Earthquake Report: Bakersfield!

Today we had a M 4.9 earthquake northwest of Bakersfield in the middle of the southern San Joaquin Valley. At first, it looked like there were no faults here because I was looking at the USGS fault and fold database. However, upon looking at the California Geological Survey Jennings (1977) map, updated by Saucedo et al. (2010), I found that there are some pre-Quaternary faults mapped in this region. Here is the online fault map. Here is the USGS website for this earthquake, that was broadly felt across the entire region. Based upon the PAGER alert, there is actually a 24% chance of between 1 and 10 casualties. The felt area may include as many as 2.2 million people.
Here is my interpretive map that shows the epicenter, along with the shaking intensity contours. These contours use 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. I plot the moment tensor and I interpret this earthquake to be a northwest striking right lateral strike-slip earthquake. This is based upon the mapped faults in the region and their sense of motion, which is synthetic to the San Andreas fault system to the west. I used this kml to make this map.
I include inset maps of the Saucedo et al. (2010) fault map, with larger scale maps for the southern San Joaquin Valley (upper center) and Bakersfield (lower left) areas. I plot the epicenter for today’s earthquake as a blue dot. In the Bakersfield scale map (lower left corner), one can see that there are several pre-Quaternary faults mapped in this region. Also, the other, more recently active faults are named and labeled with their most recent event age (e.g. the San Andreas to the west ruptured in 1857 and the White Wolf fault to the southeast ruptured in 1952).
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.


Also as an inset, I include a map that shows earthquake epicenters for earthquakes of magnitude M ≥ 4.0 in this region. Here is the query that I used to create this map. There is a paucity of seismicity in this region.


Here are the two Saucedo et al. (2010) large scale maps. I made these using this fault interface.




Here is a map from Jascha Polet (Cal Poly Pomona Seismologist). Dr. Polet plots moment tensors and seismicity in this region, showing the lack of seismicity in this region. earthquakes are colored vs. depth (most earthquakes in this region are deep).


This earthquake was felt broadly, including my friends on the east side of the Sierra Nevada mountains. H/T Cindy Pridmore (California Geological Survey) who shared with me this paper by Namson and Davis (1988). Cross section in Figure 11 is just north of this region. Pridmore suggested that this earthquake may have ruptured the deep basement (22 km epicenter).

Earthquake Report: Antarctic plate!

We just had an interesting mid-plate earthquake (not along a plate boundary). Hat Tip to Jascha Polet, who pointed out this is in the region of the 1998 M 8.1 earthquake, one of the largest strike-slip and mid-plate earthquakes ever recorded. I then learned that the seismicity in this region may be related to isostatic adjustments in the Antarctic plate! Here is the USGS website for this M 5.9 strike-slip earthquake.
Here is my interpretive map. I plot the USGS location as a yellow star. I also include some other figures as insets. I will discuss these below. I include a figure from Kreemer and Holt (2000) that shows focal mechanisms for earthquakes in the region plotted on a bathymetric map (seafloor topography). I also include a few maps from Das and Henry (2003). About a week ago, there was an earthquake along the Australia-Pacific plate boundary to the northeast of this earthquake (here is the Earthquake Report for that earthquake).
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.


Here is the earthquake report interpretive poster for the recent earthquake to the northeast.


Here is the Kreemer and Holt (2000) figure 1, showing the focal mechanisms for earthquakes along the regional plate boundary faults, as well as the focal mechanisms from the earthquakes in the region of the 1998 M 8.1 earthquake. I include their figure caption below in blockquote.

Focal mechanisms are from the Harvard CMT catalog (1/77-6/99). The black focal mechanisms indicate the 1998 Antarctic plate event with (some of) its aftershocks. Bathymetry is from Smith and Sandwell [1994]. Transform locations are derived from satellite altimetry by Spitzak and DeMets [1996]. MRC is the Macquarie Ridge Complex and TJ is the Australia-Pacific- Antarctica triple junction.

