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Earthquake Report: 1992.04.25 M 7.1 Petrolia

The 25 April 1992 M 7.1 earthquake was a wake up call for many, like all large magnitude earthquakes are.

    Here is the USGS website for these three large earthquakes.

  • 1992-04-25 18:06:05 UTC 40.335°N 124.229°W 9.9 km depth M 7.2
  • 1992-04-26 07:41:40 UTC 40.433°N 124.566°W 18.8 km depth M 6.5
  • 1992-04-26 11:18:25 UTC 40.383°N 124.555°W 21.7 km depth M 6.6

Below is my interpretive poster for this earthquake.

I plot the seismicity for a week beginning April 25, 1992, with color representing depth and diameter representing magnitude (see legend)..

  • 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 (McCrory 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 poster.

  • In the upper left corner is a map of the Cascadia subduction zone (CSZ) and regional tectonic plate boundary faults. This is modified from several sources (Chaytor et al., 2004; Nelson et al., 2004)
  • Below the CSZ map is an illustration modified from Plafker (1972). This figure shows how a subduction zone deforms between (interseismic) and during (coseismic) earthquakes.
  • In the upper left 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. To the right of this map are two panels. The upper panel shows the location and orientation of the fault plane used by Rollins and Stein (2010) to model potential changes in coulomb stress following the 1992 M 7.2 earthquake. The Lower panel shows the results from this modeling.
  • In the lower right corner is the map from Stein et al. (1993). This map shows an estimate of coseismic vertical ground motion induced by the 1992 earthquake sequence.
  • In the upper right corner is a series of USGS shakemaps. These plot intensity using the MMI scale.
  • Below the shakemaps is the “Did You Feel It?” map and attenuation relation plot.


  • 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).

  • This figure shows how a subduction zone deforms between (interseismic) and during (coseismic) earthquakes.

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

  • Here is an animation produced by the folks at Cal Tech following the 2004 Sumatra-Andaman subduction zone earthquake. I have several posts about that earthquake here and here. One may learn more about this animation, as well as download this animation here.
  • Following the earthquake, there was lots of work done by local geologists, along with help from those visiting from out of the area. One of the projects included the measurement and modeling of the ground deformation related to the earthquake. The measurements consistend of a first order survey of benchmarks, along with Global Positioning System measurements at GPS monuments. The results from these analyses were presented in a U.S. Geological Survey Open-File Report 93-383 (Stein et al., 1993). Below is a map that shows a modeled estimate of the surface deformation associated with this earthquake.

  • Here is a figure from Oppenheimer et al. (1993) that shows the shaking intensity from this earthquake sequence. Below is a colorized version.


  • Simplified tectonic map in the vicinity of the Cape Mendocino earthquake sequence. Stars, epicenters of three largest earthquakes; contours, Modified Mercalli intensities (values, Roman numerals) of main shock; open circles, strong motion instrument sites (adjacent numbers give peak horizontal accelerations in g). Abbreviations FT Fortuna; F Ferndale; RD, Rio Dell; S, Scotia; P, Petrolia; H, Honeydew; MF, Mendocino fault; CSZ, seaward edge of Cascadia subduction zone; and SAF, San Andreas fault.

  • This map shows an alternate model of earthquake ground deformation (Oppenheimer et al, 1993).

  • Observed and predicted coseismic displacements for the Cape Mendocino main shock (epicenter located at star).

  • This is a figure that shows the tsunami recorded by tide gages in California, Hawaii, and Oregon (Oppenheimer et al., 1993)

  • Here is a map from Rollins and Stein (2010), 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.

  • This figure shows the fault plane and aftershocks used in their analysis of the 1992 earthquake sequence.

  • Source models for earthquakes 25 April 1992, Mw = 6.9, open circles are from Waldhauser and Schaff ’s [2008] earthquake locations for 25 April 1992 (1806 UTC) to 26 April 1992 (0741 UTC)

  • This figure shows the change in coulomb stress imparted by the M 7.1 earthquake onto different faults: (a) the CSZ and (b) the faults that were triggered to generate the two main aftershocks.

  • (a) Coulomb stress changes imparted by the 1992 Mw = 6.9 Cape Mendocino earthquake (J) to the Cascadia subduction zone. Calculation depth is 8 km. Open circles are Waldhauser and Schaff [2008] earthquake locations for 25 April 1992 to 2 May 1992, 0–15 km depth. Seismicity data were cut off at 15 km depth to prevent interference from aftershocks of K and L. Cross section A‐A′ includes seismicity between 40.24°N and 40.36°N. Cross section B‐B′ includes seismicity between 40.36°N and 40.48°N. (b) Coulomb stress changes imparted by the 1992 Mw = 6.9 earthquake (J) to Mw = 6.5 and Mw = 6.6 shocks the next day (K and L). Stress change is resolved on the average of the orientations of K and L (strike 127°/dip 90°/rake 180°). Calculation depth is 21.5 km. (c) Calculated Coulomb stress changes imparted by M ≥ 5.9 shocks in 1983, 1987, and 1992 (C, E, and J) to the epicenters of K and L. The series of three colored numbers represent stress changes imparted by C, E, and J, respectively.

  • Here is a plot of the seismograms from the NCEDC.

    Here is the USGS website for all the earthquakes in this region from 1917-2017 with M ≥ 6.5.

  • 1922.01.31 13:17 M 7.3
  • 1923.01.22 09:04 M 6.9
  • 1934-07-06 22:48 M 6.7
  • 1941-02-09 09:44 M 6.8
  • 1949-03-24 20:56 M 6.5
  • 1954-11-25 11:16 M 6.8
  • 1954-12-21 19:56 M 6.6
  • 1980-11-08 10:27 M 7.2
  • 1984-09-10 03:14 M 6.7
  • 1984-09-10 03:14 M 6.6
  • 1991-07-13 02:50 M 6.9
  • 1991-08-17 22:17 M 7.0
  • 1992-04-25 18:06 M 7.2
  • 1992-04-26 07:41 M 6.5
  • 1992-04-26 11:18 M 6.6
  • 1994-09-01 15:15 M 7.0
  • 1995-02-19 04:03 M 6.6
  • 2005-06-15 02:50 M 7.2
  • 2005-06-17 06:21 M 6.6
  • 2010-01-10 00:27 M 6.5
  • 2014-03-10 05:18 M 6.8
  • 2016-12-08 14:49 M 6.5
  • This is the map used in the animation below. Earthquake epicenters are plotted (some with USGS moment tensors) for this region from 1917-2017 with M ≥ 6.5. I labeled the plates and shaded their general location in different colors.
  • I include some inset maps.
    • In the upper right corner is a map of the Cascadia subduction zone (Chaytor et al., 2004; Nelson et al., 2004).
    • In the upper left corner is a map from Rollins and Stein (2010). They plot epicenters and fault lines involved in earthquakes between 1976 and 2010.



