Earthquake Report: 1994 Northridge!

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Today is the anniversary of the 1994 M 6.7 Northridge Earthquake. I was living in Arcata at the time (actually in my school bus in a driveway to save money). I remember calling my mom from a pay phone to talk about the earthquake (I did not have a phone at the time; turns out Pac Bell did not want to install a phone in my bus and I probably could not afford it anyways). She lived in Long Beach, but the damage affected the entirety of southern California.

I put together a commemorative #EarthquakeReport interpretive poster to discuss the tectonics of the region. The San Andreas fault (SAF) system is the locus of ~75% of the Pacific-North America plate boundary motion. The SAF is in some places a mature fault with a single strand and in other places, there are multiple strands (e.g. the Elsinore, San Jacinto, and SAF in southern CA or the Maacama, Bartlett Springs, and SAF in northern CA). In southern CA, the SAF makes a bend (called the “Big Bend”Smilie: ;) that forms a region of compression. This compression is realized in the form of thrust faults and folds, creating uplift forming the mountain ranges like the Santa Monica Mountains. Some of these thrust faults breach the ground surface and some are blind (they don’t reach the surface).

In 1971 there was a large earthquake (M 6.7) that caused tremendous amounts of damage in southern CA. A hospital was built along one of the faults and this earthquake caused the hospital to collapse killing many people. The positive result of this earthquake is that the Alquist Priolo Act was written and passed in the state legislature. I plot the moment tensor for the 1971 earthquake (Carena and Suppe, 2002).

Then, over 2 decades later, there was the M 6.7 Northridge Earthquake. This earthquake was very damaging. Here is a page that links to some photos of the damage. I plot the moment tensor for this 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 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 compression at the “Big Bend.”
  • 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 plot the fault lines from the USGS Quaternary Fault and Fold Database. I include a legend showing how colors represent the USGS estimates for the most recent activity along each of these faults. More can be found about this database here.

    I include some inset figures in the poster.

  • In the upper right corner is a map of the faults in southern CA (Tucker and Dolan, 2001). Strike-slip faults (like the SAF) have arrows on either side of the fault desginating the relative motion across the fault. Thrust faults have triangle barbs showing the convergence direction (the triangles are on the side of the fault that is dipping into the Earth).
  • To the left of this fault map is a low-angle oblique block diagram showing the configuration of thrust faults in the region of the Big Bend. These thrust faults are forming the topography in southern CA. The 1971 and 1994 earthquakes occurred along thrust faults similar to the ones shown in this block diagram.
  • In the lower right corner is a figure that shows some historic earthquakes in this region (Hauksson et al., 1995). The upper panel shows the affected areas from these earthquakes in hatchured polygons. The lower panel shows the focal mechanisms for these earthquakes.
  • In the upper left corner I include the USGS “Did You Feel It?” shakemap. This map uses the MMI scale mentioned above. These are results from the USGS DYFI reporting website. So, these are real observations, compared to the MMI contours in the main map, which is based upon ground motion modeling of the earthquake.
  • Below the DYFI map is a cross section of seismicity associated with the 1971 and 1994 earthquakes (Tsutsumi and Yeats, 1994). 1971 main and aftershocks are in blue and 1994 main and aftershocks are in red. Note how both earthquakes occurred along blind thrust faults. Also note that these faults were dipping in opposite directions (1971 dips to the north (south vergent) and 1994 dips to the south (north vergent).
  • In the lower left corner is another figure showing the aftershocks from the 1971 and 1994 earthquakes (Fuis et al., 2003). On the left panel is their seismic velocity model (with fault interpretations) and on the right panel shows the seismicity plotted on the velocity model. I present this figure below.


Below I present some more figures that help understand the tectonics of this region. Many of these are in the poster. I include their original captions in blockquote.

  • Here is the fault map from Tucker and Dolan (2001).

  • Regional neotectonic map for metropolitan southern California showing major active faults. The Sierra Madre fault is a 75-km-long active reverse fault that extends along the northern edge of the metropolitan region. Fault locations are from Ziony and Jones (1989), Vedder et al. (1986), Dolan and Sieh (1992), Sorlien (1994), and Dolan et al. (1997, 2000b). Closed teeth denote reverse fault surface trace; open teeth on dashed lines show upper edge of blind thrust fault ramps. Strike-slip fault surface traces shown by double arrows. Star denotes location of Oak Hill paleoseismologic trench site of Bonilla (1973). CSI, Clamshell-Sawpit fault; ELATB, East Los Angeles blind thrust system; EPT, Elysian park blind thrust fault; Hol Fl, Hollywood fault; PHT, Puente Hills blind thrust fault; RMF, Red Mountain fault; SCII, Santa Cruz Island fault; SSF, Santa Susana fault; SJcF, San Jacinto fault; SJF, San Jose fault; VF, Verdugo fault; A, Altadena study site of Rubin et al. (199Smilie: 8); LA, Los Angeles; LB, Long Beach; LC, La Crescenta; M, Malibu; NB, Newport Beach; Ox, Oxnard; P, Pasadena; PH, Port Hueneme; S, Horsethief Canyon study site in San Dimas; V, Ventura. Dark shading denotes mountains.

  • This is a figure that is based upon Fuis et al. (2001) as redrawn by UNAVCO that shows the orientation of thrust faults in this region of southern CA. Below the block diagram is a map showing the location of their seismic experiment (LARSE = Line 1; Fuis et al., 2003).

  • Schematic block diagram showing interpreted tectonics in vicinity of LARSE line 1. Active faults are shown in orange, and moderate and large earthquakes are shown with orange stars and attached dates, magnitudes, and names. Gray half-arrows show relative motions on faults. Small white arrows show block motions in vicinities of bright reflective zones A and B (see Fig. 2A). Large white arrows show relative convergence direction of Pacific and North American plates. We interpret a master decollement ascending from bright reflective zone A at San Andreas fault, above which brittle upper crust is imbricating along thrust and reverse faults and below which lower crust is flowing toward San Andreas fault (brown arrows) and depressing Moho. Fluid injection, indicated by small lenticular blue areas, is envisioned in bright reflective zones A and B.


    Shaded relief map of Los Angeles region, southern California, showing Quaternary faults (thin black lines, dotted where buried), shotpoints (gray and orange filled circles), seismographs (gray and orange lines), air-gun bursts (dashed yellow lines), and epicenters of earthquakes .M 5.8 since 1933 (focal mechanisms with attached magnitudes: 6.7a—Northridge [Hauksson et al., 1995], 6.7b—San Fernando [Heaton, 1982], 5.9—Whittier Narrows [Hauksson et al., 1988], 5.8—Sierra Madre [Hauksson, 1994], 6.3—Long Beach [Hauksson, 1987]). Faults are labeled in red; abbreviations: HF—Hollywood fault, MCF—Malibu Coast fault, MHF—Mission Hills fault, NHF—Northridge Hills fault, RF—Raymond fault, SF—San Fernando surface breaks, SSF—Santa Susana fault, SMoF—Santa Monica fault, SMFZ—Sierra Madre fault zone, VF—Verdugo fault. NH is Newhall.

  • Here are the figures from Hauksson et al. (1995) showing the regions effected by earthquakes in southern CA.

  • (A) Significant earthquakes of M >= 4.8 that have occurred in the greater Los Angeles basin area since 1920. Aftershock zones are shaded with cross hatching, including the 1994 Northridge earthquake. Dotted areas indicate surface rupture, including the rupture of the 1857 earthquake along the San Andreas fault. (B) Lower hemisphere focal mechanisms (shaded quadrants are compressional) for significant earthquakes that have occurred since 1933 in the greater Los Angeles area.

  • Here is the seismicity cross section plot from Tsutsumi and Yeats (1999).

  • Cross section down to 20 km depth across the central San Fernando Valley, including the 1971 Sylmar and 1994 Northridge earthquake zones. See Figure 2 for location of the section and Figure 3 for stratigraphic abbreviations. Wells are identified in the Appendix. Aftershock data for the 1971 (blue) and 1994 (red) earthquakes within a 10-km-wide strip including the line of this section are provided by Jim Mori at Kyoto University. Abbreviation for faults: MHF, Mission Hills fault; NHF, Northridge Hills fault; SSF, Santa Susana fault.

  • Here is the figure from Fuis et al. (2003) showing their interpretation of seismic data from the region. These data are from a seismic experiment also plotted in the map above. The panel on the left is A and the panel on the right is B. This is their figure 3.

  • Cross section along part of line 2 with superposition of various data layers. A: Tomographic velocity model plus line drawing extracted from reflection data (see text); heavier black lines represent better-correlated or higher-amplitude phases. B: Velocity model plus relocated aftershocks of 1971 San Fernando and 1994 Northridge earthquakes (brown and blue dots, respectively); main shock focal mechanisms (far hemispheres) are red (San Fernando; Heaton, 1982) and blue (Northridge; Hauksson et al., 1995). Aftershocks are projected onto line 2 from up to 10 km east.

  • This is a smaller scale cross section from Fuis et al. (2003) showing a broader view of the faults in this region. This shows the velocity model color legend that also applies to the above figure. This is their figure 4.

  • Similar to Fig. 3, with expanded depth and distance frame. See caption for Fig. 3 for definition of red, magneta, and blue lines; orange line—interpreted San Andreas fault (SAF); yellow lines—south-dipping reflectors of Mojave Desert and northern Transverse Ranges; “K” —reflection of Cheadle et al. (1986), which is out of plane of this section. SAF is not imaged directly; interpretation is based on approximate northward termination of upper reflections (best constrained) in San Fernando reflective zone (magenta lines). (See similar interpretation for SAF on line 1—Fig. 5.) Wells shown in Mojave Desert are (s) H&K Exploration Co., (t) Meridian Oil Co. (Dibblee, 1967). For well color key, see caption for Fig. 3. Thin, dashed yellow-orange line—estimated base of Cenozoic sedimentary rocks in Mojave Desert based on velocity. Darker, multicolored region (above region of light violet) represents part of velocity model where resolution ≥ 0.4 (see color bar).

  • Here is a fascinating figure from Carena and Suppe (2002) showing the 3-dimensional configuration of the faults involved in the 1971 and 1994 earthquakes.

  • Perspective view, looking from the SE, of the modeled Northridge and San Fernando thrusts. The Northridge thrust stops at a depth of about 6 km, and its upper tip east of the lateral ramp (Fig. 4) terminates almost against the San Fernando thrust, as was suggested by Morti et al. (1993). The San Fernando thrust loser tip is at a depth o 13 km, whereas the Northridge thrust lower tip is at 32 km.

  • Here is a map view of the Carena and Suppe (2002) interpretation of these fault planes.

  • Schematic geological map showing the position of the main faults and folds, as well as the depth contours (contour interval = 1 km) of the Northridge (solid) and San Fernando (dashed) thrusts.

  • Here is a structural cross section across this region (Carena and Suppe, 2002).

  • Cross-section through the San Fernando Valley with projected aftershocks of the 1994 Northridge earthquake and of the 1971 Sylmar earthquake. The Northridge aftershocks are projected from a distance of 1 km or less on each side of the cross-section (main shock projected from 2 km W), whereas those of the Sylmar earthquake are projected from 1.5 km or less (main shock projected from 5 km ESE). The sources that we used for near-surface geology and structure are Dibblee (1991) and a seismic line (Fig. 11). The large N-S changes in Upper Tertiary stratigraphic thicknesses in this region (Dibblee, 1991, 1992a), prevent detailed stratigraphic correlation across fault blocks (this figure and Fig. 12). This face suggests that the shallow faults and possible the deeper San Fernando thrust itself, are reactivating old normal faults of the southern margin of the Ventura Basin (Yeats, et al., 1994; Huftle and Yeats, 1996; Tsutsumi and Yeats, 1999). Location of cross-section is in Fig. 13.

References:

  • Carena, S. and Supper, J., 2002. Three-dimensional imaging of active structures using earthquake aftershocks: the Northridge thrust, California in Journal of Structural Geology, v. 24, p. 887-904.
  • Fuis, G.S>, Ryberg, T., Godfrey, N.J>, Okaya, D.A., and Murphy, J.M., 2001. Crustal structure and tectonics from the Los Angeles basin to the Mojave Desert, southern California in Geology, v. 29, no. 1. p. 15-18.
  • Fuis, G.S. et al., 2003. Fault systems of the 1971 San Fernando and 1994 Northridge earthquakes, southern California: Relocated aftershocks and seismic images from LARSE II in Geology, v. 31, no. 2, p. 171-174.
  • Hauksson, E., Jones, L.M., and Hutton, K., 1995. The 1994 Northridge earthquake sequence in California: Seismological and tectonic aspects in Journal of Geophysical Research, v., 100, no. B7, p. 12235-12355.
  • Tsutsumi, H. and Yeats, R.S., 1999. Tectonic Setting of the 1971 Sylmar and 1994 Northridge Earthquakes in the San Fernando Valley, California in BSSA, v. 89, p. 1232-1249.
  • Tucker, A.Z. and Dolan, J.F., 2001. Paleoseismologic Evidence for a 8 Ka Age of the Most Recent Surface Rupture on the Eastern Sierra Madre Fault, Northern Los Angeles Metropolitan Region, California in BSSA, v. 91, no. 2, p. 232-249.

Posted in collision, earthquake, education, geology, HSU, los angeles, plate tectonics, San Andreas

Earthquake Report: Explorer plate!

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In the past 2 days there have been a few earthquakes in the Explorer plate region along the Pacific-North America plate boundary. On March 19 of this year there was a series of earthquakes in this same region (to the southeast of today’s earthquakes). Here is my report for the March 2016 earthquakes.