Here is the first figure from Das and Henry (2003). They plot the epicenters and focal mechanisms for earthquakes from the 1998 swarm overlain upon the gravity anomaly map. I include their figure caption below in blockquote.

The 25 March 1998 Antarctic plate earthquake (with a seismic moment of 1.3  1021 N m). (a) Relocated aftershocks [Henry et al., 2000] for the period 25 March 1998 to 25 March 1999 are shown as diamonds, with the main shock epicenter shown by a star. Only those earthquakes which are located with the semimajor axis of the 90% confidence ellipse 20 km are shown. International Seismological Centre epicenters for the period 1 January 1964 to 31 July 1997 are shown as circles. Marine gravity anomalies from an updated version of Sandwell and Smith [1997], illuminated from the east, with contours every 20 mGal, are shown in the background in the epicentral region. Selected linear gravity features are identified by white lines and are labeled F1–F6. F1, F2, and their southward continuation to join F1a compose the George V fracture zone. F4–F6 compose the Tasman fracture zone. (b) An expanded view of the region of the aftershocks. The relocated aftershocks in the first 24 hours are shown as diamonds; the rest are shown as circles. The 90% confidence ellipses are plotted for the locations; earthquakes without confidence ellipses were not successfully relocated and are plotted at the National Earthquake Information Center (NEIC) locations. The yellow star shows the NEIC epicenter for the main shock, with the CMT mechanism of solution 5 from Henry et al. [2000]. Available Harvard CMT solutions for the aftershocks are plotted, linked with lines to their centroid locations and then to their relocated epicenters, and are identified by their dates (mmddyy). The location of the linear features identified on Figure 6a are shown by black arrows. (c) Final distribution of moment release for preferred solution 8 of Henry et al. [2000]. There are the same gravity anomalies, same linear features, and same epicenters as Figure 6b except that now only earthquakes which are located with the semimajor axis of the 90% confidence ellipse 20 km are shown. Two isochrons from Mu¨ller et al. [1997] are plotted as white lines. Superimposed graph shows the final moment density, with a peak density of 1.25  1019 N m km 1. Regions of the fault with 15% of this maximum value are excluded in this plot. The baseline of the graph is the physical location of the fault. The spatial and temporal grid sizes used in the inversion for the slip were 5 km 5 km and 3 s, respectively.

This is the continuation of the above figure. This shows their interpretation of the faults that slipped during this 1998 earthquake series. In their paper, Das and Henry (2003) discuss the relations between main shocks and aftershocks. At the time, the 1998 earthquake “was the largest crustal submarine intraplate earthquake ever recorded, the largest strike-slip earthquake
since 1977, and at the time the fifth largest of any type worldwide since 1977” (Das and Henry, 2003). This M 8.1 earthquake was interesting because it crossed the fracture zones that trend N-S in the area. This is especially interesting because this is also what happened during the 2012 Sumatra Outer Rise earthquakes. Toda and Stein (2000) model the coulomb stress changes associated with different slip models from the M 8.1 earthquake to estimate if the aftershocks were triggered by the main earthquake. I include their figure caption below in blockquote.

(d) Principal features of the main shock rupture process [from Henry et al., 2000]. Arrows show location and directivity for the first and second subevents. Arrows are labeled with start and end times of rupture segments. Focal mechanisms are shown for the initiation, the first subevent plotted at the centroid obtained by Henry et al. [2000], and the second subevent. (The second subevent is not well located, and the centroid location is not indicated.) The cross shows the centroid location of moment tensor of the total earthquake obtained by Henry et al. [2000], and the triangle shows the Harvard CMT centroid. The same aftershock epicenters as Figure 6c are shown. Linear gravity features are shown as shaded lines, and probable locations of tectonic features T1a and T3a associated with the gravity features F1a and F3a are shown as shaded dashed lines. (See Henry et al. [2000] for further details.)

