Earthquake Report: Guatemala

There was a really cool earthquake sequence a few days ago on and offshore of Guatemala. Offshore of Guatemala in the Pacific Ocean, the Cocos plate subducts beneath the North America and Caribbean plates (NAP & CP). The transform plate boundary between the NAP and CP forms the Motagua-Polochic fault zone onshore, which bisects Guatemala.

From late May 2017 through mid June there were several earthquakes with the largest magnitude M = 5.5. These earthquake hypocenters have depths that are deeper and shallower than the estimated depth for the subduction zone fault (Hayes et al., 2012), but many of the earthquakes simply have a default depth of 10 km. So it is difficult to say if these are all near the megathrust or are on upper plate faults (e.g. in the accretionary prism). These earthquakes have compressional fault plane solutions. Either way, they appear to have loaded some faults down-dip along the subducting slab. This may or may not be the case, but there was a deep extensional magnitude M 6.9 earthquake (with an aftershock of M = 5.1 nearby). These along dip earthquakes are probably related.

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. 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 right 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 plotted a cross section that is adjacent (southeast) to this earthquake sequence. 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. On the map and the cross section, I place a blue stars in the location for the M 6.9 and the 6/14 M 5.5 earthquakes.
  • To the left of these figures is a comparison map and plot, showing the responses from real people who reported their observations during these two earthquakes. 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 M 5.5 earthquake seems to fit the Central-Eastern US regression much better than the California regression. However, there are very few observations. The M 6.9 earthquake seems to fit the California regression better.
  • In the lower left corner is a map that shows the plate tectonic setting for this region of Middle America (Eric Calias). Earthquake epicenters are plotted with color representing depth and circle diameter representing magnitude. Dr. Calais shaded the Caribbean plate a little darker than the surrounding plates. Relative plate motions are plotted as white arrows. I place a blue star in the general location of the M 6.9 earthquake.
  • Above the tectonic figure, is a figure that shows how Franco et al. (2012) hypothesize that the amount that the subducting fault is locked (“coupling” or the proportion of the plate convergence rate that is stored along the fault that would eventually slip during earthquakes). Note that this earthquake sequence mostly occurred in the segment of the subduction zone that has high coupling. I place a blue star in the general location of the M 6.9 earthquake.
  • In the lower right corner I plot most all the available moment tensors for the earthquakes in this sequence. I label the Polochic and Motagua faults that delineate the Motagua-Polochic fault zone, left lateral strike-slip faults that form the boundary between the Caribbean and North America plates.


Here are the USGS webpages for the earthquakes with moment tensors plotted above

  • Here is the USGS Did You Feel It comparison.

References

Posted in caribbean, earthquake, education, geology, pacific, plate tectonics, strike-slip, subduction

Earthquake Report: Westernmost Aleutian Arc

This earthquake happened a couple weeks ago, but was interesting and I have been looking forward to following up on this with a report. Here is the USGS website for this M 6.8 earthquake.

The M 6.8 earthquake happened in a region where the Pacific-North America plate boundary transitions from a subduction zone to a shear zone. To the east of this region, the Pacific plate subducts beneath the North America plate to form the Alaska-Aleutian subduction zone. As a result of this subduction, a deep oceanic trench is formed. To the west of this earthquake, the plate boundary is in the form of a shear zone composed of several strike-slip faults. The main fault that is positioned in the trench is the Bering-Kresla shear zone (BKSZ), a right-lateral strike-slip fault. In the oceanic basin to the north of the BKSZ there are a series of parallel fracture zones, also right-lateral strike-slip faults.

My initial thought is that the entire Aleutian trench was a subduction zone prior to about 47 million years ago (Wilson, 1963; Torsvik et al., 2017). Prior to 47 Ma, the relative plate motion in the region of the BKSZ would have been more orthogonal (possibly leading to subduction there). After 47 Ma, the relative plate motion in the region of the BKSZ has been parallel to the plate boundary, owing to the strike-slip motion here. However, Konstantinovskaia (2001) used paleomagnetic data for a plate motion reconstruction through the Cenozoic and they have concluded that there is a much more complicated tectonic history here (with strike-slip faults in the region prior to 47 Ma and other faults extending much farther east into the plate boundary). When considering this, I was reminded that the relative plate motion in the central Aleutian subduction zone is oblique. This results in strain partitioning where the oblique motion is partitioned into fault-normal fault movement (subduction) and fault-parallel fault movement (strike-slip, along forearc sliver faults). The magmatic arc in the central Aleutian subduction zone has a forearc sliver fault, but also appears to have blocks that rotate in response to this shear (Krutikov, 2008).

There have been several other M ~6 earthquakes to the west that are good examples of this strike-slip faulting in this area. On 2003.12.05 there was a M 6.7 earthquake along the Bering fracture zone (the first major strike-slip fault northeast of the BKSZ). On 2016.09.05 there was a M 6.3 earthquake also on the Bering fracture zone. Here is my earthquake report for the 2016 M 6.3 earthquake. The next major strike-slip fault, moving away from the BKSZ, is the right-lateral Alpha fracture zone. The M 6.8 earthquake may be related to this northwest striking fracture zone. However, aftershocks instead suggest that this M 6.8 earthquake is on a fault oriented in the northeast direction. There is no northeast striking strike-slip fault mapped in this area and the Shirshov Ridge is mapped as a thrust fault (albeit inactive). There is a left-lateral strike-slip fault that splays off the northern boundary of Bowers Ridge. If this fault strikes a little more counter-slockwise than is currently mapped at, the orientation would match the fault plane solution for this M 6.8 earthquake (and also satisfies the left-lateral motion for this orientation). The bathymetry used in Google Earth does not reveal the orientation of this fault, but the aftershocks sure align nicely with this hypothesis.

Below is my interpretive poster for this earthquake.

I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I also include USGS earthquake epicenters from 1997-2017 for magnitudes M ≥ 6.5.

