Earthquake Report: Caroline Ridge Update #1

Well, there was an earthquake this morning (my time) that may help us select a fault plane solution. Recall that these fault plane solutions (moment tensors and focal mechanisms) have two possible fault planes. We need additional information (like fault maps for the region, or aftershock patterns, etc.) in order to choose the more likely potential fault plane. Because we are scientists and we cannot make direct observations (the key part of the scientific method), we cannot ever know which fault plane is the correct one. However, we can be very certain based upon some basic reasoning.

I speculated in my initial Earthquake Report here, that the M 6.5 and M 6.4 seemed to align along a north-northeast orientation, slightly favoring the NE striking right-lateral strike slip fault plane as the principal fault plane. Today’s M 6.1 earthquake provides additional corroborating evidence to support this initial interpretation.

In the interpretive poster, I draw a dashed red line connecting the earthquake epicenters along a hypothetical fault. This is pure conjecture, other than the aftershock pattern. However, these earthquakes are aligned in a way that is sub-parallel to a submarine mountain range (oriented ~north-south; note this range to the north of the epicenters). This range could be formed by faulting in the region. If this structure is related to these earthquakes, it might be a right-lateral strike-slip fault.

I also highlight some normal faults in the subducting Caroline plate to the east of these earthquakes. As the subducting plate flexes downward (during and preceding to subduction), this causes extension in the upper part of the plate. This extension leads to normal faulting as evidenced in the bathymetry. I simply digitized these faults here as a test bed to suggest that there are similar faults along the Yap Trench to the west of the earthquakes (tho the higher resolution multibeam bathymetry here is more spotty, so more difficult to interpret).

The subduction zone associated with the Mariana Trench may continue as the Challenger Deep and then as the Yap Trench, but there may be some strike-slip motion for part of this plate boundary fault (based upon the Fujiwara et al. (2000) hypothesis mentioned in my initial report. However, this strike-slip motion may be located on a separate fault system (often plate boundary motion can be partitioned between separate fault systems, e.g. the Sumatra-Andaman subduction zone and the Sumatra forearc sliver fault). It is not altogether clear what is happening here. There is a lineament to the north of the plate boundary mapped by the USGS here, but we really need some seismic reflection data to confirm this as a possibility.

Below is my interpretive poster for this earthquake.


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1917-2017 with magnitudes M > 6.5 (though there are not many historic earthquakes in this small region).

I plot the USGS fault plane solutions (moment tensors in blue) for the M 6.5, M 6.4, and M 6.1 earthquakes.

  • 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 plots this close to the location of the fault as mapped by Hayes et al. (2012).
  • Here are the USGS pages for the main earthquake in this sequence.
  • 2017.12.08 M 6.5
  • 2017.12.08 M 6.4
  • 2017.12.09 M 6.1
  • I include some inset figures.

  • In the upper center I include a map showing the tectonics of the region to the north of these earthquakes (Stern et al., 2003). I place a blue star where these earthquakes are located and a green rectangle showing the general limit of this interpretive poster.
  • In the lower right corner I include the tectonic model from Fujuwara et al. (2000) for the subduction of the Caroline Ridge (they hypothesize that the Yap Trench was part of the Mariana Trench, but has been offset due to the attempted (and succeeded?) subduction of the Caroline Ridge).
  • In the upper left corner I include a figure from Emry et al. (2014) that shows how the bending moment normal faults (I mention these above, as outlined in orange in the interpretive poster) are evidenced by normal fault plate solutions.


  • Here is the interpretive poster from my original Earthquake Report.

  • This shows the arc and backarc tectonic elements in this region (Stern et al., 2003).

  • Generalized locality map for the Izu-Bonin-Mariana Arc system. Dashed line labeled STL = Sofugan Tectonic Line.

  • This shows, in cross-section form, the magmatic evolution of the Back-Arc along the Mariana Trench (Stern et al., 2003).

  • Simplified history of the IBM arc system. Shaded areas are magmatically inactive, cross-hatched areas are magmatically active.

  • Here is a map showing the fault plane solutions used by Emry et al. (2014) to evaluate the faulting related to the Mariana Trench.

  • Relocated GCMT earthquakes in map view. Lower hemisphere stereographic projections for earthquakes are shown with compressional P wave quadrants (containing the T axis in black) and dilatational P wave quadrants (containing the P axis in white). The event numbers next to each focal mechanism correspond to Tables 2 and 4. The red arrow shows the angle of convergence of the Pacific plate relative to the Mariana fore arc as determined by Kato et al. [2003]. High-resolution bathymetry data in Northern and Central Mariana are from 2010 Mariana Law of the Sea Cruise [Gardner, 2010] and high-resolution bathymetry data in Southern Mariana are courtesy of F.Martinez. The color scale for bathymetry is positioned below and is the same for all bathymetric maps in the paper. (inset) Tectonic setting of the Philippine Sea. Bathymetry contours are shown by thin black lines. Subduction trenches are shown in blue; spreading centers are shown in red; transforms are shown in green.

  • Here is the larger scale map from Emry et al. (2014) showing some moment tensors and high resolution multibeam bathymetry, revealing fault geomorphology. Below the map is a cross section, showing how these normal faulting earthquakes occur in the downgoing Pacific plate, beneath the subduction zone fault.

  • (top) Relocated GCMT earthquake locations in mapview. Lower hemisphere stereographic projections for earthquakes are shown with compressional quadrants (in black) and dilatational quadrants (in white). Event numbers next to each focal mechanism correspond to Tables 2 and 4. The red arrow shows the angle and rate of Pacific plate convergence relative to the fore arc as determined by Kato et al. [2003]. High-resolution bathymetry data are from Gardner [2010]. The bathymetry scale is the same as in Figure 1. Inset shows the tectonic setting of the Mariana Islands. Bathymetry contours are shown by thin black lines. The trench axis is shown in blue; back-arc spreading center is in red; transform is in green. (bottom) Trench perpendicular cross section with the location of the subduction trench at 0 km; negative distances indicate the distance landward (or west of the trench) and positive distances indicate seaward distances (or east of the trench). Thick black lines show the bathymetry along (17.25°N, 147.3577°E) to (17.2752°N, 148.9311°E). Thick red lines show depth to the Moho used in our waveform inversion. Black squares show the depth to the plate interface at ~17°N from Oakley et al. [2008]; red squares indicate the continuation of the Moho landward from the trench. Focal mechanisms for the region are rotated 90° into cross section. Dilatational quadrants are indicated by white while compressional quadrants are indicated by red. The event numbers next to each focal mechanism correspond to Tables 2 and 4. Vertical exaggeration (VE) is 1.5.

  • This is a cross section showing how Fujiwara et al. (2000) interpret how the Pacific plate (Caroline Ridge) subducts beneath the Philippine Sea plate at the Yap Trench.

  • Schematic across the axis cross section of the northern part of the Yap Trench and its tectonic interpretation.

  • This illustration shows four time steps in the evolution of the plate boundary in this area.

  • Proposed scenario of the evolution of the Yap Trench.


    References:

  • Bird, P., 2003. An updated digital model of plate boundaries in Geochemistry, Geophysics, Geosystems, v. 4, doi:10.1029/2001GC000252, 52 p.
  • Emry, E. L., Wiens, D. A., and Garcia-Castellanos, D., 2014. Faulting within the Pacific plate at the Mariana Trench: Implications for plate interface coupling and subduction of hydrous minerals, J. Geophys. Res. Solid Earth, 119, 3076–3095, doi:10.1002/2013JB010718. Fujiwara, T., Tamura, C., Nishizawa, A., Fujioka, K., Kobayashi, K., and Iwabuschi, Y., 2000. Morphology and tectonics of the Yap Trench in Marine Geophysical Researches, v. 21, p. 69-86
  • Gaina, C. and Müller, R.D., 2007. Cenozoic tectonic and depth/age evolution of the Indonesian gateway and associated back-arc basins in Earth-Science Reviews v. 83, p. 177-203
  • 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.
  • Holm, R.J., Rosenbaum, G., Richards, S.W., 2016. Post 8 Ma reconstruction of Papua New Guinea and Solomon Islands: Microplate tectonics in a convergent plate boundary setting in Eartth Science Reviews, v. 156, p. 66-81.
  • Müller, R.D., Sdrolias, M., Gaina, C., and Roest, W.R., 2008. Age, spreading rates, and spreading asymmetry of the world’s ocean crust in Geochemistry, Geophysics, Geosystems, v. 9, no. 3. Q04006, doi:10.1029/2007GC001743
  • Okino, K., Ohara, Y., Fujiwara, T., Lee, S-M., Koizumi, K., Nakamura, Y., and Wu., S., 2009. Tectonics of the southern tip of the Parece Vela Basin, Philippine Sea Plate in Tectonophysics, v. 466, p. 213-228.
  • Richards, S., Holm, R., Barber, G., 2011. When slabs collide: A tectonic assessment of deep earthquakes in the Tonga-Vanuatu region in Geology, v. 39, no. 8., p. 787-790
  • Smoczyk, G.M., Hayes, G.P., Hamburger, M.W., Benz, H.M., Villaseñor, Antonio, and Furlong, K.P., 2013, Seismicity of the Earth 1900–2012 Philippine Sea Plate and vicinity: U.S. Geological Survey Open-File Report 2010–1083-M, scale 1:10,000,000, https://dx.doi.org/10.3133/ofr20101083m.
  • Stern, R.J., 2010. The anatomy and ontogeny of modern intra-oceanic arc systems in Kusky, T. M., Zhai, M.-G. & Xiao, W. (eds) The Evolving Continents: Understanding Processes of Continental Growth. Geological Society, London, Special Publications, 338, 7–34.
  • Stern, R. J., Fouch, M. J. & Klemperer, S. 2003. An overview of the Izu–Bonin–Mariana subduction factory. In: Eiler, J. (ed.) Inside the Subduction Factory. American Geophysical Union, Geophysical Monograph, 138, 175–222.
  • Uyeda and Kanamori, 1979. Back-Arc Opening and the Mode of Subduction in JGR, v. 84, no. B3, p. 1049-1061.
  • Zahirovic et al., 2014. The Cretaceous and Cenozoic tectonic evolution of Southeast Asia in Solid Earth, v. 5, p. 227-273, doi:10.5194/se-5-227-2014.

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

Earthquake Report: Caroline Ridge

There was an earthquake sequence beginning 2017.12.08 along the northern flank of Caroline Ridge in the western Pacific Ocean, near the intersection of the Mariana and Yap trenches.

The two largest earthquakes, M 6.5 and M 6.4, are both strike-slip earthquakes. The M 6.5 is northeast of the M 6.4, so maybe this represents a northeast striking left lateral earthquake. But I rank this a low certainty interpretation. I did not find any geologic maps of this region that might show geologic structures that we might associate with this seismicity, so it is difficult to know if the alternate solution is correct (northwest striking left-lateral strike slip). However, there is possibly a transform (strike-slip) plate boundary fault associated with the southern boundary of the Caroline Ridge (this interpretation is based upon my interpretation of the “Age of Oceanic Lithosphere” map below). There may be synthetic faults to this transform boundary fault, ones sub-parallel to the main fault (the red faultline with purple arrows showing sense of motion). In this case, I would interpret the M 6.5 and M 6.4 earthquakes as northwest striking left-lateral earthquakes.

The Yap and Mariana trenches are formed by the subduction of the Pacific plate beneath the Philippine Sea plate.

The Caroline Islands are volcanic islands formed when the Pacific plate passed over a hotspot (Rehman et al., 2013). As in Hawaii (and other young hotspot volcanic island chains), the younger islands are to the east. The westward movement of this oceanic ridge, formed similar in fashion to the Ninetyeast Ridge, has interacted with the subduction zone in a way that has indented the trench. Prior to the ridge hitting the trench, the Mariana trench may have extended further south. As the ridge interacted with the subduction zone, the megathrust fault moved westwards, offsetting the trench and forming the Yap Trench. There is a great illustration from Fujiwara et al. (2000) below.

There are fossil oceanic spreading ridges within the Caroline plate. These show up in the bathymetry and evidenced by the magnetic anomaly data.

Below is my interpretive poster for this earthquake.


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1917-2017 with magnitudes M > 6.5.

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the M 6.5 and M 6.4 earthquakes, in addition to some relevant historic earthquakes.

  • 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 depth is probably not very well constrained due to the geometry and lack of seismometer coverage in the oceanic setting.
  • Here are the USGS pages for the main earthquake in this sequence.
  • 2017.12.08 M 6.5
  • 2017.12.08 M 6.4
  • I include some inset figures.

  • In the lower left corner is a great figure showing the generalized plate tectonic boundaries in this region of the equatorial Pacific Ocean (Holm et al., 2016). I place a blue star in the general location of the M 6.5 and M 6.4 earthquakes (also plotted in other inset figures).
  • In the upper right corner I include a large scale view of the regional tectonics (Zahirovic et al., 2014). Plate boundary fault symbology (and other features, like fracture zones) is shown in the legend. I place a blue star on the map in the general location of this earthquake epicenter. There are two east-west green lines in the Caroline plate. These are the axes of fossil spreading centers (‘W. Caroline Trough’ & ‘Kilssguard Trough’).
  • In the lower right corner is a map that shows the age of the oceanic crust in the western Pacific (Müller et al., 2008). The big blue bleb is the older Pacific plate crust. The CA labels the Caroline plate. Note the two east-west trending magenta lines, these are the fossil spreading centers. See how the crust gets older to the north and south of these ancient spreading ridges. I also locate these spreading ridges on the main map.
  • In the upper left corner is a view of the seismicity with a better view of the bathymetry than in the main map. Note how the Caroline Ridge is an ~east-west trending (N80W or so) lighter blue area, because the ocean depth is shallower here. This region of the Pacific plate may be overthickened during its formation. The M 6.4 had a pretty deep hypocenter. Even if there was some considerable uncertainty, this is a deep earthquake for oceanic crust (~7 km on average for oceanic crust).
  • To the right of the inset seismicity map is an illustration that shows an interpretation about how the Caroline Islands, and surrounding crust, was formed (Rehman et al., 2013). Rehman et al. (2013) show how the Pacific plate passed over a hotspot, creating the Caroline Islands (like Hawaii) and leading to a thicker crust here (see panel B).