Here is the Kreemer and Holt (2000) figure that shows their interpretation of the stress field. The first figure below shows their determination of the strain rates as modeled from tectonic stresses at the plate boundaries. Note the low strain rate in the area near the M 8.1 earthquake (plotted as a focal mechanism). The second figure below shows the averaged minimum horiztonal deviatoric stress field caused by by flexure in the crust following the last ice age. Based upon their analyses, they attribute the earthquake to possibly be the result of stresses in the Antarctic plate following the last deglaciation. I include their figure caption below in blockquote.

a) Grid in which a strain rate field is determined associated with the accommodation of relative plate motions [DeMets et al., 1994]. These motions are applied as boundary velocity conditions,
illustrated by the grey arrows. b) Principal axes of the strain rate field for the region where the Antarctic event occurred (indicated by CMT focal mechanism). Model strain rates in this
region are one order of magnitude lower than along the surrounding ridges and transforms.


Principal axes of the vertically averaged minimum horizontal deviatoric stress field caused by gravitational potential energy differences within the lithosphere. CMT focal mechanism of Antarctic plate earthquake is shown. a) ‘ice-age’ simulation. b) change in stress tensor field from ‘ice-age’ to present day determined by taking the tensorial difference between the two solutions.

Earthquake Report: Macquarie – New Zealand

There was an earthquake along the plate boundary between the Australia plate to the west and the Pacific plate to the east, just south of New Zealand. Here is the USGS website for this M 6.2 earthquake. Yesterday on 2015/02/14 there was a widely felt earthquake with a smaller magnitude of M = 5.8 near Christchurch, on the northern end of the South Island, New Zealand. The M 5.8 was in the region that is still rebuilding following the Darfield-Canterbury earthquake series in 2010-2011. Here is my earthquake report for the M 5.8 earthquake.
Below is a map that has the USGS seismicity from 1900-2016, for earthquakes of magnitude M ≥ 6.0, plotted as orange circles. Here is the USGS query that I used to make this map. I include the USGS Modified Mercalli Intensity contours, made using this USGS kml file in Google Earth. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here.
I placed the moment tensors, that exist (post ~1970), for each of the earthquakes larger than M = 7.0. I include relative motion vectors for each moment tensor, based upon my interpretation from the mapped faults along this plate boundary. There is not a USGS moment tensor for the M 6.2 earthquake, so I place a generic strike-slip focal mechanism and place it in the orientation that aligns with the strike of the plate boundary near the epicenter.
There is a north-northeast striking ridge east of the Puysegur Trench, that trends southwest and turns into the Macquarie Ridge. At this latitude, the Trench to the west is mapped as a subduction zone. The plate convergence is oblique to the plate boundary. The earthquakes that are rupturing in the region of today’s earthquake, that have USGS moment tensors, all have orientations aligned with this ridge. Some earthquakes have strike-slip moment tensors and some have compressional moment tensors. The compressional earthquakes may be along subduction zone faults or simply thrust/reverse faults in the crust. There are no deep earthquakes in this region, so the subduction zone may not be active or existent in this region. The strike-slip earthquakes may be rupturing along forearc sliver faults, possibly there due to strain partitioning from the oblique convergence.
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.

    Here are two examples of earthquakes with similar moment tensors as today’s M 6.2 earthquake.

  • 1981.05.25 M 7.6 (NE-SW strike-slip
  • 1994.01.03 M 6.1 (NE-SW strike-slip
  • 2007.09.30 M 6.6 (E-W compressional moment tensor)
  • 2008.04.26 M 6.1 (E-W compressional moment tensor)


    Inset maps in the above interpretation poster include the following:

  • In the upper left corner I include the tectonic map from Benz et al. (2011) which shows how the East Vergent subduction zone at the Hikurangi Trench transforms (pun intended) via the Alpine fault (a right lateral strike-slip fault) to the West Vergent subduction zone at the Puysegur Trench. Fault vergence refers to the up-dip direction of the fault.