  • 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. I plot moment tensors for the M 6.8 earthquake, as well as for the 2003 and 2016 earthquakes mentioned above. I also include moment tensors for earthquakes in 1999 and 2001 because these are also interesting earthquakes that I had not noticed before. It appears that perhaps the 1999 strike-slip earthquake led to an increased stress on the subduction zone, which slipped in 2001. I will need to consider this earthquake pair more later. Here are the USGS websites for the 1999 and 2001 earthquakes.
  • 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 right corner is a figure that shows the historic earthquake ruptures along the Aleutian Megathrust (Peter Haeussler, USGS). I place a yellow star in the general location of this earthquake sequence (same for other figures here).
  • In the upper left corner is a figure from Gaedicke et al. (2000) which shows some of the major tectonic faults in this region.
  • In the lower right corner is a figure from Konstantnovskaia et al. (2001) that shows a very detailed view of all the faults in this complicated region.


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

  • Here is a map that shows some of the large earthquakes in this region from 1996 through 2015. Refer to the moment tensor legend to help interpret the moment tensors for each earthquake. All, but one, are compressional solutions. Note how all the compressional earthquakes have roughly the same strike, oriented relative to the plate convergence vectors (blue arrows). Note the fault plane solution and location for the 2014.06.23 M 7.2 earthquake. Do we see a trend here? This earthquake suggests the strike-slip faulting extends at least to the Bowers Ridge.

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

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

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

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

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

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

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

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

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

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

  • Here is a figure that shows the plate age for seamounts in the Hawaii-Emperor Seamount Chain (Torsvik et al., 2017).

  • Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

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

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

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


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

Alaska | Kamchatka | Kurile


Earthquake Reports

  • 2017.06.02 M 6.8 Aleutians
  • 2017.05.08 M 6.2 Aleutians
  • 2017.05.01 M 6.3 British Columbia
  • 2017.03.29 M 6.6 Kamchatka
  • 2017.03.02 M 5.5 Alaska
  • 2016.09.05 M 6.3 Bering Kresla (west of Aleutians)
  • 2016.04.02 M 6.2 Alaska Peninsula
  • 2016.03.27 M 5.7 Aleutians
  • 2016.03.12 M 6.3 Aleutians
  • 2016.01.24 M 7.1 Alaska
  • 2015.11.09 M 6.2 Aleutians
  • 2015.11.02 M 5.9 Aleutians
  • 2015.11.02 M 5.9 Aleutians (update)
  • 2015.07.27 M 6.9 Aleutians
  • 2015.05.29 M 6.7 Alaska Peninsula
  • 2015.05.29 M 6.7 Alaska Peninsula (animations)
  • 1964.03.27 M 9.2 Good Friday
  • References

    • Gaedicke, C., Baranov, B., Seliverstov, N., Alexeiev, D., Tsukanov, N., Freitag, R., 2000. Structure of an active arc-continent collision area: the Aleutian-Kamchatka junction. Tectonophysics 325, 63–85
    • Hayes, G. P., D. J. Wald, and R. L. Johnson, 2012. Slab1.0: A three-dimensional model of global subduction zone geometries, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524.
    • Konstantnovskaia, 2001. Arc-continent collision and subduction reversal in the Cenozoic evolution of the Northwest Pacific: an example from Kamchatka (NE Russia) in Tectonophysics, v. 333, p. 75-94.
    • Krutikov, L., et al., 2008. Active Tectonics and Seismic Potential of Alaska, Geophysical Monograph Series 179, doi:10.1029/179GM07
    • Lange, D., Cembrano, J., Rietbrock, A., Haberland, C., Dahm, T., and Bataille, K., 2008. First seismic record for intra-arc strike-slip tectonics along the Liquiñe-Ofqui fault zone at the obliquely convergent plate margin of the southern Andes in Tectonophysics, v. 455, p. 14-24
    • Torsvik, T. H. et al., 2017. Pacific plate motion change caused the Hawaiian-Emperor Bend in Nat. Commun., v. 8, doi: 10.1038/ncomms15660
    • Wilson, J. Tuzo, 1963. “A possible origin of the Hawaiian Islands” in Canadian Journal of Physics. v. 41, p. 863–870 doi:10.1139/p63-094.

    Posted in alaska, earthquake, education, geology, pacific, plate tectonics, strike-slip, subduction, Transform

    Earthquake Report: Turkey!

    We had a couple of earthquakes in western Turkey today (in the Aegean Sea offshore of the Island of Lesbos, part of Greece). The M 6.3 earthquake shows evidence for extension (normal fault), based on the moment tensor (read below).

    The tectonics here are dominated by the compressional tectonics related to (1) the Alpide Belt, a convergent plate boundary formed in the Cenozoic that extends from Australia to Morocco and (2) the North Anatolia fault, a strike-slip fault system that strikes along northern Turkey and extends into Greece and the Aegean Sea.

    There is a series of normal faults in this region of the north Aegean Sea and today’s earthquakes are likely associated with that extensional regime. The M 6.3 epicenter plots near the Magiras fault, though the strike of the fault is different from the orientation of the moment tensor. Perhaps the fault is not optimally aligned to the modern tectonic strain. There was an earthquake on 1949.07.23 that had a similarly oriented fault plane solution (showing northeast-southwest extension), which probably occurred on the Northern Chios fault. See below (Papazachos et al., 1998).

    Below is my interpretive poster for this earthquake.

    I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I also include USGS seismicity from 1917-2017 for earthquakes with M ≥ 6.0.

    • 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 faults included in two fault databases. Faults in Italy are from the Instituto Nazionale di Geofisica e Vulcanologia Database of Individual Seismogenic Sources (DISS; Basili et al., 2008; DISS Working Group, 2015). This DISS is available online here. The faults in Greece are from the Greek Satabase of Seismogenic Sources (GreDaSS; Caputo et al., 2012). The GreDaSS is available online here.

      I include some inset figures in the poster.