  • Here is a map from the USGS that shows the USGS seismicity from the 20th century or so (Smocyk et al., 2010). The cross sections show the earthquake hypocenters in orientations that reveal the various types of subduction zones in this region of Earth. For example, the northern Mariana Trench shows a steeply dipping megathrust fault (and very deep seismicity), while cross section E-E’ in the southern Mariana Trench is not as deep. Click on the map to see the entire 92 MB pdf poster.

  • Here is the plate tectonic map from Zahirovic et al. (2014), which shows the Sorol Trough as a fracture zone (strike-slip).

  • 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)

  • This is a gravity anomaly map for the western Pacific north of Papua New Guinea (Gaina and Müller, 2007). Overlain on the map are magnetic anomaly isochrons (labeled C6 for example; lower chron numbers are younger). Note the fossil spreading center on the east is just younger than isochron C8, possibly the same for the West Caroline Trough on the west. Also clearly shown on this map are the subduction zones, which have negative gravity anomalies due to the downward flexing oceanic lithosphere as it approaches the trench.

  • Gravity anomaly derived from satellite altimetry (Sandwell and Smith, 2005) for the Caroline sea region. Superimposed are the interpreted magnetic lineations (C8–C16 stand for chron numbers, see detailed interpretation in Figs. 3 and 4). Locations of DSDP and ODP drilling are shown in white boxes. Abbreviations: CR—Caroline Ridge, PKR—Palau Kyushu Ridge, WCR—West Caroline Ridge, KT—Kiilsgaard Trough, PVB—Parece Vela Basin, WPB — West Philippine Basin.

  • Here is the crust age map from the poster (Müller et al., 2008).

  • Oceanic lithospheric age. the following isochrons are shown: West Philippine Basin (18y, 20o, 21o, 24o, 26o), Caroline Sea (8o, 10o, 11o, 13y, 15o, 16y), Ayu Trough (3o, 5o), Parece Vela/Shikoku Basin (5Dy, 6o, 6By, 7o, 8o, 9o), Mariana Trough (1o, 2Ao, 3o), South China Sea (5Dy, 6o, 6By, 7o, 8o, 19o, 11o), Celebes Sea (16o, 17o, 18o, 19o, 21o), Sulu Sea (3o, 5o), South Fiji Basin (7o, 8o, 9o, 10o, 11o), North Fiji Basin (1o), Solomon Sea (17o, 18o), North Loyalty Basin (16y, 17o, 18o, 20o), and Lau Basin (1o, 2Ao, 3o). SCS, South China Sea; SS, Solomon Sea; CS, Celebes Sea; BS, Bismark Sea; PS, Philippine Sea; AT, Ayu Trough; CA, Caroline Sea; PVB, Parece Vela Basin; SB, Shikoku Basin; MT,
    Marianas Trough; SO, Solomon Sea; CO, Coral Sea; LO, North Loyalty Basin; NF, North Fiji Basin; SF, South Fiji Basin; LB, Lau Basin.

  • Here is another oceanic crust age map, with more details in the region of the Caroline plate (Gaina and Müller, 2007).

  • Fig. 12. Set of tectonic reconstructions that depict the evolution of oceanic crust north of Australia since Middle Eocene (50 Ma). Light yellow represent continental blocks, rotated present day coastline are in black, island arcs are colored in pale brown. Grey areas depict regions with insufficient data to constrain paleo-age grids. Black lines are active plate boundaries (if dashed—unconstrained plate boundary), light grey lines are extinct spreading ridges. The two blue circles show the location of the Manus (west) and Caroline (east) hotspots assuming an underlying Pacific mantle. An additional position for the Manus hotspot depicts its location if part of moving Indian Ocean hotspot (green circle). Large Igneous Provinces (in this case Ontong Java Plateau, NE of Australia and Kerguelen and Broken Ridge plateaus, SWof Australia) are colored in magenta.

  • This is a cross section showing how Fujiwara et al. (2000) interpret how the Pacific plate (Caroline Ridge) subducts beneath the Philippine Sea plate at the Yap Trench.

  • Schematic across the axis cross section of the northern part of the Yap Trench and its tectonic interpretation.

  • This illustration shows four time steps in the evolution of the plate boundary in this area.

  • Proposed scenario of the evolution of the Yap Trench.

  • This shows an alternative hypothesis for the formation of the Yap Trench (Okino et al., 2009).

  • One hypothesis for the evolution of the southern tip of the PVB. (a) The landward side of the Yap Trench consists of island arc crust and remnant lithosphere inherited from old Philippine Sea. Backarc extension in E–W direction split the arc crust and new oceanic crust was formed. (b) Overlapping rifts developed at the southern tip of the PVB. (c) The southernmost overlapping rift system was abandoned and the NE–SWspreading occurred in the main PVB.

  • Here is a comparison of the two earthquakes from IPGP. Below are the links to the earthquake pages from IPGP. The IPGP depths are shallower, but still on the deep side for oceanic crust earthquakes.
  • 2017.12.08 M 6.5
  • 2017.12.08 M 6.4

  • Here is a map of the Pacific with the location of hotspots designated by orange dots (Nunn et al., 2016). The Caroline volcanic chain is ~2,800 km long.

  • Island locations within the Pacific Basin showing their relationship with principal island-forming island locations. Plate boundaries (in red) and hotspots active within the past 43 Ma (orange circles) are also shown. Active convergent plate boundaries are shown by lines with triangles pointing in the downthrust direction; all other plate boundaries are undistinguished, although most are transform except for most of the East Pacific Rise where divergence occurs. Hotspot locations are from King and Adam (2014).

  • This shows the arc and backarc tectonic elements in this region (Stern et al., 2003).

  • Generalized locality map for the Izu-Bonin-Mariana Arc system. Dashed line labeled STL = Sofugan Tectonic Line.

  • This shows, in cross-section form, the magmatic evolution of the Back-Arc along the Mariana Trench (Stern et al., 2003).

  • Simplified history of the IBM arc system. Shaded areas are magmatically inactive, cross-hatched areas are magmatically active.

  • This is the cross section from Hussong and Fryer (1982) showing the crustal structure in the region of Deep Sea Drilling Project Site 60. More about this site is available online here. The upper panel is the original figure (showing the drill sites) and the lower panel is an updated version. These figures also show the velocity model in km/second (this shows how the seismic velocity varies through different materials). Note the rough incoming oceanic crust (see the Magellan Seamounts on the Pacific plate). The rough plate is also visible in the main poster above.

  • Physiographic diagram and crustal structure across the Mariana Trench and arc and the Mariana Trough, with Leg 60 site location shown. Crustal structure generalized from IODP site survey data by D. Hussong. Physiography drawn from IPOD site survey data by W. Coulbourn. This diagram summarizes the data and structural interpretations available prior to drilling.


  • Here Stern (2010) updates the cross section of Uyeda and Kanamori (1979) even further.

  • Comparison of Andean-type arc (a) and intra-oceanic arc system (b), greatly simplified.

    • Here is the seismicity cross section prepared by Fouch also an inset in the poster above.

    • Map view of bathymetry and seismicity in the IBM subduction zone using the earthquake catalog of Engdahl, van der Hilst & Buland 1998. Circles denote epicentral locations; lighter circles represent shallower events, darker circles represent deeper events. Black lines denote cross sectional areas depicted in 6 profiles on right, organized from N to S. Black circles represent hypocentral locations in volume ~60 km to each side of the lines shown on the map at left. Large variations in slab dip and maximum depth of seismicity are apparent. Distance along each section is measured from the magmatic arc. A) Northern Izu-Bonin region. Slab dip is ~45°; seismicity tapers off from ~175 km to ~300 km depth but increases around 400 km, and terminates at ~475 km. B) Central Izu Bonin region. Slab dip is nearly vertical; seismicity tapers off from ~100 km to ~325 km but increases in rate and extends horizontally around 500 km, and terminates at ~550 km. C) Southern Izu Bonin region. Slab dip is ~50°; seismicity is continuous to ~200 km, but a very few anomalous events are evident down to ~600 km. D) Northern Mariana region. Slab dip is ~60°; seismicity is continuous to ~375 km and terminates at ~400 km, but a very few anomalous events are evident down to ~600 km. E) Central Mariana region. Slab dip is vertical; seismicity tapers off slightly between ~275 km and ~575 km, but is essentially continuous. A pocket of deep events around 600 km exists, as well as 1 deep event at 680 km. F) Southern Mariana region. Slab dip is ~55°; seismicity is continuous to ~225 km, with an anomalous event at 375 km.


      References:

    • Bird, P., 2003. An updated digital model of plate boundaries in Geochemistry, Geophysics, Geosystems, v. 4, doi:10.1029/2001GC000252, 52 p.
    • Fujiwara, T., Tamura, C., Nishizawa, A., Fujioka, K., Kobayashi, K., and Iwabuschi, Y., 2000. Morphology and tectonics of the Yap Trench in Marine Geophysical Researches, v. 21, p. 69-86
    • Gaina, C. and Müller, R.D., 2007. Cenozoic tectonic and depth/age evolution of the Indonesian gateway and associated back-arc basins in Earth-Science Reviews v. 83, p. 177-203
    • 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.
    • Holm, R.J., Rosenbaum, G., Richards, S.W., 2016. Post 8 Ma reconstruction of Papua New Guinea and Solomon Islands: Microplate tectonics in a convergent plate boundary setting in Eartth Science Reviews, v. 156, p. 66-81.
    • Müller, R.D., Sdrolias, M., Gaina, C., and Roest, W.R., 2008. Age, spreading rates, and spreading asymmetry of the world’s ocean crust in Geochemistry, Geophysics, Geosystems, v. 9, no. 3. Q04006, doi:10.1029/2007GC001743
    • Okino, K., Ohara, Y., Fujiwara, T., Lee, S-M., Koizumi, K., Nakamura, Y., and Wu., S., 2009. Tectonics of the southern tip of the Parece Vela Basin, Philippine Sea Plate in Tectonophysics, v. 466, p. 213-228.
    • Richards, S., Holm, R., Barber, G., 2011. When slabs collide: A tectonic assessment of deep earthquakes in the Tonga-Vanuatu region in Geology, v. 39, no. 8., p. 787-790
    • Smoczyk, G.M., Hayes, G.P., Hamburger, M.W., Benz, H.M., Villaseñor, Antonio, and Furlong, K.P., 2013, Seismicity of the Earth 1900–2012 Philippine Sea Plate and vicinity: U.S. Geological Survey Open-File Report 2010–1083-M, scale 1:10,000,000, https://dx.doi.org/10.3133/ofr20101083m.
    • Stern, R.J., 2010. The anatomy and ontogeny of modern intra-oceanic arc systems in Kusky, T. M., Zhai, M.-G. & Xiao, W. (eds) The Evolving Continents: Understanding Processes of Continental Growth. Geological Society, London, Special Publications, 338, 7–34.
    • Stern, R. J., Fouch, M. J. & Klemperer, S. 2003. An overview of the Izu–Bonin–Mariana subduction factory. In: Eiler, J. (ed.) Inside the Subduction Factory. American Geophysical Union, Geophysical Monograph, 138, 175–222.
    • Uyeda and Kanamori, 1979. Back-Arc Opening and the Mode of Subduction in JGR, v. 84, no. B3, p. 1049-1061.
    • Zahirovic et al., 2014. The Cretaceous and Cenozoic tectonic evolution of Southeast Asia in Solid Earth, v. 5, p. 227-273, doi:10.5194/se-5-227-2014.

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

Earthquake Report: Delaware!

Today there was an earthquake in the state of Delaware, a region that does not have many mapped surface faults (I could not find any active faults in a couple hours of lit review). This area also does not have much historic seismicity, however there is an Open File Report from the Delaware Geological Survey, published in 2001. Today’s M 4.1 earthquake matches the record for the largest earthquake of record. There was a M 4.1 in the Wilmington, Delaware region on 1871.10.09 (see OFR42 linked above). The Wilmington region seems to be the most seismically active part of Delaware. Today’s earthquake, to the northeast of Dover, Delaware, happened in a place that has only had a single earthquake in the historic record (M 3.3 on 1879.03.26).

The earthquake happened along the coast plain, where the surficial geology is mapped as marsh deposits (underlain by Quaternary sediments, then by Tertiary sediments). I include a geological map below, along with a cross section. There does not appear to be any structural control for today’s earthquake (but I have only spent a couple hours on this, and the cross section is not very deep). To the south, in Maryland, there is an impact structure (from a Bolide impact). But the structures from this probably don’t extend this far north. There is probably some structures related to the active tectonics of the past (as mapped in the Appalachans to the west), and this earthquake is probably reactivating one of those structures.

Also, there is a possible chance that this is a foreshock. But we won’t know until later if this is the case.

Here is the USGS website for this 2017.11.30 M 4.1 earthquake.

Below is my interpretive poster for this earthquake.


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1917-2017 with magnitudes M > 4.5. I include fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the larger earthquakes in the eastern USA.

