Earthquake Report: El Salvador

There were a couple interesting earthquakes offshore of El Salvador “today.” These earthquakes occurred along the Middle America Trench, a low spot in the ocean formed by the subduction of the Cocos plate beneath the Caribbean plate (a convergent plate boundary). The subduction zone here typically generates earthquakes that are the result of horizontal compression (e.g. thrust or reverse earthquakes). Due to the slightly oblique plate convergence, along with preexisting structures (?), there is a large strike-slip fault system in the upper plate (the Caribbean plate) here. These are called forearc sliver faults. As these faults step left and right, they create basins. The forearc sliver faults are also co-located with the magmatic arc (the volcanoes formed because of the subducting oceanic lithosphere).

The earthquakes today are interesting because they have extensional moment tensors (a.k.a. “dilatational”). As oceanic crust subducts, it can deform (bend) and experience extension in parts of the crust when it deforms. Also, the slab (another word for the lithosphere or crust) can also experience extension from the crust being pulled down due to gravity (probably one of the major causes of plate motions), called “slab pull.”

These earthquakes may be experiencing extension for the above two reason. Alternatively, these earthquakes could be in the upper plate where the plate is experiencing along-strike (in the direction parallel to the subduction zone fault trench) extension as a result of the forearc sliver faults stretching parts of the upper plate. The orientation of these earthquakes does not preclude either of these interpretations. These earthquakes have default depths, so it is difficult to know if these are in the downgoing slab or if they are in the upper plate (Caribbean plate).

Based on the seismicity from the past century (mostly M ~6 earthquakes in this region), these earthquakes are probably not foreshocks for a larger earthquake. But, a hundred years is far from enough data to really understand ANY fault system. Seismology (and plate tectonics for that matter) is just too young a science to understand these things. Maybe after a couple thousand years we will have enough data to be able to make meaningful forecasts.

There was an earthquake on 2016.11.24 to the southeast of today’s sequence. Here is my report for that earthquake.

Here are the USGS web pages for the earthquakes.

  • 2017-05-09 14:15 M 5.2 This earthquake may be related, but probably not.
  • 2017-05-12 10:41 M 6.2 the mainshock
  • 2017-05-12 10:51 M 5.4 aftershock
  • 2017-05-12 15:22 M 4.7 aftershock

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 MMI contours for the M 6.2. I also include USGS seismicity for the past century. Here is the kml for these USGS earthquakes for magnitudes M ≥ 7.0 from 1917-2017 that I used to make this map.

  • 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. The hypocentral depth of the M 5.5 plots this close to the location of the fault as mapped by Hayes et al. (2012).

    I include some inset figures in the poster.

  • In the lower left corner, I include a subset of figures from Benz et al. (2011). There is a map that shows USGS epicenters with dots colored by depth and magnitude represented by circle diameter. There is also a cross section for this region, just to the northwest of El Salvador. Cross section B-B’ shows the earthquake hypocenters along a profile displayed on the map. Note how the subduction zone dip steepens to the northeast. I place a green star in the general location of today’s M 6.2 earthquake.
  • In the upper right corner is a tectonic summary figure from Funk et al. (2009). The authors suggest that there is a forearc sliver in this region. A forearc sliver is a large plate boundary scale strike-slip fault bounded block that is formed due to strain partitioning. When the relative plate motion at a subduction zone are not perpendicular to the megathrust fault, the motion that is perpendicular is accommodated by the megathrust fault. The plate motion that is not perpendicular to the subduction zone fault is accommodated by strike-slip faults parallel to the strike of the subduction zone fault. The authors place red arrows showing the relative motion along the plate bounding fault along the eastern boundary of this forearc sliver (these are called forearc sliver faults for obvious reasons). A classic example of a forearc sliver fault is the Sumatra fault along the Sunda subduction zone. Forearc sliver faults do not always bound a block like this and are not always parallel to the plate boundary. The Cascadia subduction zone has a series of forearc sliver faults offshore, but these are formed oblique to the plate boundary. For the case in cascadia, the plate margin parallel strain is accommodated by rotating blocks, not rigid blocks. As these blocks rotate in response to this shear couple, they rotate and form strike-slip faults between the blocks (forming “bookshelf” faults). Another plate where relative oblique motion creates rotating blocks is along the Aleutian subduction zone.
  • In the upper left corner, I include a map from CSEM EMSC. This map was prepared for the 2016.11.24 El Salvador earthquake. They plot focal mechanisms for historic earthquakes in this region. One may observe that there are compression mechanisms associated with the megathrust and that there are strike-slip (shear) mechanisms associated with the strike-slip faults on land.


  • Here is the “seismicity of the Earth” USGS series poster for this region. Click on the thumbnail below for the pdf version (13 MB pdf).

  • Here is the focal mechanism from Anthony Lomax. This orientation and sense (compression) more aligns with what we might expect for earthquakes in this region, however, note that this is still a dilatational (extentional) mechanism.

  • Here is the seismicity map and cross sections for the region to the southeast of El Salvador. The subduction zone in the El Salvador region is depicted by cross section B. Note that the subduction zone has a low angle dip in the shallow region of the fault, then steepens to 55 degrees to the east.


  • (A) Earthquake locations are from the National Earthquake Information Center (NEIC) and, in Nicaragua, were recorded by the local Nicaragua network from 1995 to 2003 operated by the Instituto Nicaraguense de Estudios Territoriales (INETER ). (B–E) Earthquake profiles are perpendicular to the Middle America Trench (MAT) and extend to the interior volcanic highlands of Central America. These profiles merge all earthquakes within a 50-km-wide swath along each transect. Seismic activity beneath the volcanic front in Nicaragua is more evident because of more data from local stations of the Nicaraguan seismic network. These shallow crustal earthquakes commonly occur within the upper 30 km of the crust and are concentrated within ~25 km of the active Central America volcanic front (CAVF).

  • Here is the summary figure from Funk et al. (2009). This map shows the detailed fault mapping the authors prepared for their manuscript. Their observations include field mapping and seismic profiles. There are several parts of this forearc sliver fault system that show how the strike-slip system bends and steps. There are restraining bends (where the s-s fault generates compression) and releasing bends (where the s-s fault generates extension). These regions are colored red and green, respectively. In the Marabios En Exchelon segment of this map there are some plate margin obvlique extensional fault bounded basins. These appear to be formed by bookshelf style faulting.