  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. Many of the earthquakes people are familiar with in the Mendocino triple junction region are either compressional or strike slip. The following three animations are from IRIS.
  • Strike Slip:
  • Compressional:
  • Extensional:
  • Here is a primer that helps people learn how to interpret focal mechanisms and moment tensors. Moment tensors are calculated differently from focal mechanisms, but the interpretation of their graphical solution is similar. This is from the USGS.

  • For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:

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.
  • Goldfinger, C., Nelson, C.H., Morey, A., Johnson, J.E., Gutierrez-Pastor, J., Eriksson, A.T., Karabanov, E., Patton, J., Gràcia, E., Enkin, R., Dallimore, A., Dunhill, G., and Vallier, T., 2012 a. Turbidite Event History: Methods and Implications for Holocene Paleoseismicity of the Cascadia Subduction Zone, USGS Professional Paper # 1661F. U.S. Geological Survey, Reston, VA, 184 pp.
  • McCrory, P.A., 2000, Upper plate contraction north of the migrating Mendocino triple junction, northern California: Implications for partitioning of strain: Tectonics, v. 19, p. 11441160.
  • McCrory, P. A., Blair, J. L., Oppenheimer, D. H., and Walter, S. R., 2006, Depth to the Juan de Fuca slab beneath the Cascadia subduction margin; a 3-D model for sorting earthquakes U. S. Geological Survey
  • Nelson, A.R., Kelsey, H.M., Witter, R.C., 2006. Great earthquakes of variable magnitude at the Cascadia subduction zone. Quaternary Research 65, 354-365.
  • Oppenheimer, D., Beroza, G., Carver, G., Dengler, L., Eaton, J., Gee, L., Gonzalez, F., Jayko, A., Ki., W.H., Lisowski, M., Magee, M., Marshall, G., Murray, M., McPherson, R., Romanowicz, B., Satake, K., Simpson, R., Somerille, P., Stein, R., and Valentine, D., The Cape Mendocino, California, Earthquakes of April, 1992: Subduction at the Triple Junction in Science, v. 261, no. 5120, p. 433-438.
  • Patton, J. R., Goldfinger, C., Morey, A. E., Romsos, C., Black, B., Djadjadihardja, Y., and Udrekh, 2013. Seismoturbidite record as preserved at core sites at the Cascadia and Sumatra–Andaman subduction zones, Nat. Hazards Earth Syst. Sci., 13, 833-867, doi:10.5194/nhess-13-833-2013, 2013.
  • Plafker, G., 1972. Alaskan earthquake of 1964 and Chilean earthquake of 1960: Implications for arc tectonics in Journal of Geophysical Research, v. 77, p. 901-925.
  • 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.
  • Stein, R.S., Marshall, G.A., Murray, M.H., Balazs, E., Carver, G.A., Dunklin, T.A>, McLaughlin, R.J., Cyr, K., and Jayko, A., 1993. Permanent Ground Movement Associate with the 1992 M=7 Cape Mendocino, California, Earthquake: Implications for Damage to Infrastructure and Hazards to navigation, U.S. Geological Survey Open-File Report 93-383.
  • Wang, K., Wells, R., Mazzotti, S., Hyndman, R. D., and Sagiya, T., 2003, A revised dislocation model of interseismic deformation of the Cascadia subduction zone Journal of Geophysical Research, B, Solid Earth and Planets v. 108, no. 1.

Earthquake Report: Chile Update #1

Well, I thought more to compare this ongoing earthquake sequence with the 1985 M 8.0 earthquake. This, in context with the 2010 and 2015 earthquakes. My initial report based upon the M ~4-5.9 swarm is here and my report on the “current” M 6.9 mainshock is here. More information about the background for the tectonics along the plate boundary, please refer to those earlier reports.

I used the USGS epicenters for earthquakes with magnitudes M ≥ 2.5. For each earthquake (1985, 2010, and 2015) I chose a month of seismicity beginning 3 days before the mainshock. Then I digitized the general outline of the earthquakes. This is a rough approximation for the slip patch for each of these earthquakes. I separated the interface earthquakes from the triggered outer rise earthquakes into separate polygons for the 2010 and 2015 earthquakes (they both appear to have triggered earthquakes in the downgoing Nazca plate to the west of the subduction zone fault, where it flexes in response to subduction here.

This current sequence is about the same magnitude and along-strike size as the 1971 earthquake (M 7.0). This sequence also lies within the 1985 earthquake aftershock region (and also within the northernmost area of the 2010 aftershock region). The M 6.9 could still be a foreshock of a larger earthquake. The 1985 earthquake was preceded by 3 earthquakes in the M 4-5.5 range. But, looking into the past, there are instances when this part of the fault only ruptures a small patch (1971, 1873, 1851). Given that this part of the fault slipped recently (2010), it seems more probable that there won’t be a larger earthquake (M > 8.0). This is difficult to know because we don’t really know the state of stress on the fault (how ready it is to rupture in an earthquake). I still cannot stop thinking about the Juan Fernandez Ridge and how this plays a part in this story.

I include the moment tensors from each of the Great Earthquakes, as well as the 2017 M 6.9 earthquake.

  • In the interpretive poster below
    • I outline the 1985 aftershock region in black dashed lines
    • I outline the 2010 aftershock region in blue dashed lines
    • I outline the 2015 aftershock region in white dashed lines
    • I outline the 2017 aftershock region in red dashed lines

Below is my interpretive poster for this earthquake. Click on the map to enlarge.

  • 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 one inset figure in the poster.

  • In the upper right corner I present Figure 2 from Beck et al. (1998 ) on the map, the space-time plot of historic and prehistoric earthquakes associated with the Chile subduction zone. I add a green line showing my interpretation for the strike length of the 2015 M 8.3 earthquake. Originally it appeared to match the 1943 and 1880 earthquakes, though it appears to extend further along strike. The 1922 and 1880 strike lengths are not well constrained, so this 2015 earthquake may indeed be slipping the same patch of this part of the subduction zone. Indeed, Juan Fernandez Ridge may be a structural boundary that may cause segmentation in this part of the subduction zone. If it does, it does not do so every time, as evidenced by the strike-length of the 1730 AD and 1647 AD earthquakes. I placed a green triangle at the approximate location of this 2017 swarm. This M 6.9 appears to be correlative in space with the 1985 earthquake (albeit a much smaller magnitude, closer to the 1971 in size).


Some background about the heterogeneous megathrust in this region

  • Here is the first of two figures from Moreno et al., 2010. Note that the M 6.9 is close in space to the 1985 earthquake. Also note the along strike heterogeneous seismogenic coupling. I include the figure caption below in blockquote.

