Earthquake Report: Turkey

We just had a good shaker in western Turkey. At the moment, there are over 400 reports of ground shaking to the USGS “Did you Feel It?” web page. The USGS PAGER report estimates that there may be some casualties (though a low number of them), but that the economic loss estimate is higher (35% chance of between 10 and 100 million USD).

This earthquake appears to have been along a normal fault named either the Bodum fault (NOA; Helenic Seismic Network) or the Ula-Oren fault (GreDASS; Greek Database of Seismogenic Sources). The inset map shows the faults and fault planes from the GreDASS database. A third name for this fault is the Gökova fault (Kurt et al., 1999).

Here is the USGS website for this earthquake.

There is lots of information on the European-Mediterranean Seismological Centre (EMSC) page here.

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 1917-2017 for magnitudes M ≥ 6.5. This is also the time and magnitude range of earthquakes in the inset 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 plot moment tensors for the M 6.3 earthquake. Based upon the series of earthquakes and the mapped faults, I interpret this M 6.7 earthquake to be a normal fault (extensional) earthquake.
  • 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 from the (Database of Individual Seismogenic Sources (DISS), Version 3.2.0), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.

    I include some inset figures in the poster.

  • In the lower left corner I include a map of the regional tectonics (Dilek and Sandvol, 2009). I place a green star in the general location of today’s M 6.7 earthquake.
  • In the lower right corner is a figure from Jolivet et al. (2013) that shows focal mechanisms for earthquakes across the Aegean-Anatolian region. Earthquakes plotted in the region of today’s M 6.7 (the green star) are all normal (extensional) earthquakes (with one extensional oblique).
  • In the upper right corner is a tectonic map of western Eurasia and northern Africa (Dilek, 2006). Today’s earthquake lies near the cross section G-G (in yellow). I also show the general location of this cross-section on the main map.
  • Below this map is a figure showing a north-south cross section through this region (Dilek, 2006), G-G on the above map. This shows the subduction zone in the south, the transform fault (North Anatolian fault) in the north, and the Aegean Extensional Province in the center. Today’s earthquake is along the southern boundary of the core complex, which is in the center of this extensional province.
  • In the upper left corner is a larger scale map showing the same earthquakes as the main map. I also include the faults and fault planes from the GreDASS database. I also label the larger earthquakes in this region. Note the 2017 M 6.3 Lesbos earthquake in the north. Here is my earthquake report for that earthquake. Note the flare up of seismicity in the 1950s, possibly beginning in 1948.


  • Here is the same poster, but with USGS earthquake epicenters from 2007-2017 with magnitude M ≥ 4.5.

  • There was a small tsunami recorded at the Bodum tide gage. Here is the source.

  • 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).

  • Below is a series of figures from Jolivet et al. (2013). These show various data sets and analyses for Greece and Turkey.
  • Upper Panel (A): This is a tectonic map showing the major faults and geologic terranes in the region. The fault possibly associated with today’s earthquake is labeled OU on the map, for the Ula-Oren fault.
  • Lower Panel (B): This shows historic seismicity for the region. Note the general correlation with the faults in the upper panel.

  • A: Tectonic map of the Aegean and Anatolian region showing the main active structures
    (black lines), the main sutures zones (thick violet or blue lines), the main thrusts in the Hellenides where they have not been reworked by later extension (thin blue lines), the North Cycladic Detachment (NCDS, in red) and its extension in the Simav Detachment (SD), the main metamorphic units and their contacts; AlW: Almyropotamos window; BD: Bey Daglari; CB: Cycladic Basement; CBBT: Cycladic Basement basal thrust; CBS: Cycladic Blueschists; CHSZ: Central Hellenic Shear Zone; CR: Corinth Rift; CRMC: Central Rhodope Metamorphic Complex; GT: Gavrovo–Tripolitza Nappe; KD: Kazdag dome; KeD: Kerdylion Detachment; KKD: Kesebir–Kardamos dome; KT: Kephalonia Transform Fault; LN: Lycian Nappes; LNBT: Lycian Nappes Basal Thrust; MCC: Metamorphic Core Complex; MG: Menderes Grabens; NAT: North Aegean Trough; NCDS: North Cycladic Detachment System; NSZ: Nestos Shear Zone; OlW: Olympos Window; OsW: Ossa Window; OSZ: Ören Shear Zone; Pel.: Peloponnese; ÖU: Ören Unit; PQN: Phyllite–Quartzite Nappe; SiD: Simav Detachment; SRCC: South Rhodope Core Complex; StD: Strymon Detachment; WCDS: West Cycladic Detachment System; ZD: Zaroukla Detachment. B: Seismicity. Earthquakes are taken from the USGS-NEIC database. Colour of symbols gives the depth (blue for shallow depths) and size gives the magnitude (from 4.5 to 7.6).