  • Regional tectonic map of Central America emphasizing key structures described in this paper. The El Salvador fault zone (ESFZ) is characterized by a broad right-lateral shear zone accommodating transtensional motion that results in multiple pull-apart basins . A major transition zone occurs in the Gulf of Fonseca, where strike-slip fault zones along the Central American forearc sliver change strike from dominantly east-west strikes in El Salvador to northwesterly strikes in Nicaragua. A proposed restraining bend connects faults mapped in the Gulf of Fonseca with fault scarps deforming Cosiguina volcano and faults of the Central America volcanic front north of Lake Managua . Diffuse and poorly exposed faults parallel to the Central America volcanic front in northern Nicaraguan segment are inferred to represent a young fault boundary in which right-lateral shear is accommodated over a broad zone. This model proposes a young en echelon pattern of strike-slip and secondary faults based on secondary extensional features and fi ssure eruptions along the Marabios segment of the Central America volcanic front. Lake Managua and the Managua graben are interpreted to occur at a major releasing bend in the trend of the Nicaraguan depression and are marked by the curving surface trace of the Mateare fault interpreted from aeromagnetic data. Subsequent right-lateral strike-slip motion related to translation of the Central America forearc sliver may occur along these reactivated normal faults. The Lake Nicaragua segment of the Central America volcanic front is bounded by a normal fault (LNFZ—Lake Nicaragua fault zone) offsetting the Rivas anticline, the southeastward continuation of this normal fault into Costa Rica (CNFZ—Costa Rica fault zone), and a synthetic normal fault (SRFZ—San Ramon fault zone) that we discovered in our survey of Lake Nicaragua. Transverse faults (MFZ—Morrito fault zone, JMFZ—Jesus Maria fault zone) strike approximately east-west across the Central America volcanic front. North-south–trending rift zones are abundant in El Salvador but less common in Nicaragua and may also be controlled by regional east-west extension affecting the northwestern corner of the Caribbean plate.

  • Here is a figure from a recently published summary of subduction zones from Goes et al. (2017). Note the various factors and forces to which a subducting slab is exposed.

  • Schematic diagram showing the main forces that affect how slabs interact with the transition zone. The slab sinks driven by its negative thermal buoyancy (white filled arrows). Sinking is resisted by viscous drag in the mantle (black arrows) and the frictional/viscous coupling between the subducting and upper plate (pink arrows). To be able to sink, the slab must bend at the trench. This bending is resisted by slab strength (curved green arrow). The amount the slab needs to bend depends on whether the trench is able to retreat, a process driven by the downward force of the slab and resisted (double green arrow) by upper-plate strength and mantle drag (black arrows) below the upper plate. At the transition from ringwoodite to the postspinel phases of bridgmanite and magnesiowüstite (rg – bm + mw), which marks the interface between the upper and lower mantle, the slab’s further sinking is hampered by increased viscous resistance (thick black arrows) as well as the deepening of the endothermic phase transition in the cold slab, which adds positive buoyancy (open white arrow) to the slab. By contrast, the shallowing of exothermic phase transition from olivine to wadsleyite (ol-wd) adds an additional driving force (downward open white arrow), unless it is kinetically delayed in the cold core of the slab (dashed green line), in which case it diminishes the driving force. Phase transitions in the crustal part of the slab (not shown) will additionally affect slab buoyancy. Buckling of the slab in response to the increased sinking resistance at the upper-lower mantle boundary is again resisted by slab strength.

  • This is a summary figure showing subducting slabs and where they experience downdip extension vs downdip compression (Goes et al., 2017). Their cross section for the Cocos plate does not show any region of downdip extension. This slightly favors the upper plate forearc along-strike extensional interpretation for today’s earthquakes.

  • Summary of morphologies of transition-zone slabs as imaged by tomographic studies and their Benioff stress state. Arrows on the map indicate the approximate locations of the cross sections shown around the map, with their points in downdip direction. Blue shapes are schematic representations of slab morphologies (based on the extent of fast seismic anomalies that were tomographically resolvable from the references listed). Horizontal black lines indicate the base of the transition zone (~660 km depth). For flattened slabs, the approximate length of the flat section is given in white text inside the shapes. For penetrating slabs, the approximate depth to which the slabs are continuous is given in black text next to the slabs. Circles inside the slabs indicate whether the mechanisms of earthquakes at intermediate (100–350 km) and deep (350–700 km) are predominantly downdip extensional (black) or compressional (white). Stress states are from the compilations of Isacks and Molnar (1971), Alpert et al. (2010), Bailey et al. (2012), complemented by Gorbatov et al. (1997) for Kamchatka, Stein et al. (1982) for the Antilles, McCrory et al. (2012) for Cascadia, Papazachos et al. (2000) for the Hellenic zone, and Forsyth (1975) for Scotia. The subduction zones considered are (from left to right and top to bottom): RYU—Ryukyu, IZU—Izu, HON—Honshu, KUR—Kuriles, KAM—Kamchatka, ALE—Aleutians, ALA—Alaska, CAL—Calabria, HEL—Hellenic, IND—India, MAR—Marianas, CAS—Cascadia, FAR—Farallon, SUM—Sumatra, JAV—Java, COC—Cocos, ANT—Antilles, TON—Tonga, KER—Kermadec, CHI—Chile, PER—Peru, SCO—Scotia. Numbers next to the red subduction zone codes refer to the tomographic studies used to define the slab shapes: 1—Van der Hilst et al., 1991; Fukao et al., 1992; 2—Bijwaard et al., 1998; Hafkenscheid et al., 2001; Hafkenscheid et al., 2006; 3—Fukao et al., 2001; Fukao et al., 2009; Fukao and Obayashi, 2013; 4—Bunge and Grand, 2000; Grand, 2002; 5—Karason and Van der Hilst, 2000; Replumaz et al., 2004; Li et al., 2008; 6—Miller et al., 2005; Miller et al., 2006; 7—Gorbatov et al., 2000; 8—Hall and Spakman, 2015; 9—Van der Hilst, 1989; Grand et al., 1997; Ren et al., 2007; 10—Van der Lee and Nolet, 1997; Schmid et al., 2002; 11—Sigloch et al., 2008; Sigloch, 2011; 12—Simmons et al., 2012; 13—Gorbatov and Fukao, 2005; 14—Van der Hilst, 1995; Schellart and Spakman, 2012; 15—Spakman et al., 1993; Wortel and Spakman, 2000; Piromallo and Morelli, 2003.

  • Here is a figure from Dewey et al. (2004) that shows a cross section of earthquake hypocenters with symbols representing their mechanism type. There was an earthquake sequence in 2001 that had an extensional mechanism. This supports the interpretation that today’s M 6.2 sequence is in the downgoing slab.

  • Cross section of earthquake hypocenters, 1978–April 2001, classified by focal mechanism as determined by the Harvard CMT methodology (Dziewonski et al., 1981). Events are those that lie within the box labeled “Fig. 5” on Figure 4. Hypocenters of the most destructive El Salvadoran earthquakes since 1978 are labeled by their dates. Shallow-focus earthquakes for which the hypocentroid cannot be accurately determined by Harvard CMT methodology are assigned default depths of 15 km.

