Monthly Archives: April 2016

Earthquake Report: Vanuatu Update #1

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We have had several aftershocks and triggered earthquakes in the region. I provide a brief update below.

    Here are the USGS websites for the largest earthquakes plotted below.

  • 2016.04.03 M 6.9 08:23:53
  • 2016.04.06 M 6.7 06:58:48
  • 2016.04.06 M 5.9 07:57:38
  • 2016.04.06 M 5.3 06:54:54
  • 2016.04.07 M 6.7 03:32:53

Below is a map that shows some of these earthquakes with their moment tensors.
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 poster.

  • In the upper right corner is a tectonic map showing the plate boundaries (Richards et al., 2011).
  • To the left of the Richards et al. (2011) figure is a cross section of a generic ocean-ocean subduction zone, where oceanic lithosphere is subducting beneath oceanic lithosphere. Note the location of the magmatic island arc. Vanuatu and the other islands in this region are formed as part of an island arc.
  • In the lower left corner I place a subset of the USGS tectonic map poster for this region of the Pacific Ocean. This is from the USGS Open File Report 2010-1083-I (Benz et al., 2011). Hypocenters are plotted as cross sections to show the geometry of the subducting slabs.



This map is larger scale, showing the slab contours. The red-orange-yellow lines are slab contour lines from Hayes et al. (2012). These lines are a best estimate for the depth to the subduction zone fault. These are based largely upon seismicity and there is currently an effort to update these contours to integrate other data types. The hypocentral depth is 35 km, and the slab contour nearest the epicenter is 40 km (so this makes sense that it is associated with the subduction zone megathrust).

    I include some inset figures in the poster.

  • In the lower left corner, I include a map from Cleveland et al. (2011). This figure shows earthquakes and some moment tensors for larger earthquakes.
  • In the upper right corner I place a subset of the USGS tectonic map poster for this region of the Pacific Ocean. This is from the USGS Open File Report 2010-1083-I (Benz et al., 2011). Hypocenters are plotted as cross sections to show the geometry of the subducting slabs.


    Here is an animation that shows the seismicity for this region from 1960 – 2016 for earthquakes with magnitudes greater than or equal to 7.0.

  • I include some figures mentioned in the posters above, in addition to a plot from Cleveland et al. (2014). In the upper right corner, Cleveland et al. (2014) on the left plot a map showing earthquake epicenters for the time period listed below the plot on the right. On the right is a plot of earthquakes (diameter = magnitude) of earthquakes with latitude on the vertical axis and time on the horizontal axis. Cleveland et al (2014) discuss these short periods of seismicity that span a certain range of fault length along the New Hebrides Trench in this area. Above is a screen shot image and below is the video.


  • Here is a link to the embedded video below (6 MB mp4)

    Here are the two figures from Cleveland et al. (2014).

  • Figure 1. I include the figure caption below as a blockquote.


    • (left) Seismicity of the northern Vanuatu subduction zone, displaying all USGS-NEIC earthquake hypocenters since 1973. The Australian plate subducts beneath the Pacific in nearly trench-orthogonal convergence along the Vanuatu subduction zone. The largest events are displayed with dotted outlines of the magnitude-scaled circle. Convergence rates are calculated using the MORVEL model for Australia Plate relative to Pacific Plate [DeMets et al., 2010]. (right) All GCMT moment tensor solutions and centroids for Mw ≥ 5 since 1976, scaled with moment. This region experiences abundant moderate and large earthquakes but lacks any events with Mw >8 since at least 1900.

  • Figure 17. I include the figure caption below as a blockquote.


    • One hundred day aftershock distributions of all earthquakes listed in the ISC catalog for the 1966 sequence and in the USGS-NEIC catalog for the 1980, 1997, 2009, and 2013 sequences in northern Vanuatu. The 1966 main shocks are plotted at locations listed by Tajima et al. [1990]. Events of the 1997 and 2009 sequences were relocated using the double difference method [Waldhauser and Ellsworth, 2000] for P wave first arrivals based on EDR picks. The event symbol areas are scaled relative to the earthquake magnitudes based on a method developed by Utsu and Seki [1954]. Hypocenters of most aftershock events occurred at <50 km depth.

  • Figure 17. I include the figure caption below as a blockquote.


    • (right) Space-time plot of shallow (≤ 70 km) seismicity M ≥ 5.0 in northern Vanuatu recorded in the NEIC catalog as a function of distance south of 10°N, 165.25°E. (left) The location of the seismicity on a map rotated to orient the trench vertically.