  • In the lower left corner I include the tectonic interpretation map from Daczko et al. (2004). Daczko et al. (2005) use volcaniclastic rocks on Macquarie Island to interpret the tectonic history of this region as it relates to the evolution of a spreading ridge as it converts into a transform plate boundary. I include the text from their figure below as a blockquote.

  • Location map of Macquarie Island and the Australian-Pacific transform plate boundary south of New Zealand. The thickest black line shows the extent of the Macquarie Ridge Complex (MRC). Crust formed by Australian-Pacific spreading along the (now extinct) Macquarie spreading ridge between ca. 40 and ca. 10 Ma is stippled. Filled triangles along the plate boundary are subduction zones; open triangles (in the Hjort region) represent incipient subduction (Meckel, 2003). Light gray illustrates regions ≤2000 m below sea level. Present plate boundaries are shown as black lines. Past plate boundaries are shown as dashed black lines. Fracture zones (FZ) are shown as thin black lines. Azimuthal equidistant projection centered at 60°S, 180°E (after Daczko et al., 2003).

  • In the upper right corner, I include the detailed tectonic map from Daczko et al. (2005) that shows how the spreading centers are now oritented in this region. Note that they query how the plate margin converts into the subduction zone to the north (question marks on their map). I include the text from their figure below as a blockquote.

  • Plate tectonic reconstruction for Chron 5 old – C5o (10.9 Ma). Stippled crust formed at the Macquarie spreading ridge (now a transform boundary), light gray crust formed at the Southeast Indian spreading ridge (still active), medium gray crust formed at the Pacific-Antarctic spreading ridge (still active), and dark gray crust formed at the Tasman spreading ridge (extinct). Magnetic anomaly picks are represented by open circles (C13o, 33.3 Ma); filled stars (C8o, 26.6 Ma); filled triangles (C6o, 20.1 Ma); filled hexagons (C5o, 10.9 Ma); plus symbols (all other ages). Thin black lines are lineaments (continental margins and fracture zones) interpreted from the satellite-derived free-air gravity field (Sandwell and Smith, 1997). Medium black lines are the plate boundaries active at 10.9 Ma (double line = spreading ridge; single line = transform). Thickest black lines are the Warna (Australian plate) and Matata (Pacifi c plate) fracture zones (FZ). These two fracture zones are linked by the only left lateral transform fault within the Macquarie crust as shown by the offset of C18o picks (open rectangles). The dashed lines show our interpreted position of the fracture zones allowing for a minor component of deformation of the fracture zones during Neogene and Quaternary transpression along the Australian-Pacific plate boundary. The arrow points to the inside corner of the ridge-transform intersection where we interpret the crust of Macquarie Island (MI) most likely formed.

    Something else of note is the M 8.2 earthquake, one of the largest strike-slip earthquakes ever recorded. This earthquake happened on 1989.05.23. Interestingly, this earthquake has some shared characteristics with the 2012 earthquake series offshore of Sumatra in 2012. More on this later as I need to get ready for X-Files. Here are a couple report pages for the 2012 earthquakes.

  • 2012.04.11 M 8.6
  • 2012.04.11 M 8.6 (update #1)

Earthquake Report: New Zealand!

Early on Valentines, those in New Zealand got a valentine from mother Earth. Here is the USGS website for this M 5.8 earthquake, which was felt broadly across the region.
This earthquake, and an M 4.1 aftershock, occurred in the region of a M 6.1 earthquake from which the economy is still recovering.
Below is a map that includes seismicity from the past month (note the M 5.2 to the north of today’s earthquake). I plot the epicenter of the M 5.8 as an orange circle and I place the USGS moment tensor with my interpretation for the sense of motion for this earthquake. The M 5.2 is along strike, southwest of a M 6.8 earthquake from 2013. Here is my report on this M 6.8 earthquake. In the upper left corner, I also include a figure from Castelltort et al. (2012) that shows the regional tectonics. In the lower left corner I include the tectonic map from Benz et al. (2011) which shows how the East Vergent subduction zone at the Hikurangi Trench transforms (pun intended) via the Alpine fault (a right lateral strike-slip fault) to the West Vergent subduction zone at the Puysegur Trench. Fault vergence refers to the up-dip direction of the fault.
In the earthquake report poster I also include a map from GNS Science that shows the relative motion of different locations in New Zealand. Along the northern part of the South Island, the direction of motion is ardently sub parallel to the Alpine fault. In the lower right corner, I include a fault map from Mike Norton as published on Wikipedia. In the upper right corner I include a map showing the seismicity from the past century with color representing depth, from GeoNet. The subduction zones to the north and south are well evidenced by these depth plots. Also, the seismicity in the northern 2/3 of the South Island are shown to be shallow, as expected for strike slip fault systems. The shallowness of these earthquakes contributes to how well they are felt to the public.
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.