  • Upper Panel (C): These red arrows are Global Positioning System (GPS) velocity vectors. The velocity scale vector is in the lower left corner. The main geodetic (study of plate motions and deformation of the earth) signal here is the westward motion of the North Anatolian fault system as it rotates southward as it traverses Greece. The motion trends almost south near the island of Crete, which is perpendicular to the subduction zone.
  • Lower Panel (D): This map shows the region of mid-Cenozoic (Oligo-Miocene) extension (shaded orange). It just happens that there is still extension going on in parts of this prehistoric extension.

  • C: GPS velocity field with a fixed Eurasia after Reilinger et al. (2010) D: the domain affected by distributed post-orogenic extension in the Oligocene and the Miocene and the stretching lineations in the exhumed metamorphic complexes.

  • Upper Panel (E): This map shows where the downgoing slab may be located (in blue), along with the volcanic centers associated with the subduction zone in the past.
  • Lower Panel (F): This map shows the orientation of how seismic waves orient themselves differently in different places (anisotropy). We think seismic waves travel in ways that reflects how tectonic strain is stored in the earth. The blue lines show the direction of extension in the asthenosphere, green lines in the lithospheric mantle, and red lines for the crust.

  • E: The thick blue lines illustrate the schematized position of the slab at ~150 km according to the tomographic model of Piromallo and Morelli (2003), and show the disruption of the slab at three positions and possible ages of these tears discussed in the text. Velocity anomalies are displayed in percentages with respect to the reference model sp6 (Morelli and Dziewonski, 1993). Coloured symbols represent the volcanic centres between 0 and 3 Ma after Pe-Piper and Piper (2006). F: Seismic anisotropy obtained from SKS waves (blue bars, Paul et al., 2010) and Rayleigh waves (green and orange bars, Endrun et al., 2011). See also Sandvol et al. (2003). Blue lines show the direction of stretching in the asthenosphere, green bars represent the stretching in the lithospheric mantle and orange bars in the lower crust.

  • Upper Panel (G): This is the map showing focal mechanisms in the poster above. Note the strike slip earthquakes associated with the North Anatolian fault and the thrust/reverse mechanisms associated with the thrust faults.

  • G: Focal mechanisms of earthquakes over the Aegean Anatolian region.

  • Here is a figure showing a north-south cross section through this region, from ~95 million years ago until about 2 million years ago (Dilek and Sandvol, 2009). This figure shows how the regional tectonics have developed over time, with the modern subduction zone in the south, the North Anatolian transform fault in the north, and an extensional metamorphic core complex in the center (“Core Complex” on cross section). Today’s earthquake is along the southern boundary of this core complex.

  • 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.

  • 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 flow 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.

  • Finally, here is a map showing tectonic domains (Taymaz et al., 2007).

  • Schematic map of the principal tectonic settings in the Eastern Mediterranean. Hatching shows areas of coherent motion and zones of distributed deformation. Large arrows designate generalized regional motion (in mm a21) and errors (recompiled after McClusky et al. (2000, 2003). NAF, North Anatolian Fault; EAF, East Anatolian Fault; DSF, Dead Sea Fault; NEAF, North East Anatolian Fault; EPF, Ezinepazarı Fault; CTF, Cephalonia Transform Fault; PTF, Paphos Transform Fault.

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., 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
  • 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.
  • Jolivet, L., et al., 2013. Aegean tectonics: Strain localisation, slab tearing and trench retreat in Tectonophysics, v. 597-598, p. 1-33
  • Kokkalas, S., et al., 2006. Postcollisional contractional and extensional deformation in the Aegean region in GSA Special Papers, v. 409, p. 97-123.
  • Kurt, H., Demirbag, E., and Kuscu, I., 1999. Investigation of the submarine active tectonism in the Gulf of Gokova, southwest Anatolia–southeast Aegean Sea, by multi-channel seismic reflection data in Tectonophysics, v. 305, p. 477-496
  • 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.
  • Taymaz, T., Yilmaz, Y., and Dilek, Y., 2007. The geodynamics of the Aegean and Anatolia: introduction in Geological Society Special Publications, v. 291, p. 1-16.
  • 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 collision, earthquake, education, europe, Extension, geology, plate tectonics, strike-slip, Transform

Earthquake Report: Bering fracture zone: UPDATE #1

Well, based upon a comment on twitter, I thought it prudent to review the seismicity of this region at the westernmost part of the western Aleutian Islands. For many years I considered this region part of the subduction zone that forms the Aleutian Trench. In the past couple of years, there have been a number of earthquakes that reveal this to not be the case. Rather, the Pacific-North America plate boundary in this region is in the form of a shear zone, distributed across several reactivated fracture zones. I noticed a plot from Jascha Polet and found another article that evaluates this plate boundary (Davaille and Lees, 2004).