  • Tectonic setting of the study area, data, observations and results. a, Shaded relief map of the Andean subduction zone in South- Central Chile. Earthquake segmentation along the margin is indicated by ellipses that enclose the approximate rupture areas of historic earthquakes (updated from refs 4–6). The inset shows the location of panel a (rectangle) relative to the South American continent. b, Compilation of GPS-observed surface velocities (1996–2008) with respect to stable South America before the 2010 Maule earthquake (for references see online-only Methods). Ellipses attached to the arrows represent 95% confidence limits. c, GPS 1 FEM modelled interface locking (fraction of plate convergence) distribution along the Andean subduction zone megathrust in the decade before the 2010 Maule earthquake. The epicentre (white star, USGS NEIC) and focal mechanism (beach ball, GCMT, http://www.globalcmt.org) of the 2010 Maule earthquake are shown in panels a and c.

  • Here is the second of the two figures from Moreno et al. (2010).

  • Relationship between pre, co- and postseismic deformation patterns. a, Coseismic slip distribution during the 2010 (blue contours; USGS slip model26) and 1960 (green contours; from ref. 30) earthquakes overlain onto pre-seismic locking pattern (red shading $0.75), as well as early (during the first 48 h post-shock) M$5 aftershock locations (the grey circle sizes scale with magnitude; GEOFON data29). b, Histograms of early (first 48 h; total number of events, 80) and late (first 3 months; total number of events, 168) aftershock density along a north–south profile (GEOFON data29, M$5). c, Residual slip deficits since 1835 as observed after the 2010 earthquake along a north–south profile (left column, based on the USGS slip model26). The middle and right columns show the effects on slip deficit of overlapping twentieth-century earthquakes (the black lines are polynomial fits to the data). Coloured data points and dates indicate earthquakes by year of occurrence.

References:

Earthquake Report: Chile!

Well, we had another earthquake in the region of a recent (yesterday and the day before) swarm offshore of Valparaiso, Chile (almost due west of Santiago, one of the largest cities in Chile). My previous report on the M 4-5 earthquakes can be found here. The earlier swarm was a series of shallower earthquakes (though some were of intermediate depth and some were deeper). The M 6.9 earthquake, in contrast, is deeper and likely on the megathrust. The slab contours are at 20 km and the hypocentral depth is 25 km (pretty good match considering the uncertainty with the location of the megathrust). Another difference is that the M 6.9 has a greater potential (likelihood, or chance) to damage people or their belongings.

Here are the USGS websites for these earthquakes

  • 2017.04.22 22:46 M 4.9
  • 2017.04.23 01:49 M 4.5
  • 2017.04.23 02:36 M 5.9 (mainshock)
  • 2017.04.23 02:43 M 4.8
  • 2017.04.23 02:52 M 4.8
  • 2017.04.23 03:00 M 4.8
  • 2017.04.23 03:02 M 4.9
  • 2017.04.23 19:40 M 5.6
  • 2017.04.24 21:38 M 6.9 (triggered mainshock)

I took a look at the seismicity from the past century. Here are Google Earth kml files from the USGS website for earthquakes from 1917-2017 with magnitudes M ≥ 5.0, M ≥ 6.0, and M ≥ 7.0.

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 the USGS epicenters for earthquakes from 1917-2017 with magnitudes M ≥ 6.0. I outline the regions of the subduction zone that have participated in earthquake slip during the 21st century (in white dashed polygons). I include USGS moment tensors from the largest earthquakes. I plot the focal mechanism for the 1960 earthquake from Moreno et al. (2011). Note the gap in seismicity in the region of the 1960 M 9.5 earthquake, except for the 2016 M 7.6 earthquake. Also, note how the 1960 and 2010 earthquake slip patches overlap.

Much of the subduction zone has ruptured, except for some spots between the 2001 and 2015 earthquakes. In 2015, I speculated that the region north of the 2015 earthquakes constituted a seismic gap. This region may get filled by a Great subduction zone earthquake or may continue to slip in moderate sized earthquakes (or be aseismic). There was an earthquake in 1877 that spanned 19-23 degrees (overlapping with the 2014 earthquake). This is shown on the Schurr et al. (2014) figure below).

  • 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 poster.

  • In the lower left corner, I include a map and a cross section of the subduction zone just to the south of this Sept/Nov 2015 swarm (Melnick et al., 2006). I placed a green triangle at the approximate location of this 2017 swarm.
  • In the upper right corner I present Figure 2 from Beck et al. (1998 ) on the map, the space-time plot of historic and prehistoric earthquakes associated with the Chile subduction zone. I add a green line showing my interpretation for the strike length of the 2015 M 8.3 earthquake. Originally it appeared to match the 1943 and 1880 earthquakes, though it appears to extend further along strike. The 1922 and 1880 strike lengths are not well constrained, so this 2015 earthquake may indeed be slipping the same patch of this part of the subduction zone. Indeed, Juan Fernandez Ridge may be a structural boundary that may cause segmentation in this part of the subduction zone. If it does, it does not do so every time, as evidenced by the strike-length of the 1730 AD and 1647 AD earthquakes. I placed a green triangle at the approximate location of this 2017 swarm. This M 6.9 appears to be correlative in space with the 1985 earthquake (albeit a much smaller magnitude, closer to the 1971 in size).
  • In the lower right corner I include two figures from Moreno et al. (2010). The upper one shows the spatial extent of historic subduction zone earthquakes in this region, the GPS velocities, and the fraction of plate convergence attributed to fault seismogenic coupling. The lower panel shows the amount of slip that is attributed to the 1960 and 2010 earthquakes (on the left) and various measures of seismicity and slip deficit (on the right). I place a green star in the general location of the M 6.9 and a green horizontal bar that matches the latitude of this M 6.9 earthquake.
  • In the upper left corner, I include a local map showing the MMI contours for the M 6.9 earthquake. I include the USGS moment tensors from most of the earthquakes in this swarm, including the M 6.9 earthquake.


  • As mentioned above, this earthquake has the potential to cause more harm than the earlier earthquakes due to its larger magnitude. Below is the USGS report that includes estimates of damage to people (possible fatalities) and their belongings from the Rapid Assessment of an Earthquake’s Impact (PAGER) report. More on the PAGER program can be found here. An explanation of a PAGER report can be found here. PAGER reports are modeled estimates of damage. On the top is a histogram showing estimated casualties and on the right is an estimate of possible economic losses. This PAGER report suggests that there will be quite a bit of damage from this earthquake (and casualties). This earthquake has a high probability of damage to people and their belongings.

  • UPDATE: Below are some observations of the tsunami. This comes from the Pacific Tsunami Warning Center.