This M 5.8 earthquake was broadly felt. Here is a map that shows the Modified Mercalli Intensity (MMI) contours for this earthquake. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here.


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

GNS (New Zealand’s USGS) has a wonderful compilation of educational material for the tectonics of New Zealand. I include some of their maps and videos below.
Here is a video from GNS that describes the tectonics of New Zealand. This is a link to download the embedded video below (54 MB mp4).


On 2010.09.13, there was a M 7.0 earthquake (aka the Darfield Earthquake) out in a field and here is the USGS website for this earthquake. This earthquake was felt across all of New Zealand and felt strongly in Christchurch. Below is the “Did You Feel It?” map from the 2010 earthquake. There was not much damage because the location was “out in a field” (i.e. it was in rural NZ). Below is a blockquote from the USGS website for this M 7.0 earthquake.

Two people seriously injured, six bridges and many buildings damaged in the Christchurch area. About 30 km of right-lateral surface faulting with a maximum offset of more than 5 m was observed southeast of Darfield. Liquefaction caused damage at Bexley, Kaiapoi and in parts of Christchurch. Landslides were observed along the Rakaia River and the Port Hills area. Maximum intensity IX in the Christchurch-Greendale area and felt (VI) in much of Canterbury. Felt throughout New Zealand. Detailed information about this earthquake is available on the New Zealand GeoNet website at http://www.geonet.org.nz.



Here is a video that GNS put together to document the Darfield Earthquake. This is the yt link for the embedded video below.

However, on 2011.02.21, there was a M 6.1 earthquake that occurred in a location that led to strong ground shaking in the urban center of Christchurch. Here is the USGS website for this M 6.1 earthquake. The city and people are still recovering from this earthquake. The economics of the region will be hampered for decades to come. The “Canterbury Earthquake” occurred along a fault that did not rupture the surface, while the M 7.0 Darfield Earthquake had. GNS has more resources about this earthquake sequence here.
Here is a map prepared by GNS Science that shows the M 7.0 and M 6.1 mainshocks, as well as the aftershocks from this paired sequence.


Here is a map that GNS put together to help visualize their estimates of slip along the blind fault that ruptured in February of 2011. I include their text as a blockquote below the map.

Based on data from GPS stations, satellite radar images, seismographs and strong-motion recorders, the fault that caused the 22 February earthquake lies within about six kilometres of the city centre, along the southern edge of the city. The fault rupture (Fig. 1) was about 14 kilometres long, and extends east-northeast from Cashmere to the Avon-Heathcote estuary area. The fault plane extends a few kilometres offshore, but not much fault movement occurred beneath the ocean.
This Google map image shows the fault plane (rectangular area) across the southern part of Christchurch and northern Port Hills. Colours on the fault plane indicate the amount of slip between the two sides of the fault (see Fig. 2). The contour lines indicate the amount (in mm) the land has risen (blue contours) or subsided (red contours) due to the slip on the fault. The white line is the contour where there was no change in height. The red, green and yellow coloured symbols show some of the GPS stations whose displacements were used to derive the fault slip model.
The top of the fault lies at a depth of about a kilometre beneath the surface, and the rupture extends down along the fault plane for about seven kilometres. The fault is not a vertical cut through the earth, but rather it dips towards the south at an angle of about 65 degrees from the horizontal. The main part of the fault thus lies beneath the northern edge of the Port Hills.