The Cascadia subduction zone (CSZ) is an approximately 1,200-kilometer convergent plate boundary that extends from northern California to Vancouver Island, Canada (inset figure). The Explorer, Juan de Fuca, and Gorda plates are subducting eastwardly below the North American plate. Seismicity, crustal deformation, and geodesy provide evidence that the Cascadia subduction zone is locked and is capable of producing a great (magnitude greater than or equal to 8.5) earthquake (Heaton and Kanamori, 1984; McPherson, 1989; Clarke and Carver, 1992; Hyndman and Wang, 1995; Flück and others, 1997).

The Queen Charlotte fault (QCF) is a dextral (right-lateral) transform plate boundary (strike-slip) fault that forms the Pacific-North America plate boundary north of Vancouver Island. There have been a series of earthquakes along this fault system in the last 100 years, including earthquakes in the 1920s, 1940s, and 2010s. At its southern terminus it meets the CSZ and Explorer ridge (a spreading ridge system that forms oceanic lithosphere of the Explorer plate) to form the Queen Charlotte triple junction (QCTJ labeled on the interpretive poster below). I also include a map below showing the earthquakes with magnitudes M ≥ 7.0 for this time period. The southernmost part of the QCF also has a subduction zone beneath the strike-slip fault. This part of the boundary had a subduction zone earthquake in 2012.

Below is my interpretive poster for this earthquake.


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

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely. 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.
    • Here are the USGS websites for the earthquakes plotted in the interpretive poster below.

    • 2017-01-06 15:49 M 5.1
    • 2017-01-06 16:42 M 4.3
    • 2017-01-06 16:52 M 4.6
    • 2017-01-06 21:05 M 4.1
    • 2017-01-07 03:13 M 5.7

    I include some inset figures in the poster.

  • 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 place a red star in the general location of today’s seismicity.
  • 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.
  • To the left of the CSZ map is a map showing the plate boundary faults associated with the northern CSZ and to the north (including the Queen Charlotte fault; Braunmiller and Nabalek, 2002). I place a red star in the general location of today’s seismicity. These earthquakes occurred in the region east of the Explorer rift. This region of the world still contains some major tectonic mysteries and this is quite exciting. This shows the Winona Block as a microplate between the Pacific and North America plates, north of the Explorer plate. The Winona Block is labeled “WIN BLOCK” on the map. Note that there are two spreading ridges on the western and central part of this block. It is possible that the Explorer ridge-rift system extends into the Winona Block to form a third spreading ridge in the Winona Block.
  • In the upper left corner is a map from Braunmiller and Nabalek (2002) that shows focal mechanisms and bathymetry for the Pacific-Explorer plate boundary region. I place a red star in the general location of today’s seismicity.
  • In the lower left corner is a figure that shows an interpretation of how this plate boundary developed over the past 3 million years (Braunmiller and Nabalek, 2002). I place a red star in the general location of today’s seismicity.


  • Here is the same map, but with the seismicity from 1900-2017 plotted. These are USGS earthquakes with magnitudes M ≥ 5.0 for this time period. These are the same earthquakes plotted in the video below.


  • Here is a larger scale map showing these earthquakes from the last couple of days. Note how these earthquakes align along a north-northeast striking orientation. This orientation matches the lineations that are co-parallel to the spreading ridges in this area. These could be strike-slip fault earthquakes, but the orientation of the moment tensor solutions do not align with the strike-slip faults in this region. This is why I interpret these to be extensional (normal) earthquakes.



  • Here is a video showing the earthquake epicenters for the period of 1900-2017 for USGS earthquakes with magnitudes M ≥ 5.0. Here is a link to the embedded video below (2.5 MB mp4).
  • Here is the map with the seismicity from 1900-2017 plotted. These are USGS earthquakes with magnitudes M ≥ 7.0 for this time period. I include the moment tensors from the 2012 and 2013 earthquakes (the only earthquakes for this time period that have USGS moment tensors). The 2012 earthquake generated a tsunami. I discuss the 2012 “Haida Gwai” earthquake here.


  • Here is the general tectonic map of the region (Braunmiller and Nabalek, 2002). Today’s earthquakes happened in a place that suggest the Explorer ridge extends further to the north into the Winona Block. Below I include the text from the original figure caption in blockquote.

  • Map of Explorer region and surroundings. Plate boundaries are based on Riddihough’s [1984] and Davis and Riddihough’s [1982] tectonic models. Solid lines are active plate boundaries (single lines are transform faults, double lines are spreading centers, barbed lines are subduction zones with barbs in downgoing plate direction). The wide double line outlines the width of the Sovanco fracture zone, and the dots sketch the Explorer-Winona boundary. Plate motion vectors (solid arrows) are from NUVEL-1A [DeMets et al., 1994] for Pacific-North America motion and from Wilson [1993] for Pacific-Juan de Fuca and Juan de Fuca-North America motion. Open arrows are Explorer relative plate motions averaged over last 1 Myr [Riddihough, 1984] (in text, we refer to these most recent magnetically determined plate motions as the ‘‘Riddihough model’’). Winona block motions (thin arrows), described only qualitatively by Davis and Riddihough [1982], are not to scale. Abbreviations are RDW for Revere-Dellwood- Wilson, Win for Winona, FZ for fault zone, I for island, S for seamount, Pen for peninsula.

  • Here is the larger scale figure that shows the details of the plate boundary in this region (Braunmiller and Nabalek, 2002). Below I include the text from the original figure caption in blockquote.

  • Close-up of the Pacific-Explorer boundary. Plotted are fault plane solutions (gray scheme as in Figure 3) and well-relocated earthquake epicenters. The SeaBeam data are from the RIDGE Multibeam Synthesis Project (http://imager.ldeo.columbia.edu) at the Lamont-Doherty Earth observatory. Epicenters labeled by solid triangles are pre-1964, historical earthquakes (see Appendix B). Solid lines mark plate boundaries inferred from bathymetry and side-scan data [Davis and Currie, 1993]; dashed were inactive. QCF is Queen Charlotte fault, TW are Tuzo Wilson seamounts, RDW is Revere-Dellwood-Wilson fault, DK are Dellwood Knolls, PRR is Paul Revere ridge, ER is Explorer Rift, ED is Explorer Deep, SERg is Southern Explorer ridge, ESM is Explorer seamount, SETB is Southwest Explorer Transform Boundary, SAT is Southwestern Assimilated Territory, ESDZ is Eastern Sovanco Deformation Zone, HSC is Heck seamount chain, WV is active west valley of Juan de Fuca ridge, MV is inactive middle valley.

  • This is the figure that shows an interpretation of how this plate boundary formed over the past 3 Ma (Braunmiller and Nabalek, 2002). Below I include the text from the original figure caption in blockquote.

  • Schematic plate tectonic reconstruction of Explorer region during the last 3 Myr. Note the transfer of crustal blocks (hatched) from the Explorer to the Pacific plate; horizontal hatch indicates transfer before 1.5 Ma and vertical hatch transfer since then. Active boundaries are shown in bold and inactive boundaries are thin dashes. Single lines are transform faults, double lines are spreading centers; barbed lines are subduction zones with barbs in downgoing plate direction. QCF is Queen Charlotte fault, TW are the Tuzo Wilson seamounts, RDW is Revere-Dellwood-Wilson fault, DK are the Dellwood Knolls, ED is Explorer Deep, ER is Explorer Rift, ERg is Explorer Ridge, ESM is Explorer Seamount, SOV is Sovanco fracture zone, ESDZ is Eastern Sovanco Deformation Zone, JRg is Juan de Fuca ridge, and NF is Nootka fault. The question mark indicates ambiguity whether spreading offshore Brooks peninsula ceased when the Dellwood Knolls became active (requiring only one independently moving plate) or if both spreading centers, for a short time span, where active simultaneously (requiring Winona block motion independent from Explorer plate during that time).

  • Below I include some inset maps from Audet et al. (2008 ) and Dziak (2006). Each of these authors have published papers about the Explorer plate. Dziak (2006) used bathymetric and seismologic data to evaluate the faulting in the region and discussed how the Explorer plate is accommodating a reorganization of the plate boundary. Audet et al. (2008 ) use terrestrial seismic data to evaluate the crust along northern Vancouver Island and present their tectonic map as part of this research (though they do not focus on the offshore part of the Explorer plate). I include these figures below along with their figure captions. Today’s earthquakes happened at the northwestern portion of these maps from Dziak (2006).
  • Dziak, 2006

  • Bathymetric map of northern Juan de Fuca and Explorer Ridges. Map is composite of multibeam bathymetry and satellite altimetry (Sandwell and Smith, 1997). Principal structures are labeled: ERB—Explorer Ridge Basin, SSL—strike-slip lineation. Inset map shows conventional tectonic interpretation of region. Dashed box shows location of main figure. Solid lines are active plate boundaries, dashed line shows Winona-Explorer boundary, gray ovals represent seamount chains. Solid arrows show plate motion vectors from NUVEL-1A (DeMets et al., 1994) for Pacific–North America and from Wilson (1993) for Pacific–Juan de Fuca and Juan de Fuca–North America. Open arrows are Explorer relative motion averaged over past 1 m.y. (Riddihough, 1984). Abbreviations: RDW—Revere-Dellwood-Wilson,Win—Winona block, C.O.—Cobb offset, F.Z.—fracture zone. Endeavour segment is northernmost section of Juan de Fuca Ridge.

  • Dziak, 2006

  • Structural interpretation map of Explorer–Juan de Fuca plate region based on composite multibeam bathymetry and satellite altimetry data (Fig. 1). Heavy lines are structural (fault) lineations, gray circles and ovals indicate volcanic cones and seamounts, dashed lines are turbidite channels. Location of magnetic anomaly 2A is shown; boundaries are angled to show regional strike of anomaly pattern.

  • Dziak, 2006

  • Earthquake locations estimated using U.S. Navy hydrophone arrays that occurred between August 1991 and January 2002. Focal mechanisms are of large (Mw>4.5) earthquakes that occurred during same time period, taken from Pacific Geoscience Center, National Earthquake Information Center, and Harvard moment-tensor catalogs. Red mechanism shows location of 1992 Heck Seamount main shock.

  • Dziak, 2006

  • Tectonic model of Explorer plate boundaries. Evidence presented here is consistent with zone of shear extending through Explorer plate well south of Sovanco Fracture Zone (SFZ) to include Heck, Heckle, and Springfield seamounts, and possibly Cobb offset (gray polygon roughly outlines shear zone). Moreover, Pacific– Juan de Fuca–North American triple junction may be reorganizing southward to establish at Cobb offset. QCF—Queen Charlotte fault.

  • Audet et al., 2008

  • Identification of major tectonic features in western Canada. BP—Brooks Peninsula, BPfz—Brooks Peninsula fault zone, NI— Nootka Island, QCTJ—Queen Charlotte triple junction. Dotted lines delineate extinct boundaries or shear zones. Seismic stations are displayed as inverted black triangles. Station projections along line 1 and line 2 are plotted as thick white lines. White triangles represent Alert Bay volcanic field centers. Center of array locates town of Woss. Plates: N-A—North America; EXP—Explorer; JdF—Juan de Fuca; PAC—Pacific.

  • Here is another map that shows the tectonics of this region. Hyndman (2015) shows the region where the 2012 Haida Gwaii earthquake ruptured. I include two more figures below. This figure Below I include the text from the original figure caption in blockquote.

  • The Queen Charlotte fault (QCF) zone, the islands of Haida Gwaii and adjacent area, and the locations of the 2012 Mw 7.8 (ellipse), 2013 Mw 7.5 (solid line), and 1949 Ms 8.1 (dashed) earthquakes. The along margin extent of the 1949 event is not well constrained.

  • This map shows the main and aftershocks from the 2012 Haida Gwaii earthquake sequence (Hyndman, 2015). This 2012 sequence is interesting because, prior to these earthquakes, it was unclear whether the fault along Haida Gwaii was a strike-slip or a thrust fault. For example, Riddihough (1984) suggests that there is no subduction going on along the Explorer plate at all. Turns out it is probably both. When this 2012 earthquake happened, I took a look at the bathymetry in Google Earth and noticed the Queen Charlotte Terrace, which looks suspiciously like an accretionary prism. This was convincing evidence for the thrust fault earthquakes. Below I include the text from the original figure caption in blockquote.

  • Aftershocks of the 2012 Mw 7.8 Haida Gwaii thrust 13 earthquake (after Cassidy et al., 2013). They approximately define the rupture area. The normal-faulting mechanisms for two of the larger aftershocks are also shown. Many of the aftershocks are within the incoming oceanic plate and within the overriding continental plate rather than on the thrust rupture plane.

  • This is a great version of this figure that shows how there are overlapping subduction (thrust) and transform (strike-slip) faults along the Haida Gwaii region (Hyndman, 2015). Below I include the text from the original figure caption in blockquote.

  • Model for the 2012 Mw 7.8 earthquake rupture and the partitioning of oblique convergence into margin parallel motion on the Queen Charlotte transcurrent fault and nearly orthogonal thrust convergence on the Haida Gwaii thrust fault.

  • Here is a figure that shows two ways of interpreting the Queen Charlotte triple junction region (Kreemer et al., 199Smilie: 8). Note the 1900-2017 seismicity map above, which supports the interpretation in the right panel (B). Something of trivial nature is that this article is from the pre-computer illustration era (see the squiggly hand drawn arrow in the right panel B). Below I include the text from the original figure caption in blockquote.