  • Here is the figure from Lin et al. (2013) that shows the tectonic context of the 2010 Maule earthquake. I include the figure captions as blockquote.

  • (a) Regional tectonic map showing slab isodepth contours (blue lines) [Cahill and Isacks, 1992], M>=4 earthquakes from the National Earthquake Information Center catalog between 1976 and 2011 (yellow circles for depths less than 50 km, and blue circles for depths greater than 50 km), active volcanoes (red triangles), and the approximate extent of large megathrust earthquakes during the past hundred years (red ellipses) modified from Campos et al. [2002]. The large white vector represents the direction of Nazca Plate with respect to stable South America [Kendrick et al., 2003]. (b) Simplified seismo-tectonic map of the study area. Major Quaternary faults are modified after Melnick et al. [2009] (black lines). The Neogene Deformation Front is modified from Folguera et al. [2004]. The west-vergent thrust fault that bounds the west of the Andes between 32 and 38S is modified from Melnick et al. [2009]. (c) Schematic cross-section along line A–A0 (Figure 1b), modified from Folguera and Ramos [2009]. The upper bound of the coseismic slip coincides with the boundary between the frontal accretionary prism and the paleo-accretionary prism [Contreras-Reyes et al., 2010], whereas the contact between the coseismic and postseismic patch is from this study. The thick solid red line and dashed red line on top of the slab represent the approximate coseismic and postseismic plus interseismic slip section of the subduction interface. The thin red and grey lines within the overriding plate are active and inactive structures in the retroarc, modified from Folguera and Ramos [2009]. The red dashed line underneath the Andean Block represents the regional décollement. Background seismicity is from the TIPTEQ catalog, recorded between November 2004 and October 2005 [Rietbrock et al., 2005; Haberland et al., 2009].

  • Here is a cross section of the subduction zone just to the south of this Sept/Nov 2015 swarm (Melnick et al., 2006). Below I include the text from the Melnick et al. (2006) figure caption as block text.

  • (A) Seismotectonic segments, rupture zones of historical subduction earthquakes, and main tectonic features of the south-central Andean convergent margin. Earthquakes were compiled from Lomnitz (1970, 2004), Kelleher (1972), Comte et al. (1986), Cifuentes (1989), Beck et al. (1998), and Campos et al. (2002). Nazca plate and trench are from Bangs and Cande (1997) and Tebbens and Cande (1997). Maximum extension of glaciers is from Rabassa and Clapperton (1990). F.Z.—fracture zone. (B) Regional morphotectonic units, Quaternary faults, and location of the study area. Trench and slope have been interpreted from multibeam bathymetry and seismic-reflection profiles (Reichert et al., 2002). (C) Profile of the offshore Chile margin at ~37°S, indicated by thick stippled line on the map and based on seismic-reflection profiles SO161-24 and ENAP-017. Integrated Seismological experiment in the Southern Andes (ISSA) local network seismicity (Bohm et al., 2002) is shown by dots; focal mechanism is from Bruhn (2003). Updip limit of seismogenic coupling zone from heat-fl ow measurements (Grevemeyer et al., 2003). Basal accretion of trench sediments from sandbox models (Lohrmann, 2002; Glodny et al., 2005). Convergence parameters from Somoza (1998 ).

  • Here is the first of two figures from Moreno et al., 2010. Note that the M 6.9 is close in space to the 1985 earthquake. I include the figure caption below in blockquote.

  • Tectonic setting of the study area, data, observations and results. a, Shaded relief map of the Andean subduction zone in South- Central Chile. Earthquake segmentation along the margin is indicated by ellipses that enclose the approximate rupture areas of historic earthquakes (updated from refs 4–6). The inset shows the location of panel a (rectangle) relative to the South American continent. b, Compilation of GPS-observed surface velocities (1996–2008) with respect to stable South America before the 2010 Maule earthquake (for references see online-only Methods). Ellipses attached to the arrows represent 95% confidence limits. c, GPS 1 FEM modelled interface locking (fraction of plate convergence) distribution along the Andean subduction zone megathrust in the decade before the 2010 Maule earthquake. The epicentre (white star, USGS NEIC) and focal mechanism (beach ball, GCMT, http://www.globalcmt.org) of the 2010 Maule earthquake are shown in panels a and c.

  • Here is the second of the two figures from Moreno et al. (2010).

  • Relationship between pre, co- and postseismic deformation patterns. a, Coseismic slip distribution during the 2010 (blue contours; USGS slip model26) and 1960 (green contours; from ref. 30) earthquakes overlain onto pre-seismic locking pattern (red shading $0.75), as well as early (during the first 48 h post-shock) M$5 aftershock locations (the grey circle sizes scale with magnitude; GEOFON data29). b, Histograms of early (first 48 h; total number of events, 80) and late (first 3 months; total number of events, 168) aftershock density along a north–south profile (GEOFON data29, M$5). c, Residual slip deficits since 1835 as observed after the 2010 earthquake along a north–south profile (left column, based on the USGS slip model26). The middle and right columns show the effects on slip deficit of overlapping twentieth-century earthquakes (the black lines are polynomial fits to the data). Coloured data points and dates indicate earthquakes by year of occurrence.

  • Here is the Beck et al. (1998) space time diagram.

Here is an animation of seismicity from the 21st century

Useful Resources

References:

Earthquake Report: Chile!

There have been a number of earthquakes along the subduction zone offshore of Chile. These have happened near the boundary of two Great Earthquakes from 2010 and 2015. This region may be a segment boundary along the subduction zone, albeit possibly a non persistent one. The Juan Ferndandez ridge may control this segmentation.

The earthquakes from today and yesterday form a range of about 1 1/2 magnitudes (M 4.2- M 5.9). This may be considered a swarm (when there are a series of earthquakes along a fault with similar magnitudes), though there is an M 5.9 that could be considered the mainshock. But, I would not get hung up on terminology as that is not very important. However, there is a great page with a discussion about swarms, including some good examples.

Here are the USGS websites for these earthquakes

I took a look at the seismicity from the past century. Here are Google Earth kml files from the USGS website for earthquakes from 1917-2017 with magnitudes M ≥ 5.0, M ≥ 6.0, and M ≥ 7.0.

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 the USGS epicenters for earthquakes from 1917-2017 with magnitudes M ≥ 5.0. I outline the regions of the subduction zone that have participated in earthquake slip during the 21st century (in white dashed polygons). I include USGS moment tensors from the largest earthquakes. I plot the focal mechanism for the 1960 earthquake from Moreno et al. (2011). Note the gap in seismicity in the region of the 1960 M 9.5 earthquake, except for the 2016 M 7.6 earthquake. Also, note how the 1960 and 2010 earthquake slip patches overlap.