References

Posted in Uncategorized

Earthquake Report: Aleutian Trench

Well, well, well. What have we here. We have some earthquakes that are related to each other and some earthquakes that are not.

Less than 2 weeks ago there initiated a sequence of earthquakes along a fault splaying off of the Denali fault in British Columbia (with M 6.2 and M 6.3 mainshocks). Here is my report for that sequence. Over the past few days there have been a number of earthquakes along the western Aleutian Island Arc, a subduction zone formed by the convergence between the Pacific and North America plates. Initially there were some M ~5 earthquakes near the trench, in an up-dip region of the megathrust. The moment tensors (the beach balls on the map) reflect ~north-south compression, consistent with the subduction zone here. Then there popped some earthquakes to the northwest, including the mainshock (so far) of the sequence, an M 6.2. More earthquakes occurred in both small regions. The earthquakes in the more northerly patch have interesting moment tensors showing oblique slip.

The earthquakes in the southern patch mostly have hypocentral depths the are similar in depth to the megathrust fault as modeled by Hayes et al. (2012). The earthquakes in the northern patch are either shallow or have default (10 km) depths. Earthquakes are commonly given a default 10 km hypocentral depth prior to listing a calculated depth. All the larger earthquakes have a 10 km depth and the M 5.0 on 5/10 has a 9.6 km depth. There are about a dozen earthquakes (out of about 100 with M > 2.5) that have a deeper location (possibly on the megathrust). BUT the majority of earthquakes with determined depths have depths < 10 km. This suggests that these earthquakes may be in the upper North America plate. This makes sense because the upper plate has been interpreted to to have formed blocks that rotate in response to oblique subduction (see Krutikov figure below). The only problem is that if these earthquakes happened on the more northerly striking faults, then they should be right-lateral strike-slip (but the moment tensors show this nodal plan to be left-lateral). Perhaps these are along the more easterly nodal plane, which actually matches the sense of motion in the Krutikov figure below. The epicenter locations do not align along a clearly oriented north-south nor east-west trend, so this northerly patch is really difficult to interpret (at least until my colleagues send me an email telling me how they interpret these; I will update this report after that and give credit to those who have figured this out!).

Today there was an earthquake offshore of Kodiak Island. This earthquake is more simple because (1) it shows compression oriented with the subduction zone and (2) the depths align with the megathrust slab contours from Hayes et al. (2012).

SO. The Denali fault, Kodiak Island, and western Aleutian sequence earthquakes are unrelated to each other. However, the earthquakes in the western Aleutians do appear related (in time and space), and this is interesting if the northern patch is in the upper plate and the southern patch is along the megathrust. Pretty cool if that were the case.

    Here are the USGS web pages for the earthquakes.

  • Earthquakes with a moment tensor have an ‘mt’ after the link.
  • Aleutian
    • 2017-05-08 15:31 M 5.7 mt
    • 2017-05-08 15:47 M 5.9 mt
    • 2017-05-08 17:00 M 6.2 mt
    • 2017-05-08 17:08 M 5.2
    • 2017-05-08 19:53 M 5.0
    • 2017-05-09 08:59 M 5.4 mt
    • 2017-05-10 07:59 M 5.9 mt
    • 2017-05-10 19:25 M 5.0 mt
  • Kodiak
    • 2017-05-11 14:36 M 5.2 mt

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 MMI contours for the M 6.2 along the western Aleutians, the M 5.2 offshore of Kodiak Island, and the M 6.3 along the Denali fault.

  • 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. The hypocentral depth of the M 5.5 plots this close to the location of the fault as mapped by Hayes et al. (2012).

    I include some inset figures in the poster.

  • In the upper left corner is a figure that shows the historic earthquake ruptures along the Aleutian Megathrust (Peter Haeussler, USGS). I place a red star in the general location of this earthquake sequence (same for other figures here).
  • To the right of that figure are two figures from IRIS. On the left is a map showing seismicity plotted vs. depth (with color). On the right is a low-angle oblique view of a cross section and map of the plate boundary faults here.
  • In the lower right corner is a figure from Bassett and Watts (2015 B) that shows the results of their analyses using gravity data.
  • To the left of that is a schematic illustration from Bassett and Watts (2015 B) that shows how their gravity anomaly data may relate to different parts of the megathrust fault. They interpret a trench-parallel fore-arc ridge (TPFR) that may “provide insights into the dimensions, seismogenic behavior, and segmentation of subduction thrust faults.”


  • Here is a figure from Krutikov et al. (2008) that shows how blocks in the Aleutian Arc may accommodate the oblique subduction, along forearc sliver faults. Note that these blocks may also rotate to accommodate the oblique convergence. There are also margin parallel strike slip faults that bound these blocks. These faults are in the upper plate, but may impart localized strain to the lower plate, resulting in strike slip motion on the lower plate (my arm waving part of this). Note how the upper plate strike-slip faults have the same sense of motion as these deeper earthquakes.

  • Here is a figure from a GSA paper (here) that shows how the stresses from oblique convergence is partitioned along subduction zone faults and strike-slip faults (forearc slivers).

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

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

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

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

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

References

Posted in alaska, earthquake, Extension, geology, plate tectonics, strike-slip, subduction

Earthquake Report: Vanuatu!

There was an earthquake along the New Hebrides Trench this morning (my time in northern California). This earthquake is located deep, possibly below the subduction zone megathrust (but probably is a subduction zone earthquake). The hypocentral depth is 169 km, while the subducting slab is mapped at between 100-120 km in this location. While the slab location has some inherent uncertainty, today’s earthquake is within the range. We probably will never really know, until there is a movie made about this earthquake (surely Hollywood will know).

There was an earthquake on 2015.10.20 that has a similar moment tensor with a slightly larger magnitude (M 7.1). There was also a sequence of earthquakes in this region in April 2016 (here is my report from 2016.04.28).

  • Here are the USGS websites for these two earthquakes.
  • 2015.10.20 M 7.1
  • 2017.05.09 M 6.8

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). In the second poster, I include seismicity from the past century.

  • 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 upper right corner I include an illustration showing how Oceanic-Oceanic Subduction Zones are configured. One oceanic plate subducts beneath another oceanic plate. At some depth, the water dehydrated from the downgoing plate provides the flux for the mantle to melt. This molten mantle rises due to its decreased density, forming volcanoes. These volcanoes form a line subparallel to the subduction zone trench and we call this a magmatic arc, or an oceanic volcanic arc (or simply, the arc).
  • To the left of this schematic illustration is a plot from Benz et al. (2011). This plot shows a cross section of earthquake hypocenters. Hypocenters are the 3-D locations of earthquakes as calculated from seismologic records. This profile F-F’ is shown on the map as a yellow line with dots at the end of the line.
  • In the lower left corner I include two figures from Cleveland et al. (2014). The upper panel show USGS NEIC earthquake epicenters from 1973-2013 and GCMT moment tensors for earthquakes since 1976. There are some regions of this subduction zone that appear to have repeating earthquakes every decade or two (while other parts of the subduction zone do not). The lower panel shows these earthquakes plotted on a space-time diagram (on the right). This plot shows earthquakes with magnitudes represented by the circle diameter, plotted vs. latitude on the vertical axis and time on the horizontal axis. I place a red star in the approximate location of today’s M 6.8 earthquake.