Here is my initial #EarthquakeReport on this M 7.7 strike-slip earthquake.

Here is a report from the University of Alaska Fairbanks, Alaska Earthquake Center.

Below is my interpretive poster for this earthquake.

  • 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 7.7 earthquake. Based upon the series of earthquakes and the mapped faults, I interpret this M 7.7 earthquake as a right-lateral strike-slip earthquake related to slip associated with the Bering fracture zone.
  • 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 plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I also include USGS earthquake epicenters from 1917-2017 for magnitudes M ≥ 5.0. The depths for these earthquakes is represented by color. This is quite revealing when considering whether subduction is occurring in this part of the plate boundary. Note how earthquakes east of Attu Island have hypocentral depths (3-dimensional location of the earthquake focus) that extend to over 150 km. These represent earthquakes in the downgoing Pacific plate. We also observe earthquakes even deeper along the Kamchatka Trench on the west. Also note that the slab contours (Hayes et al., 2012) have decreasing maximum depths from east to west between Bowers Ridge and Attu Island. The same is true for the subduction zone beneath Kamchatka, though the cut-off of these depth contours is more abrupt (due to the tear in the Pacific plate, which marks the edge of the subduction zone beneath Kamchatka). Finally, note how the earthquake depths in the region between Attu Island and Kamchatka are largely less than 33 km.

    I include some inset figures in the poster.

  • In the upper left corner I include a regional tectonic map showing Kamchatka and the westernmost tip of the westernmost Aleutian Islands (Davaille and Lees, 2004).
  • In the lower right corner I show a schematic illustration that depicts how the subduction transitions to strike-slip in the region of the Attu Island (Davaille and Lees, 2004).
  • To the right is a large scale map of this region showing the complicated intersection of strike slip and compressional tectonics as the tip of the Aleutians intersects with Kamchatka (Gaedicke et al., 2000).
  • In the upper right corner is a figure showing seismicity in this region (Davaille and Lees, 2004). There is a map (with earthquakes plotted vs. depth in color) and several cross sections (one parallel to the Kamchatka trench and two normal/perpendicular to the trench). Note how the depths for the earthquakes shallow to the north of the Meiji Seamount, and particularly shallower to the north of the Stellar fault (a.k.a. Bering Kresla Shear zone) and the Bering fault (a.k.a. Bering fracture zone).


  • Here is my original #EarthquakeReport Interpretive Poster and report.

  • Here is the seismicity plot from Davaille and Lees (2000). They suspect that the tear in the Pacific plate is related to the interaction of a mantle plume hotspot and the oceanic lithosphere as it is formed near this hotspot. I include their figure caption below in blockquote.

  • Seismicity in Kamchatka. (a) Mapview showing earthquakes plotted with colors corresponding to depth. Projections of the Bering and Steller faults onto Kamchatka are presented for clarity and reference. (b–d) Vertical cross-sections of seismicity in Kamchatka. Magenta circles are from the Gorbatov et al. [3] catalog and yellow events are from the Engdahl et al. [52] catalog. The projection of the subducting Meiji seamounts is shaded gray.

  • Here is a plot from Jascha Polet, seismologist at Cal Poly Pomona. These data are consistent with the Davaille and Lees (2000) plot, showing shallow seismicity in this region of the plate boundary.

  • Here is the low-angle oblique map from Portnyagin and Manea (2008). This was in my original report, but is a great visualization for this interpretation of the Pacific plate tear (the edge of subduction in Kamchatka). While this is not evidence for the lack of subduction in the region of the Bering Kresla Shear zone, it does suggest that there is no active subduction beneath Kamchatka to the north. If this is the case (and I have no reason to think that it is not the case), there is little evidence for subduction west of Attu Island and east of Kamchatka. I include the figure caption as a blockquote below.