Much of the subduction zone has ruptured, except for some spots between the 2001 and 2015 earthquakes. In 2015, I speculated that the region north of the 2015 earthquakes constituted a seismic gap. This region may get filled by a Great subduction zone earthquake or may continue to slip in moderate sized earthquakes (or be aseismic). There was an earthquake in 1877 that spanned 19-23 degrees (overlapping with the 2014 earthquake). This is shown on the Schurr et al. (2014) figure below).

  • 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 poster.

  • In the lower left corner, I include a figure from Lin et al. (2013) that shows the tectonic context of the 2010 Maule earthquake. On the map are plotted extents of historic earthquakes along this convergent plate margin. On the right is a large scale map showing the active magmatic arc volcanoes associated with this subduction zone. Finally, there is a cross section showing where the coseismic slip and postseismic slip occurred as part of the 2010 earthquake sequence. I placed a green triangle at the approximate location of this 2017 swarm.
  • In the lower right corner, I include a time-space diagram from Moernaut et al. (2010). There is also a map showing the fracture zones. I placed a green triangle at the approximate location of this 2017 swarm.
  • Above the Moernaut et al. (2010) figure, I present Figure 2 from Beck et al. (1998 ) on the map, the space-time plot of historic and prehistoric earthquakes associated with the Chile subduction zone. This space-time plot overlaps slightly with the Moernaut figure. I add a green line showing my interpretation for the strike length of the 2015 M 8.3 earthquake. Originally it appeared to match the 1943 and 1880 earthquakes, though it appears to extend further along strike. The 1922 and 1880 strike lengths are not well constrained, so this 2015 earthquake may indeed be slipping the same patch of this part of the subduction zone. Indeed, Juan Fernandez Ridge may be a structural boundary that may cause segmentation in this part of the subduction zone. If it does, it does not do so every time, as evidenced by the strike-length of the 1730 AD and 1647 AD earthquakes. I placed a green triangle at the approximate location of this 2017 swarm.
  • In the upper right corner is a space-time figure showing earthquakes for the past few centuries. This diagram does not overlap with the Beck figure. This figure shows the outline of some subduction zone earthquakes and shows how the 2014 earthquake is composed of two earthquakes (an M 8.1 and an M 7.6) that ruptured different but adjacent patches of the subduction zone.
  • In the upper left corner, I include a local map showing the MMI contours for the M 5.9 earthquake. I include the USGS moment tensors from most of the earthquakes in this swarm.


  • Here is the figure from Lin et al. (2013) that shows the tectonic context of the 2010 Maule earthquake. I include the figure captions as blockquote.

  • (a) Regional tectonic map showing slab isodepth contours (blue lines) [Cahill and Isacks, 1992], M>=4 earthquakes from the National Earthquake Information Center catalog between 1976 and 2011 (yellow circles for depths less than 50 km, and blue circles for depths greater than 50 km), active volcanoes (red triangles), and the approximate extent of large megathrust earthquakes during the past hundred years (red ellipses) modified from Campos et al. [2002]. The large white vector represents the direction of Nazca Plate with respect to stable South America [Kendrick et al., 2003]. (b) Simplified seismo-tectonic map of the study area. Major Quaternary faults are modified after Melnick et al. [2009] (black lines). The Neogene Deformation Front is modified from Folguera et al. [2004]. The west-vergent thrust fault that bounds the west of the Andes between 32 and 38S is modified from Melnick et al. [2009]. (c) Schematic cross-section along line A–A0 (Figure 1b), modified from Folguera and Ramos [2009]. The upper bound of the coseismic slip coincides with the boundary between the frontal accretionary prism and the paleo-accretionary prism [Contreras-Reyes et al., 2010], whereas the contact between the coseismic and postseismic patch is from this study. The thick solid red line and dashed red line on top of the slab represent the approximate coseismic and postseismic plus interseismic slip section of the subduction interface. The thin red and grey lines within the overriding plate are active and inactive structures in the retroarc, modified from Folguera and Ramos [2009]. The red dashed line underneath the Andean Block represents the regional décollement. Background seismicity is from the TIPTEQ catalog, recorded between November 2004 and October 2005 [Rietbrock et al., 2005; Haberland et al., 2009].

  • Here is a cross section of the subduction zone just to the south of this Sept/Nov 2015 swarm (Melnick et al., 2006). Below I include the text from the Melnick et al. (2006) figure caption as block text.

  • (A) Seismotectonic segments, rupture zones of historical subduction earthquakes, and main tectonic features of the south-central Andean convergent margin. Earthquakes were compiled from Lomnitz (1970, 2004), Kelleher (1972), Comte et al. (1986), Cifuentes (1989), Beck et al. (1998), and Campos et al. (2002). Nazca plate and trench are from Bangs and Cande (1997) and Tebbens and Cande (1997). Maximum extension of glaciers is from Rabassa and Clapperton (1990). F.Z.—fracture zone. (B) Regional morphotectonic units, Quaternary faults, and location of the study area. Trench and slope have been interpreted from multibeam bathymetry and seismic-reflection profiles (Reichert et al., 2002). (C) Profile of the offshore Chile margin at ~37°S, indicated by thick stippled line on the map and based on seismic-reflection profiles SO161-24 and ENAP-017. Integrated Seismological experiment in the Southern Andes (ISSA) local network seismicity (Bohm et al., 2002) is shown by dots; focal mechanism is from Bruhn (2003). Updip limit of seismogenic coupling zone from heat-fl ow measurements (Grevemeyer et al., 2003). Basal accretion of trench sediments from sandbox models (Lohrmann, 2002; Glodny et al., 2005). Convergence parameters from Somoza (1998 ).

  • In March 2015, there was some seismicity in this September/November 2015 earthquake slip region. I put together an earthquake report about those earthquake of magnitudes M = 5.0-5.3. I speculate that the 1922 earthquake region is a seismic gap. Note that this September/November 2015 earthquake region is along the southern portion of the seismic gap that I labeled on the map below.
  • Here is a map that shows the recent swarm of ~M = 5 earthquakes. There are moment tensors for the earthquakes listed below, some recent historic subduction zone earthquakes. I placed the general along-strike distance for older historic earthquakes in green (and labeled their years). The largest earthquake ever recorded, the Mw = 9.5 Chile earthquake, had a slip patch that extends from the south of the map to just south of the 2010 earthquake swarm. The 2010 and 2014 earthquake swarm epicenters are plotted as colored circles, while most other historic earthquake epicenters are plotted as gray circles. Note how this March 2015 swarm is at the northern end of the 1922/11/11 M 8.3 earthquake. At the bottom of this page, I put a USGS graphic about what these moment tensor plots (beach balls) tell us about the earthquakes.

  • Here is the first space-time figure from Schurr et al., 2014. I include their caption as blockquote below.