    • In the upper right corner is a regional tectonic map from Dilek and Sandvol (2009). This shows all the major tectonic plate boundary faults, as well as some of the major intraplate faults for this region. Reverse/Thrust faults are labeled with triangles on the upthrown (hanging wall) side of the fault. strike slip faults show relative motion arrows on either sides of the fault. The different plates and microplates are colored. I place a cyan star in the general location of today’s earthquake (also placed in the other inset figures).
    • In the upper left corner is a map that shows focal mechanisms for historic earthquakes in this region. Note the focal mechanism for the 1949 earthquake and compare this with the M 6.3 earthquake moment tensor from today.
    • In the lower right corner I include a larger scale view of the seismicity and faults displayed in the main map. I here also include the fault planes from the active fault databases (orange rectilinear polygons). These polygons show how different faults dip in different directions. The strike slip faults have more narrow polygons becuase they dip more vertically than the normal and thrust/reverse faults. I label the two faults mentioned above (possibly related to the 2017 M 6.3 and 1949 M 6.5 earthquakes), the Magiras and Northern Chios faults (Caputo et al., 2012).
    • In the lower left corner is a figure from Ersoy et al. (2014). This shows their interpretation of the geodynamics of the Aegean Sea. They hypothesize that this region is rotating in a clockwise fashion, leading to extension in western Turkey and the northern Aegean Sea. The 1949 and 2017 earthquake fault plane solutions (focal mechanisma and moment tensors) are oriented correctly with this model.


    • Here is the tectonic map from Dilek and Sandvol (2009).

    • Tectonic map of the Aegean and eastern Mediterranean region showing the main plate boundaries, major suture zones, fault systems and tectonic units. Thick, white arrows depict the direction and magnitude (mm a21) of plate convergence; grey arrows mark the direction of extension (Miocene–Recent). Orange and purple delineate Eurasian and African plate affinities, respectively. Key to lettering: BF, Burdur fault; CACC, Central Anatolian Crystalline Complex; DKF, Datc¸a–Kale fault (part of the SW Anatolian Shear Zone); EAFZ, East Anatolian fault zone; EF, Ecemis fault; EKP, Erzurum–Kars Plateau; IASZ, Izmir–Ankara suture zone; IPS, Intra–Pontide suture zone; ITS, Inner–Tauride suture; KF, Kefalonia fault; KOTJ, Karliova triple junction; MM, Menderes massif; MS, Marmara Sea; MTR, Maras triple junction; NAFZ, North Anatolian fault zone; OF, Ovacik fault; PSF, Pampak–Sevan fault; TF, Tutak fault; TGF, Tuzgo¨lu¨ fault; TIP, Turkish–Iranian plateau (modified from Dilek 2006).

    • Here is a great map from Ersoy et al. (2014) that shows the geologic map of the region. Faults are shown also. Today’s earthquakes happened in the northwest corner of the figure 2 inset rectangle.

    • Tectonostratigraphic units and major tectonic elements of the Aegean Extensional Province (compiled from1/500,000 scaled geological maps of Greece (IGME) and Turkey (MTA), Okay and Tüysüz, 1999; Ring et al., 2001, 2010; Candan et al., 2005; van Hinsbergen et al., 2005; Ersoy and Palmer, 2013). CRCC: Central Rhodope, SRCC: Southern Rhodope, KCC: Kazdağ, CCC: Cycladic, SAC: South Aegean (Crete) core complexes. KKD: Kesebir–Kardamos Dome. MEMC1 and MEMC2 refer to first- and second-stage development of theMenderes Extensional Metamorphic Complex (MEMC). VİAS: Vardar–İzmir–Ankara suture zone, NAF: North Anatolian Fault Zone.

    • This is the Ersoy et al. (2014) map showing their interpretation of the modern deformation in the northern Aegean Sea and western Turkey.

    • Geological map showing the distribution of the Menderes Extensional Metamorphic Complex (MEMC), Oligocene–Miocene volcanic and sedimentary units and volcanic centers in the Aegean Extensional Province (compiled from geological maps of Greece (IGME) and Turkey (MTA), and adapted from Ersoy and Palmer, 2013). Extensional deformation field with rotation (rotational extension) is shown with gray field, and simplified from Brun and Sokoutis (2012), Kissel et al. (2003) and van Hinsbergen and Schmid (2012). İzmir–Balıkesir Transfer zone (İBTZ) give the outer limit for the rotational extension, and also limit of ellipsoidal structure of the MEMC. MEMC developed in two stages: the first one was accommodated during early Miocene by the Simav Detachment Fault (SDF) in the north; and the second one developed during Middle Miocene along the Gediz (Alaşehir) Detachment Fault (GDF) and Küçük Menderes Detachment Fault (KMDF). Extensional detachments were also accommodated by strike-slip movement along the İBTZ (Ersoy et al., 2011) and Uşak–Muğla Transfer Zone (Çemen et al., 2006; Karaoğlu and Helvacı, 2012). Other main core complexes in the Aegean, the Central Rhodope (CRCC), Southern Rhodope (SRCC), Kesebir–Kardamos Dome (KKD) and Cycladic (CCC) Core Complexes are also shown. The area bordered with dashed green line represents the surface trace of the asthenospheric window between the Aegean and Cyprean subducted slabs (Biryol et al., 2011; de Boorder et al., 1998). See text for detail.

    • This is a great figure showing another interpretation to explain the extension in this region (slab rollback and mantle flow) from Brun and Sokoutis (2012).

    • Mantle flow pattern at Aegean scale powered by slab rollback in rotation around vertical axis located at Scutary-Pec (Albania). A: Map view of fl ow lines above (red) and below (blue) slab. B: Three-dimensional sketch showing how slab tear may accommodate slab rotation. Mantle fl ow above and below slab in red and blue, respectively. Yellow arrows show crustal stretching.

    • The following three figures are from Dilek and Sandvol, 2006. The locations of the cross sections are shown on the map as orange lines. Cross section G-G’ is located in the region of today’s earthquake.
    • Here is the map (Dilek and Sandvol, 2006). I include the figure caption below in blockquote.

    • Simplified tectonic map of the Mediterranean region showing the plate boundaries, collisional zones, and directions of extension and tectonic transport. Red lines A through G show the approximate profile lines for the geological traverses depicted in Figure 2. MHSZ—mid-Hungarian shear zone; MP—Moesian platform; RM—Rhodope massif; IAESZ— Izmir-Ankara-Erzincan suture zone; IPS—Intra-Pontide suture zone; ITS—inner Tauride suture zone; NAFZ—north Anatolian fault zone; KB—Kirsehir block; EKP—Erzurum-Kars plateau; TIP—Turkish-Iranian plateau.