  • Here is a version showing USGS seismicity from 1917-2017 for earthquakes with magnitudes M ≥ 7.0


  • Here is an animation that shows the seismicity for this region from 1960 – 2016 for earthquakes with magnitudes greater than or equal to 7.0.
  • I include some figures mentioned in my report from 2016.04.28 for a sequence of earthquakes in the same region as today’s earthquake (albeit shallower hypocentral depths), in addition to a plot from Cleveland et al. (2014). In the upper right corner, Cleveland et al. (2014) on the left plot a map showing earthquake epicenters for the time period listed below the plot on the right. On the right is a plot of earthquakes (diameter = magnitude) of earthquakes with latitude on the vertical axis and time on the horizontal axis. Cleveland et al (2014) discuss these short periods of seismicity that span a certain range of fault length along the New Hebrides Trench in this area. Above is a screen shot image and below is the video.

  • Here is a link to the embedded video below (6 MB mp4)
  • Here are the two figures from Cleveland et al. (2014).
  • Figure 1. I include the figure caption below as a blockquote.

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

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

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

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

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

  • Here is the USGS poster showing the seismicity for this region from 1900-2010 (Benz et al., 2011). Below I include the legend (not the correct scale; click on this link for the entire poster (65 MB pdf)). Note the cross section F-F’ which I plot on the poster above.


  • Here is the cross section F-F’ again, with the legend below.


References

Posted in Uncategorized

Earthquake Report: Denali fault, British Columbia

This is an interesting earthquake for a number of reasons. The epicenters of the largest earthquakes in this series (M 6.2 and M 6.3) align just off-strike from the Dalton section of the Denali fault (DF) which was mapped as having offset Holocene features by Plafker et al (1977), though there were no numerical ages to support their interpretation. This is just north of the Chilkat River section of the DF and just north of the Chatham Strait section of the DF. These sections of the Denali fault have not been found to be active (though they may be and today’s earthquake sequence suggests that they are!). There are many faults mapped in this region based upon the British Columbia data catalogue.

The moment tensor for the M 6.3 is also slightly misaligned to the orientation (strike) of the Denali fault here. Also interesting because the USGS has been putting forth significant effort on an investigation of the Quuen Charlotte (QCF)/Fairweather fault to the south of these earthquakes. The Chatham Strait fault splinters eastwards from the QCF and connects to the Denali fault just south of this sequence. The Chatham Strait fault was recognized to have dextral slip (right-lateral strike-slip) by Hudson et al. (1982; and references therein) using offsets of geologic units. These and earlier authors found up to 150 km of separation (offset) of these post-middle Cretaceous rocks.

UPDATE: Dr. Rick Koehler (UNR) informs me that the Chilkat section is now included in the Dalton section of the Denali fault.

  • Here are the USGS websites for the two largest earthquakes.
  • 2017.05.01 12:31 M 6.2
  • 2017.05.01 14:18 M 6.3

Below is my interpretive poster for this earthquake.

I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I also plot USGS epicenters from 1917-2017 for magnitudes M ≥ 3.5. For some of the larger magnitude earthquakes, I include moment tensors (blue) and a focal mechanism (orange) that shows the sense of motion on the faults. I outline the aftershock region of this current sequence in dashed white lines. Here is the USGS kml query file that I used to create this map. Here is a USGS kml file that only includes the earthquakes M ≥ 5.5.

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely. Based upon the alignment of the two mainshocks and the regionally mapped faults, I interpret these to be right-lateral strike-slip faults.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.

    I include some inset figures in the poster.

  • In the lower left corner is a map produced by Dr. Peter Haeussler from the USGS Alaska Science Center (pheuslr at usgs.gov) that shows the historic earthquakes along the Aleutian-Alaska subduction zone. I place a blue star in the general location of today’s earthquake sequence.
  • In the upper left corner is a map from the USGS that shows the regional tectonic boundary fault system. The USGS has been working on the onshore and offshore segments of the Queen Charlotte fault for the past few years. For the region of the QCF that is in Canada waters, the Canada Geological Survey has been studying there.
  • In the upper right corner are the MMI intensity maps for the two earthquakes listed above: 2017 M 6.3 & 6.2. Below each map are plotted the reports from the Did You Feel It? USGS website for each earthquake. These reports are plotted as green dots with intensity on the vertical axes and distance on the horizontal axes. There are comparisons with Ground Motion Prediction Equation (attenuation relations) results (the orange model uses empirical data from central and eastern US earthquakes; the green model uses empirical data from earthquakes in California). The data seem to fit slightly better to the California empirical relations, which makes sense because this region of Canada/USA is a series of accreted terranes (sequences of subduction zone and oceanic crust material that has been accreted to the North America plate during subduction zone convergence).
  • To the left of these DYFI maps is a map from Walton et al. (2015) that shows extents of some of the large earthquakes along the QCF. They also plot the seismicity and focal mechanism from the 2012 Haida Gwaii earthquake and 2013 QCF sequences.
  • In the lower right corner is a map showing these accreted terranes (British Columbia Geological Survey, 2011).


  • Here is the map from Hauessler et al., 2014.

  • Here is the USGS fault map. I include their figure text below as blockquote.

  • Study region along the Queen Charlotte-Fairweather fault offshore southeastern Alaska. Rectangles show locations of the two USGS-led marine geophysical surveys in May and August 2015. The third cruise was offshore Haida Gwaii, British Columbia, and southern Alaska in September 2015 (see inset map). CSF, Chatham Strait fault; CSZ, Coastal shear zone; LIPSF, Lisianski Inlet-Peril Strait fault; QCFF, Queen Charlotte-Fairweather fault.

  • This is the figure from Walton et al. (2015) showing the extent of some historical ruptures along the QCF.