  • Kamchatka subduction zone. A: Major geologic structures at the Kamchatka–Aleutian Arc junction. Thin dashed lines show isodepths to subducting Pacific plate (Gorbatov et al., 1997). Inset illustrates major volcanic zones in Kamchatka: EVB—Eastern Volcanic Belt; CKD—Central Kamchatka Depression (rift-like tectonic structure, which accommodates the northern end of EVB); SR—Sredinny Range. Distribution of Quaternary volcanic rocks in EVB and SR is shown in orange and green, respectively. Small dots are active vol canoes. Large circles denote CKD volcanoes: T—Tolbachik; K l — K l y u c h e v s k o y ; Z—Zarechny; Kh—Kharchinsky; Sh—Shiveluch; Shs—Shisheisky Complex; N—Nachikinsky. Location of profiles shown in Figures 2 and 3 is indicated. B: Three dimensional visualization of the Kamchatka subduction zone from the north. Surface relief is shown as semi-transparent layer. Labeled dashed lines and color (blue to red) gradation of subducting plate denote depths to the plate from the earth surface (in km). Bold arrow shows direction of Pacific Plate movement.

  • Here is another great visualization of the subducting slabs in this region, prepared by Lees et al. (2007). This figure shows isosurfaces (surfaces of equal value of some parameter, like seismic velocity, or Vp:Vs ratios; Vp = P-Wave velocity, Vs = S-Wave velocity). These isosurfaces were created using seismic tomography, which is basically just like a CT scan of the earth. CT stands for “computed tomography” of X-Rays. The mathematical solutions used in these methods are the same, even though the source of the data are different (X-Rays vs. seismic waves). Coastlines are in grey outline. Kamchatka is labelled M. The lower panel may be easier to look at first because north is up. Basically, blue is old and cold and red is young and hot. So, the downgoing Pacific plate is in blue and the surrounding mantle is in red. Note how the blue blobs end in the area of interest.

  • Tomographic image of the Kamchatka Subduction zone rendered in three-dimensions. The cut-off perturbation level is 3% with blue regions being high velocity and red lower velocity perturbations. The slab is a clear high velocity zone approximately 100 km thick plunging into the upper mantle at an angle of ~50. The green plane represents the top of the subduction zone seismicity, contoured and rendered along with the tomographic images. Gold cones are active volcanoes along the Kamchatka arc and white squares are stations included for reference to the map in Figure 1. The bars represent length scales of 100 km. Points of interest discussed in the text are marked with letters (I–M).

  • This is a great visualization from IRIS that shows the seismic waves propagating through seismometers operated in the US. First is a screenshot and below is the embedded video. Here is a link to the video file (7 MN mp4). The seismic wave at the bottom of the animation is for the seismometer located in the center of the green circle on the map.

  • Finally of note (unrelated to the previous discussion) is that this M 7.7 had a really long duration (Dr. Polet pointed this out in a tweet). This plot is from IPGP here. The source time function represents how much energy is released over time. Time is on the horizontal axis.

    References:

  • Bassett and Watts, 2015. Gravity anomalies, crustal structure, and seismicity at subduction zones: 1. Seafloor roughness and subducting relief in Geochemistry, Geophysics, Geosystems, v. 16, doi:10.1002/2014GC005684.
  • Davaille, A. and Lees, J.M., 2004. Thermal modeling of subducted plates: tear and hotspot at the Kamchatka corner in EPSL, v. 226, p. 293-304.
  • 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.
  • Ikuta, R., Mitsui, Y., Kurokawa, Y., and Ando, M., 2015. Evaluation of strain accumulation in global subduction zones from seismicity data in Earth, Planets and Space, v. 67, DOI 10.1186/s40623-015-0361-5
  • 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.
  • Koulakov, I.Y., Dobretsov, N.L., Bushenkova, N.A., and Yakovlev, A.V., 2011. Slab shape in subduction zones beneath the Kurile–Kamchatka and Aleutian arcs based on regional tomography results in Russian Geology and Geophysics, v. 52, p. 650-667.
  • Krutikov, L., et al., 2008. Active Tectonics and Seismic Potential of Alaska, Geophysical Monograph Series 179, doi:10.1029/179GM07
  • Lees, J.M., VanDecar, J., Gordeev, E., Ozerov, A., Brandon, M., Park, J., and Levvin, V, 2007. Three Dimensional Images of the Kamchatka-Pacific Plate Cusp in Volcanism and Subduction: The Kamchatka Region Geophysical Monograph Series 172, DOI: 10.1029/172GM06
  • Portnyagin, M. and Manea, V.C., 2008. Mantle temperature control on composition of arc magmas along the Central Kamchatka Depression in Geology, v. 36, no. 7, p. 519-522.
  • Rhea, S., Tarr, A.C., Hayes, G., Villaseñor, A., Furlong, K.P., and Benz, H.M., 2010. Seismicity of the Earth 1900-2007, Kuril-Kamchatka arc and vicinity: U.S. Geological Survey Open-File Report 2010-1083-C, 1 map sheet, scale 1:5,000,000.