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely. The structural grain of the Appalachians are oriented in a north-northeastern orientation, so the left-lateral northeast striking solution is slightly favored. However, more analysis will need to be done (or more lit review).
  • 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 MMI contours for the earthquakes with fault plane solutions plotted (except, I do not include the MMI contours for the 2011.08.25 M 4.5 earthquake, an aftershock of the Mineral, Virginia M 5.8 earthquake.
  • I include some inset figures.

  • In the upper left corner I include an inset of the Geological Map of Kent County, Delaware (Ramsey, 2007). I include the legend for the relevant geological units on the map and on the cross section below. I place a blue star in the general location of the M 4.1 epicenter. Note that there are no mapped faults on this map (the geologic contacts are depositional contacts). Here is a link to a pdf of the map (19 MB pdf).
  • In the lower right corner I include the cross section B-B’ that shows the subsurface geology in this region (Ramsey, 2007). This cross section is constructed from well log data. The location of the cross section is shown on the geologic map as a red line. The well log locations are the vertices (labeled dots) along this red line. Only the easternmost portion of the cross section is represented on the map in the interpretive poster.
  • In the upper right corner is a plot showing “Did You Feel It?” (DYFI) responses for two earthquakes. This shows how earthquakes on the west coast attenuate faster than earthquakes on the east coast. Basically, on the west coast, due to the geology there, seismic waves are absorbed by the Earth with distance. While, on the east coast, they do so to a lesser degree. The result is that earthquakes on the east coast are felt from a greater distance than those on the west coast. This comparison is for between the 2004.09.28 M 6.0 Parkfield Earthquake in California and the 2011.08.23 M 5.8 Mineral Virginia Earthquake.
  • Below the DYFI comparison figure is a map from the 2014 USGS National Seismic Hazard Map project. This shows a simplified view of seismic hazard for the USA. “The 2014 U.S. Geological Survey (USGS) National Seismic Hazard Maps display earthquake ground motions for various probability levels across the United States and are applied in seismic provisions of building codes, insurance rate structures, risk assessments, and other public policy.” Today’s earthquake happened in a region of low seismic hazard (due to the lack of active faults in the region and the low background seismicity).


  • Here is the geologic map for the Dover, Delaware region (Ramsey, 2007) that is included in the interpretive poster above.

  • Here is cross section B-B’ for the Dover, Delaware region (Ramsey, 2007) that is included in the interpretive poster above. Note the petrophysical logs that the correlations are made with. The Tch unit is well correlated, especially in the western region of this section. The unit Tc has more variability within the unit, but it represents more time and geologic thickness. Note that there are no faults in this cross section (albeit a shallow cross section, only about 100 meters deep).

  • The Ramapo fault system is one of the best known fault systems in the Mid-Atlantic region. Today’s M 4.1 is not related to this fault system. Below is a map showing this fault system reltaed to the topography in the region. Todays’ M 4.1 earthquake is located behind the legend of this map, just to the south of the “g” in the word Furlong.

  • Here is a figure that shows a comparison between several earthquakes in this region. I plot intensity maps above and empirical relations between shaking and distance to the earthquake below. It is important to note that these maps are just models of ground shaking; the “Did You Feel It?” maps are better to show the actual felt intensities as they are based upon reports from real people. The solid and dashed lines represent the mean and 2 sigma range of the empirical relations between shaking and distance. Basically, thousands of earthquakes have been measured by seismometers. These measurements have been entered into a database and filtered by various factors. The filtered data have been regressed and the equation for these regression lines are used to estimate ground shaking at locations relative to their distance from the earthquake. In most cases, for earthquakes of smaller magnitude, the distance is measured as a point source from the epicentral location (which is not realistic). However, for larger earthquakes, where a fault can be resolved from the seismologic data (source inversions), the rectilinear fault is used as a source to model the intensities.
  • There was an aftershock to the 2011.08.23 M 5.8 Mineral, VA earthquake, an M 4.5 earthquake. This M 4.5 earthquake is more comparable to today’s M 4.1 earthquake. The maps are at different scales (unfortunately). However, the regressions are at the same scale. Note that the M 4.1 and M 4.5 have similar regression lines (due to their similar magnitude).

  • Here is the DYFI comparison map from the USGS. More can be found about the 2011.08.23 M 5.8 earthquake at the USGS website here.

  • Earthquakes in the central and eastern U.S., although less frequent than in the western U.S., are typically felt over a much broader region. East of the Rockies, an earthquake can be felt over an area as much as ten times larger than a similar magnitude earthquake on the west coast. A magnitude 4.0 eastern U.S. earthquake typically can be felt at many places as far as 100 km (60 mi) from where it occurred, and it infrequently causes damage near its source. A magnitude 5.5 eastern U.S. earthquake usually can be felt as far as 500 km (300 mi) from where it occurred, and sometimes causes damage as far away as 40 km (25 mi).

  • Here is a great video from IRIS that helps explain Earthquake Instensity.
  • Here is the generalized 2014 Seismic Hazard map for the USA.

  • The 2014 U.S. Geological Survey (USGS) National Seismic Hazard Maps display earthquake ground motions for various probability levels across the United States and are applied in seismic provisions of building codes, insurance rate structures, risk assessments, and other public policy. The updated maps represent an assessment of the best available science in earthquake hazards and incorporate new findings on earthquake ground shaking, faults, seismicity, and geodesy. The USGS National Seismic Hazard Mapping Project developed these maps by incorporating information on potential earthquakes and associated ground shaking obtained from interaction in science and engineering workshops involving hundreds of participants, review by several science organizations and State surveys, and advice from expert panels and a Steering Committee. The new probabilistic hazard maps represent an update of the seismic hazard maps; previous versions were developed by Petersen and others (2008) and Frankel and others (2002), using the methodology developed Frankel and others (1996). Algermissen and Perkins (1976) published the first probabilistic seismic hazard map of the United States which was updated in Algermissen and others (1990).

    The National Seismic Hazard Maps are derived from seismic hazard curves calculated on a grid of sites across the United States that describe the annual frequency of exceeding a set of ground motions. Data and maps from the 2014 U.S. Geological Survey National Seismic Hazard Mapping Project are available for download below. Maps for available periods (0.2 s, 1 s, PGA) and specified annual frequencies of exceedance can be calculated from the hazard curves. Figures depict probabilistic ground motions with a 2 percent probability of exceedance. Spectral accelerations are calculated for 5 percent damped linear elastic oscillators. All ground motions are calculated for site conditions with Vs30=760 m/s, corresponding to NEHRP B/C site class boundary.


Posted in Uncategorized

Earthquake Report: Loyalty Islands Update #1

I just got back from one of the best conferences that I have ever attended, PATA Days 2017 (Paleoseismology, Active Tectonics, and Archeoseismology). This conference was held in Blenheim, New Zealand and was planned to commemorate the 300 year anniversary of the 1717 AD Alpine fault earthquake (the possibly last “full” margin rupture of the Alpine fault, a strike-slip plate boundary between the Australia and Pacific plates, with a slip rate of about 30 mm per year, tapering northwwards as synthetic strike slip faults splay off from the AF). While the meeting was being planned, the 2016 M 7.8 Kaikoura earthquake happened, which expanded the subject matter somewhat.

Prior to the meeting, we all attended a one day field trip reviewing field evidence for surface rupture and coseismic deformation and landslides from the M 7.8 earthquake in the northern part of the region. The road is still cut off and being repaired, so one cannot drive along the coast between Blenheim and Christchurch (will be open in a few months). During the meeting, there were three days of excellent talks (check out #PATA17 on twitter). Following the meeting, a myajority of the group attended a three day field trip to review the geologic evidence as reviewed by earthquake geologists here of historic and prehistoric earthquakes on the Alpine fault and faults along the Marlborough fault zone (faults that splay off the Alpine fault, extracting plate boundary motion from the Alpine fault). The final day we saw field evidence of rupture from the M 7.8 earthquake, including a coseismic landslide, which blocked a creek. The creek later over-topped some adjacent landscape, down-cutting and exposing stratigraphy that reveals evidence for past rupture on that fault. The trip was epic and the meeting was groundbreaking (apologies for the pun).

This region of the southern New Hebrides subduction zone is formed by the subduction of the Australia plate beneath the Pacific plate. There has been an ongoing earthquake sequence since around Halloween (I prepared a report shortly after I arrived in New Zealand; here is my report for the early part of this sequence). Today there was the largest magnitude earthquake in the sequence. This M 7.0 earthquake generated a tsunami measured on tide gages in the region. However, there was a low likelihood of a transpacific tsunami. The sequence beginning several weeks ago included outer rise extension earthquakes and associated thrust fault earthquakes along the upper plate. I have discussed how the lower/down-going plates in a subduction zone flex and cause extension in this flexed bulge (called the outer rise because it bulges up slightly). Here is my report discussing a possibly triggered outer rise earthquake associated with the 2011 M 9.0 Tohoku-oki earthquake. Here is my report for this M 6.0 earthquake from 2016.08.20.

While looking into this further today, I found that there was a similar sequence (to the current sequence) in 2003-2004. For both sequences, there is an interplay between the upper and lower plates, with compressional earthquakes in the upper plate and extensional earthquakes in the lower plate. Based upon the 2003-2004 sequence, it is possible that there may be a forthcoming compressional earthquake. However, there are many factors that drive the changes in static stress along subduction zones and how that stress may lead to an earthquake (so, there may not be a large earthquake in the upper plate). This is just a simple comparison (albeit for a section of the subduction zone in close proximity).

  • I list below some USGS earthquake pages for earthquakes in this report.
  • These are earthquakes from this current sequence.
  • 2017.10.31 M 6.8
  • 2017.11.16 M 5.9
  • 2017.11.19 M 6.4
  • 2017.11.19 M 5.9
  • 2017.11.19 M 6.6
  • 2017.11.19 M 7.0
  • 2017.11.20 M 5.8
  • These are earthquakes from the 2003-2004 sequence.
  • 2003.12.25 M 6.5
  • 2003.12.26 M 6.8
  • 2003.12.27 M 6.7
  • 2003.12.27 M 7.3
  • 2003.12.27 M 6.3
  • 2004.01.03 M 6.4
  • 2004.01.03 M 7.1

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 include earthquake epicenters from 1917-2017 with 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. The moment tensor shows northeast-southwest compression, perpendicular to the convergence at this plate boundary. Most of the recent seismicity in this region is associated with convergence along the New Britain trench or the South Solomon trench.
  • 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 upper right corner I include the map and seismicity cross section from Benz et al. (2011). These maps plot the seismicity and this reveals the nature of the downgoing subducting slab. Shallower earthquakes are generally more related to the subduction zone fault or deformation within either plate (interplate and intraplate earthquakes). While the deeper earthquakes are not megathrust fault related, but solely due to internal crustal deformation (intraplate earthquakes). I highlight the location of the cross section with a blue line labeled G-G’ (and place this cross section in the general location on the main interpretive map.
  • In the lower left corner I include some tide gage records from the region, which are from the UNESCO IOC online Sea Level Monitoring Facility. These three records are labeled A, B, and C and the locations of these gages are designated by red dots on the main map, along with A, B, and C labels in white.
  • Above the raw tide gage records are some reported wave heights from the Pacific Tsunami Warning Center. These are basically the wave heights recorded on the tide gages, but interpreted by a subject matter expert to estimate the timing of wave arrival and the water surface elevation in excess of the ambient sea level.
  • In the lower right corner I include a comparison between the 2003-2004 sequence and the current and ongoing sequence. I plot moment tensors for earthquakes [largely] from after my initial report (though I include the largest magnitude earthquake in the upper plate). I plot USGS moment tensors for the larger magnitude earthquakes from the 2003 sequence. I also label the along strike extent for these two sequences. They overlap by a very small amount, but generally seem to be happening in adjacent sections of the fault system here. All things being equal, the 2003 sequence included M 7 earthquakes in both the upper and lower plates. If the 2017 sequence is similar, there may still be an M 7 earthquake in the upper plate. Of course, there is also a possiblity of a large subduction zone earthquake here too. We just don’t really have enough information to really know (it is difficult to know, if not impossible, the state of stress on the megathrust fault. this make it impossible to predict if there will be more or larger earthquakes here.).
  • In the upper left corner I include a figure from Lay et al. (2011) that shows the general tectonic setting at subduction zone faults. There are three examples. Lay et al. (2011) modeled the Japan trench subduction zone after the 2011 M 9.0 Tohoku-Oki earthquake and estimated the static stress changes imparted in the adjacent crust as a result of the M 9.0 earthquake. Lay et al. (2011) determined that the downgoing plate to the east of the M 9.0 earthquake experienced an increase in static stress. This was used to support the hypothesis that the M 6.0 earthquake along the outer rise, east of the M 9.0 slip patch, was statically triggered by the M 9.0 earthquake. The two sequences along the southern New Hebrides trench are probably playing out a similar fault-geometry and static stress relation.


  • Here is my poster from the beginning of this sequence.

  • Here is a map from the USGS report (Benz et al., 2011). Read more about this map on the USGS website. Earthquakes are plotted with color related to depth and circle diameter related to magnitude. Today’s M 6.8 earthquake occurred south of cross section G-G’.

  • This is the legend.

  • Here is a cross section showing the seismicity along swatch profile G-G’.

  • Craig et al. (2014) evaluated the historic record of seismicity for subduction zones globally. In particular, the evaluated the relations between upper and lower plate stresses and earthquake types (cogent for the southern New Hebrides trench). Below is a figure from their paper for this part of the world. I include their figure caption below in blockquote.