    • Here are cross sections A-D (Dilek and Sandvol, 2006). I include the figure caption below in blockquote.


    • Simplified tectonic cross-sections across various segments of the broader Alpine orogenic belt.

    • (A) Eastern Alps. The collision of Adria with Europe produced a bidivergent crustal architecture with both NNW- and SSE-directed nappe structures that involved Tertiary molasse deposits, with deep-seated thrust faults that exhumed lower crustal rocks. The Austro-Alpine units north of the Peri-Adriatic lineament represent the allochthonous outliers of the Adriatic upper crust tectonically resting on the underplating European crust. The Penninic ophiolites mark the remnants of the Mesozoic ocean basin (Meliata). The Oligocene granitoids between the Tauern window and the Peri-Adriatic lineament represent the postcollisional intrusions in the eastern Alps. Modified from Castellarin et al. (2006), with additional data from Coward and Dietrich (1989); Lüschen et al. (2006); Ortner et al. (2006).
    • (B) Northern Apennines. Following the collision of Adria with the Apenninic platform and Europe in the late Miocene, the westward subduction of the Adriatic lithosphere and the slab roll-back (eastward) produced a broad extensional regime in the west (Apenninic back-arc extension) affecting the Alpine orogenic crust, and also a frontal thrust belt to the east. Lithospheric-scale extension in this broad back-arc environment above the west-dipping Adria lithosphere resulted in the development of a large boudinage structure in the European (Alpine) lithosphere. Modified from Doglioni et al. (1999), with data from Spakman and Wortel (2004); Zeck (1999).
    • (C) Western Mediterranean–Southern Apennines–Calabria. The westward subduction of the Ionian seafloor as part of Adria since ca. 23 Ma and the associated slab roll-back have induced eastward-progressing extension and lithospheric necking through time, producing a series of basins. Rifting of Sardinia from continental Europe developed the Gulf of Lion passive margin and the Algero-Provencal basin (ca. 15–10 Ma), then the Vavilov and Marsili sub-basins in the broader Tyrrhenian basin to the east (ca. 5 Ma to present). Eastward-migrating lithospheric-scale extension and
      necking and asthenospheric upwelling have produced locally well-developed alkaline volcanism (e.g., Sardinia). Slab tear or detachment in the Calabria segment of Adria, as imaged through seismic tomography (Spakman and Wortel, 2004), is probably responsible for asthenospheric upwelling and alkaline volcanism in southern Calabria and eastern Sicily (e.g., Mount Etna). Modified from Séranne (1999), with additional data from Spakman et al. (1993); Doglioni et al. (1999); Spakman and Wortel (2004); Lentini et al. (this volume).
    • (D) Southern Apennines–Albanides–Hellenides. Note the break where the Adriatic Sea is located between the western and eastern sections along this traverse. The Adria plate and the remnant Ionian oceanic lithosphere underlie the Apenninic-Maghrebian orogenic belt. The Alpine-Tethyan and Apulian platform units are telescoped along ENE-vergent thrust faults. The Tyrrhenian Sea opened up in the latest Miocene as a back-arc basin behind the Apenninic-Maghrebian mountain belt. The Aeolian volcanoes in the Tyrrhenian Sea represent the volcanic arc system in this subduction-collision zone environment. Modified from Lentini et al. (this volume). The eastern section of this traverse across the Albanides-Hellenides in the northern Balkan Peninsula shows a bidivergent crustal architecture, with the Jurassic Tethyan ophiolites (Mirdita ophiolites in Albania and Western Hellenic ophiolites in Greece) forming the highest tectonic nappe, resting on the Cretaceous and younger flysch deposits of the Adria affinity to the west and the Pelagonia affinity to the east. Following the emplacement of the Mirdita- Hellenic ophiolites onto the Pelagonian ribbon continent in the Early Cretaceous, the Adria plate collided with Pelagonia-Europe obliquely starting around ca. 55 Ma. WSW-directed thrusting, developed as a result of this oblique collision, has been migrating westward into the peri-Adriatic depression. Modified from Dilek et al. (2005).
    • (E) Dinarides–Pannonian basin–Carpathians. The Carpathians developed as a result of the diachronous collision of the Alcapa and Tsia lithospheric blocks, respectively, with the southern edge of the East European platform during the early to middle Miocene (Nemcok et al., 1998; Seghedi et al., 2004). The Pannonian basin evolved as a back-arc basin above the eastward retreating European platform slab (Royden, 1988). Lithospheric-scale necking and boudinage development occurred synchronously with this extension and resulted in the isolation of continental fragments (e.g., the Apuseni mountains) within a broadly extensional Pannonian basin separating the Great Hungarian Plain and the Transylvanian subbasin. Steepening and tearing of the west-dipping slab may have caused asthenospheric flow and upwelling, decompressional melting, and alkaline volcanism (with an ocean island basalt–like mantle source) in the Eastern Carpathians. Modified from Royden (1988), with additional data from Linzer (1996); Nemcok et al. (1998); Doglioni et al. (1999); Seghedi et al. (2004).
    • (F) Arabia-Eurasia collision zone and the Turkish-Iranian plateau. The collision of Arabia with Eurasia around 13 Ma resulted in (1) development of a thick orogenic crust via intracontinental convergence and shortening and a high plateau and (2) westward escape of a lithospheric block (the Anatolian microplate) away from the collision front. The Arabia plate and the Bitlis-Pütürge ribbon continent were probably amalgamated earlier (ca. the Eocene) via a separate collision event within the Neo-Tethyan realm. BSZ—Bitlis suture zone; EKP—Erzurum-Kars plateau. A slab break-off and the subsequent removal of the lithospheric mantle (lithospheric delamination) beneath the eastern Anatolian accretionary complex caused asthenospheric upwelling and extensive melting, leading to continental volcanism and regional uplift, which has contributed to the high mean elevation of the Turkish-Iranian plateau. The Eastern Turkey Seismic Experiment results have shown that the crustal thickness here is ~ 45–48 km and that the Turkish-Iranian plateau is devoid of mantle lithosphere. The collision-induced convergence has been accommodated by active diffuse north-south shortening and oblique-slip faults dispersing crustal blocks both to the west and the east. The late Miocene through Plio-Quaternary volcanism appears to have become more alkaline toward the south in time. The Pleistocene Karacadag shield volcano in the Arabian foreland represents a local fissure eruption associated with intraplate extension. Data from Pearce et al. (1990); Keskin (2003); Sandvol et al. (2003); S¸engör et al. (2003).
    • (G) Africa-Eurasia collision zone and the Aegean extensional province. The African lithosphere is subducting beneath Eurasia at the Hellenic trench. The Mediterranean Ridge represents a lithospheric block between the Africa and Eurasian plate (Hsü, 1995). The Aegean extensional province straddles the Anatolide-Tauride and Sakarya continental blocks, which collided in the Eocene. NAF—North Anatolian fault. South-transported Tethyan ophiolite nappes were derived from the suture zone between these two continental blocks. Postcollisional granitic intrusions (Eocone and Oligo-Miocene, shown in red) occur mainly north of the suture zone and at the southern edge of the Sakarya continent. Postcollisional volcanism during the Eocene–Quaternary appears to have migrated southward and to have changed from calc-alkaline to alkaline in composition through time. Lithospheric-scale necking, reminiscent of the Europe-Apennine-Adria collision system, and associated extension are also important processes beneath the Aegean and have resulted in the exhumation of core complexes, widespread upper crustal attenuation, and alkaline and mid-ocean ridge basalt volcanism. Slab steepening and slab roll-back appear to have been at work resulting in subduction zone magmatism along the Hellenic arc.
    • Here is another cross section that shows the temporal evolution of the tectonics of this region in the area of cross section G-G’ above (Dilek and Sandvol, 2009).