  • Map of Northern Chile and Southern Peru showing historical earthquakes and instrumentally recorded megathrust ruptures. IPOC instruments used in the present study (BB, broadband; SM, strong motion) are shown as blue symbols. Left: historical1,2 and instrumental earthquake record. Centre: rupture length was calculated using the regression suggested in ref. 28, with grey lines for earthquakes M .7 and red lines for Mw .8. The slip distribution of the 2014 Iquique event and its largest aftershock derived in this study are colour coded, with contour intervals of 0.5 m. The green and black vectors are the observed and modelled horizontal surface displacements of the mainshock. The slip areas of the most recent other large ruptures4,5,7 are also plotted. Right: moment deficit per kilometre along strike left along the plate boundary after the Iquique event for moment accumulated since 1877, assuming current locking (Fig. 3a). The total accumulated moment since 1877 from 17u S to 25u S (red solid line) is 8.97; the remaining moment after subtracting all earthquake events with Mw .7 (grey dotted line) is 8.91 for the entire northern Chile–southern Peru seismic gap

  • Here is the Beck et al. (1998) space time diagram.

  • Finally, here is the southernmost space-time diagram from Moernaut et al. (2010). These data are largely derived from Melnick et al. (2009).

  • Setting and historical earthquakes in South-Central Chile. Data derived from Barrientos (2007); Campos et al. (2002); Melnick et al.(2009).

Here is an animation of seismicity from the 21st century

References:

Earthquake Report: New Zealand Post # 02

Here is the first update on the 2016.11.13 (UTC) Mw 7.8 Kaikoura Earthquake and associated fault ruptures, ground shaking, and other geologic effects. I will be preparing several more posts on this subject. I was invited to participate on a research cruise offshore of new Zealand. Our goal was to collect sediment cores in the deep sea so that we might test the hypothesis that strong ground shaking from earthquakes along the Hikurangi subduction zone generate submarine landslides that can be used to establish an earthquake chronology. This is a new method being used globally. I have applied this method in the northeastern Indian Ocean (offshore of Sumatra: the Andaman-Sumatra subduction zone), the northeastern Pacific (offshore of the Pacific northwest coasts of Washington, Oregon, and California: the Cascadia subduction zone and northern San Andreas fault), and in the western equatorial Atlantic Ocean (offshore of Guadeloupe: the subduction zone along the Lesser Antilles).

The chief scientists for this southwestern Pacific turbidite paleoseismology investigation are Drs. Philip Barnes (NIWA) and Jamie Howarth (GNS).

While we were at sea aboard the R/V Tangaroa, this M 7.8 and associated complicated earthquakes occurred. We initiated planning to modify our goals to include data collection in response to this earthquake series. This included (1) additional sediment coring and (2) seismic reflection and multibeam bathymetric mapping. The coring is important because this earthquake is a small earthquake for turbidite paleoseismology, so the results will be an important contribution to the global studies of trigger magnitude-distance relations. The seismic reflection data are important to determine the extent of surface rupture of the faults offshore. The multibeam mapping also helps extend our observations of surface rupture offshore. These observations will help constrain fault slip models. Some of the results from our cruise are presented below. We prepared this presentation for a press conference immediately following our cruise.

Related Posts

  • My initial Earthquake Report is posted here. I discuss and present observations made following the earthquake.
  • My report for prior to the cruise here. I present some background information about New Zealand tectonics. I have learned much more and will post more about this in future reports.
  • I presented some updates on our cruise via my blog here. I list some of these posts below (dates are local time).

Information Sources

Press Conference


    Here are the slides that we put together for our press conference.

  • We arrived at port about 8 AM and the press conference was at 2 PM. No rest for the wicked. The fearless leaders of our R/V Tangaroa research cruise were Drs. Philip Barnes from NIWA and Jamie Howarth from GNS Science.
  • Here is the digital press release as displayed blow: (pdf)












Some Cruise Videos

  • Here is the link to the embedded video below. This was taken by Dr. Howarth and shows a core from recovery to discovery. (102 MB mp4)
  • Observations Made Prior to Earthquake


    In 2000, several Humboldt State University, Department of Geology students joined a faculty member (Dr. Raymond “Bud” Burke, a soils geomorphologist) in a trip to New Zealand. They traveled to work with Russ Van Dissen from GNS, an HSU graduate (he also went to Oregon State University and worked with Dr. Bob Yeats). Their efforts were to investigate faulting along the Kekerungu fault. They established that the Kekrungu fault is the main plate boundary fault in this region. Here is the accepted abstract currently being presented at a geological conference in New Zealand. These scientists had established that this fault system was capable of doing what it just did. I remember helping to prepare some field maps for this trip, but the GIS data available at the time was scarce and my maps were of little utility to the team.

      The students included

    • Steve N. Bacon
    • Ronna Bowers
    • Harland L. Goldstein
    • Joanna R. Redwine
    • Diane G. Sutherland
    • Stephen F. Tilinghast

    Here is the abstract:



    Earthquake Report: New Britain!

    Last night there was a magnitude M 6.9 earthquake associated with the subduction zone that forms the New Britain Trench (where the Solomon Sea plate subducts northwards beneath the South Bismarck plate). The day before there was a M 6.4 earthquake to the northeast of this M 6.9 earthquake. Here is the USGS website for today’s M 6.9 earthquake.

    In the map below I plot the epicenters of earthquakes from the past 30 days of magnitude greater than M = 2.5. The epicenters have colors representing depth in km. The USGS plate boundaries are plotted vs color. The USGS modeled estimate for ground shaking is plotted with contours of equal ground shaking using the Modified Mercalli Intensity (MMI) scale. 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 placed a moment tensor / focal mechanism legend in the lower left corner of the map. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.

    I also 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. The hypocentral depth plots this close to the location of the fault as mapped by Hayes et al. (2012). So, the earthquake is either in the downgoing slab, or in the upper plate and a result of the seismogenic locked plate transferring the shear strain from a fracture zone in the downgoing plate to the upper plate.

    Today’s earthquake has an hypocentral depth of 35 km, while the slab depth estimate from Hayes et al. (2013) is between 60 & 80 km. This is a pretty good match, so the earthquake is possibly above the slab interface.

      I include some inset figures.

    • In the upper right corner is a generalized tectonic map of the region from Holm et al., 2015. This map shows the major plate boundary faults including the New Britain trench (NBT), one of the main culprits for recent seismicity of this region.
    • In the lower right corner a figure from Oregon State University, which are based upon Hamilton (1979). “Tectonic microplates of the Melanesian region. Arrows show net plate motion relative to the Australian Plate.” This is from Johnson, 1976. There is a plate tectonic map and a cross section showing the subduction of the Solomon Sea plate.
    • In the lower left corner is a figure from Baldwin et al. (2012). This figure shows a series of cross sections along this convergent plate boundary from the Solomon Islands in the east to Papua New Guinea in the west. Cross section ‘C’ is the most representative for the earthquake today. I present the map and this figure again below, with their original captions.