  • Outer-rise seismicity along the New Hebrides arc. (a) Seismicity and focal mechanisms. Seismicity at the southern end of the arc is dominated by two major outer-rise normal faulting events, and MW 7.6 on 1995 May 16 and an MW 7.1 on 2004 January 3. Earthquakes are included from Chapple & Forsyth (1979); Chinn & Isacks (1983); Liu & McNally (1993). (b) Time versus latitude plot.

  • Here is a summary figure from Craig et al. (2014) that shows different stress configurations possibly existing along subduction zones.

  • Schematic diagram for the factors influencing the depth of the transition from horizontal extension to horizontal compression beneath the outer rise. Slab pull, the interaction of the descending slab with the 660 km discontinuity (or increasing drag from the surround mantle), and variations in the interface stress influence both the bending moment and the in-plane stress. Increases in the angle of slab dip increases the dominance of the bending moment relative to the in-plane stress, and hence moves the depth of transition towards the middle of the mechanical plate from either an shallower or a deeper position. A decrease in slab dip enhances the influence of the in-plane stress, and hence moves the transition further from the middle of the mechanical plate, either deeper for an extensional in-plane stress, or shallower for a compressional in-plane stress. Increased plate age of the incoming plate leads to increases in the magnitude of ridge push and intraplate thermal contraction, increasing the in-plane compressional stress in the plate prior to bending. Dynamic topography of the oceanic plate seawards of the trench can result in either in-plane extension or compression prior to the application of the bending stresses.

  • Here is a great figure from here, the New Caledonian Seismologic Network. This shows how geologists have recorded uplift rates along dip (“perpendicular” to the subduction zone fault). On the left is a map and on the right is a vertical profile showing how these rates of uplift change east-west. This is the upwards flexure related to the outer rise, which causes extension in the upper part of the downgoing/subducting plate.

  • The subduction of the Australian plate under the Vanuatu arc is also accompanied by vertical movements of the lithosphere. Thus, the altitudes recorded by GPS at the level of the Quaternary reef formations that cover the Loyalty Islands (Ouvéa altitude: 46 m, Lifou: 104 m and Maré 138 m) indicate that the Loyalty Islands accompany a bulge of the Australian plate. just before his subduction. Coral reefs that have “recorded” the high historical levels of the sea serve as a reference marker for geologists who map areas in uprising or vertical depression (called uplift and subsidence). Thus, the various studies have shown that the Loyalty Islands, the Isle of Pines but alsothe south of Grande Terre (Yaté region) rise at speeds between 1.2 and 2.5 millimeters per decade.

  • Here are the figures from Richards et al. (2011) with their figure captions below in blockquote.
  • The main tectonic map

  • bathymetry, and major tectonic element map of the study area. The Tonga and Vanuatu subduction systems are shown together with the locations of earthquake epicenters discussed herein. Earthquakes between 0 and 70 km depth have been removed for clarity. Remaining earthquakes are color-coded according to depth. Earthquakes located at 500–650 km depth beneath the North Fiji Basin are also shown. Plate motions for Vanuatu are from the U.S. Geological Survey, and for Tonga from Beavan et al. (2002) (see text for details). Dashed line indicates location of cross section shown in Figure 3. NFB—North Fiji Basin; HFZ—Hunter Fracture Zone.

  • Here is the map showing the current configuration of the slabs in the region.

  • Map showing distribution of slab segments beneath the Tonga-Vanuatu region. West-dipping Pacifi c slab is shown in gray; northeast-dipping Australian slab is shown in red. Three detached segments of Australian slab lie below the North Fiji Basin (NFB). HFZ—Hunter Fracture Zone. Contour interval is 100 km. Detached segments of Australian plate form sub-horizontal sheets located at ~600 km depth. White dashed line shows outline of the subducted slab fragments when reconstructed from 660 km depth to the surface. When all subducted components are brought to the surface, the geometry closely approximates that of the North Fiji Basin.

  • This is the cross section showing the megathrust fault configuration based on seismic tomography and seismicity.

  • Previous interpretation of combined P-wave tomography and seismicity from van der Hilst (1995). Earthquake hypocenters are shown in blue. The previous interpretation of slab structure is contained within the black dashed lines. Solid red lines mark the surface of the Pacifi c slab (1), the still attached subducting Australian slab (2a), and the detached segment of the Australian plate (2b). UM—upper mantle;
    TZ—transition zone; LM—lower mantle.

  • Here is their time step interpretation of the slabs that resulted in the second figure above.

  • Simplifi ed plate tectonic reconstruction showing the progressive geometric evolution of the Vanuatu and Tonga subduction systems in plan view and in cross section. Initiation of the Vanuatu subduction system begins by 10 Ma. Initial detachment of the basal part of the Australian slab begins at ca. 5–4 Ma and then sinking and collision between the detached segment and the Pacifi c slab occur by 3–4 Ma. Initial opening of the Lau backarc also occurred at this time. Between 3 Ma and the present, both slabs have been sinking progressively to their current position. VT—Vitiaz trench; dER—d’Entrecasteaux Ridge.

  • Here is a figure that shows the coulomb stress changes due to the 2011 earthquake. Basically, this shows which locations on the fault where we might expect higher likelihoods of future earthquake slip. I include their figure caption below as a blockquote.

  • Maps of the Coulomb stress change predicted for the joint P wave, Rayleigh wave and continuous GPS inversion in Fig. 2. The margins of the latter fault model are indicated by the box. Two weeks of aftershock locations from the U.S. Geological Survey are superimposed, with symbol sizes scaled relative to seismic magnitude. (a) The Coulomb stress change averaged over depths of 10–15 km for normal faults with the same westward dipping fault plane geometry as the Mw 7.7 outer rise aftershock, for which the global centroid moment tensor mechanism is shown. (b) Similar stress changes for thrust faults with the same geometry as the mainshock, along with the Mw 7.9 thrusting aftershock to the south, for which the global centroid moment tensor is shown.

  • Here is a figure schematically showing how subduction zone earthquakes may increase coulomb stress along the outer rise. The outer rise is a region of the downgoing/subducting plate that is flexing upwards. There are commonly normal faults, sometimes reactivating fracture zone/strike-slip faults, caused by extension along the upper oceanic lithosphere. We call these bending moment normal faults. There was a M 7.1 earthquake on 2013.10.25 that appears to be along one of these faults. I include their figure caption below as a blockquote.

  • Schematic cross-sections of the A) Sanriku-oki, B) Kuril and C) Miyagi-oki subduction zones where great interplate thrust events have been followed by great trench slope or outer rise extensional events (in the first two cases) and concern about that happening in the case of the 2011 event.

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

  • Here is a link to the embedded video below (6 MB mp4)

    References:

  • Benz, H.M., Herman, M., Tarr, A.C., Hayes, G.P., Furlong, K.P., Villaseñor, A., Dart, R.L., and Rhea, S., 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.
  • Bird, P., 2003. An updated digital model of plate boundaries in Geochemistry, Geophysics, Geosystems, v. 4, doi:10.1029/2001GC000252, 52 p.
  • Craig, T.J., Copley, A., and Jackson, J., 2014. A reassessment of outer-rise seismicity and its implications for the mechanics of oceanic lithosphere in Geophysical Journal International, v. 197, p/ 63-89.
  • Geist, E.L., and Parsons, T., 2005, Triggering of tsunamigenic aftershocks from large strike-slip earthquakes: Analysis of the November 2000 New Ireland earthquake sequence: Geochemistry, Geophysics, Geosystems, v. 6, doi:10.1029/2005GC000935, 18 p. [Download PDF (6.5 MB)]
  • 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.
  • Lay, T., and Kanamori, H., 1980, Earthquake doublets in the Solomon Islands: Physics of the Earth and Planetary Interiors, v. 21, p. 283-304.
  • Lay, T., Ammon, C.J., Kanamori, H., Kim, M.J., and Xue, L., 2011. Outer trench-slope faulting and the 2011 Mw 9.0 off the Pacific coast of Tohoku Earthquake in Earth Planets Space,
    v. 63, p. 713-718.
  • Richards, S., Holm, R., Barber, G., 2011. When slabs collide: A tectonic assessment of deep earthquakes in the Tonga-Vanuatu region in Geology, v. 39, no. 8., p. 787-790
  • Schwartz, S.Y., 1999, Noncharacteristic behavior and complex recurrence of large subduction zone earthquakes: Journal of Geophysical Research, v. 104, p. 23,111-123,125.
  • Schwartz, S.Y., Lay, T., and Ruff, L.J., 1989, Source process of the great 1971 Solomon Islands doublet: Physics of the Earth and Planetary Interiors, v. 56, p. 294-310.

Posted in earthquake, education, Extension, geology, pacific, plate tectonics, subduction, tsunami

Earthquake Report: Papua New Guinea!

Well. As I was preparing a job application at the library public wifi (the Airbnb I was staying at did not have wifi in my cabin, nor electricity for that matter), I prepared an interpretive poster for this earthquake. Interestingly, the library prevented any ftp connections, so I had to wait until today to upload my files.

Note: not sure why, but when I prepared this report, I initially entitled it as being along the Tonga subduction zone. This was not correct and I fixed it. There was a recent earthquake along the Tonga subduction zone and I had that on my mind.

This M 6.5 earthquake (here is the USGS website for this earthquake) happened in a region of Papua New Guinea (PNG) that has a long record of different types of tectonic deformation (including subduction, strike-slip, and several fold and thrust belts). To the northwest, in 1998, there was an earthquake that triggered a submarine landslide, which generated a large, devastating, and deadly tsunami. Here is the USGS website for this 1998 M 7.0 earthquake.

There are historic earthquakes to the west of this M 6.5 that are associated with the fold and thrust belt, but this M 6.5 earthquake is too deep to be associated with a possibly eastern extension of this fold and thrust belt. womp womp.

However, there have been a few earthquakes that are more closely (spatially) related to the 2017.11.07 M 6.5 earthquake. On 1986.06.24 there was an M 7.2 earthquake (here is the USGS website for this earthquake) to the southeast. These two earthquakes both have similar fault plane solutions (a moment tensor for the 2017 earthquake and a focal mechanism for the 1986 earthquake) and nearly identical depths. These deep earthquakes are deeper than we would expect for a subduction zone fault, so are possibly related to internal deformation within the downgoing slab. The subduction zone associated with the New Guinea (NG) trench (associated with the 1998 landslide tsunami earthquake) may or may not extend into this region (The Holm et al. (2016) figure below shows it does now). There is a fossil subduction zone (the Melanesian or Manus Trench) to the east of the NG trench, but this is also probably unrelated.

The best candidate is the downgoing slab associated with the New Britain Trench. This subduction zone is formed by the northward motion of the Solomon Sea plate beneath the South Bismarck plate (in the region of New Britain), but to the west, this fault splays into three mapped thrust faults that trend on land in eastern PNG. This is the slab imaged beneath where the epicenters are plotted for both 1986 and 2017 earthquakes. The Holm et al. (2016) figure has an inactive splay that more optimally (geometrically) is suited to fit these two earthquakes. There is a figure in the poster (and plotted below) that shows the geometry of this downgoing slab.

Below is my interpretive poster for this earthquake.


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1917-2017 with 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 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 upper left corner is a great figure showing the generalized plate tectonic boundaries in this region of the equatorial Pacific Ocean (Holm et al., 2016). I place a blue star in the general location of the M 6.5 earthquake (also plotted in other inset figures).
  • In the lower left corner is another plate tectonic map (Sapiie and Cloos, 2004) showing a slightly different interpretation of the faults in this region.
  • In the upper right corner are two figures from Holm and Richards (2013). Their paper discusses the back-arc spreading in the Bismarck Sea. They use hypocenter data to construct this 3-D model of the slab. On the right is a forecast of how the slab will be consumed along these subduction zones in the future.
  • In the lower right corner is another figure from Holm and Rihards (2013) where they present a low angle oblique view of the slab that they modeled in their paper.
  • To the left of the Holm and Richards figure is a map only has plate motions plotted (from Paul Tregoning at The Australian National University). The map plots, “linear velocities of GPS sites in PNG, showing absolute motions of the numerous tectonic plates.” Go to his website where he presents some related papers.


  • In 2015 there were a few small earthquakes to the northeast of the 1986 and 2017 deep earthquakes, but they were much shallower. However, they show a similarly oriented fault place solution. Though, these 2015 earthquakes are probably associated with the current strain being accumulated and released associated with plate tectonic boundaries (while the 1986 and 2017 deep earthquakes are not; their shared orientation is probably just coincidental?).
  • Here is the report from 2015. Below is the interpretive poster for those earthquakes (note how my posters are seeing a realized improvement over time).

  • Here is the Holm et al. (2016) figure.

  • Topography, bathymetry and regional tectonic setting of New Guinea and Solomon Islands. Arrows indicate rate and direction of plate motion of the Australian and Pacific plates (MORVEL, DeMets et al., 2010); Mamberamo thrust belt, Indonesia (MTB); North Fiji Basin (NFB)

  • Here is the tectonic map figure from Sappie and Cloos (2004). Their work was focused on western PNG, so their interpretations are more detailed there (and perhaps less relevant for us for these eastern PNG earthquakes).

  • Seismotectonic interpretation of New Guinea. Tectonic features: PTFB—Papuan thrust-and-fold belt; RMFZ—Ramu-Markham fault zone; BTFZ—Bewani-Torricelli fault zone; MTFB—Mamberamo thrust-and-fold belt; SFZ—Sorong fault zone; YFZ—Yapen fault zone; RFZ—Ransiki fault zone; TAFZ—Tarera-Aiduna fault zone; WT—Waipona Trough. After Sapiie et al. (1999).