  • (A) Major tectonic features describing the micro-plate model for the Explorer region. The Explorer plate (EXP) is an independent plate and is in convergent motion towards the North American plate (NAM). V.I. D Vancouver Island; PAC D the Pacific plate; JdF D the Juan the Fuca plate. The accentuated zone between the Explorer and JdF ridges is the Sovanco transform zone and the two boundary lines do not indicate the presence of faults but define the boundaries of this zone of complex deformation. (B) The key features of the pseudo-plate model for the region are a major plate boundary transform fault zone between the North American and Pacific plates and the Nootka Transform, a left-lateral transform fault north of the Juan the Fuca plate.

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.
  • Braunmiller, J. and Nabelek, J., 2002. Seismotectonics of the Explorer region in JGR, v. 107, NO. B10, 2208, doi:10.1029/2001JB000220, 2002
  • 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.
  • Audet, P., Bostock, M.G., Mercier, J.-P., and Cassidy, J.F., 2008., Morphology of the Explorer–Juan de Fuca slab edge in northern Cascadia: Imaging plate capture at a ridge-trench-transform triple junction in Geology, v. 36, p. 895-898.
  • Clarke, S. H., and Carver, G. C., 1992. Late Holocene Tectonics and Paleoseismicity, Southern Cascadia Subduction Zone, Science, vol. 255:188-192.
  • Dziak, R.P., 2006. Explorer deformation zone: Evidence of a large shear zone and reorganization of the Pacific–Juan de Fuca–North American triple junction in Geology, v. 34, p. 213-216.
  • Flück, P., Hyndman, R. D., Rogers, G. C., and Wang, K., 1997. Three-Dimensional Dislocation Model for Great Earthquakes of the Cascadia Subduction Zone, Journal of Geophysical Research, vol. 102: 20,539-20,550.
  • Heaton, f f., Kanamori, F. F., 1984. Seismic Potential Associated with Subduction in the Northwest United States, Bulletin of the Seismological Society of America, vol. 74: 933-941.
  • Hyndman, R. D., and Wang, K., 1995. The rupture zone of Cascadia great earthquakes from current deformation and the thermal regime, Journal of Geophysical Research, vol. 100: 22,133-22,154.
  • Keemer, C., Govers, R., Furlong, K.P., and Holt, W.E., 1998. Plate boundary deformation between the Pacific and North America in the Explorer region in Tectonophysics, v. 293, p. 225-238.
  • McPherson, R. M., 1989. Seismicity and Focal Mechanisms Near Cape Mendocino, Northern California: 1974-1984: M. S. thesis, Arcata, California, Humboldt State University, 75 p
  • 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
  • Plafker, G., 1972. Alaskan earthquake of 1964 and Chilean earthquake of 1960: Implications for arc tectonics in Journal of Geophysical Research, v. 77, p. 901-925.
  • Riddihough, R., 1984. Recent Movements of the Juan de Fuca Plate System in JGR, v. 89, no. B8, p. 6980-6994.
  • Wang, K., Wells, R., Mazzotti, S., Hyndman, R. D., and Sagiya, T., 2003. A revised dislocation model of interseismic deformation of the Cascadia subduction zone Journal of Geophysical Research, B, Solid Earth and Planets v. 108, no. 1.

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

Earthquake Report: North Fiji Basin!

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We just had a large earthquake along the West Fiji Ridge, one of the spreading ridges that forms the North Fiji Basin. Here is the USGS website for this M 7.2 earthquake.

This earthquake was relatively shallow and, probably since it was an extensional earthquake with a relatively low magnitude, did not pose a tsunami hazard or risk. There was a tsunami with a height of ~10 cm recorded in Fiji. Here is the final tsunami threat message from the Pacific Tsunami Warning Center in Hawaii.

Below is my interpretive poster for this earthquake.


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

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely. 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. Today’s M 7.2 earthquake is not directly related to the subduction zones in this region (it is genetically related to a spreading ridge), but they do play an important role in the region.
  • I also include moment tensors for several recent earthquakes in the region. Below are links to the USGS websites and the Earthquake Report pages for these earthquakes. I had not prepared a report for the M 6.3, but will briefly discuss this earthquake in this report. The M 7.8 and M 7.9 subduction zone earthquake slip patches are outlined as dashed white lines.

    I include some inset figures in the poster.

  • In the lower left corner I include map that shows the historic seismicity for this region (Martin, 2014). The color shows well how the earthquakes that happen along the Tonga Trench get deeper along with the subducting slab. Shallow earthquakes are generally subduction zone earthquakes and deeper earthquakes are related (generally) to processes happening withing the downgoing slab. The 2017.01.02 M 6.3 earthquake is one of these deep earthquakes. I will briefly compare this M 6.3 earthquake with an earthquake from the region that occurred in 1932 (Okal, 1997).
  • In the center top I include a figure that shows a small scale map of the southwestern Pacific (a) and a large scale map of the North Fiji Basin (b) from Martin, 2013. The various spreading ridges are indicated as double lines. I present this figure below.
  • In the upper right corner I include a figure from Schellart et al. (2002) that shows a conceptual model for the development of the North Fiji Basin formed by extension in the plate as the Basin rotated clockwise towards the New Hebrides Trench. I present this below.
  • In the lower right corner I include a figure from Richards et al. (2011) that shows their model of how the subducting slabs have interacted through time. These authors think that there is a stalled out and torn slab at depth below the North Fiji Basin. The M 7.2 earthquake occurred near the cross section c-c’.


Below is additional background material about the tectonics in this region.

  • Here is the seismicity map from Martin (2014). Below I include the text from the original figure caption in blockquote.

  • Earthquake hypocentres from the USGS catalogue (earthquakes from 0 to 70 km depth excluded for clarity), overlain on shaded relief bathymetry showing tectonic elements of the Vanuatu/Tonga area. FP= Fiji Platform. HFZ= Hunter Fracture Zone. NC = New Caledonia. S = Samoa.

  • Here is the general tectonic map for the western Pacific and the North Fiji Basin (Martin, 2013). Below I include the text from the original figure caption in blockquote.

  • (a) The North Fiji Basin in its regional setting in the southwest Pacific Ocean (after Hall and Spakman, 2002; Mann and Taira, 2004; Schellart et al., 2006; Whattam et al., 200Smilie: 8). Light grey¼oceanic crust. Dark grey = oceanic plateaus or island-arc crust. NZ = New Zealand; PNG = Papua New Guinea. CR = Colville Ridge; FP = Fiji Platform; KR = Kermadec Ridge; LHR = Lord Howe Rise; LR = Lau Ridge; MBP = Melanesian Border Plateau; NFB = North Fiji Basin; NC = New Caledonia; NR = Norfolk Ridge; OJP = Ontong Java Plateau; S = Samoa; SCT = San Cristobal Trench; SI = Solomon Islands; TR = Tonga Ridge; VA = Vanuatu Arc; VT = Vitiaz Trench. Black arrows show direction and rate in cm/yr of motion of the Pacific Plate relative to the Australian Plate (DeMets et al., 1994; Mann and Taira, 2004). (b) Main tectonic elements of the North Fiji Basin (NFB) (after Auzende et al., 1995a; Lagabrielle et al., 1996; Pelletier et al., 2001; Ruellan and Lagabrielle, 2005). White areas outlined in black = island arc crust. Black = Islands: A = Aneityum; Ef = Efate; Es = Espirito Santo; M = Malekula; Ta = Tanna. Ridges: Ba, Bl and Br¼Balmoral, Bligh and Braemar (after Jarvis et al., 1994); D = D’Entrecasteaux; WT = West Torres Platform. Spreading Ridges: CSR = Central; FSR = Futuna; HH = Hazel Holmes; SP = South Pandora; Tr = Tripartite; WFR = West Fiji; Fracture Zones: EFZ = Epi (after Greene and Collot, 1994; Raos and Crawford, 2004); HFZ = Hunter; NFFZ = North Fiji. Thin arrows show representative GPS convergence rates between the Vanuatu Arc and the Australian Plate (Calmant et al., 1995, 2003; Taylor et al., 1995; Wallace et al., 2005, 2009). In the Aneityum Tanna area rates are 116–124 mm/yr, in Efate 86–94 mm/yr, while in Espirito Santo and Malekula they are 17–43 mm/yr. Small dashed square shows the location of Fig. 10.

  • Here is the figure from Schellart et al. (2002) that shows their model of tectonic development of the North Fiji Basin. Schellart et al. (2002) include a long list of references for the tectonics in this region here. Below I include the text from the original figure caption in blockquote.

  • Tectonic reconstruction of the New Hebrides – Tonga region (modified and interpreted from Auzende et al. [1988], Pelletier et al. [1993], Hathway [1993] and Schellart et al.(2002a)) at (a) ~ 13 Ma, (b) ~ 9 Ma, (c) 5 Ma and (d) Present. The Indo-Australian plate is fixed. DER = d’Entrcasteaux Ridge, HFZ = Hunter Fracture Zone, NHT = New Hebrides Trench, TT = Tonga Trench, WTP = West Torres Plateau. Arrows indicate direction of arc migration. During opening of the North Fiji Basin, the New Hebrides block has rotated some 40-50° clockwise [Musgrave and Firth 1999], while the Fiji Plateau has rotated some 70-115° anticlockwise [Malahoff et al. 1982]. During opening of the Lau Basin, the Tonga Ridge has rotated ~ 20° clockwise [Sager et al. 1994]. (Click for enlargement)

  • Here is a great illustration from Martin (2013) that shows the “Double Saloon Door” model of backarc spreading that may have led to the development of the North Fiji Basin. Below I include the text from the original figure caption in blockquote.

  • Double-saloon-door rifting and seafloor spreading model (after Martin, 2006, 2007). Double wavy lines¼island arc crust created at a pre-existing accretionary wedge/magmatic arc. Motifs on the rotating terranes = island arc volcanoes. Dotted area¼extended island arc crust. Thin lines within light shaded area = isochrons within oceanic crust. Thick line with black triangles¼subduction zone. Plate West 2 rotates clockwise about pole P1, whereas plate East 2 rotates counter clockwise about pole P2. (a) 201 of Rotation: arc-parallel rifts have developed, and an arc-perpendicular rift has initiated as West 2 separates from East 2 in an east west direction. (b) 401 of Rotation: oceanic crust has extended both to the east and west (compare Fig. 3b with a), and the oceanic rift tips propagate both east and west, shown by isochrons abutting rifted island arc crust. Relative to the southwesterly motion of the east end of plate West 2, and the southeasterly motion of the west end of plate East 2, the arc-perpendicular oceanic rift tip propagates north. Simultaneously, the arc-perpendicular oceanic rift also extends south (compare the NS extent of oceanic crust in Fig. 3b and c). The arc-perpendicular rift therefore propagates both north and south. (c) 601 of Rotation: similar to the development of the North Fiji Basin (NFB), where the situation described in Fig. 3b above has evolved further. (d) For comparison to the model, a compilation of identified magnetic anomalies and lineations in the NFB (after Malahoff et al., 1982a; Auzende et al., 1988a, 1988b,1990, 1994a, 1995a, 1995b; Charvis and Pelletier, 1989; Maillet et al., 1989; Tanahashi et al., 1991; Pelletier et al., 1993; Huchon et al., 1994; Jarvis et al., 1994; Joshima et al., 1994; Tanahashi et al., 1994; Lagabrielle et al., 1996; Pelletier et al., 2000, 2001; Ruellan and Lagabrielle, 2005). Numbers and letter J¼identified magnetic anomalies. Fracture zones: HFZ¼Hunter, FFZ¼Futuna. Note that Fig. 3a–c is symmetrical, whereas the NFB is asymmetrical: Vanuatu Arc is 1163 km long, whereas Fiji Platform extends 510 km.

  • Here is a time series of maps and cross sections from Martin (2014) that shows the development of these plate margins for the past 12 Ma. Below I include the text from the original figure caption in blockquote.

  • Evolution of slabs underlying the NFB in plan and two cross-sections, based on reconstructions of Martin (2013). Red = combined Vanuatu Fiji Platform slab. Grey=Tonga slab. Toroidal flows around the Vanuatu Arc rotation pole shown in brown arrows, around the Fiji platform rotation pole in green and around the northern end of Tonga slab in blue (lighter colours indicate flows below the slab). Cross-sections are intended to show upwelling and downwelling components of toroidal as opposed to poloidal flows. See Funiciello et al. (2006), Stegman et al. (2006), Schellart (200Smilie: 8) and Faccenna et al. (2010) for the full complexity of flows generated in analogue and numeric models.

  • a) Vitiaz-parallel reconstruction when NEdirected subduction of the Australian Plate started 12/10 Ma ago. Toroidal flow radii shown as 500 km except around the Fiji Platform pole which is shown as 270 km because it may have been restricted by the northern end of the reconstructed Tonga slab (cf Fig. 2), whose hingeline was essentially stationary from 12/10Ma to 6 Ma. F=Fiji Platform. L=Lau Ridge. T=Tonga Ridge. V=Vanuatu Arc.
  • b) 7.5Ma reconstruction. Thick black arrows showclockwise rollback of Vanuatu Arc and counterclockwise rollback of Fiji Platform, creating a concave slab (cf Fig. 3).
  • c) 5Ma reconstruction. Rollback of Tonga slab beginning 6Ma ago (indicated inwhite,marking initial opening of overlying Lau Basin) initiates toroidal flowaround northern end of Tonga slab (blue arrows). Separate flow cell around Fiji Platform rotation pole is omitted for clarity. Flow likely entered both NFB and Lau Basin mantle wedges.
  • d) 1.5 Ma reconstruction when Fiji Platformstopped rotating (Martin, 2013), as it collided with Lau Ridge.
  • e) Present day. Extent of the combined Vanuatu/Fiji Platformslab based on seismicity outboard of the Tonga slab in zones I and II of Bonnardot et al. (2009). Where Richards et al. (2011) show extensive slab tears, limited tears are shown based on tomography (Hall and Spakman, 2002; Schellart and Spakman, 2012), with gaps indicating possible tears under the Hunter and Epi Fracture Zones. The modelled slab (using rotations of Martin, 2013) extends SE to the Hunter Fracture Zone. Note that Chen and Brudzinski (2001) and Richards et al. (2011) include more hypocentres to the southeast, but these have alternatively been interpreted as extensional or compressional earthquakes related to the upper surface of a Tonga slab double seismic zone (Bonnardot et al., 2009). Lau Basin has expanded, northern Tonga slab is further east and flow into the NFB is reduced or curtailed.
  • Here is an animated gif that I made from martin (2013) that shows their tectonic reconstruction from 12 Ma to ~1.5 Ma. I also prepared this as a short video clip here (4 MB mp4). I include the figure captions from the individual panels below as blockquotes.