    • In earlier earthquake reports, I discussed seismicity from 2000-2015 here. The seismicity on the west of this region appears aligned with north-south shortening along the New Britain trench, while seismicity on the east of this region appears aligned with more east-west shortening. Here is a map that I put together where I show these two tectonic domains with the seismicity from this time period (today’s earthquakes are not plotted on this map, but one may see where they might plot).

    • Here is the generalized tectonic map of the region from Holm et al., 2015. I include the figure caption below as a blockquote.

    • Tectonic setting and mineral deposits of eastern Papua New Guinea and Solomon Islands. The modern arc setting related to formation of the mineral deposits comprises, from west to east, the West Bismarck arc, the New Britain arc, the Tabar-Lihir-Tanga-Feni Chain and the Solomon arc, associated with north-dipping subduction/underthrusting at the Ramu-Markham fault zone, New Britain trench and San Cristobal trench respectively. Arrows denote plate motion direction of the Australian and Pacific plates. Filled triangles denote active subduction. Outlined triangles denote slow or extinct subduction. NBP: North Bismarck plate; SBP: South Bismarck plate; AT: Adelbert Terrane; FT: Finisterre Terrane; RMF: Ramu-Markham fault zone; NBT: New Britain trench.

    • Here is the slab interpretation for the New Britain region from Holm and Richards, 2013. Note the tear in the slab where the New Britain and South Solomon trenches intersect. This feeds into the tectonic domains discussed in my map above and also here. I include the figure caption below as a blockquote.

    • 3-D model of the Solomon slab comprising the subducted Solomon Sea plate, and associated crust of the Woodlark Basin and Australian plate subducted at the New Britain and San Cristobal trenches. Depth is in kilometres; the top surface of the slab is contoured at 20 km intervals from the Earth’s surface (black) to termination of slabrelated seismicity at approximately 550 km depth (light brown). Red line indicates the locations of the Ramu-Markham Fault (RMF)–New Britain trench (NBT)–San Cristobal trench (SCT); other major structures are removed for clarity; NB, New Britain; NI, New Ireland; SI, Solomon Islands; SS, Solomon Sea; TLTF, Tabar–Lihir–Tanga–Feni arc. See text for details.

    • This map shows plate velocities and euler poles for different blocks. Note the counterclockwise motion of the plate that underlies the Solomon Sea (Baldwin et al., 2012). I include the figure caption below as a blockquote.

    • Tectonic maps of the New Guinea region. (a) Seismicity, volcanoes, and plate motion vectors. Plate motion vectors relative to the Australian plate are surface velocity models based on GPS data, fault slip rates, and earthquake focal mechanisms (UNAVCO, http://jules.unavco.org/Voyager/Earth). Earthquake data are sourced from the International Seismological Center EHB Bulletin (http://www.isc.ac.uk); data represent events from January 1994 through January 2009 with constrained focal depths. Background image is generated from http://www.geomapapp.org. Abbreviations: AB, Arafura Basin; AT, Aure Trough; AyT, Ayu Trough; BA, Banda arc; BSSL, Bismarck Sea seismic lineation; BH, Bird’s Head; BT, Banda Trench; BTFZ, Bewani-Torricelli fault zone; DD, Dayman Dome; DEI, D’Entrecasteaux Islands; FP, Fly Platform; GOP, Gulf of Papua; HP, Huon peninsula; LA, Louisiade Archipelago; LFZ, Lowlands fault zone; MaT, Manus Trench; ML, Mt. Lamington; MT, Mt. Trafalgar; MuT, Mussau Trough; MV, Mt. Victory; MTB, Mamberamo thrust belt; MVF, Managalase Plateau volcanic field; NBT, New Britain Trench; NBA, New Britain arc; NF, Nubara fault; NGT, New Guinea Trench; OJP, Ontong Java Plateau; OSF, Owen Stanley fault zone; PFTB, Papuan fold-and-thrust belt; PP, Papuan peninsula; PRi, Pocklington Rise; PT, Pocklington Trough; RMF, Ramu-Markham fault; SST, South Solomons Trench; SA, Solomon arc; SFZ, Sorong fault zone; ST, Seram Trench; TFZ, Tarera-Aiduna fault zone; TJ, AUS-WDKPAC triple junction; TL, Tasman line; TT, Trobriand Trough;WD, Weber Deep;WB, Woodlark Basin;WFTB, Western (Irian) fold-and-thrust belt; WR,Woodlark Rift; WRi, Woodlark Rise; WTB, Weyland thrust; YFZ, Yapen fault zone.White box indicates the location shown in Figure 3. (b) Map of plates, microplates, and tectonic blocks and elements of the New Guinea region. Tectonic elements modified after Hill & Hall (2003). Abbreviations: ADB, Adelbert block; AOB, April ultramafics; AUS, Australian plate; BHB, Bird’s Head block; CM, Cyclops Mountains; CWB, Cendrawasih block; CAR, Caroline microplate; EMD, Ertsberg Mining District; FA, Finisterre arc; IOB, Irian ophiolite belt; KBB, Kubor & Bena blocks (including Bena Bena terrane); LFTB, Lengguru fold-and-thrust belt; MA, Mapenduma anticline; MB, Mamberamo Basin block; MO, Marum ophiolite belt; MHS, Manus hotspot; NBS, North Bismarck plate; NGH, New Guinea highlands block; NNG, Northern New Guinea block; OKT, Ok Tedi mining district; PAC, Pacific plate; PIC, Porgera intrusive complex; PSP, Philippine Sea plate; PUB, Papuan Ultramafic Belt ophiolite; SB, Sepik Basin block; SDB, Sunda block; SBS, South Bismarck plate; SIB, Solomon Islands block; WP, Wandamen peninsula; WDK, Woodlark microplate; YQ, Yeleme quarries.

    • This figure incorporates cross sections and map views of various parts of the regional tectonics (Baldwin et al., 2012). The New Britain region is in the map near the A and B sections. I include the figure caption below as a blockquote.