  • This is the two panel figure from Holm and Richards (2013) that shows how the New Britain trench megathrust splays into three thrust faults as this fault system heads onto PNG. They plot active thrust faults as black triangles (with the triangles on the hanging wall side of the fault) and inactive thrust faults as open triangles. So, either the NG trench subduction zone extends further east than is presented in earlier work or the Bundi Fault Zone is the fault associated with this deep seismicity.

  • Topography, bathymetry and major tectonic elements of the study area. (a) Major tectonic boundaries of Papua New Guinea and the western Solomon Islands; CP, Caroline plate; MB, Manus Basin; NBP, North Bismarck plate; NBT, New Britain trench; NGT, New Guinea trench; NST, North Solomon trench; PFTB, Papuan Fold and Thrust Belt; PT, Pocklington trough; RMF, Ramu-Markham Fault; SBP, South Bismarck plate; SCT, San Cristobal trench; SS, Solomon Sea plate; TT, Trobriand trough; WB,Woodlark Basin; WMT,West Melanesian trench. Study area is indicated by rectangle labelled Figure 1b; the other inset rectangle highlights location for subsequent figures. Present day GPS motions of plates are indicated relative to the Australian plate (from Tregoning et al. 1998, 1999; Tregoning 2002; Wallace et al. 2004). (b) Detailed topography, bathymetry and structural elements significant to the South Bismarck region (terms not in common use are referenced); AFB, Aure Fold Belt (Davies 2012); AT, Adelbert Terrane (e.g. Wallace et al. 2004); BFZ, Bundi Fault Zone (Abbott 1995); BSSL, Bismarck Sea Seismic Lineation; CG, Cape Gloucester; FT, Finisterre Terrane; GF, Gogol Fault (Abbott 1995); GP, Gazelle Peninsula; HP, Huon Peninsula; MB, Manus Basin; NB, New Britain; NI, New Ireland; OSF, Owen Stanley Fault; RMF, Ramu-Markham Fault; SS, Solomon Sea; WMR, Willaumez-Manus Rise (Johnson et al. 1979); WT, Wonga Thrust (Abbott et al. 1994); minor strike-slip faults are shown adjacent to Huon Peninsula (Abers & McCaffrey 1994) and in east New Britain, the Gazelle Peninsula (e.g. Madsen & Lindley 1994). Circles indicate centres of Quaternary volcanism of the Bismarck arc. Filled triangles indicate active thrusting or subduction, empty triangles indicate extinct or negligible thrusting or subduction.

  • Here is the slab interpretation for the New Britain region from Holm and Richards, 2013. I include the figure caption below as a blockquote.

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

  • Here are the forward models for the slab in the New Britain region from Holm and Richards, 2013. I include the figure caption below as a blockquote.

  • Forward tectonic reconstruction of progressive arc collision and accretion of New Britain to the Papua New Guinea margin. (a) Schematic forward reconstruction of New Britain relative to Papua New Guinea assuming continued northward motion of the Australian plate and clockwise rotation of the South Bismarck plate. (b) Cross-sections illustrate a conceptual interpretation of collision between New Britain and Papua New Guinea.

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

  • This map shows plate velocities and euler poles for different blocks. I include the figure caption below as a blockquote.

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

  • This figure incorporates cross sections and map views of various parts of the regional tectonics (Baldwin et al., 2012). These deep earthquakes are nearest the cross section D (though are much deeper than these shallow cross sections). I include the figure caption below as a blockquote.

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

References:

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

Earthquake Report: Tonga!

Well, I was just getting ready for bed and saw the PTWC email notification. It took a couple minutes before the USGS email notification came through, but the earthquake was already listed on the website. By the time the ENS email came in, the magnitude was adjusted, as well as the location.

This M 6.8 earthquake was originally reported as a deep earthquake at 80 km, but after real people took a look at the data, the depth was also adjusted.

It is interesting that this earthquake happened just a few days after the sequence to the west, though these earthquakes are probably too far away to be related. Here is my report from a couple days ago. I include the interpretive poster from that earthquake below today’s interpretive poster. I use some of the same figures for both of these posters since they are each in a similar region of the world. Then, moments later, there was an M 5.5 in the Loyalty Islands region. Hmmmmm.

Today’s M 6.8 earthquake did occur near the earthquake from 2009 (almost immediately down-dip of the 2009 earthquake), an M 8.1 earthquake in the downgoing slab, that caused a large and damaging tsunami in the Samoa Islands.

  • Here are the USGS websites for the M 6.8 earthquake.
  • 2017.11.04 M 6.8 Tonga
  • Here are the USGS earthquake pages for the earthquakes for which I plot fault plane solutions in the inset figure below.
  • 1981-09-01 M 7.7
  • 1985-06-03 M 6.7
  • 1990-01-04 M 6.5
  • 1993-05-16 M 6.6
  • 1995-04-07 M 7.4
  • 1996-08-05 M 6.7
  • 2009-08-30 M 6.6
  • 2009-09-29 M 8.1
  • 2015-03-30 M 6.5

Below is my interpretive poster for this earthquake.


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1917-2017 with 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 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 upper right corner is a map from Benz et al. (2011) that shows seismicity with color representing depth and circle diameter representing magnitude. This report is available online for more information. I include the general location of this M 6.8 earthquake as a blue star. Cross section H-H’ is plotted on the left of the map. I include the location of this x-sec on the main map as a blue line.
  • In the lower left corner are several more cross sections as determined from seismic tomography (de Paor et al., 2012). Seismic tomography is basically an X-ray of Earth’s interior that uses seismic waves rather than X-rays. More on seismic tomography can be found here or here. The cross section A-A’ is shown on the main map as a green line.
  • In the upper left corner is a low-angle oblique view of the megathrust fault (the slab). This is from Green (2003).
  • In the lower left corner I plot the USGS moment tensors (blue) and focal mechanisms (orange) for earthquakes in this region. These earthquakes fit two distinct domains (outlined in dashed white).
    The easternmost quakes are in the downgoing Pacific plate and are the result of extension (either slab tension or due to bending in the upper part of the plate). The westernmost quakes are either near the megathrust (subduction zone) or in the upper plate (most appear to be in the upper plate). Something that all earthquakes share is that their principal strain direction (the orientation of maximum or minimum stress) rotates with the orientation of the subduction zone fault. This may represent strain partitioning at this plate boundary. Basically, thrust earthquakes tend to be oriented perpendicular to the megathrust fault strike, and there are other strike-slip earthquakes that accommodate the non-perpendicular plate motion. There are no examples of these strike-slip earthquakes here. The subduction zone here is quite complicated as there is backarc spreading and a complex interaction between mantle flow at the edge of the subduction zone and a hotspot plume.


  • Here is the oblique view of the slab from Green (2003).

  • Earthquakes and subducted slabs beneath the Tonga–Fiji area. The subducting slab and
    detached slab are defined by the historic earthquakes in this region: the steeply dipping surface descending from the Tonga Trench marks the currently active subduction zone, and the surface lying mostly between 500 and 680 km, but rising to 300 km in the east, is a relict from an old subduction zone that descended from the fossil Vitiaz Trench. The locations of the mainshocks of the two Tongan earthquake sequences discussed by Tibi et al. are marked in yellow (2002 sequence) and orange (1986 series). Triggering mainshocks are denoted by stars; triggered mainshocks by circles. The 2002 sequence lies wholly in the currently subducting slab (and slightly extends the earthquake distribution in it),whereas the 1986 mainshock is in that slab but the triggered series is located in the detached slab,which apparently contains significant amounts of metastable olivine

  • Here is a great figure showing an interpretation of how the mantle flow and hotspot plume may interact here (Chang et al., 2015).

  • Illustrated time history of the plume–slab interaction and a cartoon for summarizing current features. (a) The Samoan plume is generated at the Mega ULVZ19 at the core–mantle boundary and is ascending to the surface. (b) The Samoan plume collided with the Tonga slab at the transition zone at about 10 Myr. (c) The upward stress by the collision has caused the stagnant slab and intense seismicity (cross marks), which is further enhanced by fast slab retreat (red arrow) due to the subduction of the Hikurangi plateau. (d) A schematic diagram illustrating the slab–plume interaction beneath the Tonga–Kermadec arc. Cyan lines on the surface show trenches, as shown in Fig. 1. HP, Hikurangi Plateau; KT, Kermadec Trench; NHT, New Hebrides Trench; TT, Tonga Trench; VT, Vitiaz Trench. The Samoan plume originates from a Mega ULVZ at the core–mantle boundary (CMB). The buoyancy caused by large
    stress from the plume at the bottom of the Tonga slab may contribute to the slab stagnation within the mantle transition zone, while the Kermadec slab is penetrating into the lower mantle directly. At the northern end of the Tonga slab, plume materials migrate into the mantle wedge, facilitated by strong toroidal flow around the slab edge induced by fast slab retreat.

  • Here are figures from Richards et al. (2011) with their figure captions below in blockquote.
  • The main tectonic map

  • bathymetry, and major tectonic element map of the study area. The Tonga and Vanuatu subduction systems are shown together with the locations of earthquake epicenters discussed herein. Earthquakes between 0 and 70 km depth have been removed for clarity. Remaining earthquakes are color-coded according to depth. Earthquakes located at 500–650 km depth beneath the North Fiji Basin are also shown. Plate motions for Vanuatu are from the U.S. Geological Survey, and for Tonga from Beavan et al. (2002) (see text for details). Dashed line indicates location of cross section shown in Figure 3. NFB—North Fiji Basin; HFZ—Hunter Fracture Zone.

  • Here is the map showing the current configuration of the slabs in the region.

  • Map showing distribution of slab segments beneath the Tonga-Vanuatu region. West-dipping Pacifi c slab is shown in gray; northeast-dipping Australian slab is shown in red. Three detached segments of Australian slab lie below the North Fiji Basin (NFB). HFZ—Hunter Fracture Zone. Contour interval is 100 km. Detached segments of Australian plate form sub-horizontal sheets located at ~600 km depth. White dashed line shows outline of the subducted slab fragments when reconstructed from 660 km depth to the surface. When all subducted components are brought to the surface, the geometry closely approximates that of the North Fiji Basin.

  • This is the cross section showing the megathrust fault configuration based on seismic tomography and seismicity.

  • Previous interpretation of combined P-wave tomography and seismicity from van der Hilst (1995). Earthquake hypocenters are shown in blue. The previous interpretation of slab structure is contained within the black dashed lines. Solid red lines mark the surface of the Pacifi c slab (1), the still attached subducting Australian slab (2a), and the detached segment of the Australian plate (2b). UM—upper mantle;
    TZ—transition zone; LM—lower mantle.

  • Here is their time step interpretation of the slabs that resulted in the second figure above.

  • Simplifi ed plate tectonic reconstruction showing the progressive geometric evolution of the Vanuatu and Tonga subduction systems in plan view and in cross section. Initiation of the Vanuatu subduction system begins by 10 Ma. Initial detachment of the basal part of the Australian slab begins at ca. 5–4 Ma and then sinking and collision between the detached segment and the Pacifi c slab occur by 3–4 Ma. Initial opening of the Lau backarc also occurred at this time. Between 3 Ma and the present, both slabs have been sinking progressively to their current position. VT—Vitiaz trench; dER—d’Entrecasteaux Ridge.

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

#Earthquake Report: Loyalty Islands

BOO! Happy Halloween/Samhain….

I am on the road and worked on this report while on layovers with intermittent internets access… Though this earthquake sequence spanned a day or so, so it is good that it took me a while to compile my figures.

Below is my interpretive poster for this earthquake.


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1917-2017 with magnitudes M > 7.5. I also plot the moment tensors for some earthquakes to the southeast of the current sequence. Also, there was a sequence in December of 2016. Here is my report for that series of earthquakes. There are other earthquakes in this region listed at the bottom of this page above the references. Note the special symbology that I used for the 1920 earthquake epicenter.

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely. The moment tensor shows northeast-southwest compression, perpendicular to the convergence at this plate boundary. Most of the recent seismicity in this region is associated with convergence along the New Britain trench or the South Solomon trench.
  • 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 plots this close to the location of the fault as mapped by Hayes et al. (2012). The M 6.8 is plotted really close to the megathrust and is also very shallow. The depth is probably not very well constrained due to the geometry and lack of seismometer coverage in the oceanic setting.
  • Here is the USGS page for the main earthquake in this sequence.
  • 2017.10.31 M 6.8
  • I include some inset figures.

  • In the upper right corner I include a figure from Richards et al. (2011) that shows the major plate boundary faults in the region. They also plot seismicity with color representing depth. This allows us to visualize the subduction zone fault as it dips (eastward for the New Hebrides and westwards for the Tonga subduction zones). The cross section in the panel on the right is designated by the black dashed line. I also place this line as a dashed green line in the interpretive poster below. I place a yellow star in the general location of the M 6.8 earthquake.
  • In the upper right corner I include the Richards et al. (2011) cross section showing earthquake hypocenters as colored circles and the megathrust subduction zone faults as red lines.
  • To the left of the cross section is a panel that shows how Richards et al. (2011) hypothesize about how the New Hebrides subducting slab (Australia plate) and the Fiji Basin (the upper plate) interacted to create the configuration of the plates and faults in this region. Note how shallow the New Hebrides fault is.
  • In the lower left corner I plot the USGS moment tensors for the main earthquakes from this sequence. Note how the mainshock is a thrust (compressional) earthquake, while the earthquakes in the downgoing Australia plate, to the west of the subduction zone, are mostly normal (extensional) earthquakes. There are many examples of this globally (Samoa, Marianas, Kuril, etc.). I will follow up by linking other reports of mine that discuss these at a later time. I am working on very little sleep from my travels.