I put together an animation that shows the earthquakes from New Zealand with magnitudes greater than or equal to M 6.5. Above is a screen shot of all these earthquakes and below is the animation. This is the kml file that I used to make this animation. Here is the query that I used to make this animation.


Here is a link to the mp4 file embedded below (2 MB mp4).


Here is an interesting (long) video about the Canterbury earthquake sequence. This is the yt link for the embedded video below.

Earthquake Report: Chile!

The aftershocks from the 2015.09.16 M 8.3 Chile earthquake continue. Today was an earthquake with magnitude M = 6.3. Here is the USGS website for today’s earthquake. I presented a comprehensive review of Chile’s historic seismicity, as well as the seismicity from 2015, here. I also presnt a summary of the seismicity in this region from 2000-2015 here.
Below is my interpretive map for today’s earthquake. I have plotted the epicenters (using the USGS earthquake feed kml) for the past 30 days with magnitudes 2.5 or greater, with color representing depth. I also include the slab depth contours from Hayes et al. (2012). These are the depth contours for the fault interface of the subduction zone. I discuss this further in my Chile 2015 Earthquake Report. The M 6.3 earthquake has a depth of ~31.5 km, which modestly fits the slab contours (the three shallowest contours are labeled in red). I also plot the historic earthquake slip regions in green (Beck et al., 1998). I include the earthquake intensity, using the USGS Modified Mercalli Intensity Scale. The largest MMI intensity contour is VI. I plot the location of the Juan de Fernandez Ridge in yellow, which may be controling the segmentation of faulting along this subduction zone (von Huene et al., 1997).
I placed a moment tensor / focal mechanism legend in the upper right corner of the map. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.


Here is the map that I include as an inset map. This map shows the extent of the 2010 and 2015 subduction zone earthquakes in Chile.


I also placed a map from the USGS earthquake summary poster in the lower left corner (Hayes et al., 2015). The red rectangle shows the general location of the main map. The USGS plot epicenters for historic earthquakes on the map, along with the slab contours (Hayes et al., 2012). There are two cross sections plotted on the map (C-C’ on the north and D-D’ on the south). The USGS plot these hypocenters along these two cross sections and I include those. The 2015 earthquake patch is just south of cross section C-C’.

    Here is the legend for the hypocentral cross sections.

In the lower right corner I place a cross section from Melnick et al. (2006). This shows the prehistoric earthquake history on the left and a cross section of the subduction zone on the right. This cross section is in the region of the 2010 subduction zone earthquake. Above the Melnick et al. (2006) figure I include the space-time diagram from Beck et al. (1998 ) showing the along strike length of prehistoric earthquakes in the central subduction zone. The map above shows these prehistoric rupture strike lengths as green lines (labeled with green labels). The 2015 earthquake series ruptured past the southern boundary of the 1943 earthquake and about 30% into the 1922 earthquake region. There is a small gap between the 2010 and 2015 earthquake series, which aligns with the Juan Fernandez Ridge (a fracture zone in the Nazca plate; von Huene et al., 1997; Rodrigo et al., 2014). This fracture zone appears to be a structural boundary to earthquake slip patches (subduction zone segmentation), at least for some earthquakes. Beck et al. (1998 ) show that possibly two earthquakes ruptured past this boundary (1730 A.D. and possibly 1647 A.D., though that is queried). This segment boundary appears to be rather persistent for the past ~500 years.

Here is the 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 ).

References:

Earthquake Report: Taiwan Update #1

People have been busy documenting the damage from the M 6.4 earthquake in Taiwan. Here is my initial Earthquake Report for this earthquake.

    Here are other blogs on this subject:

  • Temblor

Here is the interpretive map that I put together for my initial Earthquake Report.