    • Oblique block diagram of New Guinea from the northeast with schematic cross sections showing the present-day plate tectonic setting. Digital elevation model was generated from http://www.geomapapp.org. Oceanic crust in tectonic cross sections is shown by thick black-and-white hatched lines, with arrows indicating active subduction; thick gray-and-white hatched lines indicate uncertain former subduction. Continental crust, transitional continental crust, and arc-related crust are shown without pattern. Representative geologic cross sections across parts of slices C and D are marked with transparent red ovals and within slices B and E are shown by dotted lines. (i ) Cross section of the Papuan peninsula and D’Entrecasteaux Islands modified from Little et al. (2011), showing the obducted ophiolite belt due to collision of the Australian (AUS) plate with an arc in the Paleogene, with later Pliocene extension and exhumation to form the D’Entrecasteaux Islands. (ii ) Cross section of the Papuan peninsula after Davies & Jaques (1984) shows the Papuan ophiolite thrust over metamorphic rocks of AUS margin affinity. (iii ) Across the Papuan mainland, the cross section after Crowhurst et al. (1996) shows the obducted Marum ophiolite and complex folding and thrusting due to collision of the Melanesian arc (the Adelbert, Finisterre, and Huon blocks) in the Late Miocene to recent. (iv) Across the Bird’s Head, the cross section after Bailly et al. (2009) illustrates deformation in the Lengguru fold-and-thrust belt as a result of Late Miocene–Early Pliocene northeast-southwest shortening, followed by Late Pliocene–Quaternary extension. Abbreviations as in Figure 2, in addition to NI, New Ireland; SI, Solomon Islands; SS, Solomon Sea; (U)HP, (ultra)high-pressure.

    Earthquake Report: New Zealand!

    This was a very busy week for me, so I missed reporting on this series of earthquakes offshore the North Island of New Zealand. I did put together an interpretive Earthquake Report poster for these earthquakes for my general education earthquakes class, of which I present below. Initially there was a M 5.8 earthquake (USGS website for M 5.8), with a M 7.1 about 18 hours later (USGS website for M 7.1). Here is a kml that includes these, as well as the aftershocks to date. We have a great deal of information about the plate tectonics of this region. These earthquakes are along the northern Hikurangi margin, which is a convergent plate boundary formed by the subduction of the Pacific plate westward beneath the Australia plate.

    The M 5.8 earthquake had an hypocentral depth of ~22 km, which (based upon the slab contours) places this earthquake in the downgoing Pacific plate. As plates subduct, they undergo extension as the deeper part of the plate pulls down on the shallower part of the plate. This “slab tension” can result in extensional (normal) earthquakes. Also, due to various reasons, the downgoing plate can be bent or flexed downwards. This can happen oceanward of the subduction zone or beneath the tip of the subduction zone fault, after the downgoing plate has subducted. If this downgoing plate flexes downward, the upper part of the lithosphere undergoes extension and extensional/normal earthquakes occur.

    Initially, the M 7.1 earthquake was given an hypocentral depth of ~300 km. This was confusing as it did not seem to be anywhere near oceanic lithosphere as estimated by people who study this region. Within minutes, the USGS hypocentral depth was re-determined to be ~19 km. Comparing this depth with the slab contours (Hayes et al., 2012), this earthquake also occurred in the Pacific plate. The M 5.8 is probably a foreshock for the M 7.1, but we also might consider that the M 5.8 triggered the M 7.1 earthquake. Others will need to analyze these earthquake more before this detail can be worked out.

    Below is my interpretive poster for this earthquake. This map shows the slab contours (an estimate of the subduction zone plate interface). These contours are estimated by Hayes et al., (2012). The hypocentral depth is 19.0 km, which is deeper than the slab depth according to Hayes et al. (2012), which is probably about 15 km. This earthquake is clearly in one of the downgoing slabs of the Pacific plate.

    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. The moment tensor shows northwest-southeast tension. If this is due to slab tension, then the slab would be dipping to the northwest, or the southeast. This tension may also be due to some form of bending in the slab, but it is difficult to tell given these limited data. Most of the recent seismicity in this region is associated with convergence along the New Britain trench or the South Solomon trench.

      I include some inset figures.

    • In the upper right corner is a generalized tectonic map from Mike Norton (see poster for attribution).
    • In the lower right corner is a cross section of the southern Kermadec Trench. This was produced by Jack Cook at the Woods Hole Oceanographic Institution. The Lousiville Seamount Chain is clearly visible in this graphic.
    • In the upper left corner I include a map from the USGS Open File Report for this region of the world (Benz et al., 2011). The map shows seismicity colored vs. depth, some slab contours, and the location of the two cross sections that are also included in this poster (J-J’ and K-K’).


      Here are two figures from Jascha Polet, a seismologist at Cal Poly Pomona.

    • Here is a map showing seismicity and moment tensors for historic earthquakes, as well as the m 7.1 earthquake.

    • Here is a cross section showing the same seismicity and moment tensor data.

    • Here are some cross sections from Scherwath et al. (2010), located close to where the M 7.1 earthquake occurred. Note how the M 7.1 occurred at a location where the downgoing Pacific plate is indeed bending downward near the M 7.1 hypocenter.

    For more on the graphical representation of moment tensors and focal mechanisms, check this IRIS video out:

    Earthquake in Izu-Bonin!

    Early this morning (luckily I was not awake at the time) we had a really deep (~677km) M = 7.8 extensional earthquake along the Bonin Trench in the western Pacific. Here is the USGS page for this earthquake.

    Here is a map that shows the epicenter in red (due to the depth), along with the moment tensor for this earthquake. I also plot the general location of the 1944 Tōnankai and 1946 and Nankai Earthquakes. I also include a cross section for this subduction zone (also from the wikipedia page).

    Here I have plotted the slab contours (Hayes et al., 2012).

    Here is a regional tectonic map from the wikipedia site on this region.

    There is more about the regional tectonics on the USGS page here.

    Here is a map showing the general locations of the 1946 Tōnankai and 1944 Nankai Earthquakes.

    Animation of Seismicity along the Alaska/Aleutian Arc

    This took me just a few minutes to put together. I searched the USGS earthquakes website with a rectangular search area for earthquakes greater than M = 6.0, downloaded the google Earth kml file with epicenters colored by depth (and animated), and recorded this animation to a video capture application. Here is the search that I used to get these data.

    Here is a map showing the epicenters in the following animations. Check out my post about the M 6.7 earthquake from early this morning. I include more information about the regional tectonics on that page (and provide links to other sources too).

    Here are two videos that show animations of the seismicity from 1960 until today. The first one leaves the epicenters on the screen for the entire animation.

    This animation has a moving time window (~1 year), so that 1 year after the earthquake, it is removed from the map.

    HSU Geology 106 Earthquake Country 2013.05.03

    It was a great honor to be a guest lecturer today in the class that initially inspired me to get into the field of geology.

    I have placed the presentation and the videos associated with the presentation here.

    If you were in the class and have more questions, please contact me via email here: quakejay @ gmail.com (i put the spaces in my email to prevent bots from reading it).

    Thanks for your permitting me to tell you about my research project and how it relates to geologic hazards globally. Remember, if I can do it, so can you. Just take more classes, ones that you are excited to take, and there is no limit to what you can do.