      Here are some plots from Jascha Polet showing focal mechanisms for this subduction zone. The top figure is a map and the bottom figure is a cross section.

Earthquake Report: Vanuatu!

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We just had another M 6.9 earthquake about 40 km from another M 6.9 earthquake from a few days ago. Here is my earthquake report from that earlier earthquake. They are both about the same depth. While the depths probably have considerable uncertainty, the new earthquake is west of the earlier earthquake. If these were on the subduction zone fault, we would consider the newer earthquake to be “up-dip” from the earlier earthquake. Indeed, the newer earthquake has a shallower depth (~33 vs ~35 km).This is probably just a coincidence, given the errors probably associated with these earthquakes. Later there was another earthquake of magnitude M = 5.5.

    Here are the three USGS web pages for these earthquakes.

  • 2016.04.03 M 6.9
  • 2016.04.06 M 6.9
  • 2016.04.06 M 5.5

Below is my interpretive earthquake report poster. I plot the epicenters for earthquakes from the past 7 days. The epicenter for the M 6.9 earthquake is labeled and I have placed the USGS moment tensor for reference. I include the moment tensor from the earlier earthquake as well.
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 poster.

  • In the upper right corner is a tectonic map showing the plate boundaries (Richards et al., 2011).
  • To the left of the Richards et al. (2011) figure is a cross section of a generic ocean-ocean subduction zone, where oceanic lithosphere is subducting beneath oceanic lithosphere. Note the location of the magmatic island arc. Vanuatu and the other islands in this region are formed as part of an island arc.
  • In the lower left corner I place a subset of the USGS tectonic map poster for this region of the Pacific Ocean. This is from the USGS Open File Report 2010-1083-I (Benz et al., 2011). Hypocenters are plotted as cross sections to show the geometry of the subducting slabs.


    References:

  • 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.
  • 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.

Earthquake Report: Vanuatu!

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We just had an earthquake along the New Hebrides Trench in the western Pacific. Here is the USGS webpage for this M 6.9 earthquake.
Below is my interpretive earthquake report poster. I plot the epicenters for earthquakes from the past 20 days. The epicenter for the M 6.9 earthquake is labeled and I have placed the USGS moment tensor for reference.
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 poster.

  • In the upper right corner is a tectonic map showing the plate boundaries (Richards et al., 2011).
  • To the left of the Richards et al. (2011) figure is a cross section of a generic ocean-ocean subduction zone, where oceanic lithosphere is subducting beneath oceanic lithosphere. Note the location of the magmatic island arc. Vanuatu and the other islands in this region are formed as part of an island arc.
  • In the lower left corner I place a subset of the USGS tectonic map poster for this region of the Pacific Ocean. This is from the USGS Open File Report 2010-1083-I (Benz et al., 2011). Hypocenters are plotted as cross sections to show the geometry of the subducting slabs.



Here is a map that shows (1) Modified Mercalli Intensity contours and (2) slab contours for the subduction zone. The MMI scale is a qualitative scale of the ground motions. There is more about the MMI here.
Note how the MMI contours are not perfect circles. This is the result of topography affecting the distance to the earthquake. Note the along the western part of Vanuatu, the MMI contours trend to the south at the coast and then to the north. At the coast, the ground surface is closer to the earthquake, so the MMI contours extend to a greater horizontal distance from the epicenter. In contrast, along the ridge the region is at a further distance from the earthquake, so the MMI contours extend to a lesser distance from the epicenter.
The red-orange-yellow lines are slab contour lines from Hayes et al. (2012). These lines are a best estimate for the depth to the subduction zone fault. These are based largely upon seismicity and there is currently an effort to update these contours to integrate other data types. The hypocentral depth is 35 km, and the slab contour nearest the epicenter is 40 km (so this makes sense that it is associated with the subduction zone megathrust).


The New Hebrides subduction zone dips to the east and turns into a transform fault (Richards et al., 2011).


This figure shows Richards et al. (2011) Figure 4, that displays their interpretation of how the plates came to be configured here. They propose that the Australia plate detached and collided with the Pacific slab about 4 million years ago.


Here is a map from the USGS report linked above. Read more about this map on the USGS website. Earthquakes are plotted with color related to depth and circle diameter related to magnitude. Today’s M 7.1 earthquake occurred midway between these two cross sections F-F’ and G-G’.