  • Here is my interpretive poster for the 2016.12.08 earthquake.

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

  • Here is a link to the embedded video below (6 MB mp4)
  • Here is a map from the USGS report (Benz et al., 2011). Read more about this map on the USGS website. Earthquakes are plotted with color related to depth and circle diameter related to magnitude. Today’s M 6.8 earthquake occurred south of cross section G-G’.

  • This is the legend.

  • Here is a cross section showing the seismicity along swatch profile G-G’.

  • Here are the figures from Richards et al. (2011) with their figure captions below in blockquote.
  • The main tectonic map

  • bathymetry, and major tectonic element map of the study area. The Tonga and Vanuatu subduction systems are shown together with the locations of earthquake epicenters discussed herein. Earthquakes between 0 and 70 km depth have been removed for clarity. Remaining earthquakes are color-coded according to depth. Earthquakes located at 500–650 km depth beneath the North Fiji Basin are also shown. Plate motions for Vanuatu are from the U.S. Geological Survey, and for Tonga from Beavan et al. (2002) (see text for details). Dashed line indicates location of cross section shown in Figure 3. NFB—North Fiji Basin; HFZ—Hunter Fracture Zone.

  • Here is the map showing the current configuration of the slabs in the region.

  • Map showing distribution of slab segments beneath the Tonga-Vanuatu region. West-dipping Pacifi c slab is shown in gray; northeast-dipping Australian slab is shown in red. Three detached segments of Australian slab lie below the North Fiji Basin (NFB). HFZ—Hunter Fracture Zone. Contour interval is 100 km. Detached segments of Australian plate form sub-horizontal sheets located at ~600 km depth. White dashed line shows outline of the subducted slab fragments when reconstructed from 660 km depth to the surface. When all subducted components are brought to the surface, the geometry closely approximates that of the North Fiji Basin.

  • This is the cross section showing the megathrust fault configuration based on seismic tomography and seismicity.

  • Previous interpretation of combined P-wave tomography and seismicity from van der Hilst (1995). Earthquake hypocenters are shown in blue. The previous interpretation of slab structure is contained within the black dashed lines. Solid red lines mark the surface of the Pacifi c slab (1), the still attached subducting Australian slab (2a), and the detached segment of the Australian plate (2b). UM—upper mantle;
    TZ—transition zone; LM—lower mantle.

  • Here is their time step interpretation of the slabs that resulted in the second figure above.

  • Simplifi ed plate tectonic reconstruction showing the progressive geometric evolution of the Vanuatu and Tonga subduction systems in plan view and in cross section. Initiation of the Vanuatu subduction system begins by 10 Ma. Initial detachment of the basal part of the Australian slab begins at ca. 5–4 Ma and then sinking and collision between the detached segment and the Pacifi c slab occur by 3–4 Ma. Initial opening of the Lau backarc also occurred at this time. Between 3 Ma and the present, both slabs have been sinking progressively to their current position. VT—Vitiaz trench; dER—d’Entrecasteaux Ridge.

  • Here is a screenshot of my seat’s screen showing where my plane flight was today (last night). I actually flew right overhead of these earthquakes! Interestingly, I was on a ship collecting piston cores for a seismoturbidite study offshore the North Island last year when the M 7.8 Kaikoura earthquake ruptured. I did not feel the earthquakes in either case (these Halloween earthquakes nor the Kaikoura earthquakes.


    References:

  • Benz, H.M., Herman, M., Tarr, A.C., Hayes, G.P., Furlong, K.P., Villaseñor, A., Dart, R.L., and Rhea, S., 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.
  • Bird, P., 2003. An updated digital model of plate boundaries in Geochemistry, Geophysics, Geosystems, v. 4, doi:10.1029/2001GC000252, 52 p.
  • Geist, E.L., and Parsons, T., 2005, Triggering of tsunamigenic aftershocks from large strike-slip earthquakes: Analysis of the November 2000 New Ireland earthquake sequence: Geochemistry, Geophysics, Geosystems, v. 6, doi:10.1029/2005GC000935, 18 p. [Download PDF (6.5 MB)]
  • 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.
  • Lay, T., and Kanamori, H., 1980, Earthquake doublets in the Solomon Islands: Physics of the Earth and Planetary Interiors, v. 21, p. 283-304.
  • Richards, S., Holm, R., Barber, G., 2011. When slabs collide: A tectonic assessment of deep earthquakes in the Tonga-Vanuatu region in Geology, v. 39, no. 8., p. 787-790
  • Schwartz, S.Y., 1999, Noncharacteristic behavior and complex recurrence of large subduction zone earthquakes: Journal of Geophysical Research, v. 104, p. 23,111-123,125.
  • Schwartz, S.Y., Lay, T., and Ruff, L.J., 1989, Source process of the great 1971 Solomon Islands doublet: Physics of the Earth and Planetary Interiors, v. 56, p. 294-310.

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

Earthquake Report: Chiapas Earthquake Update #2

Well, we had a really interesting earthquake today. There was a M 6.1 earthquake in the North America plate (NAP) to the north of the sequence offshore of Chiapas, with the M 8.1 mainshock. Here is the USGS website for the M 6.1 earthquake. There was also an M 5.8 earthquake that was a more typical aftershock (USGS website).

Why is this earthquake interesting? It is outside the region of aftershocks from the M 8.1 earthquake and it is in the upper plate (the NAP). This is not altogether groundbreaking (pardon the pun) as there are many examples of earthquakes in one plate triggering earthquakes in other plates. For example, the recent sequence just to the south of the M 8.1 sequence (which may have led partly to the M 8.1 earthquake).

This earthquake also triggered (sorry for the pun, another one) a debate about the difference between triggered earthquakes and aftershocks. This discussion is largely semantic and does not really matter from a natural hazards perspective. The rocks behave to physics, not how we classify them. So, we don’t need to get caught up in this lexicon (as long as we all have a general understanding of what is happening). In the classic sense, I interpret this M 6.1 (and the few nearby earthquakes) to be triggered, but they are in the region that may have an increased coulomb stress.

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 epicenters from 1917-2017 for magnitudes M ≥ 8.0. I include fault plane solutions for the 1985 and 1995 earthquakes (along with the MMI contours for those earthquakes, see below for a discussion of MMI contours).

  • 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 left center I include a generalized plate tectonic map from Wikimedia Creative Commons here.
  • In the lower left corner I include a map from Dr. Jascha Polet. Dr. Polet plots focal mechanisms for historic earthquakes. Dr. Polet notes the M 8.1, M7.1, M 6.1, and M 5.8 earthquakes too. Purple dots are epicenters after the M 8.1 earthquake and black dots are SSN (the Mexico seismic network data).
  • In the upper right corner is a map and cross section from Dr. Gavin Hayes. The upper cross section, oriented perpendicular to the subduction zone fault, shows focal mechanisms. Note how the M 8.1 (large green symbol) is in the downgoing Cocos plate and the M 6,.1 (small red symbol at about 300 km) is in the North America plate.
  • In the upper left corner is a figure Dr. Hayes also prepared. This shows the change in static coulomb stress associated with the M 8.1 earthquake. Tremblor prepared this analysis and I presented that in the M 7.1 earthquake report here. Basically, areas of warm color show an increased stress and regions of cool color show a decreased stress. So, areas in warm color are more likely to trigger an earthquake. Though this is a simple run as different faults can respond differently.


  • Here is the initial report poster as presented in my initial Earthquake Report here.

  • Here is the update #1 report poster as presented in my initial Earthquake Report here.

  • Here is the update #1 report poster for the M 7.1 Puebla, Mexico earthquake (which shows the coulomb stress modeling from Tremblor).

  • Here is Dr. Polet’s tweet of this map.
  • AND an updated map and cross section.
  • Here is Dr. Hayes’ tweet of his map and cross section.
  • Here is Dr. Hayes’ tweet of his static coulomb change map.
  • The discussion about what is a triggered earthquake and what is an aftershock, as I mentioned above, is the topic of discussion between experts in the field. The debate will probably be enduring for a quite a while, especially since classification systems are a social construct and have no real basis in reality (or physics). Classification systems are an excellent example of how science is subjective (science is fundamentally subjective, but that is a longer discussion, read some Karl Popper for more insight). This discussion led me to a web post following the 2011 Christchurch earthquake sequence. Here is a website with one view on this debate, prepared by Dr. Chris Rowan. I present two of their figures below.


References:

  • Benz, H.M., Dart, R.L., Villaseñor, Antonio, Hayes, G.P., Tarr, A.C., Furlong, K.P., and Rhea, Susan, 2011 a. Seismicity of the Earth 1900–2010 Mexico and vicinity: U.S. Geological Survey Open-File Report 2010–1083-F, scale 1:8,000,000.
  • Benz, H.M., Tarr, A.C., Hayes, G.P., Villaseñor, Antonio, Furlong, K.P., Dart, R.L., and Rhea, Susan, 2011 b. 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
  • Franco, S.I., Kostoglodov, V., Larson, K.M., Manea, V.C>, Manea, M., and Santiago, J.A., 2005. Propagation of the 2001–2002 silent earthquake and interplate coupling in the Oaxaca subduction zone, Mexico in Earth Planets Space, v. 57., p. 973-985.
  • 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.
  • Lay et al., 2011. Outer trench-slope faulting and the 2011 Mw 9.0 off the Pacific coast of Tohoku Earthquake in Earth Planets Space, v. 63, p. 713-718.
  • Manea, M., and Manea, V.C., 2014. On the origin of El Chichón volcano and subduction of Tehuantepec Ridge: A geodynamical perspective in JGVR, v. 175, p. 459-471.
  • Mann, P., 2007, Overview of the tectonic history of northern Central America, in Mann, P., ed., Geologic and tectonic development of the Caribbean plate boundary in northern Central America: Geological Society of America Special Paper 428, p. 1–19, doi: 10.1130/2007.2428(01). For
  • McCann, W.R., Nishenko S.P., Sykes, L.R., and Krause, J., 1979. Seismic Gaps and Plate Tectonics” Seismic Potential for Major Boundaries in Pageoph, v. 117
  • 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, education, Extension, geology, mexico, pacific, plate tectonics

Earthquake Report: Mendocino fault! (northern California)

I was driving around Eureka today, running to the appliance center to get an appliance (heheh). I got a message from a long time held friend (who lives in Salinas, CA). They asked me if I was OK, given that there was an earthquake up here. I thought I had not felt it because I was driving around. However, after looking at the USGS website, I learned the earthquake happened earlier, while I was back working on my house. The main reason I did not feel it is because it was too far away.

Once I got home, after work, I noticed that lots of people were discussing how they were confused about the earthquake notifications from the USGS. Apparently, there were two M 5.X earthquakes in the USGS earthquake online system for a while. Then there was one. This is a common occurrence and I prepared an explanation for some people Here is what I wrote for these people on social media:

this happens regularly. earthquake notifications are automatic as epicenter locations are identified from incoming seismic waves in the seismic network. sometimes the named arrivals (eg. p wave, s wave, and the many other arrivals) are miss-correlated between stations. this miss-correlation then leads to earthquakes in the database that are not real.

seismologists are monitoring the process and review these data for quality, looking for mistakes, and refining magnitude estimates, moment tensor and focal mechanism solutions, location estimates, casualty estimages (PAGER alerts), and all the derivative data products (intensity, PGA, PGV, etc. maps and data).

sometimes these earthquakes are from data in the same location as the real earthquake (like today) and sometimes they are “picked” from seismic data from remote earthquakes.

some of these earthquakes are listed here:
https://earthquake.usgs.gov/earthquakes/errata.php

Today’s M 5.7 earthquake was along the western part of the Mendocino fault (MF), a right-lateral (dextral) transform plate boundary. This plate boundary connects the Gorda ridge and Juan de Fuca rise spreading centers with their counterparts in the Gulf of California, with the San Andreas strike-slip fault system. Transform plate boundaries are defined that they are strike-slip and that they connect spreading ridges. In this sense of the definition, the Mendocino fault and the San Andreas fault are part of the same system. Here is the USGS website for this earthquake.

See the figures from Rollins and Stein (2010) below. More on earthquakes in this region can be found in Earthquake Reports listed at the bottom of this page above the appendices.

The San Andreas fault is a right-lateral strike-slip transform plate boundary between the Pacific and North America plates. The plate boundary is composed of faults that are parallel to sub-parallel to the SAF and extend from the west coast of CA to the Wasatch fault (WF) system in central Utah (the WF runs through Salt Lake City and is expressed by the mountain range on the east side of the basin that Salt Lake City is built within).

The three main faults in the region north of San Francisco are the SAF, the MF, and the Bartlett Springs fault (BSF). I also place a graphical depiction of the USGS moment tensor for this earthquake. The SAF, MF, and BSF are all right lateral strike-slip fault systems. There are no active faults mapped in the region of Sunday’s epicenter, but I interpret this earthquake to have right-lateral slip. Without more seismicity or mapped faults to suggest otherwise, this is a reasonable interpretation.

The Cascadia subduction zone is a convergent plate boundary where the Juan de Fuca and Gorda plates (JDFP and GP, respectively) subduct norteastwardly beneath the North America plate at rates ranging from 29- to 45-mm/yr. The Juan de Fuca and Gorda plates are formed at the Juan de Fuca Ridge and Gorda Rise spreading centers respectively. More about the CSZ can be found here.

There was a good sized (M 6.5) MF earthquake late last year on 2016.12.08. I present my poster for that earthquake below. Here is my report for that earthquake. Here is the updated report.