    • Late Mesozoic–Cenozoic geodynamic evolution of the western Anatolian orogenic belt as a result of collisional
      and extensional processes in the upper plate of north-dipping subduction zone(s) within the Tethyan realm. See text
      for discussion.

    References

    • Basili R., G. Valensise, P. Vannoli, P. Burrato, U. Fracassi, S. Mariano, M.M. Tiberti, E. Boschi (2008), The Database of Individual Seismogenic Sources (DISS), version 3: summarizing 20 years of research on Italy’s earthquake geology, Tectonophysics, doi:10.1016/j.tecto.2007.04.014
    • Brun, J.-P., Sokoutis, D., 2012. 45 m.y. of Aegean crust and mantle flow driven by trench retreat. Geol. Soc. Am., v. 38, p. 815–818.
    • Caputo, R., Chatzipetros, A., Pavlides, S., and Sboras, S., 2012. The Greek Database of Seismogenic Sources (GreDaSS): state-of-the-art for northern Greece in Annals of Geophysics, v. 55, no. 5, doi: 10.4401/ag-5168
    • Dilek, Y. and Sandvol, E., 2006. Collision tectonics of the Mediterranean region: Causes and consequences in Dilek, Y., and Pavlides, S., eds., Postcollisional tectonics and magmatism in the Mediterranean region and Asia: Geological Society of America Special Paper 409, p. 1–13
    • DISS Working Group (2015). Database of Individual Seismogenic Sources (DISS), Version 3.2.0: A compilation of potential sources for earthquakes larger than M 5.5 in Italy and surrounding areas. http://diss.rm.ingv.it/diss/, Istituto Nazionale di Geofisica e Vulcanologia; DOI:10.6092/INGV.IT-DISS3.2.0.
    • Ersoy, E.Y., Cemen, I., Helvaci, C., and Billor, Z., 2014. Tectono-stratigraphy of the Neogene basins in Western Turkey: Implications for tectonic evolution of the Aegean Extended Region in Tectonophysics v. 635, p. 33-58.
    • Papazachos, B.C., Papadimitrious, E.E., Kiratzi, A.A., Papazachos, C.B., and Louvari, E.k., 1998. Fault Plane Solutions in the Aegean Sea and the Surrounding Area and their Tectonic Implication, in Bollettino Di Geofisica Terorica Ed Applicata, v. 39, no. 3, p. 199-218.
    • Wouldloper, 2009. Tectonic map of southern Europe and the Middle East, showing tectonic structures of the western Alpide mountain belt. Only Alpine (tertiary) structures are shown.

    Posted in earthquake, europe, Extension, geology, plate tectonics

    Earthquake Report: Trinidad, California

    Early this morning, I was awakened by a mild jolt. I thought, well, seems like a M 3+- nearby. I did not get out of bed. The main shaking lasted a couple of seconds, though it seemed that there was some additional shaking for several more seconds afterwards (secondary shaking? I live in the Manila Dunes, which overlie several kms of water saturated sediment.

    This earthquake is quite interesting. The hypocentral depth is about 20 km. The subduction zone fault has been modeled to be between 15 and 20 km depth at this location (McCrory et al., 2006, 2012). There is considerable uncertainty associated with this slab model (the “slab” refers to the downgoing oceanic lithosphere of the Gorda plate). If this earthquake were an interface event (on the subduction zone), the moment tensor would probably be a thrust fault solution. However, the USGS moment tensor is for a strike-slip earthquake. There was an M 4.8 earthquake on 2016.07.21 that had a similar orientation. Here are my two earthquake reports for that earthquake: (1) initial report and (2) update # 1. I also spoke with Bob McPherson about this earthquake and, without speaking for him, we agreed that this is indeed an interesting earthquake.