This is the legend.


Here are two cross sections showing the seismicity along swatch profiles F-F’ and G-G’.
F-F’


G-G’

Earthquake Report: Alaska Peninsula!

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Last night we had a shallow crustal earthquake along the Alaska Peninsula. Here is the USGS website for this M = 6.2 earthquake.
Below is my interpretive map where I use Google Earth and the kml (keyhole markup language) files from the USGS to plot epicenters, Modified Mercalli Intensity Scale (MMI) contours, and the subduction zone slab contours for this region (Hayes et al., 2012). I plot the USGS moment tensor from this M 6.2 earthquake, along with moment tensors for earthquakes further west along the Aleutian Trench (see below).
The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here.
The red-orange-yellow lines are slab contour lines from Hayes et al. (2012). These lines are a best estimate for the depth to the subduction zone fault. These are based largely upon seismicity and there is currently an effort to update these contours to integrate other data types. This M 6.2 earthquake is too shallow to be on the megathrust, so it is clearly a crustal earthquake in the North America plate. The USGS depth is 17.9 km, but the slab depth is between 80 and 100 km at this location (the slab contours have 20 km intervals).
There is a legend that shows how moment tensors can be interpreted. Moment tensors are graphical solutions of seismic data that show two possible fault plane solutions. One must use local tectonics, along with other data, to be able to interpret which of the two possible solutions is correct. The legend shows how these two solutions are oriented for each example (Normal/Extensional, Thrust/Compressional, and Strike-Slip/Shear). There is more about moment tensors and focal mechanisms at the USGS.

    I include some inset figures.

  • In the upper left corner, I place a map created by Dr. Peter Haeussler, USGS, which shows the historic earthquakes along the Alaska and Aleutian subduction zones. I place the epicenter from today’s earthquake as a red star. Note that this earthquake is near the possible segment boundary shown as the boundary between the 1938 and 1964 earthquakes.
  • To the right of Hauessler’s map is a figure from Atwater et al. (2005) that shows the earthquake cycle and how a tsunami can be generated at a subduction zone.
  • Below the Atwater figure I show a cross section of a subduction zone through the two main parts of the earthquake cycle. The interseismic part (in-between earthquakes) and the coseismic part (during earthquakes). This was developed by George Plafker and published in his 1972 paper on the Good Friday Earthquake.
  • In the lower right corner, I include the moment tensor legend.
  • To the left of that is from Saltus and Barnett (2000) shows an oblique cross section of the Aleutian subduction zone that is a part of the “Eastern Aleutian Volcanic Arc Digital Model.’



Here is a map from Michael West at the Alaska Earthquake Center. This shows today’s earthquake in reference to the Quaternary faults and folds in Alaska: A digital database (Koehler et al., 2012). Dr. Rick Koehler is currently at the University of Nevada Reno and The Nevada Bureau of Mines and Geology.


Here is the map poster from Koehler et al. (2012). The link is to a 63 MB pdf file.

    Here are the Earthquake Reports for those Aleutian Trench earthquakes. I include report posters for each of those reports below.

  • 2016.03.12 M 6.3 near the Amlia fracture zone


  • 2016.03.27 M 5.7 near Nikolski


    •

I recently prepared a summary Earthquake Report for the 1964/03/27 Great Alaska Earthquake here.
More Alaska-Aleutian related earthquakes can be found in my earthquake reports posted here.

Earthquake Report: Japan!

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Today there was an earthquake offshore of southern Japan. Here is the USGS website for the M 6.0 earthquake. It is difficult to tell if this earthquake was on the megathrust or not.
Below is my interpretive map. I plot the USGS moment tensor for this earthquake. I show the general location for the 1944 and 1946 subduction zone earthquakes mentioned below. I also include the Modified Mercalli Intensity (MMI) contours. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here.
There is a legend that shows how moment tensors can be interpreted. Moment tensors are graphical solutions of seismic data that show two possible fault plane solutions. One must use local tectonics, along with other data, to be able to interpret which of the two possible solutions is correct. The legend shows how these two solutions are oriented for each example (Normal/Extensional, Thrust/Compressional, and Strike-Slip/Shear). There is more about moment tensors and focal mechanisms at the USGS.

    I include some inset figures.