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

Earthquake Report: Western Aleutians

We just had an earthquake along the western Aleutian Islands, very close to the international date line. In this region places often have more than two names, depending upon who drew the map.

The majority of the Aleutian Islands are volcanic arc islands formed as a result of the subduction of the Pacific plate beneath the North America plate. To the west, there is another subduction zone along the Kuril and Kamchatka volcanic arcs. These subduction zones form deep sea trenches (the deepest parts of the ocean are in subduction zone trenches). Between these 2 subduction zones is another linear trough, but this does not denote the location of a subduction zone. The plate boundary between the Kamchatka and Aleutian trenches is the Bering Kresla shear zone (BKSZ).

The oceanic basin, Komandorsky Basin, to the north of the BKSZ has been mapped with northwest trending fracture zones, most of which are fossil or inactive. However, due to the oblique convergence west of Bowers Ridge, some of these fossil fracture zones are being reactivated. Based upon offsets in magnetic anomalies in the oceanic crust forming the basement of Komandorsky Basin, these fracture zones show left-lateral offsets. However, the active strike-slip faults (and BKSZ) are right lateral. This is a great example of strike-slip faults reactivating as strike-slip faults.

The mainshock was preceded earlier this day with several foreshocks and occurred very close to a M 6.3 earthquake from 9 months ago (2016.09.05). In addition, there was seismic activity to the east about 6 weeks ago (2017.06.02 M 6.8).

Some place a tear in the downgoing Pacific plate (beneath Kamchatka) in the position of the Bering Kresla Shear zone. This marks the end of the modern Kamchatka subduction zone. However, there was a subduction zone further to the west of the active arc, which extended further to the north and possibly involved subduction of the oceanic crust forming the Komandorsky Basin. The combination of offsets along the right-lateral strike-slip faults to the north of the BKSZ, along with the convergence along the Kamchatka Trench, there is a fold and thrust belt (a zone of compressional tectonics). There was an M 6.6 earthquake earlier this year (2017.03.29), which is somewhat related to this fold and thrust belt.

Finally, of note, is the M 8.3 deep extensional earthquake in the downgoing Pacific plate crust (2013.05.24).

There was a tsunami recorded at the tide gage on Shemya Island, with an observed maximum wave height of 0.3 ft! This is interesting given the strike slip earthquake.

  • Here are the USGS website for today’s earthquakes.
  • 2017-07-17 11:05 M 6.2
  • 2017-07-17 11:23 M 5.1
  • 2017-07-17 23:34 M 7.7
  • Here are my #EarthquakeReport pages for some of these related earthquakes.
  • 2016.09.05 M 6.3
  • 2017.03.29 M 6.6
  • 2017.06.02 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). I also include USGS earthquake epicenters from 2007-2017 for magnitudes 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 plot moment tensors for the M 7.7 earthquake, as well as for the 2013, 2016, and 2017 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. Based upon the series of earthquakes and the mapped faults, I interpret this M 7.7 earthquake as a right-lateral strike-slip earthquake related to slip associated with the Bering fracture zone.
  • 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 left corner I include a regional tectonic map showing Kamchatka and the westernmost tip of the westernmost Aleutian Islands (Bindeman et a., 2002). Today’s M 7.7 earthquake is just off the map to the east.
  • To the right of the Bindeman map is a map that shows a larger scale view of the faults in this region (Gaedicke et al., 2000). I include an orange star designating the rough location of today’s M 7.7 earthquake.
  • In the lower left corner are the earthquake intensity regression plots for the M 6.2 and M 7.7 earthquakes. These are plots based upon Attenuation Relation equations (formerly called Ground Motion Prediction Equations, GMPEs). Using seismic data from thousands of earthquakes with a range of magnitudes, distances, fault types, etc., people have developed empirical relations between each of these parameters. These plots are based upon these empirical relations and show how earthquake intensity (how strong it shakes) diminishes with distance.
  • In the upper right corner is a series of tectonic maps showing one interpretation of how the plates have moved relative to each other through time (Konstantinovskaia, 2001). The time spans 65, 55, and 37 millions of years into the present. Begin by looking at the present configuration and move backwards in time. These maps show an interesting story (that appears consistent with other published data).


  • 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 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.

  • Here is the low-angle oblique map from Portnyagin and Manea (2008). I include the figure caption as a blockquote below.