  • Regional tectonic setting of the Queen Charlotte fault (QCF), including major fault traces. The inset shows regional location. Because of the angle of the Pacific plate vector and the geometry of the QCF, convergence along the fault increases to the south. The bold X on the QCF marks a 10° change in strike of the QCF at 53.2° N, south of which is an obliquely convergent segment of the QCF undergoing convergence rates up to ∼20 mm=yr. The QCF is bounded to the north by the Yakutat block and to the south by the Explorer triple junction. Rupture zones defined by aftershocks for major historic earthquakes along the margin are indicated by dashed black outlines (Plafker et al., 1994). Aftershocks
    circles) and focal mechanisms for the 2013 Mw 7.5 Craig earthquake and the 2012 Mw 7.8 Haida Gwaii earthquake are also included, along with a magnitude scale for aftershocks (derived from the Global Centroid Moment Tensor catalog; Ekström et al., 2012; see Data and Resources). DE, Dixon Entrance; TWS, Tuzo Wilson seamounts; DK, Dellwood Knolls; TF, Transition fault; FF, Fairweather fault; PSF, Peril Strait fault; CSF, Chatham Strait fault.

  • Here is a figure from some of the early work on the Chatham Strait fault (Lathram, 1964).

  • Map showing geologic features displaced by the Chatham Strait fault.

  • Here is a great map showing the Denali fault system and some of the paleoseismologic sites on the eastern sections of this fault system (Redfield and Fitzgerald, 1993).

  • Simplified schematic terrane map of Alaska after Coe et. al. [1985] and Howell et al. [1985], showing the locations of major tectonostratigraphic terranes, structures, relative motion vector(RMV) analysis sites, and other localities mentioned in the text. Note that the Yakutat terrane extends offshore. RMV sites on the DFS are marked with a solid dot. Site 1 of the DFS is on the Togiak/Tikchik fault. Sites 2 and 3 of the DFS are on the Holitna fault. Sites 4-7 of the DFS are on the Farewell fault. Sites 8-10 of the DFS are on the McKinley strand of the Denali fault. Sites 11 and 12 of the DFS is on the Shakwak fault. Site 13 of the DFS is on the faiton Fault. Site 14 of the DFS is on the Chatham Strait fault.

  • Here is the map showing the Terranes in British Columbia. This map shows a more regional view than the one on the interpretive poster above.

  • The framework of the Cordilleran orogen of northwestern North America is commonly depicted as a ‘collage’ of terranes – crustal blocks containing records of a variety of geodynamic environments including continental fragments, pieces of island arc crust and oceanic crust. The series of maps available here are derived from a GIS compilation of terranes based on the map published by Colpron et al. (2007) and Nelson and Colpron (2007), and include modifications from recent regional mapping. These maps are presented in a variety of digital formats including ArcGIS file geodatabase (.gdb), shapefiles (.shp and related files) and Map Packages (.mpk), as well as in a number of graphic formats (Adobe Illustrator CS3, CorelDraw X3, PDF and JPEG). The GIS data include individual terrane layers for British Columbia, Yukon and Alaska, as well as a layer showing selected major Late Cretaceous and Tertiary strike-slip faults. Graphic files derived from the GIS compilation were prepared for the Northern Cordillera (Alaska, Yukon and BC), the Canadian Cordillera (BC and Yukon), Yukon, and British Columbia. These maps are intended for page-size display (~1:5,000,000 and smaller). Polygons are accurate to ~1 km for Yukon and BC, and ~5 km for Alaska. More detailed geological data are available from both BC Geological Survey and Yukon Geological Survey websites. Descriptions of the terranes, their tectonic evolution and metallogeny can be found in Colpron et al. (2007), Nelson and Colpron (2007), Colpron and Nelson (2009).

  • Here is a map showing all the mapped faults in this region. I drew a dashed blue line connecting the M 6.3 and M 6.2 epicenters. Note (if one clicks on the map to see a larger version) that the orientation of this alignment is oblique to the fault orientations (which are generally aligned sub parallel to the Denali fault). This map comes from the British Columbia data catalogue.

  • UPDATE: Dr. Sean Bemis posted this model on Sketchfab. Dr. Bemis used 1948 aerial imagery, in Agisoft Photoscan software, to create this 3-D model using the method called “Structure For Motion” (SFM). He then exported the model to Sketchfab. Agisoft has academic pricing for their software. However, there is also free software available that does the stuff the Photoscan does (though one may need to use multiple apps).
  • There is a north-south lineation that aligns nicely with the strike-line formed between the M 6.3 & M 6.2 epicenters (not shown on the 3-D model).

References:

  • Benz, H.M., Tarr, A.C., Hayes, G.P., Villaseñor, Antonio, Hayes, G.P., Furlong, K.P., Dart, R.L., and Rhea, Susan, 2010. Seismicity of the Earth 1900–2010 Aleutian arc and vicinity: U.S. Geological Survey Open-File Report 2010–1083-B, scale 1:5,000,000.
  • Haeussler, P., Leith, W., Wald, D., Filson, J., Wolfe, C., and Applegate, D., 2014. Geophysical Advances Triggered by the 1964 Great Alaska Earthquake in EOS, Transactions, American Geophysical Union, v. 95, no. 17, p. 141-142.
  • Hudson, Travis, Plafker, George, and Dixon, Kirk, 1982, Horizontal offset history of the Chatham Strait fault, in Coonrad, W.L., ed., The United States Geological Survey in Alaska, Accomplishments during 1980, Geological Survey Circular 844, p. 128-131
  • Lathram, E.H., 1964. Apparent Right-Lateral Separation on Chatham Strait Fault, Southeastern Alaska in GSA Bulletin, v. 75, p. 249-252.
  • Plafker, G., 1969. Tectonics of the March 27, 1964 Alaska earthquake: U.S. Geological Survey Professional Paper 543–I, 74 p., 2 sheets, scales 1:2,000,000 and 1:500,000, http://pubs.usgs.gov/pp/0543i/.
  • Redfield, T.F. and Fitzgerald, P.G., 1993. Denali Fault System of Southern Alaska: An Interior Strike_slip Structure Responding to Dextral and Sinistral Shear Coupling in Tectonics, v. 12, no. 5, p. 1195-1208
  • West, M.E., Haeussler, P.L., Ruppert, N.A., Freymueller, J.T., and the Alaska Seismic Hazards Safety Commission, 2014. Why the 1964 Great Alaska Earthquake Matters 50 Years Later in Seismological Research Letters, v. 85, no. 2, p. 1-7.

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Earthquake Report: Cotobato Trench, Philippines

Earlier in April (2017) there was some activity in 4 different regions of the Philippines. Based upon the low magnitudes and large epicentral distances, these earthquakes were most unlikely to be directly related to each other. A couple days ago, there was an earthquake along-dip from one of these earlier swarms. Here is the USGS web page for this M 6.9 earthquake. There does not appear to have been an observed tsunami based upon a quick look at gages posted to this IOC site (though the closest 2 gages seem to have intermittent records). The along dip seismicity earlier this month was along the Philippine trench subduction zone fault. The M 6.9 earthquake appears to be related to subduction along the Cotobato trench.