    • So, we can probably rule out this as a subduction zone interface earthquake. Then lets consider the other two options: (1) Gorda plate intraplate earthquake or (2) North America plate intraplate earthquake.
    1. The Gorda plate has a structural grain associated with its initial formation at the Gorda Rise. These faults initially form as ~north-south striking normal faults. As the plate is deformed with time, the faults in the southern half of the plate rotate in a clockwise fashion. As a result of the north-south compression (from the Pacific plate moving northwards,
      crushing the Gorda plate), these northeast striking faults slip with a left-lateral strike-slip motion. Today’s M 3.5 earthquake is not oriented with a northeast orientation. However, as these faults extend northwards, the strike of the faults tend to rotate back with a more northerly strike. It is possible that the faults in the Gorda plate have a north-south strike in the region of today’s earthquake. If this were the case, this would be a north-south striking left-lateral strike-slip earthquake.
    2. The North America plate (NAP) in this region has been sliced and diced by a suite of different tectonic forces that have changed with time. Prior to about 0.5 million years ago, the dominant tectonic regime was simply the subduction zone. The subduction zone exerted stresses into the NAP that resulted in thrust faults (and possibly forearc sliver faults). After that, the San Andreas fault (and the Mendocino triple junction, MTJ) came on the scene. Tertiary rocks have been uplifted and tilted northwards because of this influence. Also, the earlier formed thrust faults may rotate around to a more east-west orientation in the Humboldt Bay and south region. As the MTJ migrates north (which may not be the best way to view this motion), some San Andreas oriented fault motion has penetrated into the region north of the MTJ. The Trinidad and Big Lagoon faults are mapped as strike-slip faults offshore. These faults may have formed this sense of motion prior to the MTJ arrival (due to oblique plate motion on the subduction zone, formed as forearc sliver faults; Lange et al., 2008). One of the strands of the Big Lagoon fault zone is oriented north-south. The only (major) problem with this possibility is that these NAP strike-slip faults are all right-lateral. Today’s moment tensor, if using the north-south solution, is left-lateral. So, this is not a reasonable interpretation.

    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 highlighted the north-south striking Big Lagoon fault with a yellow line. I also labeled Mt. Shasta. I placed labels for the three major thrust fault systems in this region (Big Lagoon fault zone, Mad River fault zone, and the Little Salmon fault zone). The Big Lagoon and Mad River fault zones have offshore strike-slip motion. Also, the Little Salmon fault probably also has significant strike-slip motion (Pollitz et al., 2010).

    • 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., 2006, 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 label the shallowest contours.

      I include some inset figures in the poster.

    • In the lower right corner I include a map of the Cascadia subduction zone (Chaytor et al., 2004; Nelson et al., 2004, 2006). I mention more about this below.
    • In the upper left corner I include a map from Rollins and Stein (2010) that show some historic earthquakes in the context of the regional tectonics. Their paper documents how these different earthquakes impose increased and decreased coulomb stress upon different faults following these earthquakes.
    • Below the Rollins and Stein (2010) figure is a figure from Chaytor et al. (2004) that shows 7 different models to explain the internal deformation in the Gorda plate.
    • In the upper right corner is a larger scale map showing the USGS Quaternary fault and fold database faults overlain upon Google Earth imagery (just like the main map). I also include labels like in the main map.


    Here is the interpretive poster for the 2016.07.21 Bayside Earthquake.


    • Here is a map of the Cascadia subduction zone, modified from Nelson et al. (2004). The Juan de Fuca and Gorda plates subduct norteastwardly beneath the North America plate at rates ranging from 29- to 45-mm/yr. Sites where evidence of past earthquakes (paleoseismology) are denoted by white dots. Where there is also evidence for past CSZ tsunami, there are black dots. These paleoseismology sites are labeled (e.g. Humboldt Bay). Some submarine paleoseismology core sites are also shown as grey dots. The two main spreading ridges are not labeled, but the northern one is the Juan de Fuca ridge (where oceanic crust is formed for the Juan de Fuca plate) and the southern one is the Gorda rise (where the oceanic crust is formed for the Gorda plate).

    • Here is a version of the CSZ cross section alone (Plafker, 1972). This shows two parts of the earthquake cycle: the interseismic part (between earthquakes) and the coseismic part (during earthquakes). Regions that experience uplift during the interseismic period tend to experience subsidence during the coseismic period.

    • Here is a figure from Chaytor et al. (2004) that shows how they interpret the different faults based upon bathymetric data. Note the north-south striking faults in the northern part of the Gorda plate. However, they are normal faults, not strike slip. So, this makes it more difficult (again) to interpret today’s M 3.5 earthquake.

    • A: Mapped faults and fault-related ridges within Gorda plate based on basement structure and surface morphology, overlain on bathymetric contours (gray lines—250 m interval). Approximate boundaries of three structural segments are also shown. Black arrows indicated approximate location of possible northwest- trending large-scale folds. B, C:
      Uninterpreted and interpreted enlargements of center of plate showing location of interpreted second-generation strike-slip faults and features that they appear to offset. OSC—overlapping spreading center.

    • Here is another figure from Chaytor et al. (2004) that shows the different models for the Gorda plate faults.

    • Models of brittle deformation for Gorda plate overlain on magnetic anomalies modified from Raff and Mason (1961). Models A–F were proposed prior to collection and analysis of full-plate multibeam data. Deformation model of Gulick et al. (2001) is included in model A. Model G represents modification of Stoddard’s (1987) flexural-slip model proposed in this paper.

    • Here is a map showing a number of data sets. Seismicity is plotted versus depth (NCEDC). Tremor is plotted (Pacific Northwest Seismic Network). Vertical Deformation rates are plotted (unpublished). Slab depth contours (km) are plotted (McCrory et al., 2006). Fault locking zones are plotted (Wang et al., 2003; Burgette et al., 2009). Bob McPherson (Humboldt State University, Department of Geology) is currently working on a research paper where he will discuss how the seismicity reveals the location of the seismogenically locked fault zone.

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

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

    • As mentioned above, Pollitz et al. (2010) modeled interseismic deformation along faults in the Pacific northwest and fit this deformation to GPS geodetic data. The authors evaluated how San Andreas type fault motion penetrates into the southern Cascadia subduction zone. Below are two figures from their paper that helps us understand their interpretations. The upper figure shows the GPS velocity field and the strain rate field for this region of northern California. The lower panel shows an estimate of right-lateral strike-slip rates for the Little Salmon fault.