  • >12 Ma (Fig. 4)

  • Pre-12 Ma pre-rift reconstruction. Fiji Platform is rotated 126.4° (after Taylor et al., 2000) about a rotation pole at 15°37S 179°W (PFP), thereby positioning it NS and aligned with the Lau Ridge. The Vanuatu Arc is rotated 631 about a pole at 10°22S 166°E (PVA). Stars with OJP and MBP mark Ontong Java Plateau and Melanesian Border Plateau collisions which choke the SW-directed subduction zone (thick line with triangles). Dotted lines: VT = Vitiaz Trench; NDZ = northern deformation zone (after Pelletier and Auzende, 1996). Tonga Ridge, outlined in red, is rotated against the Lau Ridge to its position prior to opening of the Lau Basin.

  • 10-12 Ma (Fig. 5)

  • Vitiaz-parallel reconstruction. Vanuatu Arc rotated 52°, and Fiji platform 58.4° from their present-day positions. This is similar to the pre-rift reconstruction of Auzende et al. (1988b, 1995a) which they date as 10 and 12 Ma respectively. Dotted line¼ = Vitiaz Trench. Black line with triangles = NE-directed subduction zone. Thin arrows indicate initial movements of the Vanuatu Arc and the Fiji Platform. Depending on their relative rate of motion (Martin, 2006), initial movement between the east end of the Vanuatu Arc and the west end of the Fiji Platform is strike–slip, implying an R–R–F triple junction. Dashed line¼first magnetic lineation southwest of Fiji Platform, which in its rotated position is sub-parallel to a magnetic lineation off the tip of the Vanuatu Arc. Inset shows subdivision of magnetic lineations into three regions (after Auzende et al., 1988b, 1990). Double dashed line¼axial anomaly. Note that region 2 is divided into two sub-regions, separated by region 3.

  • 7.5 Ma (Fig. 6)

  • Figs. 6–9 are drawn taking the Vitiaz-parallel reconstruction (Fig. 5) to be 12Ma (Auzende et al., 1995a), and assuming 521 rotation of the Vanuatu Arc pro-rata over 12 Ma, and 58.41 of the Fiji Platform pro-rata from 12 Ma to 1.56 Ma (see Sections 2 and 4). 7.5 Ma reconstruction. Vanuatu Arc rotated 32.51 and Fiji Platform rotated 33.21 from their present-day positions. Thin lines are rotated magnetic lineations northeast and southeast of the Vanuatu Arc (Fig. 3d), whereas dashed thin lines are rotated lineations north and southwest of Fiji Platform. Paired thin lines represent a cartoon of the R–R–R triple junction which develops as a result of the WNW–ESE separation of the east end of the Vanuatu Arc and the west end of the Fiji Platform (compare Fig. 3). Note that the first lineation to the southwest of Fiji, which was oriented slightly east of due north in Fig. 5, has been rotated and is slightly west of due north.

  • 5 Ma (Fig. 7)

  • 5.0 Ma reconstruction. Vanuatu Arc rotated 21.7°, Fiji Platform rotated 19.3°. Dotted lines = EW-oriented anomalies (after Pelletier et al., 1993). Other lineations as in Fig. 6. With initial movement having started at 6 Ma, Tonga Ridge and Lau Ridge have begun to separate (after Parson and Hawkins, 1994; Parson and Wright, 1996; Taylor et al., 1996).

  • 2.5 Ma (Fig. Smilie: 8)

  • 2.5 Ma reconstruction. Vanuatu Arc rotated 10.91, Fiji Platform rotated 5.31. 2A = anomaly 2A (3.6–2.6 Ma). Note that anomalies 2A are superimposed at 17°S, but are overlapped at 17°30′–19°30′S.

  • 1.5 Ma (Fig. 9)

  • 1.5 Ma reconstruction. Vanuatu Arc rotated 6.51, Fiji Platform present-day position. 2 = anomaly 2 (1.95–1.78 Ma).

  • Martin (2014) suggests that mantle flow at the edges of the subducting slabs play a part in the “Double Saloon Door” tectonic model. Here is an illustration that shows how these flows (toroidal in shape) may be configured. Below I include the text from the original figure caption in blockquote.

  • Proposed geodynamic mechanism for concave slab under the NFB. Rollback induces slab curvature and opposite toroidal flows with upwelling and downwelling components (Faccenna et al., 2010; Funiciello et al., 2006; Schellart, 200Smilie: 8), the latter influenced by slab curvature (Kneller and Van Keken, 200Smilie: 8). Mantle flow is concentrated towards the slab in a central location (curved solid lines with arrows, shown dashed when seaward of the slab). Gravitational forces on either flank of curved subduction slab shown by open white arrows. Rifted island arc crust in overlying plate shown in white with fault block pattern. Oceanic crust lightly shaded.

  • Auzende et al. (1994) posit that the Central Spreading Ridge and the West Fiji Ridge have been spreading and “functioning synchronously” for the past 1-1.5 Ma. Here is a figure where they present their estimates of spreading rates for these ridges. These spreading rates are based upon the distances between magnetic anomalies and the ridges. Below I include the text from the original figure caption in blockquote.

  • Kinematic sketch of twin ridges. NFFZ = north Fiji fracture zone; CFZ = central Fiji fracture zone; SFFZ = south Fiji fracture zone; CSR = central spreading ridge; WFR = west Fiji ridge; 5-6 = spreading rate (in cm/yr) calculated from magnetic data; 0-2? = inferred spreading rate (in cm/yr). Arrows at tips of ridge segments indicate direction of propagation. Contour interval = 1 km. A = western North Fiji Basin plate; B = intermediate microplate; C = north Fiji (Pacific?) plate; D = southeast Fiji (Australian?) plate.

  • Okal (1997) conducted an analysis of seismological records from a deep earthquake that happened in the region of the M 6.3 earthquake. This earthquake occurred on 26 May 1932, long before modern seismometers made it to the scene. Okal estimated the magnitude to be similar in size to earthquakes in the mid M 7 range. Here is a figure from Okal (1997) that shows some focal mechanisms for the earthquakes from 1932. Compare the mainshock (the largest focal mechanism) with the moment tensor for the 2016.01.02 M 6.3 earthquake. Below I include the text from the original figure caption in blockquote.
  • 1932.05.26 M 7.6 (USGS)

  • Focal mechanism of the 1932 earthquake, as determined in this study. We also show CMT solutions in the immediate vicinity of the event, as available from Dziewonski et al. (1983, and subsequent quarterly updates) and Huang et al. (1997). Their spatial distribution is shown in map view. The background map at the upper right sets the study area (shaded) into the familiar bathymetry of the Fiji-Tonga-Kermadec region. The separation of isobaths is 1000 m.

  • Interestingly, deep focus earthquakes take up ~66% of the deep earthquakes globally. From Yu and Wen (2012), we can see some moment tensors for deep earthquakes in this region. The 1994.07.30 earthquake is just west of the 2017 M 6.3 earthquake and also has a similar moment tensor to the 2017 M 6.3 earthquake.

  • Regional map of deep-focus similar earthquake pairs and seismicity near the Tonga–Fiji subduction zone. Deep similar earthquake pairs (black stars) and their available Global Centroid Moment Tensor (CMT) (Dziewonski et al., 1981; Ekstrom et al., 2003) are labeled with event date and doublet/cluster ID where applicable. Source parameters of the doublets/clusters are listed in Tables 1, 2. Background deep seismicity is shown as gray dots. Black lines indicate the slab contours below 300 km depth (Gudmundsson and Sambridge, 199Smilie: 8), with an interval of 100 km. Regional map of the Tonga–Fiji–Kermadec subduction zone is shown in the inset, with gray dotted box indicating the region blow-up in the main figure. Black lines are the slab contours below 300 km depth and the Tonga–Kermadec trench (Bird, 2003). The color version of this figure is available only in the electronic edition.

UPDATE: 2017.01.03 20:15 PST:

References:

  • Auzende, J-M., Pelletier, B., Lafoy, Y., 1994. Twin active spreading ridges in the North Fiji Basin (southwest Pacific) in Geology, v. 22, p. 63-66.
  • Benz, H.M., Herman, Matthew, Tarr, A.C., Furlong, K.P., Hayes, G.P., Villaseñor, Antonio, Dart, R.L., and Rhea, Susan, 2011. Seismicity of the Earth 1900–2010 eastern margin of the Australia plate: U.S. Geological Survey Open-File Report 2010–1083-I, scale 1:8,000,000.
  • Hayes, G.P., Wald, D.J., and Johnson, R.L., 2012. Slab1.0: A three-dimensional model of global subduction zone geometries in, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524
  • Martin, A.K., 2013. Double-saloon-door tectonics in the North Fiji Basin in EPSL, v. 374, p. 191-203.
  • Martin, A.K., 2014. Concave slab out board of the Tonga subduction zone caused by opposite toroidal flows under the North Fiji Basin in Tectonophysics, v. 622, p. 56-61.
  • Okal, 1997. A reassessment of the deep Fiji earthquake of 26 May 1932 in Tectonophysics v., 275, p. 313-329.
  • Richards, S., Holm., R., Barber, G., 2011. When slabs collide: A tectonic assessment of deep earthquakes in the Tonga-Vanuatu region, Geology, v. 39, pp. 787-790.
  • Schellart, W., Lister, G. and Jessell, M. 2002. Analogue modelling of asymmetrical back-arc extension. In: (Ed.) Wouter Schellart, and Cees W. Passchier, Analogue modelling of large-scale tectonic processes, Journal of the Virtual Explorer, Electronic Edition, ISSN 1441-8142, volume 7, paper 3, doi:10.3809/jvirtex.2002.00046
  • Yu, W. and Wen, L., 2012. Deep-Focus Repeating Earthquakes in the Tonga–Fiji Subduction Zone, BSSA, v. 102, no. 4, pp. 1829-1849

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

Earthquake Report: 2016 Summary Cascadia

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Here I summarize the seismicity for Cascadia in 2016. I limit this summary to earthquakes with magnitude greater than or equal to M 4.0. I reported on all but five of these earthquakes. I put this together a couple weeks ago, but wanted to wait to post until the new year (just in case that there was another earthquake to include).

I prepared a 2016 annual summary for Earth here.

    I include summaries of my earthquake reports in sorted into three categories. One may also search for earthquakes that may not have made it into these summary pages (use the search tool).

  • Magnitude
  • Region
  • Year

Earthquake Summary Poster (2016)

  • Here is the map where I show the epicenters as circles with colors designating the age. I also plot the USGS moment tensors for each earthquake, with arrows showing the sense of motion for each earthquake.
  • I placed a moment tensor / focal mechanism legend in the lower left corner of the map. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • In some cases, I am able to interpret the sense of motion for strike-slip earthquakes. In other cases, I do not know enough to be able to make this interpretation (so I plot both solutions).

    I include some inset figures in the poster.

  • In the upper left corner is a map of the Cascadia subduction zone (CSZ) and regional tectonic plate boundary faults. This is modified from several sources (Chaytor et al., 2004; Nelson et al., 2004)
  • Below the CSZ map is an illustration modified from Plafker (1972). This figure shows how a subduction zone deforms between (interseismic) and during (coseismic) earthquakes. 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.
  • To the lower right of the Cascadia map and cross section is a map showing the latest version of the Uniform California Earthquake Rupture Forecast (UCERF). Let it be known that this is not really a forecast, and this name was poorly chosen. People cannot forecast earthquakes. However, it is still useful. The faults are colored vs. their likelihood of rupturing. More can be found about UCERF here. Note that the San Andreas fault, and her two sister faults (Maacama and Bartlett Springs), are orange-red.
  • To the upper right of the Cascadia map and cross section is a map showing the shaking intensities based upon the USGS Shakemap model. Earthquake Scenarios describe the expected ground motions and effects of specific hypothetical large earthquakes. The color scale is the same as found on many of my #EarthquakeReport interpretive posters, the Modified Mercalli Intensity Scale (MMI). The latest version of this map is here.
  • In the upper right corner I include generalized fault map of northern California from Wallace (1990).
  • To the left of the Wallace (1990) map is a figure that shows the evolution of the San Andreas fault system since 30 million years ago (Ma). This is a figure from the USGS here.
  • In the lower right corner I include the Earthquake Shaking Potential map from the state of California. This is a probabilistic seismic hazard map, basically a map that shows the likelihood that there will be shaking of a given amount over a period of time. More can be found from the California Geological Survey here. I place a yellow star in the approximate location of today’s earthquake.