These earthquakes are ~300 km from each other. Also, the Philippine trench swarm appears to possibly have reduced stress on the Cotobato trench, but I might be wrong. Regardless, it is probably a coincidence that these earthquakes are along dip to each other. Another coincidence is another earthquake along-dip to these earthquakes, a deep M 7.3 earthquake in the Celebes Sea in January 2017. Here is my earthquake report for this earthquake.

” target=”_blank”>Earthquake Report for these earlier earthquakes.

  • Here is another blog about the earthquakes near Manila (including the M 5.9): Stephen Hicks
  • Here is the Berkeley Seismo Blog page for these earthquakes.
  • Below is my interpretive poster for this earthquake.

    I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend).

    • I 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 upper right corner is a figure from Hall (2011). This shows the plate tectonic configuration in the equatorial Pacific with a low angle oblique perspective. Note how the upper panel shows a west dipping slab on the east side of the Philippines. Note the contrast in the center panel (Halmahera), where the eastern fault is dipping to the east (westward vergent) and the western fault is dipping to the west (eastward vergent). This region near Halmahera forms the Molucca Strait, one of the most tectonically active areas in this region.
    • In the upper left corner is a map showing the regional tectonics from Smoczyck et al. (2013). Earthquakes are plotted with color representing depth and diameter representing magnitude (see legend). The Philippine trench is an eastward vergent (dipping to the west) subduction zone on the east side of the Philippines. The Manila trench and Cotoban trench are westward vergent (dipping to the east) subduction zone faults on the western side of the Philippines. Below the map I include the cross section showing earthquake hypocenters from this Smoczyck et al. (2013) publication (see legend). I placed a blue star in the general location of the M 6.9 Cotobato trench earthquake and a transparent blue star in the region of the M 5.6 and M 5.7 Philippine trench earthquakes on both the map and the cross section.
    • In the lower right corner I include a map showing the tectonics from the Molucca Sea (Waltham et al., 2008). I highlit the Cotobato trench in red. This map overlaps with the southern 75% of the Hall (2011) low-angle oblique figure above. This is a really interesting configuration. To the north, the subduction zones oppose each other (eastward vergent on the east and westward vergent on the west). Along the latitude of the Molucca Strait, the uppermost thrust faults have a similar opposing vergence. In contrast, the molucca sea plate is below these more shallow thrusts and forms opposing subduction zones with different polarity (eastward vergent on the east and westward vergent on the west).


    • This is the low-angle oblique view of the region (Hall, 2011).

    • 3D cartoon of plate boundaries in the Molucca Sea region modified from Hall et al. (1995). Although seismicity identifies a number of plates there are no continuous boundaries, and the Cotobato, North Sulawesi and Philippine Trenches are all intraplate features. The apparent distinction between different crust types, such as Australian continental crust and oceanic crust of the Philippine and Molucca Sea, is partly a boundary inactive since the Early Miocene (east Sulawesi) and partly a younger but now probably inactive boundary of the Sorong Fault. The upper crust of this entire region is deforming in a much more continuous way than suggested by this cartoon.

    • This is the tectonic map from Waltham et al. (2008)

    • (A) Location and major tectonic features of the Molucca Sea region. Small, black-filled triangles are modern volcanoes. Bathymetric contours are at 200, 2000, 4000, and 6000 m. Large barbed lines are subduction zones, and small barbed lines are thrusts. (B) Cross section across the Halmahera and Sangihe Arcs on section line B. Thrusts on each side of the Molucca Sea are directed outward toward the adjacent arcs, although the subducting Molucca Sea plate dips east beneath Halmahera and west below the Sangihe Arc. (C) Inset is the restored cross section of the Miocene–Pliocene Weda Bay Basin of SW Halmahera on section line C, fl attened to the Pliocene unconformity, showing estimated thickness of the section.

    • This is smaller scale tectonic map of the region (Zahirovic et al., 2014).

    • Regional tectonic setting with plate boundaries (MORs/transforms = black, subduction zones = teethed red) from Bird (2003) and ophiolite belts representing sutures modified from Hutchison (1975) and Baldwin et al. (2012). West Sulawesi basalts are from Polvé et al. (1997), fracture zones are from Matthews et al. (2011) and basin outlines are from Hearn et al. (2003). ANI – Andaman and Nicobar Islands, BD– Billiton Depression, Ba – Bangka Island, BI – Belitung (Billiton) Island, BiS – Bismarck Sea, BP – Benham Plateau, CaR – Caroline Ridge, CS – Celebes Sea, DG– Dangerous Grounds, EauR – Eauripik Ridge, FIN – Finisterre Terrane, GoT – Gulf of Thailand, GR– Gagua Ridge, HAL– Halmahera, HBa – Huatung Basin, KB–Ketungau Basin, KP – Khorat Platform, KT – Kiilsgaard Trough, LS – Luconia Shoals, MacB – Macclesfield Bank, ManTr – Manus Trench, MaTr – Mariana Trench, MB– Melawi Basin, MDB– Minami Daito Basin, MG– Mangkalihat, MIN – Mindoro, MN– Mawgyi Nappe, MoS – Molucca Sea, MS– Makassar Straits, MTr – Mussau Trench, NGTr – New Guinea Trench, NI – Natuna Islands, ODR– Oki Daito Ridge, OJP –Ontong Java Plateau, OSF – Owen Stanley Fault, PAL – Palawan, PhF – Philippine Fault, PT – Paternoster Platform, PTr – Palau Trench, PVB – Parece Vela Basin, RB – Reed Bank, RMF– Ramu-Markham Fault, RRF – Red River fault, SEM– Semitau, ShB – Shikoku Basin, Sol. Sea – Solomon Sea, SPK – Sepik, SPT – abah–Palawan Trough, STr – Sorol Trough, Sul – Sulawesi, SuS – Sulu Sea, TPAA– Torricelli–Prince Alexander Arc, WB–West Burma, WCT–W Caroline Trough, YTr –Yap Trough.

    • This is a map and series of cross sections showing subducting plates in blue (Zahirovic et al., 2014). The cross sections are based upon seismic wave tomography, which is similar to CT scans (Computed Tomography of X-Rays). These two processes use the same general methods to investigate the 3-dimensional views of internal structures (bodies vs. the Earth). More can be found in their paper, but basically, the blue regions represent areas that have higher seismic velocity. Oceanic lithosphere has higher seismic velocities than the surrounding mantle. So, the subducting oceanic slabs show up as blue. The corss section G-G’ is at about the same latitude as the M 5.6-7 and M 6.9 earthquakes. Note that the Philippine sea plate subducting at the Philippine trench (dipping to the west/left) is evident, while the slab associated with the Cotobato trench does not appear visible. Compare this with the seismicity cross section from Smoczyck et al. (2013), where the Cotobato trench seismicity is much more shallow than the Philippine trench.