  • Kamchatka subduction zone. A: Major geologic structures at the Kamchatka–Aleutian Arc junction. Thin dashed lines show isodepths to subducting Pacific plate (Gorbatov et al., 1997). Inset illustrates major volcanic zones in Kamchatka: EVB—Eastern Volcanic Belt; CKD—Central Kamchatka Depression (rift-like tectonic structure, which accommodates the northern end of EVB); SR—Sredinny Range. Distribution of Quaternary volcanic rocks in EVB and SR is shown in orange and green, respectively. Small dots are active vol canoes. Large circles denote CKD volcanoes: T—Tolbachik; K l — K l y u c h e v s k o y ; Z—Zarechny; Kh—Kharchinsky; Sh—Shiveluch; Shs—Shisheisky Complex; N—Nachikinsky. Location of profiles shown in Figures 2 and 3 is indicated. B: Three dimensional visualization of the Kamchatka subduction zone from the north. Surface relief is shown as semi-transparent layer. Labeled dashed lines and color (blue to red) gradation of subducting plate denote depths to the plate from the earth surface (in km). Bold arrow shows direction of Pacific Plate movement.

  • Here is a map that shows the seismicity (1960-2014) for this plate boundary. This is the spatial extent for the videos below.

  • Here is a link to the file to save to your computer.

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

Earthquake Report: Ecuador

Earlier today there was a moderate sized earthquake (M 6.0) along coast of Ecuador. This earthquake happened in the region of the 2016.04.16 M 7.8 subduction zone earthquake. Based upon the depth and our knowledge of this region, this earthquake may also be on the megathrust. However, the depth is poorly resolved (initially depth ~ 7 km, but now set at 10 km, the default depth). Here is the USGS website for this earthquake.

More information about this earthquake can be found here at earthquake report dot com.

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 lower left corner I include a clipping of the map and cross section from the USGS Open File Report for the historic seismicity of this region (Rhea et al., 2010). I include the seismicity cross section in the upper left corner. This cross section shows earthquakes related to the downgoing Nazca plate.
  • Between the map and cross section, I include the MMI intensity maps for both the M 7.8 earthquake and this M 6.0 earthquake.
  • In the upper right corner, I include a map that shows the regional tectonics as published by Gutscher et al. (1999). These authors pose that the Carnegie Ridge exerts a control for the segmentation of the subduction zone.
  • In the lower right corner, I include a figure from Chlieh et al. (2014) that shows their coupling model. This model informs us about how strongly the subduction zone fault is seismogenically “locked” and how this varies spatially. They also plot historical earthquake locations and their “moment rate deficit” calculation (i.e. how much the plate motion rate has been accumulated as tectonic strain, which would presumably lead to earthquake slip). I include blue stars in the general location of these two earthquakes. The M 7.8 lies within the seismic gap hypothesized by Chlieh et al. (2014).


  • Here is the same poster but only with the seismicity from the past month and the MMI contours from the 2016 M 7.8 earthquake. (wait for now to get this done… need to restart software)

  • Here are my two interpretive posters for the 2016 M 7.8 earthquake. Here is my initial report. Here is my update.
  • First is the initial interpretive poster.
  • Here is the updated interpretive poster.

  • Below is the tectonic setting map from Gutscher et al. (1999). I include their figure caption as a blockquote.

  • Tectonic setting of the study area showing major faults, relative plate motions according to GPS data [7] and the NUVEL-1 global kinematic model [8], magnetic anomalies [13] and active volcanoes [50]. Here and in Fig. 4, the locations of the 1906 (Mw D 8:8, very large open circle) and from south to north, the 1953, 1901, 1942, 1958 and 1979 (M  7:8, large open circles) earthquakes are shown. GG D Gulf of Guayaquil; DGM D Dolores–Guayaquil Megashear.

  • Below is a low angle oblique view of the structures in the downgoing Nazca plate, from Gutscher et al. (1999). I include their figure caption as a blockquote.

  • 3-D view of the two-tear model for the Carnegie Ridge collision featuring: a steep ESE-dipping slab beneath central Colombia; a steep NE-dipping slab from 1ºS to 2ºS; the Peru flat slab segment south of 2ºS; a northern tear along the prolongation of the Malpelo fossil spreading center; a southern tear along the Grijalva FZ; a proposed Carnegie flat slab segment (C.F.S.) supported by the prolongation of Carnegie Ridge.

  • The 2016 M 7.8 earthquake is near two historic earthquakes with similar magnitudes. Below I plot a map showing the seismicity from 1900-2016 for earthquakes with magnitudes greater than or equal to M 6.0. Here is the USGS query that I used to make this map.
    • 1906.01.31 M 8.3 occurred ~100 km to the northeast.
    • 1942.05.14 M 7.8 occurred <50 km to the southwest.


  • I prepared an animation that shows the seismicity of this region from 1900-2016 for earthquakes with a magnitude greater than or equal to M 6.0. Here is the kml file that I used to prepare this animation. First I provide a screen shot and then a link and the embedded video.