    Cascadia subduction zone: General Overview

  • Cascadia’s 315th Anniversary 2015.01.26
  • Cascadia’s 316th Anniversary 2016.01.26
  • Earthquake Information about the CSZ 2015.10.08


The big player this year was an M 6.5 along the Mendocino fault on 2016.12.08. Here I present an inventory of 8 earthquakes with M ≥ 5.0. There are a few additional earthquakes with smaller magnitudes that are of particular interest.

Please visit the #EarthquakeReport pages for more information about the figures that I include in the Earthquake Report interpretive posters below.


    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.
  • 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/
  • 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, humboldt, mendocino, mendocino, oregon, pacific, plate tectonics, San Andreas, strike-slip, subduction, Transform, tsunami

Earthquake Report: 2016 Summary

2016_summary_interpretation_thumb

Here I summarize the global seismicity for 2016. I limit this summary to earthquakes with magnitude greater than or equal to M 7.0. I reported on all but two of these earthquakes. There were no earthquakes as large as an M 8.0 for the entire year of 2016. However, we had an inventory of 17 earthquakes with M ≥ 7.0. Here is the 2015 Earthquake Summary Page. I initially prepared this a couple weeks ago, but wanted to wait until January 1 before I presented it. Good thing I waited as there was an earthquake in Chile on 12/25 and a swarm in Nevada on 12/28. Happy New Year! Waiting to post this was challenging, sort of like waiting to open wrapped holiday gifts.

I prepared a 2016 annual summary for the Cascadia region here.

    I include summaries of my earthquake reports in sorted into three categories. One may also search for earthquakes that may not have made it into these summary pages (use the search tool).

  • Magnitude
  • Region
  • Year

Earthquake Summary Poster (2016)

  • Here is the map where I show the epicenters as circles with colors designating the age. I also plot the USGS moment tensors for each earthquake, with arrows showing the sense of motion for each earthquake.
  • I placed a moment tensor / focal mechanism legend in the lower left corner of the map. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • In some cases, I am able to interpret the sense of motion for strike-slip earthquakes. In other cases, I do not know enough to be able to make this interpretation (so I plot both solutions).

  • Compare with last year’s summary poster. Here is the 2015 Earthquake Summary Page. Note how the subduction zones in the southwestern Pacific are highly active in both 2015 and 2016.

    2016 Highlights from others

  • Here is a summary showing a running total and mean of earthquakes for different magnitude ranges. This came from Chris Rowan @Allochthonous. Here is an update to the graphic below, coming with an explanation.

  • Here is a summary showing the epicenters from earthquakes in 2016 with symbol sorted vs. magnitude. This came from Susan Hough @SeismoSue.


ALL Earthquake Reports – 2016

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

Earthquake Report: Nevada!

20161228_nevada_interpretation_thumb

Well, this is an interesting series of earthquakes. They occurred in a region that has not had any earthquakes (given the USGS NEIC database). However, as Jascha Polet pointed out on twitter, there was a swarm to the east in 2011 (here is the University of Nevada Reno Seimological Laboratory page on these 2011 Hawthorne Earthquake Sequence). These two swarms are slightly different in this complicated region of the Pacific-North America plate boundary. A portion of the Pacific-North America plate boundary motion is partitioned along the eastern side of the Sierra Nevada mountains along the Eastern California shear zone, the Walker Lane, and eventually zones like the Mohawk Valley fault zone. Further to the east exists Basin and Range (B&R) extension and along the boundary there is some combination of dextral (right-lateral) shear and B&R extension.

Below I present an interpretive poster for this earthquake series.

I plot the seismicity from the past week, with color representing time and diameter representing magnitude (see legend). I include the active faults from the USGS active fault and fold database. I include moment tensors for the four largest earthquakes in this series.

  • 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.

At first, I saw the strike-slip moment tensors and thought about the Mina deflection, which is actually further to the south of this swarm. The Mina deflection is an interesting area where the plate boundary motion is diverted to the east along sinistral (left lateral) faults formed by rotating blocks (Lee et al., 2009; Wesnousky et al., 2010, 2012; Rinke et al., 2012; Bormann et al., 2016). There is still debate about how plate motion is accommodated in this region.

What I found interesting is that these 2016 earthquakes do not (at first) appear to fit any existing tectonic model, but are consistent with some faults mapped that may be associated with the 2011 earthquakes. The Wassuk fault to the east accommodates dextral slip associated with the plate boundary (Adams and Sawyer, 1999 (A)). Other faults in the region are characterized a little differently. The mapped faults that probably were activated in 2011 (Adams and Sawyer, 1998) are considered left-lateral and normal. The existing mapped faults to the south of this 2016 swarm are mapped as normal faults based upon offset topography (Adams and Sawyer, 1998 (B)). These unnamed faults west of Aurora Crater, possibly activated today, are classified as younger than 1.6 Ma and have a low slip rate (< 0.2 mm/yr). It is possible that these Aurora Crater faults are more active than 1.6 Ma or that other, more active faults, have not yet been mapped yet. Unfortunately, the M 5.7 probably won't produce surface rupture. What is interesting is that the strike-slip faults in the Mina deflection (MD) to the south are either northwesterly trending dextral or easterly trending sinistral faults. I will discuss this below when I present others' figures. Both interpretations seem possible, but I favor a northeast striking left-lateral interpretation.

    Here are the USGS websites for the larger earthquakes in this 2016 swarm.

  • 2016.12.28 08:22 M 5.7
  • 2016.12.28 08:18 M 5.7
  • 2016.12.28 09:13 M 5.5
  • 2016.12.28 12:18 M 4.1

    I include some inset figures in the poster.

  • In the upper right corner, I include a map that shows the plate boundary scale tectonics for the western US (Nagorsen-Rinke et al., 2012). I placed a yellow star in the general location of the 2016 earthquake series.
  • To the left of this map I include an illustration from Lee et al. (2009) that shows how the faults in the Mina deflection may have formed. Another version of this figure is found in Nagorsen-Rinke et al. (2012). Note how the sinistral faults form as the blocks rotate between faults formed through dextral shear formed by the Pacific-North America plate boundary motion.
  • In the lower right corner I include a smaller scale map that shows the regional scale faulting in the region (Nagorsen-Rinke et al., 2012). I place a yellow star in the location of today’s earthquake swarm.
  • In the upper left corner I include a map from Bormann et al. (2016) that shows tectonic domains as devised by the authors based upon Global Positioning System (GPS) geodetic observations. GPS geodesy is the study of the deformation of Earth’s surface through the use of GPS observations for 3-D positions. I place a red star where today’s earthquakes occurred. Note how these earthquakes are along the boundary of the Mono Basin and Walker Range tectonic domains. There are many short faults in this region forming a complicated tectonic region. The low slip rates for these faults make it difficult to use GPS geodesy to resolve the strain and sense of motion along these faults.
  • Below this map I include the USGS Did You Feel It? maps for the 4 largest earthquakes. These maps show estimates of ground motions that is actually based on real observations as submitted to the USGS website for each of these earthquakes. Note how the M 4.1 was felt in slightly different regions than the other earthquakes. The M 4.1 earthquake was an extensional earthquake, compared to strike-slip for the others. This most likely contributed to these variations in ground motions, in addition to the magnitude differences.
  • In the lower left corner I include a Google Earth map that shows the seismicity from 1900-2016 for earthquakes with magnitudes M ≥ 4.0. I outline the 2011 and 2016 regions of seismicity in dashed white lines. Here is the query that I used to find these earthquakes. I label the major fault zones in this region. Note how the 2011 swarm was mostly extensional, but there were a few strike-slip earthquakes. If these are related to the unnamed faults near Alkali Valley, then they are probably northeast striking sinistral (left-lateral) strike-slip earthquakes.


  • Here are the two figures from Rinke et al. (2012) that show the global and regional tectonics here. I include the figure captions below as blockquotes. The first map shows the plate boundary scale tectonic regions. This is a generalized map (e.g. don’t pay attention to where the San Andreas and Cascadia faults are located). The second map shows the regional fault systems.

  • Simplified tectonic map of the western U.S. Cordillera showing the modern plate boundaries and tectonic provinces. Basin and Range Province is in medium gray; Central Nevada seismic belt (CNSB), eastern California shear zone (ECSZ), Intermountain seismic belt (ISB), and Walker Lane belt (WLB) are in light gray; Mina deflection (MD) is in dark gray.


    Shaded relief map of the WLB and northern part of the ECSZ showing the major Quaternary faults. Solid ball is located on the hanging wall of normal faults; arrow pairs indicate relative motion across strike-slip faults; white dashed box outlines location of Figure 2; light gray shaded areas show the Mina deflection and the Carson domain. BSF—Benton Springs fault; CF—Coaldale fault; DSF—Deep Springs fault; DVFCFLVFZ—Death Valley–Furnace Creek–Fish Lake Valley fault zone; GHF—Gumdrop Hills fault; HLF—Honey Lake fault; HMF—Hunter Mountain fault; MVF—Mohawk Valley fault; OVF— Owens Valley fault; PLF—Pyramid Lake fault; PSF—Petrifi ed Springs fault; QVF—Queen Valley fault; SLF—Stateline fault; SNFFZ—Sierra Nevada frontal fault zone; WMFZ—White Mountains fault zone; WRF—Wassuk Range fault; WSFZ—Warm Springs fault zone.

  • Here is a figure from Wesnousky et al. (2012) where a wax block model is used to illustrate their interpretations of the tectonic deformation along the Walker Lane region. Today’s earthquakes occurred in the basin to the west of the circled number “7.”

  • (Left) Model to visualize accommodation of strain and development of basins in northern Walker lane. The upper is a block of wax has been heated to become ductile and subjected to transtensional right-lateral shear. Ice has been applied to the surface of lower wax block to create brittle upon ductile layer, and then subjected to same shear. The transtensional shear results in a zone of deformation displaying rotation of ‘crustal blocks’, an en echelon arrangement of asymmetric ‘basins,’ observable extension along the axis of shear, and the ability to locally traverse the entire zone of shear without encountering a major fault structure. (Right) Oblique view of study area illustrates the en echelon arrangement and triangular shape of basins nested along the east edge of the Sierra Nevada. Black and colored lines are portions of Walker Lane faults shown in Fig. 1 (Wesnousky et al., 2012)

  • Here are the geodetic observations for each of these blocks along the Walker Lane (Wesnousky et al., 2012). GPS rates are plotted as red vectors. Geologic rates are in the white boxes and are plotted as vectors in black, purple, and blue. Today’s earthquake series happened in the basin where the label “LUCK” is. Note how the GPS site on the northeast side of the basin is moving slightly faster than the GPS site on the western side of the basin. A northwesterly striking right-lateral strike-slip fault could produce this if it ran between these two GPS sites.

  • Physiographic and fault map of area of interest in northern Walker Lane shows major structural basins (numbered), active basin-bounding faults (thick black lines), and geodetic displacement field (red arrows). Shown in white boxes are geologically determined values of fault-normal extension (black-upper text), geodetic estimates of fault-normal extension (magenta-middle text) and geodetic estimates of fault-parallel strike-slip (blue-lower text) rates along each of the basin bounding faults. Two-headed arrows schematically show ranges of same values and correspond in arrangement and color to the values in boxes. The geologically determined extension rate arrows are placed adjacent to the sites of studies except for Lake Tahoe where the estimate is an average value across several submarine faults. Dotted (yellow) lines define paths AB, CD, and EF.

  • This is the tectonic domain figure from Bormann et al. (2016). Some faults have arrows that show their relative sense of motion and blocks have arrows that show their relative sense of rotation. Note the east-west sinistral strike-slip faults that bound the northern and southern boundaries of the blocks in the Mina deflection. Today’s earthquakes happened along the eastern boundary of the Bodie Hills tectonic domain (BH). The BH domain has clockwise rotation like in the Mina deflection. This would place sinistral strain along the southern boundary of the BH domain, creating left-lateral strike-slip faults oriented northeast striking. This is consistent with the sense of motion along the “unnamed faults near Alkali Valley.” If these 2016 earthquakes are associated with these faults, then they are along northeast striking structures and would be left-lateral.

  • Regional map showing the block model boundaries (yellow lines) in relation to the topography and faults of the Central Walker Lane. The Central Walker Lane (region within the dashed black lines) lies between the northeast striking normal faults of the Basin and Range and the Sierra Nevada microplate. Black lines delin-eate major normal faults of the Central Walker Lane, and red lines mark the location of strike slip faults (arrows indicate slip direction). Paleomagnetic observations in-dicate that crustal blocks in the Carson Domain, Bodie Hills, and Mina Deflection accommodate dextral shear through clockwise vertical axis rotations (Cashman and Fontaine, 2000; Petronis et al., 2009; Rood et al., 2011b; Carlson et al., 2013). Orange lines mark the locations of surface rupture that resulted from historic earthquakes in the Central Nevada Seismic Belt. Faults traces are modified from the USGS Qua-ternary Fault and Fold database (U.S. Geological Survey, California Geological Survey, Nevada Bureau of Mines and Geology, 2006). Inset map shows the location of the study area in relation to other elements of the Pacific/North America Plate boundary zone.

  • Here Bormann et al. (2016) present their estimates of rotation and fault slip rates for this region. The caption is below the figure. I place a red star where today’s earthquakes happened. This map helps us visualize an alternate interpretation of these earthquakes. The 2016 swarm is along the eastern boundary of the BH domain, which would suggest a northwest striking dextral (right-lateral) strike slip fault would be involved. Given that the currently mapped faults in this region are northeast striking, I interpret these to be along structures that are also northeast striking. Note how the BH domain is rotating clockwise about 1.75 °/Ma, while the MD is rotating clockwise about 2.75 °/Ma.