    • Left-hand panel: velocity field obtained after correcting the observed GPS velocity field (Fig. 3) for the effect of deformation associated with all GDZ, Juan de Fuca, and Explorer plate boundaries. The sources that contribute to the correction are faults #30–46 and 81 of Table 1. Right-hand panel: strain rate fields corresponding to the plotted velocity fields, represented by the amplitudes and directions of the principal strain rate axes (thick and thin line segments denoting a principal contractile or tensile strain rate axis, respectively) and rotation rate (indicated by color shading). It is derived from the velocity field using the velocity-gradient determination method described in appendix A of Pollitz & Vergnolle (2006).


      Estimated right-lateral strike-slip rate on the Little Salmon fault as a function of strike-slip rate on the Russ fault. Reverse slip rate on the Mad River fault is held fixed at 10 mmyr−1. Slip rates are plotted with ±1 SD.

    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.
    • Burgette, R. et al., 2009. Interseismic uplift rates for western Oregon and along-strike variation in locking on the Cascadia subduction zone in Journal of Geophysical Research, v. 114, B01408, doi:10.1029/2008JB005679
    • Chaytor, J.D., Goldfinger, C., Dziak, R.P., and Fox, C.G., 2004. Active deformation of the Gorda plate: Constraining deformation models with new geophysical data: Geology v. 32, p. 353-356
    • Lange, D., Cembrano, J., Rietbrock, A., Haberland, C., Dahm, T., and Bataille, K., 2008. First seismic record for intra-arc strike-slip tectonics along the Liquiñe-Ofqui fault zone at the obliquely convergent plate margin of the southern Andes in Tectonophysics, v. 455, p. 14-24
    • 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
    • McCrory, P. A., Blair, J. L., Waldhauser, F., and Oppenheimer, D. H., 2012. Juan de Fuca slab geometry and its relation to Wadati-Benioff zone seismicity in JGR, v. 117, doi:10.1029/2012JB009407
    • Nelson, A.R., Asquith, A.C., and Grant, W.C., 2004. Great Earthquakes and Tsunamis of the Past 2000 Years at the Salmon River Estuary, Central Oregon Coast, USA: Bulletin of the Seismological Society of America, Vol. 94, No. 4, pp. 1276–1292
    • Nelson, A.R., Kelsey, H.M., and Witter, R.C., 2006. Great earthquakes of variable magnitude at the Cascadia subduction zone: Quaternary Research, doi:10.1016/j.yqres.2006.02.009, p. 354-365.
    • 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.
    • Pollitz, F.F., McCrory, P., Wilson, D., Svarc, J., Puskas, C., and Smith, R.B., 2010. Viscoelastic-cycle model of interseismic deformation in the northwestern United States in GJI, v. 181, p. 665-696, doi: 10.1111/j.1365-246X.2010.04546.x
    • Rollins, J.C., 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 115, 19 pp.
    • USGS Quaternary Fault Database: http://earthquake.usgs.gov/hazards/qfaults/
    • 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, geology, gorda, pacific, plate tectonics, San Andreas, strike-slip, subduction

    Earthquake Report: Sulawesi, Indonesia

    There was a series of earthquakes in Sulawesi, Indonesia earlier today, with a mainshock having a magnitude of M 6.8. This series of earthquakes is interesting as it does not occur on the main plate boundary fault, but on upper plate faults in the region. There is a major left-lateral strike-slip fault system to the west of these earthquakes (the Palu-Koro fault).

    Part of this being interesting is that the orientation of the earthquake is oblique to some estimates of the orientation of extension in this region. The M 6.8 earthquake shows an extensional earthquake with extension oriented ~north-south. Some estimate extension in the upper plate to be northeast-southwest (Bellier et al., 2006), while others estimate extension in the upper plate to be oriented parallel to the M 6.8 earthquake (e.g. Walpersdorf et al., 1998). Spencer (2010) also documented normal faults in the upper plate that may also be correctly oriented for this M 6.8 earthquake. However, looking at the SRTM topographic data using the GeoMapApp, there is a structural grain that appears oriented to the extension estimated by Bellier et al., 2006.

    • Here are the USGS earthquake websites for this sequence.
    • 2017.05.29 14:35 M 6.8
    • 2017.05.29 14:53 M 4.7
    • 2017.05.29 15:04 M 5.1
    • 2017.05.29 15:18 M 5.1

    Below is my interpretive poster for this earthquake.

    I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I also include seismicity from 1917-2017 for earthquakes with magnitudes M ≥ 7.5. Here is the USGS derived Google Earth kml file I used to create this map. I show the fault plane solutions for one of these earthquakes (1996 M 7.9). The 1996 M 7.9 earthquake is oriented with the subduction fault on the north side of Sulawesi. Interestingly, there is no seismicity M ≥ 7.5 along the strike-slip systems here.

    • 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 did not include the slab contours plotted (Hayes et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault. The hypocentral depth plots this close to the location of the fault as mapped by Hayes et al. (2012).

      I include some inset figures in the poster.

    • In the lower left corner is a general tectonic map for this part of the world (Zahirovic et al., 2014). I placed a green star in the location of this M 6.8 earthquake.
    • In the lower right corner is a low-angle oblique view of the plate boundaries in the northern part of this region (Hall, 2011). The upper part of the diagram shows the opposing vergent subduction zones along that strike north-south along the Molucca Strait (Halmahera, Philippines). The lower panel shows the downgoing Australia plate along the Timor Trench and Seram Trench.. I placed a green star in the location of this M 6.8 earthquake.
    • In the upper right corner I include an inset of a seismic hazard map for this region of Indonesia. This map is from the Indonesian National Agency for Disaster Management (2011). Note the high seismic hazard associated with the Palu-Koro and Matano faults.
    • In the upper right corner I include a tectonic map showing the major fault systems and generalized plate motions (Bellier et al., 2006). Note the northeast-southwest orientation of extension in the Central Sulawesi block. I present another figure from this publication below.
    • To the right of this Bellier et al. (2006) map is another figure from that same publication. This is more generalized and shows the orientation of the faults in this region.



    • Here is the tectonic map from Bellier et al., 2006. I include their caption below in blockquote.