  • Here is a link to the video file embedded below (4 MB mp4)
  • Here are a couple maps from Chlieh et al. (2014). I include their figure captions below. Chlieh et al. (2014) use GPS data to infer the spatial variation and degree to which the subduction zone megathrust is seismogenically coupled. They consider plate motion rates and estimate the moment (earthquake energy) deficit along this fault (how much strain that plate convergence has imparted upon the fault over time). Then they compare this moment deficit to regions of the fault that have slipped historically.
  • Tectonics and GPS motion rates.

  • Seismotectonic setting of the oceanic Nazca plate, South America Craton (SoAm) and two slivers: the North Andean Sliver (NAS) and the Inca Sliver (IS). The relative Nazca/SoAm plate convergence rate in Ecuador is about 55mm/yr (Kendrick et al., 2003). Black arrows indicate the diverging forearc slivers motions relative to stable SoAm are computed from the pole solutions of Nocquet et al.(2014). The NAS indicates a northeastward long-term rigid motion of about 8.5 ±1mm/yr. The ellipse indicates the approximate rupture of the great 1906 Mw=8.8 Colombia–Ecuador megathrust earthquake. The Carnegie Ridge intersects the trench in central Ecuador and coincides with the southern limit of the great 1906 event. Plate limits (thick red lines) are from Bird(2003). DGFZ =Dolores–Guayaquil Fault Zone; GG =Gulf of Guayaquil; GR =Grijalva Ridge; AR =Alvarado Ridge; SR =Sarmiento Ridge. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

  • GPS velocities along with historic earthquake patches.

  • Interseismic GPS velocity field in the North Andean Sliver reference frame. The relative Nazca/NAS convergence rate is 46 mm/yr. The highest GPS velocity of 26 mm/yr is found on La Plata Island that is the closest point to the trench axis. The GPS network adequately covers the rupture areas of the 1998 Mw=7.1, 1942 Mw=7.8and 1958 Mw=7.7 earthquakes but only 1/4th of the 1979 Mw=8.2 and 2/3rd of the great 1906 Mw=8.8 rupture area. The black star is the epicenter of the great 1906 event and white stars are the epicenters of the Mw>7.01942–1998 seismic sequence. Grey shaded ellipses are the high slip region of the 1942, 1958, 1979 and 1998 seismic sources (Beck and Ruff, 1984;Segovia, 2001; Swenson and Beck, 1996). Red dashed contours are the relocated aftershocks areas of the 1942, 1958 and 1979 events (Mendoza and Dewey, 1984). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

  • Moment deficit along strike and historic earthquake locations (Chlieh et al., 2014). The 2016 M 7.8 earthquake may have occurred in the region marked “gap” in these figures.

  • (A) Along-strike variations of the annual moment deficit for all the interseismic models shown in Fig.5. (B)Maximum ISC model and (C)Minimum ISC model. (A)The blue, green and red lines correspond to the along-strike variation of the annual moment deficit rate respectively for models with smoothing coefficient λ1 =1.0, 0.25 and 0.1. (B) Smoother solution of Fig.5 ith a maximum moment deficit rate of 4.5 ×1018N m/yr. (C)Rougher solution of Fig.5 with a minimum moment deficit rate of 2.5 ×1018N m/yr. Yellow stars are the epicenters of subduction earthquakes with magnitude Mw>6.0 from the last 400 yr catalogue (Beauval et al., 2013). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

References

Posted in Uncategorized

Earthquake Report: Guatemala

This morning we had an earthquake offshore of Guatemala with a magnitude M = 6.8. Here is the USGS website for this earthquake. This earthquake occurred to the east of a sequence from about a week ago. Here is my earthquake report for that sequence.

Offshore of Guatemala is a subduction zone thrust fault, where the Cocos plate dives east beneath the North America (in the north) and Caribbean plates (in the south). Subduction zone faults are capable of generating the largest magnitude earthquakes possible because the fault width is wider than other faults. The seismogenic zone, the region of the crust that can store elastic strain and experience brittle rupture during earthquakes, extends into the earth several tens of kms. Strike-slip faults generally dip vertically, giving them the narrowest fault width. While subduction zones dip at an angle, so their fault width is wider. Earthquake magnitude is a measure of energy released during the earthquake and the moment magnitude (the magnitude most people use) is based on three factors: (1) fault area, (2) fault slip, and (3) shear modulus (how flexible, or rigid, the crust/lithosphere is). Fault area is length times width. Length is the distance on the ground surface that the fault ruptures and width is the distance into the earth that the fault ruptures. Because subduction zone faults dip into the earth at an angle, the distance that they extend before reaching a given depth is larger, owing to a larger possible magnitude.