  • Block motions, slip rates, and velocity residuals for the best fitting GPS model. (A) Rigid block rotation and translation exaggerated by a factor of 107(representing 10 million years of deformation). Color of block indicates vertical axis rotation rate. (B) Predicted fault slip rates represented by the thickness of black (red) line for dextral (sinistral) strike-slip motion and the length of blue (cyan) bar for fault normal extension (compression).

  • Here is the illustrative model presented by Lee et al. (2009) to explain the faulting in the MD (which may also partially explain the seismicity in this region northeast of the Bodie Hills).

  • Schematic block diagrams illustrating two fault-slip transfer mechanisms between subparallel strike-slip faults proposed for the eastern California shear zone and Walker Lane belt. (A) Displacement transfer model whereby the magnitude of extension along the connecting normal faults is proportional to the amount of strike-slip motion transferred (modified from Oldow et al., 1994). (B) Block rotation model in which clockwise rotation of blocks, bounded by dextral faults, is accommodated by sinistral faults (model of McKenzie and Jackson, 1983, 1986).

  • Here is an updated figure to show how these fault systems may have evolved through time (Nagorsen-Rinke et al., 2012).

  • Block diagrams illustrating models proposed to explain fault slip transfer across the Mina deflection. (A) Displacement transfer model in which normal slip along connecting faults transfers fault slip (modified from Oldow, 1992; Oldow et al., 1994). (B) Transtensional model showing a combination of sinistral and normal slip along connecting faults. (C) Clockwise block rotation model in which sinistral slip along connecting faults, combined with vertical axis rotation of intervening fault blocks, transfers fault slip (modified from McKenzie and Jackson, 1983, 1986). Single-barbed arrows show dextral fault motion across faults of the Eastern California shear zone (ECSZ) and Walker Lane belt (WLB) and sinistral motion along faults in the Mina deflection; half-circle double-barbed arrows indicate clockwise rotating fault blocks; solid ball is located on the hanging wall of normal slip faults; thin short lines indicate slip direction on fault surfaces.

  • Here is the map from the UNR Seismological Laboratory website. This shows the earthquakes recorded during the 2011 swarm along the “unnamed faults along Alakali Valley.” Here is the UNRSL website for this earthquake. I include the UNRSL description of the 2011 Hawthorne Sequence below.

    • Over the past nine weeks 42 earthquakes of Magnitude 3.0 and larger earthquakes (listed below) have been located in a sequence about 12 miles southwest of Hawthorne, Nevada. The first of these occurred on March 15th at 11:14 AM PDT and the latest at 10:23 AM PDT on May 19th. The preliminary magnitude for the largest event is M 4.6.
    • In all, there have been several hundred events of Magnitude 1 and larger; only a small fraction of the entire sequence has been reviewed. There have been 1000’s of smaller magnitude events.
    • The Nevada Seismological Laboratory deployed 3 temporary telemetered instruments in the source area on April 17-19 including a NetQuakes instrument at the Court House in Hawthorne. These temporary telemetered instruments deliver real-time data to the data center in Reno and are configured with 3-channel broadband sensors and 3-channel accelerometers.
  • Here is a map from Nagorsen-Rinke et al. (2012) with regional faults mapped. Note how the sisnistral faults that bound blocks in the Mina deflection are each slightly more counterclockwise rotated with the fault at the base of the southeastern Excelsior Mountains being the most northerly striking of these faults. If this configuration of faulting were in the basin to the NE of the Bodie Hills, it would explain the northeast striking sinistral interpretation for the 2016 series.

  • Shaded relief map of the southern part of the Mina deflection and northern part of the eastern California shear zone showing the major Quaternary faults. Solid black ball is located on the hanging wall of normal faults; arrow pairs indicate relative motion across strike-slip faults. Heavy arrow in northwest corner of map shows the present-day motion of the Sierra Nevada (SN) with respect to North America (NA) (Dixon et al., 2000). Location of the Adobe Hills geologic map shown in Figure 4A is outlined with a dashed line and location of this map is shown in Figure 1. PS—Pizona Springs; CF—Coaldale fault; CSF— Coyote Springs fault; DSF—Deep Springs fault; FLVFZ—Fish Lake Valley fault zone; HCF—Hilton Creek fault; OVF—Owens Valley fault; QVF—Queen Valley fault; RVF— Round Valley fault; WMFZ—White Mountains fault zone.

  • Here is the comparison map for the 2011 and 2016 earthquakes. This is the same data set as presented in the poster above, but I include the 2016 moment tensors here. If the two M 5.7 and one M 5.5 earthquakes are along a NW striking fault, it would be dipping at ~42° and daylight ~4km to the southwest. There is an east-west striking fault mapped at this distance and with this strike immediately to the east of the M 5.7 (2) moment tensor. The strike of this mapped fault is actually more aligned with the moment tensors than the other north striking Aurora Crater faults. What do you think?

References:

  • Adams, K., and Sawyer, T.L., compilers, 1998 (A), Fault number 1299, Unnamed faults near Alkali Valley, in Quaternary fault and fold database of the United States: U.S. Geological Survey website, https://earthquakes.usgs.gov/hazards/qfaults, accessed 12/28/2016 11:51 AM.
  • Adams, K., and Sawyer, T.L., compilers, 1998 (B), Fault number 1298, Unnamed faults west of Aurora Crater, in Quaternary fault and fold database of the United States: U.S. Geological Survey website, https://earthquakes.usgs.gov/hazards/qfaults, accessed 12/28/2016 10:11 AM.
  • Adams, K., and Sawyer, T.L., compilers, 1999, Fault number 1300, Wassuk Range fault zone, in Quaternary fault and fold database of the United States: U.S. Geological Survey website, https://earthquakes.usgs.gov/hazards/qfaults, accessed 12/28/2016 11:52 AM.
  • Bormann, J.M., Hammond, W.C., Kreemer, C., and Blewitt, G., 2016. Accommodation of missing shear strain in the Central Walker Lane, western North America: Constraints from dense GPS measurements in EPSL, v. 440, p. 169-177.
  • Lee, J., Garwood, J., Stockli, D.F., and Gosse, J., 2009. Quaternary faulting in Queen Valley, California-Nevada: Implications for kinematics of fault-slip transfer in the eastern California shear zone Walker Lane belt in GSA Bulletin, v. 121, no. 3/4, p. 599-614.
  • Nagorsen-Rinke, S., Lee, J., and Clavert, A., 2012. Pliocene sinistral slip across the Adobe Hills, eastern California–western Nevada: Kinematics of fault slip transfer across the Mina deflection in Geosphere, v. 9, no. 1, p. 37-53.
  • Stockli, D.F., Dumitru, T.A., McWilliams, M.O., and Farley, K.A., 2003. Cenozoic tectonic evolution of the White Mountains,California and Nevada in GSA Bulletin, v. 115, no. 7, p/ 788-816.
  • Wesnouskky, S.G., Bormann, J.M., Kreemer, C., Hammond, W.C., and Brune, J.N., 2012. Neotectonics, geodesy, and seismic hazard in the Northern Walker Lane of Western North America: Thirty kilometers of crustal shear and no strike-slip? in EPSL, v. 329-330, p. 133-140.

Posted in Uncategorized

Earthquake Anniversary: Sumatra-Andaman 2004 M 9.2 & 2005 M 8.6

20041226_20050328_sumatra_interpretation_thumb

On 26 December 2004 there was an earthquake with a magnitude of M 9.2 along the Sumatra-Andaman subduction zone (SASZ). This earthquake is the third largest earthquake ever recorded by modern seismometers and ruptured nearly 2,000 km of the megathrust fault offshore of Sumatra, the Andaman Isles, and the Nicobar Isles. (The 22 may 2960 Chile M 9.5 and 27 March 1964 M 9.2 Good Friday earthquake in Alaska are the first and second largest.) This 2004 M 9.2 earthquake triggered submarine landslides and deformed the seafloor to generate a trans-oceanic tsunami that killed almost a quarter of a million people. A few months later, on 28 March 2005, there was another megathrust earthquake, further to the south, with a magnitude of M 8.6. The M 8.6 earthquake ruptured in a region of the megathrust that had an increase in coulomb stress imparted to it by the M 9.2 earthquake to the north. The increase in stress is small, so for the stress increase to be able to trigger an earthquake, the fault must be within a margin of critical stress prior to the first earthquake in order to be triggered.

In prior years, I have written some material about the 2004 earthquake, including some observations made by others. Today I prepared an interpretive poster for the 2004 and 2005 SASZ earthquakes (while waking up at my mom’s house, where I was for the holiday; I was driving home to Arcata in 2004 when I heard about the SASZ earthquake.).

    Here are the USGS websites for these two SASZ earthquakes.

  • 2004.12.26 M 9.2
  • 2005.03.28 M 8.6

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 the region of the fault slip solution as modeled by the USGS (slightly transparent blue polygons). Note how the 2005 earthquake slips along a section of the fault that is further down-dip compared to the 2004 earthquake. This probably owes to the smaller tsunami triggered by the 2005 earthquake (and the smaller turbidite; Patton et al., 2015).

  • 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 MMI contours for both 2004 and 2005 earthquakes.
  • 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).

    I include some inset figures in the poster.

  • In the upper left corner, I include a map that shows the extent of historic earthquakes along the SASZ offshore of Sumatra. This map is a culmination of a variety of papers (summarized and presented in Patton et al., 2015).
  • In the upper right corner I include a figure that is presented by Chlieh et al. (2007). These figures show model results from several models. Each model is represented by a map showing the amount that the fault slipped in particular regions. I present this figure below.
  • In the lower right corner I present a figure from Prawirodirdjo et al. (2010). This figure shows the coseismic vertical and horizontal motions from the 2004 and 2005 earthquakes as measured at GPS sites.
  • In the lower left corner are the MMI intensity maps for the two SASZ earthquakes. Note these are at different map scales. I also include the MMI attenuation curves for these earthquakes below the maps. These plots show the reported MMI intensity data as they relate to two plots of modeled estimates (the orange and green lines). These green dots are from the USGS “Did You Feel It?” reports compared to the estimates of ground shaking from Ground Motion Prediction Equation (GMPE) estimates. GMPE are empirical relations between earthquakes and recorded seismologic observations from those earthquakes, largely controlled by distance to the fault, ray path (direction and material properties), and site effects (the local geology). When seismic waves propagate through sediment, the magnitude of the ground motions increases in comparison to when seismic waves propagate through bedrock. The orange line is a regression of data for the central and eastern US and the green line is a regression through data from the western US.


  • Here is the USGS poster for this earthquake. These results were put out very soon after the earthquake and later reports made more refined analyses. For example, there are over a dozen earthquake slip models for this earthquake, most all are better than this initial USGS version.

  • Here are the map and attenuation plots as a single figure.
  • Here is a figure that shows the wave height observations from satellites that happened to be passing over the Indian Ocean as the tsunami crossed towards India and Sri Lanka (Shearer and Burgman 2010).

  • This figure from Meltzner et al. (2010) shows measurements of vertical deformation collected from coral microatolls (which are sensitive to the tides, basically, they cannot survive above a certain level of tidal elevation. Read his and related papers to learn more about this method.). These are observations that are independent of GPS data.

  • Here is the source time function from Ishi et al. (2005). Note the similarity between this plot and the above one from Chlieh et al. (2007). These results are more comparable that the slip models we saw earlier.

  • This is from Subarya et al. (2006), an earlier plot, but still similar to Chlieh et al. (2007) and Ishii et al. (2007).

  • This is another estimate published also in 2006 (Tolstoy and Bohnenstiehl, 2006), again showing similarities with the other plots (though this is the most different). There are a number of other examples as well (e.g. Okal).

  • These next two figures from Singh et al. (2008 ) show a map and cross section at the location of the earthquake. The 2004 SASZ earthquake ruptured very deep in a location previously thought to not harbor strain to be accumulated and released during an earthquake.

  • In 2007 Dr. Chris Goldfinger and myself led a coring expedition in this region within Indonesian EEZ and international waters offshore of Sumatra. Our goal was to evaluate the sedimentary record of earthquakes in the form of submarine landslide deposits (called turbidites). We collected over 100 sediment cores and have prepared several papers documenting some of our results (Patton et al., 2013, 2015).
  • I will be preparing a website that documents this 2007 cruise aboard the R/V Roger Revelle. Here is the website. On this website, I provide a link to my research cruise blog, where I documented my cruise in real time. This is the first blog post for the RR07056 cruise.
  • Below is a figure where I present evidence for a sedimentary deposit from the 2004 SASZ earthquake (Patton et al., 2015). Sumner et al. (2013) also present sedimentary evidence for the 2004 earthquake. Especially convincing because they observed computer paper within the turbidite! I include a figure caption below the image in blockquote.

  • The uppermost (2004?) turbidite from cores 96PC and 96TC, plotted as a composite core. A. From left to right: mean particle size, point magnetic susceptibility, CT density, gamma density, turbidite classification, RGB imagery, CT imagery, turbidite structure classification division, depth (cm), turbidite structure (lithologic log), texture, and the lithologic notes are plotted vs. depth. Geophysical logs symbolized as in Figure 2. B. Detailed turbidite structure based on CT imagery. From left to right: i. CT imagery uninterpreted, ii. CT imagery interpreted, iii. Turbidite structure interpretation, iv. Turbidite structure division classification, and v. turbidite structure description. C. Results from smear slide based vertical biostratigraphic transects for core 96PC. Percent biogenic and percent lithologic are plotted vs. depth in m. D. The mean, minimum, and maximum particle size distribution for sediments collected within the uppermost turbidite (in purple) and within hemipelagic sediments underlying the uppermost turbidite (in green) are plotted. These are compared with the combined distributions (in blue).