    • Regional geodynamic sketch that presents the present day deformation model of Sulawesi area (after Beaudouin et al., 2003) and four main deformation systems around the Central Sulawesi block, highlighting the tectonic complexity of Sulawesi. Approximate location of the Central Sulawesi block rotation pole (P) [compatible with both GPS measurements (Walpersdorf et al., 1998a) and earthquake moment tensor analyses (Beaudouin et al., 2003)], as well as the major active structures are reported. Central Sulawesi Fault System (CSFS) is formed by the Palu–Koro and Matano faults. Arrows correspond to the compression and/or extension directions deduced from both inversion and moment tensor analyses of the focal mechanisms; arrow size being proportional to the deformation rate (e.g., Beaudouin et al., 2003).We also represent the focal mechanism provided by the Harvard CMT database [CMT data base, 2005] for the recent large earthquake (Mw=6.2; 2005/1/23; lat.=0.928S; long.=120.108E). The box indicates the approximate location of the Fig. 6 that corresponds to the geological map of the Palu basin region. The bottom inset shows the SE Asia and Sulawesi geodynamic frame where arrows represent the approximate Indo-Australian and Philippines plate motions relative to Eurasia.

    • Here is the larger scale map showing the fault configuration in this region (Bellier et al., 2006)


    • Sketch map of the Cenozoic Central Sulawesi fault system. ML represents the Matano Lake, and Leboni RFZ, the Leboni releasing fault zone that connects the Palu–Koro and Matano Faults. Triangles indicate faults with reverse component (triangles on the upthrown block). On this map are reported the fault kinematic measurement sites (geographic coordinates in Table 3).

    • The extension shown in the Bellier et al. (2006) map above is largely the result of analyses conducted by and presented in Beaudouin et al. (2003). Here I present their figure where they summarize their results of block modeling using historic seismicity to drive the strain in this region. It is possible that the century of seismicity data is insufficient to account for the strain here. This may explain why the orientation of the M 6.8 earthquake is not oriented like suggested in this map below.




    • Here is a figure from Walpersdorf et al. (1998) that shows regional plate motions and the tectonic faults in the region. Note that the extension is oriented parallel to the M 6.8 extension. These data are based upon their analyses of GPS geodetic data. So, given the orientation of the M 6.8 earthquake and these data, I suspect this is the correct orientation of extension. Though, this is not consistent with the topographic data I present below.


    • Distribution of the calk alkalic potassic (CAK) volcanism in Sulawesi. In the west arm this volcanism is restricted to the central part of the arm, while east of the Palu–Koro fault zone CAK volcanism is distributed across a NW–SE 200 km wide belt extending from north Sulawesi to the Una-Una Island. The two synthetic cross sections illustrate the contrasting distribution of this volcanism on both sides of the Palu–Koro fault zone. Extension of the Sula-Buton=north Sulawesi arc is speculative. The double arrow illustrates extension in the Gulf of Gorontalo. Dashed lines in cross sections indicate the presence at depth of the remnant subducted Tethys oceanic crust.

    • 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.

    • Here is a map from Spencer (2010). Today’s M 6.8 occurred along the cross section A-A.’


    • Elevation and shaded-relief maps and topographic cross sections derived from the SRTM DEM using GeoMapApp©. (A) Map of the Sulawesi and surrounding areas, with bathymetry derived from the Marine Geoscience Data System bathymetry database. Geologic features from Hamilton (1979) and Silver et al. (1983). (B) Map of central Sulawesi (location in A) showing inferred detachment faults (double ticks on hanging wall) and high-angle faults (red lines). (C) Map of the Tokorondo massif (location in B) showing inferred detachment fault and high-angle faults. (D) Topographic cross sections (location in C) of Tokorondo massif.

    • Here is a map from the GeoMapApp, using Global Multi-Resolution Topography (GMRT) topographic data (Ryan et al., 2009). Note the north-northwest structural grain. These appear to be normal faults oriented with a east-northeast/west-southwest extension from Bellier et al. (2006). This is the same general region as presented in the Spencer (2010) map above. Note the two large rounded plateau-highlands and the low lying basins (lakes are not outlined in this map).


    References:

    • Bellier, O., Se´brier, M., Seward, D., Beaudouin, T., Villeneuve, M., and Putranto, E., 2006. Fission track and fault kinematics analyses for new insight into the Late Cenozoic tectonic regime changes in West-Central Sulawesi (Indonesia) in Tectonophysics, v. 413, p. 201-220.
    • Benz, H.M., Herman, Matthew, Tarr, A.C., Hayes, G.P., Furlong, K.P., Villaseñor, Antonio, Dart, R.L., and Rhea, Susan, 2011. Seismicity of the Earth 1900–2010 New Guinea and vicinity: U.S. Geological Survey Open-File Report 2010–1083-H, scale 1:8,000,000.
    • Hall, R., 2011. Australia-SE Asia collision: plate tectonics and crustal flow in Geological Society, London, Special Publications 2011; v. 355; p. 75-109 doi: 10.1144/SP355.5
    • 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
    • Hayes, G.P., Smoczyk, G.M., Benz, H.M., Villaseñor, Antonio, and Furlong, K.P., 2015. Seismicity of the Earth 1900–2013, Seismotectonics of South America (Nazca Plate Region): U.S. Geological Survey Open-File Report 2015–1031–E, 1 sheet, scale 1:14,000,000, http://dx.doi.org/10.3133/ofr20151031E.
    • Ryan, W.B.F., S.M. Carbotte, J.O. Coplan, S. O’Hara, A. Melkonian, R. Arko, R.A. Weissel, V. Ferrini, A. Goodwillie, F. Nitsche, J. Bonczkowski, and R. Zemsky, 2009. Global Multi-Resolution Topography synthesis, Geochem. Geophys. Geosyst., 10, Q03014, doi: 10.1029/2008GC002332
    • Walpersdorf, A., Rangin, C., and Vigny, C., 1998. GPS compared to long-term geologic motion of the north arm of Sulawesi in EPSL, v. 159, p. 47-55.
    • Zahirovic, S., Seton, M., and Müller, R.D., 2014. The Cretaceous and Cenozoic tectonic evolution of Southeast Asia in Solid Earth, v. 5, p. 227-273, doi:10.5194/se-5-227-2014

    Posted in asia, earthquake, education, Extension, Indonesia, plate tectonics

    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.

    Posted in Uncategorized

    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