Here is a large scale (zoomed in) view of the region with the Modified Mercali Intensity (MMI) contours plotted. Find out more about the MMI scale here and here.


Here is a great general overview of the tectonics of the region from Shyu et al. (2005). I include their figure caption below the image as a blockquote.

A neotectonic snapshot of Taiwan and adjacent regions. (a) Taiwan is currently experiencing a double suturing. In the south the Luzon volcanic arc is colliding with the Hengchun forearc ridge, which is, in turn, colliding with the Eurasian continental margin. In the north both sutures are unstitching. Their disengagement is forming both the Okinawa Trough and the forearc basins of the Ryukyu arc. Thus, in the course of passing through the island, the roles of the volcanic arc and forearc ridge flip along with the flipping of the polarity of subduction. The three gray strips represent the three lithospheric pieces of Taiwan’s tandem suturing and disarticulation: the Eurasian continental margin, the continental sliver, and the Luzon arc. Black arrows indicate the suturing and disarticulation. This concept is discussed in detail by Shyu et al. [2005]. Current velocity vector of the Philippine Sea plate relative to the Eurasian plate is adapted from Yu et al. [1997, 1999]. Current velocity vector of the Ryukyu arc is adapted from Lallemand and Liu [1998]. Black dashed lines are the northern and western limits of the Wadati-Benioff zone of the two subducting systems, taken from the seismicity database of the Central Weather Bureau, Taiwan. DF, deformation front; LCS, Lishan-Chaochou suture; LVS, Longitudinal Valley suture; WF, Western Foothills; CeR, Central Range; CoR, Coastal Range; HP, Hengchun Peninsula. (b) Major tectonic elements around Taiwan. Active structures identified in this study are shown in red. Major inactive faults that form the boundaries of tectonic elements are shown in black: 1, Chiuchih fault; 2, Lishan fault; 3, Laonung fault; 4, Chukou fault. Selected GPS vectors relative to the stable Eurasian continental shelf are adapted from Yu et al. [1997]. A,Western Foothills; B, Hsueshan Range; C, Central Range and Hengchun Peninsula; D, Coastal Range; E, westernmost Ryukyu arc; F, Yaeyama forearc ridge; G, northernmost Luzon arc; H, western Taiwan coastal plains; I, Lanyang Plain; J, Pingtung Plain; K, Longitudinal Valley; L, submarine Hengchun Ridge; M, Ryukyu forearc basins.

This figure from Shyu et al. (2005) shows their interpretation of the different tectonic domains in Taiwan. This is a complicated region that includes collision zones in different orientations as the Okinawa Trough, Ryukyu Trench, and Manila Trench (all subduction zones) each intersect beneath and adjacent to Taiwan. I include their figure caption below the image as a blockquote.

Map of major active faults and folds of Taiwan (in red) showing that the two sutures are producing separate western and eastern neotectonic belts. Each collision belt matures and then decays progressively from south to north. This occurs in discrete steps, manifested as seven distinct neotectonic domains along the western belt and four along the eastern. A distinctive assemblage of active structures defines each domain. For example, two principal structures dominate the Taichung Domain. Rupture in 1999 of one of these, the Chelungpu fault, caused the disastrous Chi-Chi earthquake. The Lishan fault (dashed black line) is the suture between forearc ridge and continental margin. Thick light green and pink lines are boundaries of domains.

This map from Shyu et al. (2005) shows the earthquake slip regions for proposed earthquake scenarios. I include their figure caption below the image as a blockquote.

Proposed major sources for future large earthquakes in and around Taiwan. Thick red lines are proposed future ruptures, and the white patches are rupture planes projected to the surface. Here we have selected only a few representative scenarios from Table 1. Earthquake magnitude of each scenario is predicted value from our calculation.

Here is Table 1 from Shyu et al. (2005).




Here is a map from Jascha Polet, Seismologist at Cal Poly Pomona. Dr. Polet plotted focal mechanism from historic earthquakes from this region, with the M 6.4 earthquake enlarged. These focal mechanisms are colored for depth. More on focal mechanisms is presented on my initial Earthquake Report here.