  • In 1944 and 1946 there was a pair of M 8.1 damaging large subduction zone earthquakes, the Tonankai and Nankai earthquakes respectively. These earthquakes were the impetus for the efforts in Japan to learn and mitigate for earthquake and tsunami hazards. On the left are two maps. The upper right map shows the segments of the Nankai subduction zone that have ruptured historically and prehistorically. On the left is a panel that shows how these fault segments have ruptured through the years (684, 887, 1096, 1099, 1360, 1361, 1498, 1605, 1707, 1854, 1944, and 1946). This is the source for these images.
  • In the upper right corner is a figure that shows the 3-D view of the plate configuration in this region. This is from d’après L. Jolivet, ISTO, Orléans, France (source here).
  • In the lower right corner I place a seismic hazard map for Japan. This map shows the probability of exceedance for ground motion (percent g, where g = gravitational acceleration of 9.8 m/s^2) within the next 30 years. If the ground motions exceed 100% g, then objects can be thrown into the air. Here is the source of this map, from the Japan Seismic Hazard Information Station (JSHIS).



Here is the same map that shows the slab contours. The red-orange-yellow lines are slab contour lines from Hayes et al. (2012). These lines are a best estimate for the depth to the subduction zone fault. These are based largely upon seismicity and there is currently an effort to update these contours to integrate other data types.


Here is a USGS poster than summarizes the earthquake history and plate geometry for this region. This is the USGS Open File Report 2010-1083-D (Rhea et al., 2010).


I put together an animation that shows the earthquake epicenters in Japan from 1900-2016/04/01. I include earthquakes with magnitude ≥ 6.0. Below is a screenshot of all these earthquakes, followed by the video. Here is the kml that I made using a USGS earthquake query. Here is the query that I used. The animation has an additional cross section showing the Japan trench, where the 2011/03/11 Tohoku-Oki M 9.0 subduction zone earthquake occurred. Here is a summary of the observations made following that 2011 earthquake.

    There have been many IODP investigations along the Nankai Trough. These investigations include 3-D seismic and scientific drilling in this region. Here are a couple reports from Moore et al. (2009) and Kinoshita, et al. (2007).

  • Here is an image that shows a 3-D view of the seafloor and seismic data. This comes from Moore et al. (2007). First is a map showing the location of this 3-D seismic survey, then the survey results, then their interpretation of the evolution of this margin. I include their figures caption below as a blockquote.


    • Location map showing the regional setting of the Nankai Trough (upper right inset). PSP, Philippine Sea Plate; KPR, Kyushu-Palau Ridge; IBT, Izu-Bonin Trench; KP, Kii Peninsula. Conver gence direction between the Philippine Sea Plate and Japan is shown at the lower right.


    3D seismic data volume depicting the location of the megasplay fault (black lines) and its relationship to older in sequence thrusts of the frontal accretionary prism (blue lines). Steep sea-floor topography and numerous slumps above the splay fault are shown.


    (A to C) Summary diagram showing the development of the Nankai accretionary prism in the Kumano Basin area. After “normal” in-sequence thrusting and building of an accretionary prism, an out-of-sequence (splay) fault system broke through at the back of the prism, a, b, and c refer to sequential sedimentary sequences.

  • This is an image from Jin-Oh Park (University of Tokyo) that shows the Decollement (the megathrust fault) and the seafloor. I include their figure caption below as a blockquote.


    • 3-D prestack depth migration images (inline slice, crossline slice, and depth slice) of the Nankai accretionary wedge off Shikoku Island. Miocene to Pliocene Shikoku Basin sediments underthrusts the overlying accretionary prism along a decollement as the Philippine Sea Plate subducts beneath the Eurasian Plate. The oceanic crust of the subducting Philippine Sea Plate (PSP) is traceable over the entire inlines. Several imbricate thrust faults are observed in the overlying accretionary wedge. The Décollement steps down on the top of subducting oceanic crust around ~30 km landward from the deformation front.

  • Some think that the 1944 earthquake slipped along a splay fault, a fault that splays off of the megathrust. Here is an image that shows how the megasplay fault is configured. This is the source of this figure. I include their figure caption below as a blockquote.


    • A: Tectonic setting of Nankai Trough subduction zone. Large earthquakes (magnitude 8-class) have repeatedly occurred along the Nankai Trough. The orange shaded segment caused the 1944 Tonankai earthquake. The NanTroSEIZE project is underway along the red line B.
    B: Cross section (red line in A). Detailed seismic profiles illustrate the plate boundary fault and the megasplay fault.
    C: Locations of core samples from Sites C0004 and C0008, taken from hanging wall and the footwall, respectively.