hotspot activity at Grande Comore, measured along the trend of the Comores Islands to Seychelles Islands lineament, is plotted against ages of initial volcanism at several localities (Table 1, and [7,9]). The solid circles represent best age estimates of initial volcanism, whereas the open circles represent minimum ages of volcanism at each site. A rate of migration of volcanism of 50 mm/yr best fits the new K-Ar ages for shield-building lavas at Grande Comore and Mayotte and the minimum age of volcanism in northern Madagascar. Igneous activity in the Seychelles at about 40 m.y. B.P. is consistent with this trend. Generalized topography from reference 4. B. Reported ;adiometric ages along the Reunion hotspot trend [24,25] yield a rate of migration of volcanism of 44 mm/yr. An early Oligocene age for DSDP site 238 on the southern end of the Chagos-Laccadive Ridge 1261 provides a minimum age for the Nazarene Bank region of the Mascarene Plateau, which was sundered from the Chagos-Laccadive Ridge by spreading across the Central Indian Ridge about 32 m.y. ago.

    • This is a fantastic plot that shows how hotspot volcanism has a finite time at the surface (for any given location) as the plate moves across the hotspot (Emerick and Duncan, 1982).

    • Duration of volcanism at oceanic islands is proportional to the inverse of plate velocity over mantle hotspots, which determines how long magmas are available for eruption. The dashed curve fits the maximum observed eruptive histories. Other data fall below this line due to incomplete sampling or unfinished volcanism.

    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:

    • Franke, D., W. Jokat, S. Ladage, H. Stollhofen, J. Klimke, R. Lutz, E. S. Mahanjane, A. Ehrhardt, and B. Schreckenberger (2015), The offshore East African Rift System: Structural framework at the toe of a juvenile rift, Tectonics, 34, 2086–2104, doi:10.1002/2015TC003922.
    • Phethean, Jordan J.J. and Kalnins, Lara M. and van Hunen, Jeroen and Biffi, Paolo G. and Davies, Richard J. and McCaffrey, Ken J.W., 2016. Madagascar’s escape from Africa : a high-resolution plate reconstruction for the Western Somali Basin and implications for supercontinent dispersal in Geochemistry, geophysics, geosystems., 17 (12). pp. 5036-5055.
    • Reeves, C. 2014. The position of Madagascar within Gondwana and its movements during Gondwana dispersal. J. Afr. Earth Sci., http://dx.doi.org/10.1016/j.jafrearsci.2013.07.011

    Posted in africa, earthquake, education, geology, Indian Ocean, plate tectonics, strike-slip

    Earthquake-Volcanic Eruption Report: Hawai’i

    My USGS Earthquake Notification Service email inbox has been going on overtime.

    There has been a swarm of earthquakes on the southeastern part of the big island, with USGS volcanologists hypothesizing about magma movement and suggesting that an eruption may be imminent. Here is a great place to find official USGS updates on the volcanism in Hawaii (including maps). I had been following this on social media.

    Here is a temblor blogpost that I wrote. Here is a Spanish version.

    Hawaii is an active volcanic island formed by hotspot volcanism. The Hawaii-Emperor Seamount Chain is a series of active and inactive volcanoes formed by this process and are in a line because the Pacific plate has been moving over the hotspot for many millions of years.

    As these volcanoes grow with time, the flanks of the volcanoes become covered in new volcanic rock. The flanks become unstable and collapse as landslides. There is evidence that some of these landslides trigger some of the largest tsunami ever found.

    The seismicity started in the central part of the “East Rift Zone” (ERZ), a region of extension probably caused by flank collapse. This extension lowers pressure in the magma chamber, leading to eruptions. Magma migrates around for various reasons, including changes in pressure in the magma chamber. These motions of magma and fluids can cause earthquakes.

    This part of Hawaii is the locus of the most recent volcanism, with the newest volcanic center formed to the southeast of the island.

    Southeast of the main Kilauea vent, the Pu‘u ‘Ö‘ö crater saw an elevation of lava into the crater, leading to overtopping of the crater (on 4/30/2018). Seismicity migrated eastward along the ERZ. This morning, there was a M 5.0 earthquake in the region of the Hilina fault zone (HFZ). I was getting ready to write something up, but I had other work that I needed to complete. Then, this evening, there was a M 6.9 earthquake between the ERZ and the HFZ.

    There have been earthquakes this large in this region in the past (e.g. the 1975.1.29 M 7.1 earthquake along the HFZ). This earthquake was also most likely related to magma injection (Ando, 1979). The 1975 M 7.1 earthquake generated a small tsunami (Ando, 1979). These earthquakes are generally compressional in nature (including the earthquakes from today).

    Today’s earthquake also generated a tsunami as recorded on tide gages throughout Hawaii. There is probably no chance that a tsunami will travel across the Pacific to have a significant impact elsewhere.

    So. This M 6.9 may be the largest earthquake. There may be a larger one in store (the USGS suggests that these fault systems could produce a M 8 earthquake). The eruptions may be done for now. There may be more.