    • Vertical sections from MIT-P (Li et al., 2008) and GyPSuM-S (Simmons et al., 2009) seismic tomography models along profiles A to E (magenta lines). The first-order differences between the P- and S-wave models is that the amplitude of the positive seismic velocity anomalies significantly diminishes away from continental coverage (e.g., dashed lines in profiles A and B). A depth slice at 746 km from MIT-P isprovided for reference with super-imposed present-day coastlines and plate boundaries. Interpreted slab sources are labeled: GI-BA= Greater India–Neo-Tethyan back-arc slab, M/N-T – Meso- and Neo-Tethyan slabs, W-S –Woyla–Sunda slabs, S – Sunda slab, PSCS – proto-South China Sea slab, PAC – Pacific slab, PMOL– proto-Molucca slab, PSOL – proto-Solomon slab, CS – Caroline slab, PSP – Philippine Sea Plate slab, S-C = Sulu–Celebes slabs.

    • However, here is a figure that shows isosurfaces from their tomography models (Zahirovic et al., 2014). This shows what may be slabs related to the Cotobato trench (western part of G-G’ cross section). These slabs show up better on the lower figure.

    • 3-D visualization of +0.2% seismic velocity anomaly isosurfaces in MIT-P (top) and +0.9% seismic velocity perturbation in GyPSuM-S (bottom) models. Profiles A to G represent the vertical profiles (see Fig. 10) that capture the convergence and subduction histories of the region since the Cretaceous. Present-day coastlines are translucent grey shades, and present-day plate boundaries are translucent black lines. Slab volumes are colored by their depth, while the light blue color represents the interior surface of these slabs. PSCS – proto-South China Sea slab.

    • In January of this year (2017), there was an M 7.3 earthquake in the Celebes Sea south of the Philippines. Below is my interpretive map for that earthquake. I also present the same poster with 1917-2017 seismicity for earthquakes M ≥ 6.5. Here is my earthquake report for this M 7.3 earthquake. I include more background information for the Molucca Strait region on this page.


    References:

    • Bock et al., 2003. Crustal motion in Indonesia from Global Positioning System measurements in JGR, v./ 108, no. B8, 2367, doi:10.1029/2001JB000324
    • Hall, R., 2011. Australia–SE Asia collision: plate tectonics and crustal flow in Hall, R., Cottam, M. A. &Wilson, M. E. J. (eds) The SE Asian Gateway: History and Tectonics of the Australia–Asia Collision. Geological Society, London, Special Publications, 355, 75–109.
    • Hayes, G.P., Wald, D.J., and Johnson, R.L., 2012. Slab1.0: A three-dimensional model of global subduction zone geometries in, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524
    • McCaffrey, R., Silver, E.A., and Raitt, R.W., 1980. Crustal Structure of the Molucca Sea Collision Zone, Indonesia in The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands-Geophysical Monograph 23, p. 161-177.
    • Nelson, A.R., Personius, S.F., Rimando, R.E., Punongbayan, R.S., Tungol, N, Mirabueno, H., and Rasdas, A., 2000. Multiple Large Earthquakes in the Past 1500 Years on a Fault in Metropolitan Manila, the Philippines in BSSA vol. 90, p. 73-85.
    • Noda, A., 2013. Strike-Slip Basin – Its Configuration and Sedimentary Facies in Mechanism of Sedimentary Basin Formation – Multidisciplinary Approach on Active Plate Margins http://www.intechopen.com/books/mechanism-of-sedimentarybasin-formation-multidisciplinary-approach-on-active-plate-margins http://dx.doi.org/10.5772/56593
    • Smoczyk, G.M., Hayes, G.P., Hamburger, M.W., Benz, H.M., Villaseñor, Antonio, and Furlong, K.P., 2013. Seismicity of the Earth 1900–2012 Philippine Sea plate and vicinity: U.S. Geological Survey Open-File Report 2010–1083-M, 1 sheet, scale 1:10,000,000.
    • Waltham et al., 2008. Basin formation by volcanic arc loading in GSA Special Papers 2008, v. 436, p. 11-26.
    • Zahirovic et al., 2014. The Cretaceous and Cenozoic tectonic evolution of Southeast Asia in Solid Earth, v. 5, p. 227-273, doi:10.5194/se-5-227-2014.

    Posted in earthquake, geology, pacific, plate tectonics

    Earthquake Report: Chile Update #2

    Today the swarm has reminded us to stay vigilant. This region of the Chile subduction zone is pretty active and adjacent to the most active part of the Chile subduction zone. Today there was a series of earthquakes with a maximum magnitude of M 5.9. Here is the USGS webpage for this M 5.9 earthquake.

    • I have prepared several other reports for the recent seismicity here. More background information about the subduction zone history can be found there.
    • 2017.04.23 M 5.9
    • 2017.04.24 M 6.9
    • 2017.04.24 M 6.9 Update #1

    The current sequence is just to the south of the 1971 M 7.0 earthquake and has a similar along-strike distance. This may be all that we will see, but there is a small chance this will lead to an earthquake with a larger magnitude. I suspect this chance is not very high (low likelihood). If we consider the 1985 earthquake as an analog, there were only a few earthquakes prior to the mainshock. The 2017 swarm has had many earthquakes to date. I suspect that the M 6.9 is the mainshock for this series. Also, consider that the 1985 region overlaps slightly with the 2010 earthquake. While this region was an area of low slip in 2010, there might be a reason for this (e.g. the Juan Fernandez Ridge, JFR). The JFR may act as an asperity. Asperities have 2 main definitions: (1) region of largest slip during an earthquake (2) region of a fault across which strain is accumulated due to the material properties of the fault and crust. I use the second definition. Smooth subduction zone faults may be responsible for large magnitude earthquakes and rough subduction zone faults (with asperities like the JFR) may be responsible for smaller, more frequent, earthquakes. If we look at the Métois et al. (2016) figure below, this region of the subduction zone has a high rate of seismicity. If this seismicity is the result of a rough fault, it seems that this part of the megathrust may not store as much strain as other parts of the subduction zone. However, this part of the fault has slipped during Great earthquakes (M > 8.0) in the past. So, it is difficult to say. While I might be wrong, given what we know about this subduction zone, it seems like this swarm is not going to result in a Great earthquake this time. Others disagree with me and that is great! I am only looking at the seismicity from the past and others are actively testing the cycling of seismic strain (and coulomb stress) in this region.

    I include the moment tensors from each of the Great Earthquakes, as well as the 2017 M 5.9, 6.9, and 5.9 earthquakes.