The Institute of Earth Sciences, Academia Sinica, has produced some excellent visualizations from this devastating earthquake. Below are a series of their visualizations.
This map shows their estimate of deformation due to displacement on the fault that ruptured during this earthquake. This is not a measurement of vertical deformation, but a numerical model.


This map shows their modeled estimates for ground shaking using the Modified Mercalli Intensity Scale. More on the MMI can be found here.


Another measure of the effects from an earthquake is a quantification of Peak Ground Acceleration (PGA). This refers to the maximum accelerations possibly felt at the ground surface due to shaking from the earthquake. PGA is measured in units of gravity (g) where 1 g (gal) = acceleration of gravity at sea level. While gravity varies spatially, so is generally not this value at sea level, is defined to be 9.8 m/sec^2. In the figure below, 1000 = 1 g.


Here is a cool low angle oblique version of the MMI shake map.


The following three images show the east, north, and vertical components of motion measured on seismopraphs in Taiwan.






And, my favorite, shows the seismic waves propagating across the landscape.

This map from here shows the basement geology of Taiwan. Note the accretionary belts, including the forearc basin. This is a compilation from Teng et al. (2001) and Hsiao et al. (1998) as presented in Ustaszewski et al. (2012).


This shows how as additional DYFI reports were submitted, there were more reports from further distances. The upper image was acquired yesterday morning (Pacific Time) and the lower image was acquired last night (Pacific Time).



Earthquake Report: Taiwan!

We just had an earthquake in southern Taiwan. Here is the USGS website for this M 6.4 earthquake. In April 2015, there was a series of earthquakes in the northeast of Taiwan. Here is my earthquake report for those earthquakes. In 1999 there was a devastating M 7.7 earthquake in Taiwan called the Chi Chi earthquake. Here is a brief summary of this earthquake from the USGS.
Below is a map that has the epicenter located as a yellow star, with Modified Mercalli Intensity Scale contours plotted. Here is a map showing the shaking intensity that uses the Modified Mercalli Intensity (MMI) scale. The MMI scale is a qualitative scale of the ground motions. There is more about the MMI here. I place the USGS moment tensor on the map. This moment tensor suggests that this is a compressional earthquake with slight oblique motion. It is well resolved as a double couple (98%), probably due to its shallow depth. Currently, the depth is listed as 10 km (though this is a default depth, so it is probably not 10 km).
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.
Here is an update to this report, with more background material about the tectonics in the region.
More on the complicated tectonics of this region can be found here.


This earthquake was felt broadly, including the China mainland. The colors in this map are also using the MMI color scale and are based upon reports from people who used the USGS “Did You Feel It?” (DYFI) web site. There is more about this DYFI system here.


The MMI contours in the above map are based upon numerical simulations of ground motions based upon “Ground Motion Prediction Equations.” The GMPEs are empirical relations between ground shaking intensity and distance from the earthquake. These relations are regressed for thousands of earthquakes and filtered for different geological settings. Below is a plot of the DYFI report responses (green dots are individual reports). Also shown are the GMPE estimates for ground motions based upon the models developed for the central and eastern US (orange) and for California (green). Does Taiwan “behave” more like the central and eastern US, or more like California?


Here is an oblique view of the plate configuration in this region. This is from Chang (2001).


Here is a great interpretation showing how the Island of Taiwan is being uplifted and exhumed. This is from Lin (2002).


Needless to say, this is an excellent map showing the complicated faulting of this region. This is from Theunissen et al. (2012).


Here is another tectonic interpretation map from here.

Here is a map that shows the three earthquake epicenters from 2015 as they relate to these plate boundary faults.


Here is the USGS poster for seismicity from 1900-2012 in this region. USGS Open File Report 2010-1083-M, Smoczyk et al. (2013).


Here is a plot of focal mechanisms from this region, as prepared by Jascha Polet, Professor of Geophysics at Cal Poly Pomona.