Below I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I use the USGS Quaternary fault and fold database for the faults.

  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I 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 include the slab contours plotted (McCrory et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.

This is a preliminary report and I hope to prepare some updates as I collect more information.

    I have placed several inset figures.

  • In the upper right corner is a map of the Cascadia subduction zone (CSZ) and regional tectonic plate boundary faults. This is modified from several sources (Chaytor et al., 2004; Nelson et al., 2004). I placed a blue star in the general location of today’s M 5.7 earthquake.
  • Below the CSZ map is an illustration modified from Plafker (1972). This figure shows how a subduction zone deforms between (interseismic) and during (coseismic) earthquakes. Today’s earthquake did not occur along the CSZ, so did not produce crustal deformation like this. However, it is useful to know this when studying the CSZ.
  • In the lower left corner is a figure from Rollins and Stein (2010). In their paper they discuss how static coulomb stress changes from earthquakes may impart (or remove) stress from adjacent crust/faults. This map shows the major earthquakes that have occurred in this region, prior to their publication in 2010. I place a blue star in the general location of today’s earthquake.
  • Above the Rollins and Stein (2010) map are two illustrations showing the difference between a right-lateral and a left-lateral strike slip fault. This is from California Institute of Technology (Caltech).
  • To the right of the Rollins and Stein (2010) map, is a generalized illustration showing an interpretation of the results from these authors. They suggest that, for a variety of earthquake sources in this region, which types of faults have inhibited or promoted earthquake likelihood. The relevant part is C, which tests whether there is an increased or decreased likelihood (chance) of an earthquake on the left-lateral strike-slip faults in the Gorda plate. Based upon today’s M 5.7, there is a slight increase in the chance of a Gorda plate earthquake to the northwest of today’s M 5.7 earthquake. This is the distant side of the M 5.7 earthquake, so any potential GP earthquake would be further away.
    • In the upper right corner is a figure that many people in Humboldt and Del Norte counties might be interested in (the two most northwesterly counties in CA). These two panels both show the same general result (as relevant to this discussion), the increased or decreased chance of an earthquake on two types of faults (north of the dashed line, the chance on GP left-lateral faults; south of dashed line, the chance on the MF. The region of this figure is outlined in dashed white transparent box on the main poster. We can see that the CSZ is just to the east of this figure. People always want to know if there is an increased chance of a megathrust earthquake on the CSZ. This M 5.7 will not have a direct impact upon the CSZ. Over time, earthquakes like this actually bring the CSZ closer to an earthquake (they do not relieve stress, but increase it). But the deformation of the Gorda and Pacific plates is localized near the earthquake. So, it does not change the stress on the megathrust. But, hundreds of earthquakes like this, over time, do increase the stress on the megathrust.
    • The figure here helps us evaluate this concept for this M 5.7 earthquake. The 1994 earthquake, represented in this figure, caused an increase in stress along faults generally in the region of this figure (extending outwards more a little to the south, less more to the west, and very little more to the north and east. The take away is that the 1994 did not change the stress on faults very much in the region of the megathrust. Because today’s M 5.7 earthquake is even further to the west, there is not a possibility that this M 5.7 had any affect on the megathrust.


  • Here is the 2016.12.08 earthquake report poster from this report.

  • For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:
  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

  • Here is a map from Rollins and Stein, showing their interpretations of different historic earthquakes in the region. This was published in response to the January 2010 Gorda plate earthquake. The faults are from Chaytor et al. (2004). The 1980, 1992, 1994, 2005, and 2010 earthquakes are plotted and labeled. I did not mention the 2010 earthquake, but it most likely was just like 1980 and 2005, a left-lateral strike-slip earthquake on a northeast striking fault.

  • Tectonic configuration of the Gorda deformation zone and locations and source models for 1976–2010 M ≥ 5.9 earthquakes. Letters designate chronological order of earthquakes (Table 1 and Appendix A). Plate motion vectors relative to the Pacific Plate (gray arrows in main diagram) are from Wilson [1989], with Cande and Kent’s [1995] timescale correction.

  • Here is the Rollins and Stein (2010) figure that is in the report above. I include their figure caption as blockquote below.

  • Coulomb stress changes imparted by our models of (a) a bilateral rupture and (b) a unilateral eastward rupture for the 1994 Mw = 7.0 Mendocino Fault Zone earthquake to the epicenters of the 1995 Mw = 6.6 southern Gorda zone earthquake (N) and the 2000 Mw = 5.9 Mendocino Fault Zone earthquake (O). Calculation depth is 5 km.

  • Here is a large scale map of the 1994 earthquake swarm. The mainshock epicenter is a black star and epicenters are denoted as white circles.

  • Here is a plot of focal mechanisms from the Dengler et al. (1995) paper in California Geology.

  • In this map below, I label a number of other significant earthquakes in this Mendocino triple junction region. Another historic right-lateral earthquake on the Mendocino fault system was in 1994. There was a series of earthquakes possibly along the easternmost section of the Mendocino fault system in late January 2015, here is my post about that earthquake series.

  • The Gorda and Juan de Fuca plates subduct beneath the North America plate to form the Cascadia subduction zone fault system. In 1992 there was a swarm of earthquakes with the magnitude Mw 7.2 Mainshock on 4/25. Initially this earthquake was interpreted to have been on the Cascadia subduction zone (CSZ). The moment tensor shows a compressional mechanism. However the two largest aftershocks on 4/26/1992 (Mw 6.5 and Mw 6.7), had strike-slip moment tensors. These two aftershocks align on what may be the eastern extension of the Mendocino fault.
  • There have been several series of intra-plate earthquakes in the Gorda plate. Two main shocks that I plot of this type of earthquake are the 1980 (Mw 7.2) and 2005 (Mw 7.2) earthquakes. I place orange lines approximately where the faults are that ruptured in 1980 and 2005. These are also plotted in the Rollins and Stein (2010) figure above. The Gorda plate is being deformed due to compression between the Pacific plate to the south and the Juan de Fuca plate to the north. Due to this north-south compression, the plate is deforming internally so that normal faults that formed at the spreading center (the Gorda Rise) are reactivated as left-lateral strike-slip faults. In 2014, there was another swarm of left-lateral earthquakes in the Gorda plate. I posted some material about the Gorda plate setting on this page.
  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. Many of the earthquakes people are familiar with in the Mendocino triple junction region are either compressional or strike slip. The following three animations are from IRIS.
  • Strike Slip:

    Compressional:

    Extensional:

  • This figure shows what a transform plate boundary fault is. Looking down from outer space, the crust on either side of the fault moves side-by-side. When one is standing on the ground, on one side of the fault, looking across the fault as it moves… If the crust on the other side of the fault moves to the right, the fault is a “right lateral” strike slip fault. The Mendocino and San Andreas faults are right-lateral (dextral) strike-slip faults. I believe this is from Pearson Higher Ed.

Update

  • Here is a video of the seismograph for this M 5.7 earthquake. The video was captured by Cindy Scammell, an undergraduate student at the Humboldt State University, Department of Geology. She is lovingly caring for her newborn. Here is a link to download the video. (3 MB mp4)
  • Abbreviations
  • BSF – Bartlett Springs fault
  • CA – California
  • CSZ – Cascadia subduction zone
  • GP – Gorda plate
  • JDFP – Juan de Fuca plate
  • MF – Mendocino fault
  • MMI – Modified Mercalli Intensity Scale
  • SAF – San Andreas fault
  • USGS – U.S. Geological Survey
  • WF – Wasatch fault

    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.
  • 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.
  • Dengler, L.A., Moley, K.M., McPherson, R.C., Pasyanos, M., Dewey, J.W., and Murray, M., 1995. The September 1, 1994 Mendocino Fault Earthquake, California Geology, Marc/April 1995, p. 43-53.
  • Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
  • Geist, E.L. and Andrews D.J., 2000. Slip rates on San Francisco Bay area faults from anelastic deformation of the continental lithosphere, Journal of Geophysical Research, v. 105, no. B11, p. 25,543-25,552.
  • Irwin, W.P., 1990. Quaternary deformation, in Wallace, R.E. (ed.), 1990, The San Andreas Fault system, California: U.S. Geological Survey Professional Paper 1515, online at: http://pubs.usgs.gov/pp/1990/1515/
  • McCrory, P.A.,. Blair, J.L., Waldhauser, F., kand Oppenheimer, D.H., 2012. Juan de Fuca slab geometry and its relation to Wadati-Benioff zone seismicity in JGR, v. 117, B09306, doi:10.1029/2012JB009407.
  • McLaughlin, R.J., Sarna-Wojcicki, A.M., Wagner, D.L., Fleck, R.J., Langenheim, V.E., Jachens, R.C., Clahan, K., and Allen, J.R., 2012. Evolution of the Rodgers Creek–Maacama right-lateral fault system and associated basins east of the northward-migrating Mendocino Triple Junction, northern California in Geosphere, v. 8, no. 2., p. 342-373.
  • 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
  • Rollins, J.C. and Stein, R.S., 2010. Coulomb stress interactions among M ≥ 5.9 earthquakes in the Gorda deformation zone and on the Mendocino Fault Zone, Cascadia subduction zone, and northern San Andreas Fault: Journal of Geophysical Research, v. 115, B12306, doi:10.1029/2009JB007117, 2010.
  • Stoffer, P.W., 2006, Where’s the San Andreas Fault? A guidebook to tracing the fault on public lands in the San Francisco Bay region: U.S. Geological Survey General Interest Publication 16, 123 p., online at http://pubs.usgs.gov/gip/2006/16/
  • Wallace, Robert E., ed., 1990, The San Andreas fault system, California: U.S. Geological Survey Professional Paper 1515, 283 p. [http://pubs.usgs.gov/pp/1988/1434/].

Posted in cascadia, earthquake, education, geology, gorda, HSU, mendocino, strike-slip, Transform

Earthquake Report: Puebla, Mexico Update #1

Well, the responses of people who are in the midst of a deadly disaster have been inspiring, bringing tears to my eyes often. Watching people searching and helping find survivors. This deadly earthquake brings pause to all who are paying attention. May we learn from this disaster with the hopes that others will suffer less from these lessons.

I have been discussing this earthquake with other experts, both online (i.e. the twitterverse, where most convo happens these days) and offline. Many of these experts are presenting their interpretations of this earthquake as it may help us learn about plate tectonics. While many of us are interested in learning these technical details, I can only hope that we seek a similar goal, to reduce future suffering. Plate tectonics is a young science and we have an ultra short observation period (given that the recurrence of earthquakes can be centuries to millenia, it may take centuries or more to fully understand these processes).

Here I present a review of the material that I have seen in the past day and how I interpret these data. The main focus of the poster is a comparison of ground shaking for three earthquakes. Also of interest is the ongoing discussion about how the 2019.09.08 M 8.1 Chiapas Earthquake and this M 7.1 Puebla Earthquake relate to each other. My initial interpretation holds, that the temporal relations between these earthquakes is coincidental (but we now have the analysis to support this interpretation!).

  • There are some reasons why these earthquakes are unrelated.
    1. They are too distant (static triggering is often limited to 1-2 fault lengths from the first earthquake).
    2. The Cocos plate (CP) changes shape between these two earthquakes, so it is complicated. The CP dips at a steep angle in the Chiapas region, while it dips at a shallow angle (about flat in places) further north. The Tehuantepec Ridge (TR) has an age offset and this may affect how the CP behaves differently on either side of the TR (mostly a fracture zone, but I need to look into this more, it may be thickened crust for some reason other than simply due to the fracture zone here).
    3. Dynamic triggering is when faults slip because they have increased stress as seismic waves travel through them. There is some work suggesting that these seismic waves can change the fluid pressures for a transient time period, possibly triggering earthquakes for a period after the seismic waves have already passed. The M 7.1 did not happen while the seismic waves were traveling following the M 8.1, so the M 7.1 is probably not due to dynamic triggering.
  • There is one major reason the ground shaking is amplified in the region of Mexico City. Prior to the arrival of the Spanish, the first peoples here lived at the shores of a large lake. They farmed on floating islands made of reeds and other material. Eventually the lake filled with sediment and turned into land, until the lake was gone. Given that Mexico City has the largest population of any city on Earth, as it was developed, the ground water was probably drained to facilitate the construction of large buildings (but I don’t know as much about this part of the history). I include a video about why water saturated sediments (i.e. sand and mud) can amplify seismic waves and intensify ground shaking.