    • 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 lower right corner I present the space-time diagram from Métois et al. (2016). They plot seismicity vs. depth on the right. I placed a green bar with the approximate latitudinal range of the 2017 sequence.
    • In the lower left corner I present the figure from Métois et al. (2016) that shows their estimates of seismic coupling (the proportion of the fault that is “locked” and accumulating strain). Regions of low coupling do not accumulate strain, do not resist earthquake rupture, and do not contribute to the release of energy during earthquakes.
      I place a green bar as for the space time figure on the right. Note the high rate of seismicity in this region. Also, note that the region of the fault that is participating in this 2017 sequence is a region of lower coupling between the 2015 and 2010 patches. One problem is that the 2015 patch did not release much strain in the higher coupled area (though this area has more earthquakes, so that might release strain, limiting the possibility of future larger earthquakes.
    • In the upper right corner, I plot seismicity for the past 2.5 months along with the 30 day seismicity surrounding the 1985 earthquake. I also outline the 1985 and 2017 sequences.


    • Here is the space time figure from Métois et al. (2016).

    • Left estimated extent of large historical or instrumental ruptures along the Chilean margin adapted from ME´ TOIS et al. (2012). Gray stars mark major intra-slab events. The recent Mw[8 earthquakes are indicated in red. Gray shaded areas correspond to LCZs defined in Fig. 3. Right seismicity recorded by the Centro Sismologico Nacional (CSN) during interseismic period, color-coded depending on the event’s depth. Three zones have been defined to avoid including aftershocks and preshocks associated with major events: (1) in North Chile, we plot the seismicity from 2008 to january 2014, i.e., between the Tocopilla and Iquique earthquakes; (2) in Central Chile, we plot the seismicity on the entire 2000–2014 period; (3) in South-Central Chile, we selected events that occurred between 2000 and 2010, i.e., before the Maule earthquake.

    • Here is the seismic coupling figure from Métois et al. (2016).

    • a Histogram depicts the rate of Mw[3 earthquakes registered by the CSN catalog during the interseismic period defined for each zone (see Fig. 2) on the subduction interface, on 0.2 of latitude sliding windows. Stars are swarm-like sequences detected by HOLTKAMP et al. (2011) depending on their occurrence date. Swarms located in the Iquique LCZ and Camarones segment are from RUIZ et al. (2014). Empty squares are significant intraplate earthquakes. b Red curve variations of the average coupling coefficient on the first 60 km of depth calculated on 0.2 of latitude sliding windows for our best model including an Andean sliver motion. Dashed pink curves are alternative models with different smoothing options that fit the data with nRMS better than 2 (see supplementary figure 6): the pink shaded envelope around our best model stands for the variability of the coupling along strike. Green curves coseismic distribution for Maule (VIGNY et al. 2011), Iquique (LAY et al. 2014) and Illapel earthquakes (RUIZ et al. 2016). Gray shaded areas stand for the identified low coupling zones (LCZs). LCZs and high coupling segments are named on the left. The apparent decrease in the average coupling North of 30S is considered as an artifact of the Andean sliver motion (see Sect. 5.2). c Best coupling distribution obtained inverting for Andean sliver motion and coupling amount simultaneously. The rupture zones for the three major earthquakes are indicated as green ellipses. White shaded areas are zones where we lack resolution

    • Here is a figure from Conteras-Reyes and Carrizo, 2011 that shows how the structure of the Nazca plate may exert heterogeneous forces along the subduction zone fault.

    • (upper) Structure and interpretation of the (A) Nazca Ridge, (B) easternmost portion of the Juan Fernández Ridge, and (C) Mocha FZ based on the 2D seismic velocity model of Hampel et al. (2004), Kopp et al. (2004), and Contreras-Reyes et al. (2008), respectively. Map locations of seismic profiles are shown in Fig. 1A. (below) Direct comparison of these HOF’s structure with typical Nazca oceanic crust (6.5km thick). The anomalous normal stress n (buoyancy force) depends on the thickness of the corresponding anomalous crustal thickness (Hc) and on thickness of the underplated magmatic material beneath the crust and/or thickness of the serpentinized mantle (Hm). n also depends on the mantle–crust density contrast (mc = 530 kg/m3), “normal” mantle–serpentinized mantle density contrast (m= 230 kg/m3) and “normal” crust-altered crust density contrast (c = 50 kg/m3). See further details in Table 1. Density values are taken from the density model of Tassara et al. (2006).

    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:

    Posted in earthquake, plate tectonics, subduction

    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 my personal story.

    I was driving my girlfriend’s car (Jen Guevara) with her and some housemates up to attend a festival at Redwood Park in Arcata. She lived in the old blue house at the base of the bridge abutment on the southwest side of HWY 101 as it crosses Mad River. The house burned down a couple of years ago, but these memories remain. We were driving along St. Louis and about to turn east to cross the 101 towards LK Wood. The car moved left and right. I pulled over as I thought we might have just gotten a flat tire. I got out, inspected the wheels, and there was no flat. We returned to our journey. When we arrived at the park, everyone was talking about how the redwood trees were flopping around like wet spaghetti during the earthquake. I then looked back in my memory and realized that, at the lumber mill that I had parked by when I got the imaginary flat tire, there were tall stacks of milled lumber flopping around. I had dismissed it that they were blowing in the wind. Silly me.

    Later that night, I was at a reggae concert at the Old Creamery Building in Arcata. At some point, the lights flickered off and on. I figured that someone had accidentally brushed up against the light switch on the wall. BUT, this was the first of two large aftershocks.

    Even later that night, actually the following morning, I was laying in bed with Jen. The house typically shook when large semi trucks crossed the 101 bridge. However, this time, the shaking had a much longer duration. This was the second of the two major aftershocks. I finally recognized this earthquake as an earthquake and not something else. To my credit, I was dancing during the first major aftershock.

      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.
    • Here is a link to the embedded video below, showing the week-long seismicity in April 1992.
    • 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.


    • Here is a link to the embedded video below, showing these earthquakes.

    • 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.

    Posted in cascadia, earthquake, education, geology, humboldt, mendocino, pacific, plate tectonics, San Andreas, subduction, tsunami

    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

    Other Blogs about this earthquake sequence

    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).


    • Here is a great visualization from IRIS (here) that shows the seismic waves transmitting across the USA. Each dot represents a seismometer. When the seismic waves exert an upward motion, the dot turns red. When the seismic waves exert a downward motion, the dot turns blue. I present an screenshot of the animation above the video. Here is the video file as a downloadable file. (10 MB mp4).

    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:

    Posted in Uncategorized

    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

    • Here is a download link to the embedded video below. (7 MB mp4)

    Useful Resources

    References:

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    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. While this is in a region of low slip for the 2010 and 2015 earthquakes, due to the proximity of the Juan Fernandez fracture zone (which possibly promotes smaller earthquakes), there may not be a larger earthquake here. If there is, it might look something like the 1971 earthquake, with a magnitude of low 7 or so. (which could still be quite damaging).

    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

    • Here is a download link to the embedded video below. (7 MB mp4)

    Useful Resources

    References:

    Posted in Uncategorized