Today’s M 6.8 earthquake has a USGS hypocentral depth of ~47 km, which is very close to the depth of the subduction zone fault. Also, the fault plane solution (moment tensor, read below) is compressional. Thus, I interpret this earthquake to be a subduction zone earthquake at or near the megathrust. This earthquake is different from the sequence from a week ago. Those earthquakes had two populations: (1) earthquakes in the accretionary prism of the subduction zone and (2) earthquakes in the downgoing Cocos plate. Those earthquakes were not subduction zone earthquakes (though the shallower earthquakes may not have had well located hypocenters, so their depths are suspect… and could have been on the subduction zone). I suspect that this M 6.8 earthquake is related to the earthquakes from last week. I include the moment tensors for 2 of the significant earthquakes from last week. See my report for more on that sequence.

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 6.8 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.
  • In the lower right corner is a tectonic map showing the regional tectonics highlighting various study sites from sedimentary, magmatic, metamorphic and ore deposit studies of the subduction zones here (Garcia-Casco et al., 2011).
  • In the lower left corner is a map that shows the plate tectonic setting for this region of Middle America (Symithe et al., 2015). Earthquake epicenters are plotted as circles with color representing depth. Earthquake focal mechanisms are plotted with thrust earthquakes plotted in blue and other mechanisms plotted in red. Today’s earthquake happened just off this map on the left.


  • Here is an explanation from IPGP that helps us visualize the two potential fault planes (the principal and the auxiliary). I interpret this earthquake to have occurred on fault plane 1, the subduction zone.

  • Here is a map that shows today’s earthquake (with MMI contours) in the context of the seismicity from the past 100 years. I plot earthquakes from 1917-2017 for magnitudes M > 6.5.

  • Here is my poster for the earthquake sequence from last week.

  • Here is the tectonic map from Symithe et al. (2015). I include their figure caption below in blockquote.

  • Seismotectonic setting of the Caribbean region. Black lines show the major active plate boundary faults. Colored circles are precisely relocated seismicity [1960–2008, Engdahl et al., 1998] color coded as a function of depth. Earthquake focal mechanism are from the Global CMT Catalog (1976–2014) [Ekstrom et al., 2012], thrust focal mechanisms are shown in blue, others in red. H = Haiti, DR = Dominican Republic, MCS = mid-Cayman spreading center, WP = Windward Passage, EPGF = Enriquillo Plaintain Garden fault.

  • Here is the tectonic map from Garcia-Casco et al. (2011). I include their figure caption below in blockquote.

  • Plate tectonic configuration of the Caribbean region showing the location of the study cases presented in this issue (numbers refer to papers, arranged as in the issue), and other important geological features of the region (compiled from several sources).

  • Here is the Benz et al. (2011) Seismicity of the Earth poster for this region.

References

  • Benz, H.M., Tarr, A.C., Hayes, G.P., Villaseñor, Antonio, Furlong, K.P., Dart, R.L., and Rhea, Susan, 2011. Seismicity of the Earth 1900–2010 Caribbean plate and vicinity: U.S. Geological Survey Open-File Report 2010–1083-A, scale 1:8,000,000.
  • Franco, A., C. Lasserre H. Lyon-Caen V. Kostoglodov E. Molina M. Guzman-Speziale D. Monterosso V. Robles C. Figueroa W. Amaya E. Barrier L. Chiquin S. Moran O. Flores J. Romero J. A. Santiago M. Manea V. C. Manea, 2012. Fault kinematics in northern Central America and coupling along the subduction interface of the Cocos Plate, from GPS data in Chiapas (Mexico), Guatemala and El Salvador in Geophysical Journal International., v. 189, no. 3, p. 1223-1236. DOI: https://doi.org/10.1111/j.1365-246X.2012.05390.x
  • Garcia-Casco, A., Projenza, J.A., Iturralde-Vinent, M.A., 2011. Subduction Zones of the Caribbean: the sedimentary, magmatic, metamorphic and ore-deposit records UNESCO/iugs igcp Project 546 Subduction Zones of the Caribbean in Geologica Acta, v. 9, no., 3-4, p. 217-224
  • 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.
  • Symithe, S., E. Calais, J. B. de Chabalier, R. Robertson, and M. Higgins, 2015. Current block motions and strain accumulation on active faults in the Caribbean in J. Geophys. Res. Solid Earth, v. 120, p. 3748–3774, doi:10.1002/2014JB011779.

Posted in earthquake, geology, pacific, plate tectonics, subduction

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