  • Here is the cross section showing where the earthquake hypocenter is compared to where we think the mantle exists. We have not been here, so nobody actually knows… These interpretations are based on industry deep seismic data (Singh et al., 2008 ).

  • Here is the historic rupture map again. I include a figure caption below that I wrote as blockquote.

  • India-Australia plate subducts northeastwardly beneath the Sunda plate (part of Eurasia) at modern rates (GPS velocities are based on regional modeling of Bock et al, 2003 as plotted in Subarya et al., 2006). Historic earthquake ruptures (Bilham, 2005; Malik et al., 2011) are plotted in orange. 2004 earthquake and 2005 earthquake 5 meter slip contours are plotted in orange and green respectively (Chlieh et al., 2007, 200Smilie: 8). Bengal and Nicobar fans cover structures of the India-Australia plate in the northern part of the map. RR0705 cores are plotted as light blue. SRTM bathymetry and topography is in shaded relief and colored vs. depth/elevation (Smith and Sandwell, 1997).

  • Here is a cross section showing the differences of vertical deformation between the coseismic (during the earthquake) and interseismic (between earthquakes). This diagram was created to explain the deformation observed during the Good Friday Alaska earthquake, but these observations are observed during earthquakes at subduction zones globally.

  • This figure, from Atwater et al. (2005) shows the earthquake deformation cycle and includes the aspect that the uplift deformation of the seafloor can cause a tsunami.

  • Here is the inset figure from Chlieh et al. (2007). I include a figure caption below in blockquote.

  • Observed (black) and Predicted (red) vertical displacements associated to model Ammon-III [Ammon et al. 2005] (A, See figure 5 in the main text), model G-M9.12 (B, figure 6), model G-M9.22 (C, figure 7) and our preferred coseismic model G-M9.15 (D, figure 9 in the main text).

    Here are some pages where I present information about these SASZ earthquakes.

  • 2014.12.21 General Overview of the regional tectonics and SASZ earthquakes
  • 2014.12.25 Sumatra-Andaman subduction zone 2014/12/26: Slip, Deformation, and Energy

    Additional Static Stress Triggering!

  • The 2004/2005 SASZ earthquakes also tended to load strain in the crust in different locations. On 2012.04.11 there was a series of strike-slip earthquakes in the India plate crust to the west of the 2004/2005 earthquakes. The two largest magnitudes for these earthquakes were M 8.6 and M 8.2. The M 8.6 is the largest strike-slip earthquake ever recorded.
  • On 2016.03.22 there was another large strike-slip earthquake in the India-Australia plate. This is probably related to this entire suite of subduction zone and intraplate earthquakes. I presented an interpretive poster about this M 7.8 earthquake here. Below is my interpretive poster for the M 7.8 earthquake. Here is the USGS website for this earthquake.
  • I include a map in the upper right corner that shows the historic earthquake rupture areas.

  • Here is a poster that shows some earthquakes in the Andaman Sea. This is from my earthquake report from 2015.11.08.

  • This map shows the fracture zones in the India-Australia plate.

References:

  • Atwater, B.F., Yamaguchi, D.K., Bondevik, S., Barnhardt, W.A., Amidon, L.J., Benson, B.E., Skjerdal, G., Shulene, J.A., and Nanalyama ,F., 2001. Rapid resetting of an estuarine recorder of the 1964 Alaska earthquake in Geology, v. 113, no. 9, p. 1193-1204.
  • Bilham, R., 2005. Partial and Complete Rupture of the Indo-Andaman Plate Boundary 1847 – 2004: Seismological Research Letters, v. 76, p. 299-311.
  • Bock, Y., Prawirodirdjo, L., Genrich, J.F., Stevens, C.W., McCaffrey, R., Subarya, C., Puntodewo, S.S.O., Calais, E., 2003. Crustal motion in Indonesia from Global Positioning System measurements: Journal of Geophysical Research, v. 108, no. B8, 2367, doi: 10.1029/2001JB000324.
  • Briggs, R.W., Sieh, K., Meltzner, A.J., Natawidjaja, D., Galetzka, J., Suwargadi, B., Hsu, Y.-j., Simons, M., Hananto, N., Suprihanto, I., Prayudi, D., Avouac, J.-P., Prawirodirdjo, L., Bock, Y., 2006. Deformation and Slip Along the Sunda Megathrust in the Great 2005 Nias-Simeulue Earthquake: Science, v. 311, p. 1,897-1,901.
  • Chlieh, M., Avouac, J.-P., Hjorleifsdottir, V., Song, T.-R.A., Ji, C., Sieh, K., Sladen, A., Hebert, H., Prawirodirdjo, L., Bock, Y., Galetzka, J., 2007. Coseismic Slip and Afterslip of the Great (Mw 9.15) Sumatra-Andaman Earthquake of 2004: Bulletin of the Seismological Society of America, v. 97, no. 1A, p. S152-S173, doi: 10.1785/0120050631.
  • Chlieh, M., Avouac, J.P., Sieh, K., Natawidjaja, D.H., Galetzka, J., 2008. Heterogeneous coupling of the Sumatran megathrust constrained by geodetic and paleogeodetic measurements: Journal of Geophysical Research, v. 113, B05305, doi: 10.1029/2007JB004981.
  • Hayes, G.P., Wald, D.J., and Johnson, R.L., 2012. Slab1.0: A three-dimensional model of global subduction zone geometries in, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524
  • Ishii, M., Shearer, P.M., Houston, H., Vidale, J.E., 2005. Extent, duration and speed of the 2004 Sumatra-Andaman earthquake imaged by the Hi-Net array. Nature 435, 933.
  • Malik, J.N., Shishikura, M., Echigo, T., Ikeda, Y., Satake, K., Kayanne, H., Sawai, Y., Murty, C.V.R., Dikshit, D., 2011. Geologic evidence for two pre-2004 earthquakes during recent centuries near Port Blair, South Andaman Island, India: Geology, v. 39, p. 559-562.
  • Meltzner, A.J., Sieh, K., Chiang, H., Shen, C., Suwargadi, B.W., Natawidjaja, D.H., Philobosian, B., Briggs, R.W., Galetzka, J., 2010. Coral evidence for earthquake recurrence and an A.D. 1390–1455 cluster at the south end of the 2004 Aceh–Andaman rupture. Journal of Geophysical Research 115, 1-46.
  • Patton, J. R., Goldfinger, C., Morey, A. E., Romsos, C., Black, B., Djadjadihardja, Y., Udrekh, 2013, Seismoturbidite Record as Preserved at Core Sites at the Cascadia and Sumatra‐Andaman Subduction Zones: : The Offshore Search of Large Holocene Earthquakes: Obergurgl, Austria, Natural Hazards and Earth System Sciences, 13, p. 833‐867
  • Patton, J. R., Goldfinger, C., Morey, A. E., Ikehara, K., Romsos, C., Stoner, J., Djadjadihardja, Y., Udrekh, Ardhyastuti, S., Gaffar, E.Z., and Viscaino, A. A 6500 year earthquake history in the region of the 2004 Sumatra‐Andaman subduction zone Earthquake, Geosphere, vol. 11, no. 6, p. 1‐62, doi:10.1130/GES01066.1
  • Plafker, G., 1969. Tectonics of the March 27, 1964 Alaska earthquake: U.S. Geological Survey Professional Paper 543–I, 74 p., 2 sheets, scales 1:2,000,000 and 1:500,000, http://pubs.usgs.gov/pp/0543i/.
  • Prawirodirdjo, P., McCaffrey,R., Chadwell, D., Bock, Y, and Subarya, C., 2010. Geodetic observations of an earthquake cycle at the Sumatra subduction zone: Role of interseismic strain segmentation, JOURNAL OF GEOPHYSICAL RESEARCH, v. 115, B03414, doi:10.1029/2008JB006139
  • Rajendran, C.P., Rajendran, K., Anu, R., Earnest, A., Machado, T., Mohan, P.M., Freymueller, J., 2007. Crustal Deformation and Seismic History Associated with the 2004 Indian Ocean Earthquake: A Perspective from the Andaman–Nicobar Islands: Bulletin of The Seismological Society of America, v. 97, S174-S191, doi: 10.1785/0120050630.
  • Shearer, P., and Burgmann, R., 2010. Lessons Learned from the 2004 Sumatra-Andaman Megathrust Rupture, Annu. Rev. Earth Planet. Sci. v. 38, pp. 103–31
  • Singh, S.C., Carton, H.L., Tapponnier, P, Hananto, N.D., Chauhan, A.P.S., Hartoyo, D., Bayly, M., Moeljopranoto, S., Bunting, T., Christie, P., Lubis, H., and Martin, J., 2008. Seismic evidence for broken oceanic crust in the 2004 Sumatra earthquake epicentral region, Nature Geoscience, v. 1, pp. 5.
  • Smith, W.H.F., Sandwell, D.T., 1997. Global seafloor topography from satellite altimetry and ship depth soundings: Science, v. 277, p. 1,957-1,962.
  • Sumner, E., Siti, M., McNeil, L.C., Talling, P.J., Henstock, T., Wynn, R., Djadjadihardja, Y., Permana, H., 2013. Can turbidites be used to reconstruct a paleoearthquake record for the central Sumatran margin?: Geology, v. 41, p. 763-766.
  • Subarya, C., Chlieh, M., Prawirodirdjo, L., Avouac, J., Bock, Y., Sieh, K., Meltzner, A.J., Natawidjaja, D.H., McCaffrey, R., 2006. Plate-boundary deformation associated with the great Sumatra–Andaman earthquake: Nature, v. 440, p. 46-51.
  • Tolstoy, M., Bohnenstiehl, D.R., 2006. Hydroacoustic contributions to understanding the December 26th 2004 great Sumatra–Andaman Earthquake. Survey of Geophysics 27, 633-646.

Posted in asia, earthquake, education, geology, HSU, Indian Ocean, plate tectonics, subduction, sumatra, tsunami

Earthquake Report: Chile!

201225_chile_interpretation_19602010_thumb

Last night we had a large earthquake in the southern region of the 1960 M 9.5 Valdivia earthquake. Below are some USGS websites for the three large earthquakes for this region that I mention in my interpretive poster. Based on the hypocentral depth of ~35 km and the slab contours from Hayes et al. (2012), this appears to be a subduction zone interface earthquake. The epicenter plots between the 20 and 40 km slab contours.

Below is my interpretive poster for this earthquake.


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

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely. 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).
  • I also include the latitudinal ranges of historic earthquakes in this region (labeled in green).

    I include some inset figures in the poster.

  • In the upper left corner, I include a time-space diagram from Moernaut et al. (2010).
  • To the right of that space time figure is a plot showing the tsunami recorded at a tide gage 30-40 km to the south of the epicenter. This comes from the Intergovernmental Oceanographic Commission here. Here is the NWS National Tsunami Warning Center tsunami announcement.
  • In the upper right corner I include an inset map from the USGS Seismicity History poster for this region (Rhea et al., 2010). There is one seismicity cross section with its locations plotted on the map. The USGS plot these hypocenters along this cross section and I include that below (with the legend).
  • In the lower left corner are the MMI intensity maps for the three earthquakes listed above: 1960 M 9.5, 2010 M 8.8, and 2016 M 7.6. Note these are at different map scales.


  • This map shows the MMI contours for the 1960 and 2010 earthquakes in addition to this 2016 earthquake. This helps us visualize the spatial extent for these earthquakes with a large range of magnitudes. Recall that an M 9.5 earthquake releases about 32 times the energy that an M 8.5 earthquake releases. Note how the 1960 and 2010 earthquakes span a region between the Juan Fernandez fracture zone and where the Chile Rise intersects the trench, where the 4 fracture zones (Guamblin, Darwin, Taitao, and Tres Montes) intersect the trench.


  • Here is the USGS fault slip model. Note the short rupture length of this earthquake, proportional to the magnitude.

  • Below are some figures from Moreno et al. (2011) that show estimates of locking along the plate interface in this region. I include the figure captions as blockquote.
  • The first figure shows how the region of today’s earthquake is in an area of higher locking.

  • a) Optimal distribution of locking rate in the plate interface. Predicted interseismic velocities and GPS vectors corrected by the postseismic signals are shown by green and blue arrows, respectively. b) Tradeoff curve for a broad range of the smoothing parameter (β). The optimal value for β is 0.0095 located at the inflection of the curve.


  • a) Latitudinal distribution of the coseismic moment (Mc) released by the 1960 Valdivia (Moreno et al., 2009) (red line) and 2010 Maule (Tong et al., 2010) (blue line) earthquakes, and of accumulated deficit of moment (Md) due to interseismic locking of the plate interface 50 (orange line) and 300 (gray line) years after the 1960 earthquake, respectively. The range of errors of the Md rate is depicted by dashed lines. High rate of Md was found in the earthquake rupture boundary, where slip deficit accumulated since 1835 seems to be not completely released by the 2010 Maule earthquake. b) Schematic map showing the deformation processes that control the observed deformation in the southern Andes and the similarity between coseismic and locking patches. Blue and red contours denote the coseismic slip for the 2010 Maule (Tong et al., 2010) and 1960 Valdivia (Moreno et al., 2009) earthquakes, respectively. Patches with locking degree over 0.75 are shown by brown shaded areas. The 1960 earthquake (red star) nucleated in the segment boundary, area that appears to be highly locked at present. The 2011 Mw 7.1 aftershock (gray) may indicate that stress has been transmitted to the southern limit of the Arauco peninsula.

  • Here is the space-time diagram from Moernaut et al., 2010. I include their figure caption below in blockquote.