    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 1918-2018 with magnitudes M ≥ 4.5 in a second poster (and down to M ≥ 3.5 in a third poster).

    I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the M 6.9 earthquake, in addition to some relevant historic earthquakes.

    I placed green circles in the locations of the (a) 4/30 lava lake filling event and (b) 5/3-4 fissure eruption.

    • 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 include some inset figures.

    • In the upper right corner is a geologic map with color representing the relative age of volcanic deposits (Sherrod et al., 2007). (red = youngest, orange next youngest) I placed a blue circle in the location of the vents that erupted on 5/3-4.
    • In the upper left corner is a map that shows the rift zones (active extensional volcanism) and the region is divided by the major sources for the volcanic rocks (e.g. Mauna Kea, Mauna Loa, and Kilauea; Tilling et al., 2010). I placed a blue circle in the location of the vents that erupted on 5/3-4.
    • In the lower left corner is a visualization showing the magma reservoir hypothesized to be the source of lava along the Southwest and East Rift zones, as well as for Kilauea (Tilling et al., 2010).
    • In the lower right corner is a map that shows the relative severity of volcanic hazard for the island of Hawaii (Tilling, et al., 2010).
    • To the left of the hazard map is a geological cross section showing the subsurface structures in the region (USGS).


    • This version includes earthquakes M ≥ 4.5


    • This version includes earthquakes M ≥ 3.5 (note the seismicity offshore to the south, this is where the youngest Hawaii volcano is).


    USGS Earthquake Pages

    • I put together a short video that shows seismicity from the past month. This reveals how the magma may have moved throughout the region between 4/27 and 5/4. Here is a link to download the ~8 MB mp4 video file.

    Some Relevant Discussion and Figures

    • Here is the map showing the rifts (Tilling et al., 2010)

    • Shaded relief map of the southeastern part of the Island of Hawai‘i, showing the principal features and localities of Mauna Loa, Kïlauea, and Lö‘ihi Volcanoes discussed in the text.

    • This is the figure that shows an hypothetical configuration of the magma reservoir beneath Kilauea (Tilling et al., 2010).

    • Cut-away view looking deep beneath Kïlauea Volcano, showing the shallow magma reservoir and the principal magma passageways. Areas in yellow are the most favorable zones for magma movement (arrows show direction) and storage. Though greatly generalized, this depiction of Kïlauea’s “plumbing system” is compatible with all known scientific information. (Simplified from technical illustration of Michael P. Ryan, USGS.)

    • Below are a series of plots from tide gages installed at several sites in the Hawaii Island Chain. These data are all posted online here and here.
    • Hilo, Hawaii

    • Kawaihae, Hawaii

    • Here is a plot showing the tsunami from the 1975 M 7.1 earthquake (Ando, 1979). On the left are modeled tsunami wave height based on two different fault models (each with fault dips of 20 degrees, but widths of 20km and 30km).

    • This is a timeline of historic volcanism on Hawaii (Tilling et al., 2010).

    • Graph summarizing the eruptions of Mauna Loa and Kïlauea Volcanoes during the past 200 years. The Pu‘u ‘Ö‘ö- Kupaianaha eruption has continued into the 21st century. Information is sketchy for eruptions before 1823, when the first missionaries arrived on the Island of Hawai‘i. The total duration of eruptive activity in a given year, shown by the length of the vertical bar, may be for a single eruption or a combination of several separate eruptions.

    • Here is the volcanic hazard severity map from Tilling et al. (2010).

    • Map of Island of Hawai‘i showing the volcanic hazards from lava flows. Severity of the hazard increases from zone 9 to zone 1. Shaded areas show land covered by flows erupted in the past two centuries from three of Hawai‘i’s five volcanoes (Hualälai, Mauna Loa, and Kïlauea).

    • Below is a series of maps that shows the recent volcanism in the region (Orr et al., 2012).

    • The first 3½ years of the Pu‘u ‘Öÿö eruption of Kïlauea Volcano (January 1983–June 1986) were dominated by episodic lava fountains that constructed the Pu‘u ‘Öÿö cone and fed ‘a‘ä flows (the less fluid of the two types of Hawaiian lava flows) (USGS photo by J.D. Griggs, June 1984). The map shows lava flows erupted from Kïlauea Volcano in the 19th and 20th centuries (gray). These flows originated from the summit caldera, the East Rift Zone, or the Southwest Rift Zone (not shown). Flows erupted during the first 3½ years of the Pu‘u ‘Öÿö eruption are shown in red. The Island of Hawai‘i (see inset map) is composed of five volcanoes—Kohala, Mauna Kea, Hualälai, Mauna Loa, and Kïlauea


      In 1986, the Pu‘u ‘Öÿö eruption shifted to the Kupaianaha vent. This photo shows lava flows erupted from Kupaianaha entering the community of Kalapana on the Island of Hawai‘i’s southeast coast in May 1990 (USGS photo by J.D. Griggs). Over the following months, Kalapana was almost completely destroyed, and lava filled beautiful nearby Kaimü Bay. The map shows lava flows erupted from Kupaianaha and nearby fissures during 1986–1992 in red. Older flows from the Pu‘u ‘Öÿö eruption are shown in orange.