Below is my interpretive poster for this earthquake

I plot the USGS seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I also include USGS epicenters from 1917-2017 for magnitudes M ≥ 7.0. I include the USGS fault plane solution for the 1985 earthquake. I also include the USGS moment tensor for the 2017.09.08 M 8.1 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 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 generalized plate tectonic map from Wikimedia Creative Commons here.
  • In the upper right corner are two map insets. The upper one is a map that includes the USGS MMI contours for the M 7.1 earthquake and the lower one is the same for the 1985 M 8.0 earthquake. I have outlined the area of Mexico City with a white dashed line. I created polygons for the higher MMI contours in the region of Mexico City and colored them with respect to these MMI valaues. For the M 7.1 earthquake, MMI VI is shown in yellow and MMI VI.5 is shown in darker yellow. For the 1985 earthquake, MMI VI is shown, but MMI VI.5 is not modeled for Mexico City. The take away: the M 7.1 potentially caused greater ground shaking in the Mexico City region than did the 1985 earthquake.
  • In the lower left corner is a comparison of three ground motion model results from the Instituto De Ingenieria. From left to right are the 1985 M 8.0, the 2017 M 8.1, and the 2017 M 7.1 earthquakes. There are a variety of model results for these earthquakes, but I selected the results shown for a 1 second period (the period of seismic waves) because this is a frequency of seismic waves that multi story buildings can be sensitive to (see educational video about resonance below for more on this). Note that the largest ground motions are from the M 7.1 earthquake. The 1985 was quite deadly and damaging, with between 6,000 and 12,000 deaths. If this M 7.1 earthquake had occurred in 1985, there probably would have been even more damage and a higher casualty number.
  • Above these comparison maps is a figure prepared by Temblor here, a company that helps people learn and prepare to be more resilient given a variety of natural hazards. This figure is the result of numerical modeling of static coulomb stress changes in the lithosphere following the 2017 M 8.1 earthquake. This basically means that regions that are red have an increased stress (an increased likelihood for an earthquake) following the earthquake, while blue represents a lower stress, or likelihood. The change in stress are very very small compared to the overall stress on any tectonic fault. This means that an earthquake may be triggered from this change in stress ONLY IF the fault is already highly strained (i.e. that the fault is about ready to generate an earthquake within a short time period, like a day, month, or year or so). The take away: the M 8.1 earthquake did not increase the stress on faults in the region of the M 7.1 (Temblor suggests the amount of increased stress near the M 7.1 is about the amount of force it takes to snap one’s fingers.


  • Here is my original interpretive poster.

  • Here is a video of the seismic waves, as they are being recorded, on the Baby Benioff seismograph in Van Matre Hall at Humboldt State University, Department of Geology. This was provided via the unofficial HSU geology facebook page, uploaded by Dr. Mark Hemphill-Haley. Here is a link to the 10 MB mp4 file for downloading.
  • Here is what Dr. Hemphill-Haley wrote about this video.
  • M 7.1 Puebla, Mexico earthquake at 11:14 AM PDT as recorded on the HSU Baby Benioff. The video is showing the surface waves arriving at campus, preceded by the P-waves at the beginning and S-waves immediately prior to the large amplitude waves. Our thoughts are with people in Mexico.

  • Here is one of the ground motion visualizations from IRIS here. Above the video is a screenshot preview. Here is a link to the video for downloading. (7 MB mp4)

  • As I mentioned the lake basin, here are some figures addressing that.
  • Here is a figure showing the thickness of the lake sediments here (Cruz-Atienza et al., 2016).

  • Topographic setting of Mexico City (MC) and the Valley of Mexico. Color scale corresponds to the basin thickness (i.e., the basin contact with the Oligocene volcanics of the Transmexican Volcanic Belt, TMVB). Stars show the epicenters for the vertical body forces applied at the free surface (green) and the magnitude 3.4 earthquake of December 1, 2014 (red). This figure has been created using the Generic Mapping Tools (GMT) Version 5.3.0, http://gmt.soest.hawaii.edu.

  • This is also from Cruz-Atienza et al. (2016) which shows their modeled seismic waves traveling through the basin.

  • Snapshots of the Green’s function for the vertical body force S6 (see Fig. 1) described by the inset time history with flat spectrum up to 1 Hz. Notice the topographic scattering, the generation and propagation of wave trains at different speeds within the basin, and their multiple diffractions. This figure has been created using the Matlab software Version R2016a, http://www.mathworks.com/.

  • Finally, here is a compilation of their model results showing how the lake basin sediments both amplify the ground motions (upper right panel) and increase their duration (lower right panel). Basically, the lake acts like a bowl of Jello.

  • (a,c) Comparison of average eigenfunctions for the 8 sources with standard deviation bars for both elastic (blue solid) and viscoelastic (red solid) simulations at two representative sites, P1 and P2, and different frequencies. Dashed lines show theoretical eigenfunctions for the vertical component of Rayleigh waves in the model of Figure A1a (Table A1) for the fundamental mode (blue) and the first (red) and second (green) overtones. Normalized peak vertical displacements observed in different boreholes (green dots in Fig. 1) are shown with black circles and error bars (after Shapiro et al., 2001). (b) Fourier spectral amplifications (geometric mean of both horizontal components) at 0.5 Hz with respect to the CUIG site (Fig. 1) averaged for the 8 sources. The black contour corresponds to the 2 s dominant-period. (d) Duration of the strong shaking phase for f < 1 Hz averaged for the 8 sources.

  • Here is an educational animation from IRIS that helps us learn about how different earth materials can lead to different amounts of amplification of seismic waves. Recall that Mexico City is underlain by lake sediments with varying amounts of water (groundwater) in the sediments.
  • Here is an educational video from IRIS that helps us learn about resonant frequency and how buildings can be susceptible to ground motions with particular periodicity, relative to the building size.
  • So, bringing this work as applied to this earthquake, Dr. Jascha Polet prepared this map that shows the outline of the lake and the locations of damaged and collapsed buildings. Note the correlation. Below the map, I include her tweet.

  • Here are some figures that show how the subduction zone varies across the Tehuantepec Ridge. More about this in my initial report, as well as in my reports for the M 8.1 earthquake.
  • This is a figure showing the location of the Tehuantepec Ridge (Quzman-Speziale and Zunia, 2015).

  • Tectonic framework of the Cocos plate convergent margin. Top- General view. Yellow arrows indicate direction and speed (in cm/yr) of plate convergence, calculated from the Euler poles given by DeMets et al. (2010) for CocoeNoam (first three arrows, from left to right), and CocoeCarb (last four arrows). Length of arrow is proportional to speed. Red arrow shows location of the 96 longitude. Box indicates location of lower panel. Bottom- Location of features and places mentioned in text. Triangles indicate volcanoes of the Central American Volcanic Arc (CAVA) with known Holocene eruption (Siebert and Simkin, 2002).

  • Here is another figure, showing seismicity for this region (Quzman-Speziale and Zunia, 2015).

  • Seismicity along the convergent margin. Top: Map view. Blue circles are shallow (z < 60 km) hypocenters; orange, intermediate-depth (60 < z < 100 km); yellow, deep (z > 100 km). Next three panels: Earthquakes as a function of longitude and magnitude for shallow (blue dots), intermediate (orange), and deep (yellow) hypocenters. Numbers indicate number of events on each convergent margin, with average magnitude in parenthesis. Gray line in this and subsequent figures mark the 96 deg longitude.

  • This shows the location of the cross sections. The cross sections show how the CP changes dip along strike (from north to south) (Quzman-Speziale and Zunia, 2015).

  • Location of hypocentral cross-sections. Hypocentral depths are keyed as in previous figures.

  • Here are the cross sections showing the seismicity associated with the downgoing CP (Quzman-Speziale and Zunia, 2015).

  • Hypocentral cross-sections. Depths are color-coded as in previous figures. Dashed lines indicate the 60-km and 100-km depths. Tick marks are at 100-km intervals, as shown on the sections. There is no vertical exaggeration and Earth’s curvature is taken into account. Number of sections refers to location on Fig. 3.

  • This figure shows thrust and normal earthquakes for three ranges of depth (Quzman-Speziale and Zunia, 2015).

  • Earthquake fault-plane solutions from CMT data. a. Shallow (z < 60 km), thrust-faulting mechanisms. b. Intermediate-depth (60 < z < 100 km) thrust-faulting events. c. Deep (z > 100 km), thrust-faulting earthquakes. d. to f. Normal-faulting events, in same layout as for thrust-faulting events.

  • Here are three figures from Tremblor.net, one of which is in the interpretive poster. These are the analyses I was discussing that we needed to see in my initial report. More detailed discussion can be found here.

  • This figure shows that there are not many earthquakes in the region between the M 8.1 and M 7.1 earthquakes. This is supporting evidence that there was not a significant increase in stress in this region (independent negative evidence for static triggering of the M 7.1 from the M 8.1).

  • This figure shows their modeling of the subduction zone in the region of the M 8.1 earthquake. I queried whether the megathrust had an increased stress following the M 8.1 earthquake. Part of the megathrust here ruptured in 1902, but the rest of the “Tehuantepec Gap” does not have an historic record (since ~1600 AD). Note how the megathrust is mostly blue, suggesting a lower likelihood of rupture. There is a narrow band of increased stress (in red). This model uses the finite fault model from Dr. Gavin Hayes (USGS).

  • As far as the likelihood of dynamic triggering (increased stress on faults while seismic waves are travelling through them), here is an analysis that helps us visualize this. This analysis (Pollitz et al., 2012) shows regions of increased dynamic stress following the 2012 Wharton Basin earthquakes. The lower spheres show seismicity for a time period following the earthquakes and note how they align with the red areas, areas of increased dynamic stress.

  • The 2012 M = 8.6 mainshock and M = 8.2 aftershock fault ruptures and maps of strain duration tstrain at a threshold value of 0.1 microstrain. a, Inferred fault ruptures of the 11 April 2012 M = 8.6 east Indian Ocean earthquake and an M = 8.2 aftershock that occurred 2 h later. Superimposed are the first 20 d of M > 4.5 aftershocks of 0–100-km depth. These earthquakes probably ruptured a complex set of subparallel and conjugate faults with the indicated sense of motion (arrows). Parts of the rupture areas of the 2004 M = 9.2 and 2005 M = 8.7 Nias earthquakes on the Sunda megathrust are indicated. b, c, Global maps of tstrain (colour scale). Superimposed are the epicentres of M>5.5 events that occurred during the 6 d preceding the mainshock (2 epicentres) and following the mainshock (24 epicentres, 16 of which are remote, that is, .1,500km from the mainshock). Focal mechanisms of six post-mainshock events with near-vertical strike-slip mechanisms (plunge of neutral axis, >60 deg) are indicated with red beachballs. The 9:00:09 11 April 2012 M = 5.5 event (in the western Aleutian Islands) occurred 21 min 33 s after the mainshock between the direct P- and S-wave arrivals from the mainshock; all others are delayed by hours to days. The focal mechanism of the mainshock is plotted at its epicentre.

  • Here is the comparison I put together for the ground motion modeling presented in the poster above.

  • Here is a really cool video that shows the seismic record of Hurricane Maria and the M 7.1 earthquake are recorded by seismometers (prepared by . The top panel shows the seismograph. The middle panel shows a spectrogram of these seismic data (showing the frequency content of the seismic waves). The lower panel shows the position of the Hurricane and M 8.1 earthquake epicenter (they should have shown the M 7.1, but that is not important. The audio is a conversion of the seismic data into sound. Here is the 1 MB mp4 file for downloading. This was prepared by Zhigang Peng from Georgia Tech for the station IU.SJG — San Juan, Puerto Rico. This is posted on the IRIS special event page. note: the hurricane and this earthquake are NOT RELATED!

References:

  • Benz, H.M., Dart, R.L., Villaseñor, Antonio, Hayes, G.P., Tarr, A.C., Furlong, K.P., and Rhea, Susan, 2011 a. Seismicity of the Earth 1900–2010 Mexico and vicinity: U.S. Geological Survey Open-File Report 2010–1083-F, scale 1:8,000,000.
  • Benz, H.M., Tarr, A.C., Hayes, G.P., Villaseñor, Antonio, Furlong, K.P., Dart, R.L., and Rhea, Susan, 2011 b. 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.
  • Cruz-Atienza et al., 2016. Long Duration of Ground Motion in the Paradigmatic Valley of Mexico in Scientific Reports, v. 6, DOI: 10.1038/srep38807
  • 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
  • Franco, S.I., Kostoglodov, V., Larson, K.M., Manea, V.C>, Manea, M., and Santiago, J.A., 2005. Propagation of the 2001–2002 silent earthquake and interplate coupling in the Oaxaca subduction zone, Mexico in Earth Planets Space, v. 57., p. 973-985.
  • 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
  • Gérault, M., Husson, L., Miller, M.S., and Humphreys, E.D., 2015. Flat-slab subduction, topography, and mantle dynamics in southwestern Mexico in Tectonics, v. 34, p. 1892-1909, doi:10.1002/2015TC003908.
  • Quzman-Speziale, M. and Zunia, F.R., 2015. Differences and similarities in the Cocos-North America and Cocos-Caribbean convergence, as revealed by seismic moment tensors in Journal of South American Earth Sciences, http://dx.doi.org/10.1016/j.jsames.2015.10.002
  • 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.
  • Lay et al., 2011. Outer trench-slope faulting and the 2011 Mw 9.0 off the Pacific coast of Tohoku Earthquake in Earth Planets Space, v. 63, p. 713-718.
  • Manea, M., and Manea, V.C., 2014. On the origin of El Chichón volcano and subduction of Tehuantepec Ridge: A geodynamical perspective in JGVR, v. 175, p. 459-471.
  • Mann, P., 2007, Overview of the tectonic history of northern Central America, in Mann, P., ed., Geologic and tectonic development of the Caribbean plate boundary in northern Central America: Geological Society of America Special Paper 428, p. 1–19, doi: 10.1130/2007.2428(01). For
  • McCann, W.R., Nishenko S.P., Sykes, L.R., and Krause, J., 1979. Seismic Gaps and Plate Tectonics” Seismic Potential for Major Boundaries in Pageoph, v. 117
  • Pérez-Campos, Z., Kim, Y., Husker, A., Davis, P.M. ,Clayton, R.W., Iglesias,k A., Pacheco, J.F., Singh, S.K., Manea, V.C., and Gurnis, M., 2008. Horizontal subduction and truncation of the Cocos Plate beneath central Mexico in GRL, v. 35, doi:10.1029/2008GL035127
  • Polltz, F.F., Stein, R.S., Sevigen, V., Burgmann, R., 2012. The 11 April 2012 east Indian Ocean earthquake triggered large aftershocks worldwide in Nature, v. 000, doi:10.1038/nature11504
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