Earthquake Report: Japan!

While I was returning from my research cruise offshore of New Zealand, there was an earthquake offshore of Japan in the region of the 2011.01.11 M 9.0 Tohoku-Oki Earthquake. Japan is one of the most seismically active regions on Earth. Below is a series of earthquake reports for the region of Japan. Here is the USGS website for this M 6.9 earthquake.

Here is my interpretive poster for the extensional earthquake that is in the upper North America plate. This earthquake has a shallow depth and produced a small tsunami run-up. I include two versions: (1) the first one has seismicity from the past 30 days and (2) the second one includes earthquakes with magnitudes M ≥ 5.5. The second map is useful to view the aftershock region of the 2011.03.11 M 9.0 earthquake. The M 9.0 Tohoku-Oki Earthquake was a subduction zone earthquake, while this M 6.9 earthquake is a shallow depth extensional earthquake. I label the location of the 1944 Tonanki and 1946 Nankai subduction zone earthquakes (both M 8.1). These earthquakes spawned decades of research that continues until this day. I discuss the recurrence of earthquakes in this region of Japan in my earthquake report here.

I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.

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

I include the slab contours plotted (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.

    Inset Figures

    I include some inset figures. Here is some information about them. Below I include the original figures with the figure captions as blockquotes.

  • In the upper right corner is a map showing the tectonics of the region (Kurikami et al., 2009). I include this map below.
  • In the lower right corner is a figure from the USGS that shows seismicity along the subduction zone that forms the Japan trench.
  • To the left of the cross section shows a low angle oblique view of the plate configuration in this region (from AGU).
  • In the upper left corner is a comparison of the USGS “Did You Feel It?” report maps. The map on the right is from the M 9.0 Tohoku-Oki earthquake and the map on the left is from this M 6.9 earthquake.




  • Here is the upper figure showing the tectonic setting (Kurikami et al., 2009). I include their figure caption as a blockquote.


    • Active faults in southwest Japan from the Active Fault Research Centre’s active fault database (http://www.aist.go.jp/RIODB/activefault/cgi-bin/index.cgi). The faults are color coded by sense of movement (green = dextral; blue = normal, red = reverse, yellow = sinistral).

  • Here is another figure showing the tectonic setting (Kurikami et al., 2009). I include their figure caption as a blockquote.


    • Current tectonic situation of Japan and key tectonic features.

  • The upper slope of the accretionary prism for this part of the subduction zone that forms the Japan trench has well developed normal faults. Tsuji et al. (2013) present seismic reflection profiles that for this region. I present their figure and include their figure citation below as a blockquote. The first figure is a map showing the locations of the cross sections and the locations of sites with direct observations of sea floor surface displacements (surface ruptures).


    • Index maps for the 2011 Tohoku-oki earthquake in the Japan Trench (JCG, JAMSTEC, 2011). (a) Blue and white contour lines are subsidence and uplift, respectively, estimated from tsunami inversion (Fujii et al., 2011), with contour intervals of 0.5 m (subsidence) and 1.0 m (uplift).Blue arrows indicate dynamic seafloor displacements observed at seafloor observatories (Kido et al., 2011; Sato et al., 2011). Red lines are locations of seismic profiles (SR101, MY101, and MY102) shown in Fig. 2. Stars indicate diving sites and are labeled with dive numbers of pre-earthquake observations (blue numerals) and post-earthquake observations in 2011 (red numerals) and in 2012 (orange numerals). Background heatflow values measured before the 2011 earthquake are displayed as colored dots (Yamano et al.,2008; Kimura et al., 2012). (b) Enlarged map around the diving sites, corresponding to the yellow rectangle in panel (a). Red dashed lines indicate seafloor traces of normal faults (i.e.,ridge structures). Yellow dashed lines indicate estimated locations of the backstop interface. The white dashed line indicates the boundary of the area of significant seafloor uplift (49 m uplift)and also the tsunami generation area (Fujii et al., 011), corresponding to the reddish-brown area in panel (a). Observations made during the post-earthquake dives are described in panel(b).


    Reflection seismic profiles obtained in the central part of tsunami source area(line MY102 in panels f–h), at its northern edge (line MY101 in panels c–e), and its outside (line SR101 in panels a,b). Original profiles of (a) line SR101, (c) line MY101, and (f) line MY102. Composite seismic reflection profiles with geological interpretations of(b) line SR101,(d) line MY101, and (g) line MY102 (Tsuji et al.,2011). Red arrows in panel (d) and (g) indicate seafloor displacements (Ito et al.,2011; Kido et al.,2011; Sato et al.,2011). Enlarged profiles around (e) Site 2W on line MY101, and (h) Site 3W on line MY102.

  • Here is a figure from Tsuji et al. (2013) that shows some images of the seafloor. These show views of ruptured sea floor.


    • (a) Diving tracks on seafloor bathymetry at Site 2W. Stars indicate locations of seafloor photographs displayed in panels (b)–(f). (b) Photograph of an open fissure representative of those commonly observed after the earthquake. (d) An open fissure was observed during post-earthquake observations where (c) no fissure had been before the earthquake.(g,h) Photographs taken in (g) 2011 and (h) 2012 showing the heat flow measurements being made at the same location by SAHF probe.


    (a) Diving tracks on seafloor bathymetry at Site 1E. The white dashed line indicates the location of the interpreted fault. Stars indicate locations of seafloor images displayed in panels(b)–(f).(b) Photograph of an open fissure representative of those commonly observed after the earthquake. (d) Open fissure seen during post-earthquake observations where (c) a clam colony (1 m wide) was observed before the earthquake. (e,f) Photographs taken in (e) 2011 and (f) 2012,showing the heatflow measurements at the same location by SAHF probe. (g) Dive track on seafloor bathymetry at Site 3E. The star indicates the location of (h) a seafloor photograph showing a steep cliff.

  • Here is an explanation for the extension generated during the 2011 earthquake.


    • Schematic images of coseismic fault ruptures and the tsunami generation model (a) at the northern edge (and outside) and (b) in the central part of the tsunami source area. Soft slope sediments covering the continental crust are not shown in these images. (a) Collapse of the continental framework occurred mainly at the backstop interface north of the large tsunami source area. (b) Anelastic deformation around the normal fault allowed large extension of the overriding plate in the tsunami source area.

  • These are some observations posted by the Pacific Tsunami Warning Center.


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