      Lava flows erupt from new vents on the south flank of the Pu‘u ‘Öÿö cone (right side of photo) that opened after Pu‘u ‘Öÿö Crater filled to overflowing in early 2004 (USGS photo by Richard Hoblitt, January 2004). Collapse of the southwest side of the cone formed a scallop-shaped scar, revealing red layers of welded spatter (deposited as clots of molten lava) that under-lie loose tan-colored pyroclastic deposits (hot debris ejected during an eruption). The map shows flows erupted from Pu‘u ‘Öÿö and from fissures in Näpau Crater during 1992–2007 in red. Older flows from the Pu‘u ‘Öÿö eruption are shown in orange.


      A lava channel, elevated as much as 150 feet (45 meters) above the adjacent terrain, transports lava away from the Fissure D vent, which opened in July 2007 (USGS photo by James Kauahikaua, October 2007). The “perched” (elevated) lava channel was the main path for lava until November 2007, when lava was diverted from the vent to the southeast. Pu‘u ‘Öÿö is at upper right. The map shows lava flows erupted in Pu‘u ‘Öÿö and from the Fissure D vent between Pu‘u ‘Öÿö and Kupaianaha during 2007–2011 in red. Older flows from the Pu‘u ‘Öÿö eruption are shown in orange.


      In March 2011, lava broke to the surface between Pu‘u ‘Öÿö and Näpau Crater marking the start of the Kamoamoa fissure eruption. In this photo lava erupts from the fissure shortly after the beginning of the eruption and pours into a deep, older crack (USGS photo by Tim Orr). The map shows flows erupted during the Kamoamoa eruption and from Pu‘u ‘Öÿö during 2011–2012 in red. The Kamoamoa flows are to the left, flows from the August 2011 Pu‘u ‘Öÿö flank breakout are at center, flows from a fissure high on Pu‘u ‘Öÿö’s northeast flank are to the right. Older flows from the Pu‘u ‘Öÿö eruption are shown in orange.

    • This map highlights the seismicity associated with volcanism related to the youngest volcano in the Hawaii Islands (Tilling et al., 2010).

    • Map showing the locations of earthquakes that occurred during the 1970s and in the July–August 1996 period in the vicinity of Lö‘ihi. These earthquake swarms, plus similar occurrences in 1984–85 and the early 1990s, provide seismic evidence that Lö‘ihi is an active submarine volcano.

    • The ERZ and HFZ are also actively deforming between earthquakes. Below are two maps that show (a) regional vertical land motion and (b) results from block modeling to resolve the differential motion across this area (Shirzaei et al., 2013).

    • The linear velocity field in the line of sight of the descending-orbit Envisat satellite (track 200) over the Kilauea south flank from 2003 till 2010. Area of the study, Hilina Fault System (HFS), is outlined by dashed box. Location of GPS stations used is marked by their names next to filled squares colored by the mean rate of motion in the LOS direction. Station PGF1 is the reference for both GPS and InSAR datasets. WHP = western Hilina Pali, HP = Holei Pali.


      Colored panels represent relatively coherently moving blocks based on the InSAR deformation over the HFS according to Figs. 2–4 and traces of mapped faults, which are used to compare with GPS data. Each block is labeled by its average LOS velocity.

    Geologic Fundamentals

    • 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 educational video from the USGS that discusses eruptions in 2011, which were similar in type and style of eruptions as the current phase of eruption. Here is a link to the 4 MB mp4 video file.
    • 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).

    • 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:

    Social Media

    UPDATE 5/5

      References:

    • Ando, M., 1979. The Hawaii Earthquake of November 29, 1975: Low Dip Angle Faulting Due to Forceful Injection of Magma in JGR, v. 84, no. B13
    • Orr, T.R., et a., 2012. The Ongoing Pu‘u ‘Ö‘ö Eruption of Kïlauea Volcano, Hawai‘i—30 Years of Eruptive Activity in USGS Fact Sheet 2012-3127, 2013.
    • Sherrod, D.R., Sinton, J.M., Watkins, S.E., and Brunt, K.M., 2007, Geologic Map of the State of Hawaii: U.S. Geological Survey Open-File Report 2007-1089, 83 p., 8 plates, scales 1:100,000 and 1:250,000, with GIS database
    • Tilling, R.I., Keliker, C., and Swanson, D.A., 2010. Eruptions of Hawaiian Volcanoes—Past, Present, and Future, U.S. Geological Survey, General Information Product 117, 72 pp.
    • Torsvik, T.H., et al., 2017. Pacific plate motion change caused the Hawaiian-Emperor Bend in Nature Communications, DOI: 10.1038/ncomms15660

    Posted in earthquake, education, geology, pacific, plate tectonics, volcanoes