  • Fig.: Setting and historical earthquakes in South-Central Chile. Data derived from Barrientos (2007); Campos et al. (2002); Melnick et al.(2009)

  • Here is the cross section of the subduction zone just to the south of this Sept/Nov 2015 swarm (Melnick et al., 2006). Below I include the text from the Melnick et al. (2006) figure caption as block text.

  • (A) Seismotectonic segments, rupture zones of historical subduction earthquakes, and main tectonic features of the south-central Andean convergent margin. Earthquakes were compiled from Lomnitz (1970, 2004), Kelleher (1972), Comte et al. (1986), Cifuentes (1989), Beck et al. (1998 ), and Campos et al. (2002). Nazca plate and trench are from Bangs and Cande (1997) and Tebbens and Cande (1997). Maximum extension of glaciers is from Rabassa and Clapperton (1990). F.Z.—fracture zone. (B) Regional morphotectonic units, Quaternary faults, and location of the study area. Trench and slope have been interpreted from multibeam bathymetry and seismic-reflection profiles (Reichert et al., 2002). (C) Profile of the offshore Chile margin at ~37°S, indicated by thick stippled line on the map and based on seismic-reflection profiles SO161-24 and ENAP-017. Integrated Seismological experiment in the Southern Andes (ISSA) local network seismicity (Bohm et al., 2002) is shown by dots; focal mechanism is from Bruhn (2003). Updip limit of seismogenic coupling zone from heat-fl ow measurements (Grevemeyer et al., 2003). Basal accretion of trench sediments from sandbox models (Lohrmann, 2002; Glodny et al., 2005). Convergence parameters from Somoza (1998 ).

  • In September through November of 2015, there was a M 8.3 earthquake further to the north. Below is my interpretive poster for that earthquake and here is my report, where I discuss the relations between the 2010, 2015, and other historic earthquakes in this region. Here is my report from September.

  • Here is a space time diagram from Beck et al. (1998 ). The 2015 earthquake occurs in the region of the 1943 and 1880 earthquakes. I updated this figure to show the latitudinal extent of the 2010 and 2015 earthquakes.

References:

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

Earthquake Report: Bougainville!

20161217_bougainville_interpretation_thumb

Early this morning my time there was a large earthquake along the San Cristobal Trench, a segment of a convergent plate boundary formed by the subduction of the Solomon Sea plate eastwards beneath the Pacific plate. There was an M 6.1 earthquake in the slip patch region about a week ago that may have imparted stress across the fault in this region. The M 6.1 earthquake was very deep, as is this one. The M 6.1 appears to have been deeper than the megathrust, though the M 7.9 is much closer to the Hayes et al. (2012) fault. It seems possible that the M 6.1 loaded the fault and led to the M 7.9. Someone would need to conduct some coulomb static stress modeling to test this hypothesis. However, it would only be a model result and would likely have an unequivocal result (i.e. we still would not “know” if these earthquakes were related. after all, there has not been a movie about this. heheh).

I also include the swarm that happened about a week ago further to the south, along the South Solomon Trench. There was an M 7.8 earthquake there. Here is my Earthquake Report for this 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 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).
  • Here are the USGS web pages for the larger magnitude earthquakes. The earthquakes with plotted moment tensors are in bold. I include a link for the earthquake that happened further along this plate boundary, near the Solomon Islands. Here is a Google Earth kml file that has the earthquakes plotted in the map below.
  • I include some inset figures.

  • In the lower left corner is a generalized tectonic map of the region from Holm et al., 2015. This map shows the major plate boundary faults including the New Britain trench (NBT), one of the main culprits for recent seismicity of this region.
  • In the lower right corner is the USGS fault plane solution plotted in place on a map. This figure shows their model results with color representing the spatial variation in earthquake fault slip for this M 7.8 earthquake. This model is calibrated using seismologic observations.
  • In the upper left corner a figure from Oregon State University, which are based upon Hamilton (1979). “Tectonic microplates of the Melanesian region. Arrows show net plate motion relative to the Australian Plate.” This is from Johnson, 1976. There is a plate tectonic map and a cross section showing the subduction of the Solomon Sea plate.
  • In the upper right corner is a figure from Baldwin et al. (2012). This figure shows a series of cross sections along this convergent plate boundary from the Solomon Islands in the east to Papua New Guinea in the west. Cross section ‘C’ is the most representative for the earthquake today. I present the map and this figure again below, with their original captions.


  • There have been several observations of tsunami in the region. The likelihood for a trans Pacific tsunami as reported by the Pacific Tsunami Warning Center has passed.

  • In earlier earthquake reports, I 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).

  • Here is the generalized tectonic map of the region from Holm et al., 2015. I include the figure caption below as a blockquote.

  • Tectonic setting and mineral deposits of eastern Papua New Guinea and Solomon Islands. The modern arc setting related to formation of the mineral deposits comprises, from west to east, the West Bismarck arc, the New Britain arc, the Tabar-Lihir-Tanga-Feni Chain and the Solomon arc, associated with north-dipping subduction/underthrusting at the Ramu-Markham fault zone, New Britain trench and San Cristobal trench respectively. Arrows denote plate motion direction of the Australian and Pacific plates. Filled triangles denote active subduction. Outlined triangles denote slow or extinct subduction. NBP: North Bismarck plate; SBP: South Bismarck plate; AT: Adelbert Terrane; FT: Finisterre Terrane; RMF: Ramu-Markham fault zone; NBT: New Britain trench.

  • Here is the slab interpretation for the New Britain region from Holm and Richards, 2013. Note the tear in the slab where the New Britain and South Solomon trenches intersect. This feeds into the tectonic domains discussed in my map above and also here. 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.

  • This map shows plate velocities and euler poles for different blocks. Note the counterclockwise motion of the plate that underlies the Solomon Sea (Baldwin et al., 2012). 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). The New Britain region is in the map near the A and B 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.


Background Videos

  • Here is an educational video (from IRIS) for this part of the western Pacific. There are many plate boundaries and these margins are very active. This is a link to the embedded video below (10.4 MB mp4)
  • I put together an animation that shows the seismicity for this region from 1900-2016 for earthquakes with magnitude of M ≥ 6.5. Here is the USGS query I used to search for these data used in this animation. I show the location of the Benz et al. (2011) cross section E-E’ as a yellow line on the main map.
  • In this animation, I include some figures from the interpretive poster above. I also include a tectonic map based upon Hamilton (1979). Music is from copyright free music online and is entitled “Sub Strut.” Above the video I present a map showing all the earthquakes presented in the video.
  • Here is a link to the embedded video below (5 MB mp4)



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

Earthquake Report: Solomons!

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Yesterday there began a swarm of seismic activity along the southern Solomon trench. What began with a M 7.8 earthquake on 2016.12.08, there have been many aftershocks including a M 6.9 this morning (my time).

Below is my interpretive poster for this earthquake.


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I also plot the moment tensor and rupture slip patch for the 2007.04.01 M 8.1 subduction zone tsunamigenic 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. 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). So, the earthquake is either in the downgoing slab, or in the upper plate and a result of the seismogenic locked plate transferring the shear strain from a fracture zone in the downgoing plate to the upper plate.
  • Here are the USGS web pages for the larger magnitude earthquakes. The earthquakes with plotted moment tensors are in bold.
  • I include some inset figures.

  • In the upper right corner is a generalized tectonic map of the region from Holm et al., 2015. This map shows the major plate boundary faults including the New Britain trench (NBT), one of the main culprits for recent seismicity of this region.
  • To the left of this map is the USGS fault plane solution. This figure shows their model results with color representing the spatial variation in earthquake fault slip for this M 7.8 earthquake. This model is calibrated using seismologic observations. The MMI contours reflect the rectangular fault plane used in their model. Smaller earthquake are modeled using a point source (imagine an hypocenter as a source instead of a rectangular fault slip source). These point source MMI contours are generally more circular looking (except where they intersect topography or regions with bedrock that might amplify seismic waves).
  • Below the Holm et al. (2015) map is a map that shows the region of this subduction zone that ruptured in 2007 (USGS, 2007). This M 8.1 earthquake caused a damaging tsunami. Here is the USGS website for this earthquake.
  • In the lower left corner is another generalized tectonic map of the region from Benz et al., 201. This map shows historic seismicity for this region. Earthquake epicenters are colored to represent depth and sized to represent magnitude. There is a yellow star located approximately in the location of today’s M 7.8. This earthquake has a similar down-dip distance as the 2007.04.01 M 8.1 earthquake (the large red dot labeled 2007 near the intersection of the Woodlark Spreading Center and the South Solomon Trench). The 2007 earthquake generated a large damaging tsunami. I include the cross section that shows hypocentral depths for earthquakes in the region between the 2007 April and 2016 December earthquakes.


  • This is an image of the HSU Baby Benioff seismograph showing the M 7.8 earthquake.


  • This swarm of earthquake may be the result of a a static coulomb stress change along the subduction fault as imparted by the strike-slip earthquakes from mid-May of 2015. There is a transform plate boundary connecting the southern South Solomon Trench with the northern New Hebrides Trench. There were three earthquakes with upper M 6 magnitudes. These were left lateral strike slip faults that may have increased the stress along the subduction zone faults to the east and west of these earthquakes. This current swarm of earthquakes may be in the region of increased stress, but I have not modeled this myself (too much other stuff to do right now). There is no way to know what the state of stress along these subduction zone faults was prior to the May 2007 earthquakes, but for there to be static triggering like this, the fault would need to be at a high state of stress.
    • Here are the USGS web pages for the three largest earthquakes in this May 2007 series:
  • Here is a map that I put together. I plot the epicenters of the earthquakes, along with the moment tensors for the three largest magnitude earthquakes. I also place a transparent focal mechanism over the swarm, showing the sense of motion for this plate boundary fault. More is presented in my Earthquake Report for this May 2007 series.
  • I also note that these three largest earthquakes happen in a time order from east to west, unzipping the fault over three +- days. I label them in order (1, 2, 3) and place an orange arrow depicting this temporal relation). Very cool!

  • In addition, there was a subduction zone earthquake to the north of the current swarm. There was a M 6.0 in 2016.09.14. Here is my report for that earthquake. Below is my interpretive poster for that earthquake. This was a much smaller earthquake, but still may have contributed slightly. Coulomb modeling might help, but that would only produce estimates of stress change.

  • In 2015 November, there was a strike slip earthquake even further to the north. It seems improbable that this earthquake would have directly affected the fault to encourage rupture for the current swarm. Here is my Earthquake Report for the November earthquake.


Background Figures

  • Here is the generalized tectonic map of the region from Holm et al., 2015. I include the figure caption below as a blockquote.

  • Tectonic setting and mineral deposits of eastern Papua New Guinea and Solomon Islands. The modern arc setting related to formation of the mineral deposits comprises, from west to east, the West Bismarck arc, the New Britain arc, the Tabar-Lihir-Tanga-Feni Chain and the Solomon arc, associated with north-dipping subduction/underthrusting at the Ramu-Markham fault zone, New Britain trench and San Cristobal trench respectively. Arrows denote plate motion direction of the Australian and Pacific plates. Filled triangles denote active subduction. Outlined triangles denote slow or extinct subduction. NBP: North Bismarck plate; SBP: South Bismarck plate; AT: Adelbert Terrane; FT: Finisterre Terrane; RMF: Ramu-Markham fault zone; NBT: New Britain trench.


Background Videos

  • Here is an educational video (from IRIS) for this part of the western Pacific. There are many plate boundaries and these margins are very active. This is a link to the embedded video below (10.4 MB mp4)
  • I put together an animation that shows the seismicity for this region from 1900-2016 for earthquakes with magnitude of M ≥ 6.5. Here is the USGS query I used to search for these data used in this animation. I show the location of the Benz et al. (2011) cross section E-E’ as a yellow line on the main map.
  • In this animation, I include some figures from the interpretive poster above. I also include a tectonic map based upon Hamilton (1979). Music is from copyright free music online and is entitled “Sub Strut.” Above the video I present a map showing all the earthquakes presented in the video.
  • Here is a link to the embedded video below (5 MB mp4)

  • Here is an animation from IRIS that shows the seismic waves propagating through a national seismometer network. Here is a link to the video embedded below (10 MB mp4)
    In this region, there was a subduction zone earthquake that generated ground deformation and a tsunami on 2007.04.10. Below is some information about that earthquake and tsunami.

  • Here is a map from the USGS that shows the rupture area of the 2007 earthquake with a hashed polygon. The epicenter is shown as a red dot. The USGS preliminary analysis of the tsunami is here. I include their text as a blockquote below.

  • The M=8.1 earthquake that occurred in the Solomon Islands on April 1, 2007 (UTC), was located along the Solomon Islands subduction zone, part of the Pacific “Ring of Fire”. A subduction zone is a type of plate tectonic boundary where one plate is pulled (subducted) beneath another plate. For most subduction zones that make up the western half of the Ring of Fire, the Pacific plate is being subducted beneath local plates. In this case, however, the Pacific plate is the overriding or upper plate. There are three plates being subducted along the Solomon Islands subduction zone: the Solomon Sea plate, the Woodlark plate, and the Australian plate (see figure below). A spreading center separates the Woodlark and Australian plates. More detailed information on the plate tectonics of this region can be found in Tregoning and others (199Smilie: 8) and Bird (2003).

  • I put together this map to show how the New Britain and Solomon trenches meet. Earthquakes along the New Britain trench have principal stress aligned perpendicular to the New Britain trench and earthquakes along the Solomon trench have principal stresses aligned perpendicular to the Solomon trench due to strain partitioning in the upper plate. I provide more links and explanations about these earthquakes on this page.


Below are some Earthquake Reports for this region of the western Pacific


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