Spatial distribution of earthquakes around the southern end of the co-seismic slip of the 2011 Tohoku Earthquake

The 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0) is thought to be ruptured 200km wide and 500 km long segment of the subduction fault along the Japan Trench. Many aftershocks occurred following the mainshock in the southern part of Japan Trench. To obtain a precise aftershock activity is important for understanding the mechanism of earthquake generation, and the recovery of plate coupling at a ruptured plate boundary. However, it is difficult to determine the precise aftershock distribution by land seismic station data. In order to study the aftershock activity, we had deployed 66 long-term ocean bottom seismometers(LTOBS) off the coast of Ibaraki and the Boso Peninsula from October 2011 to November 2012. Most of the hypocenter locations have a depth shallower than 40km. The earthquakes form a plane dipping landward in the study area. Comparing the hypocenter locations with crustal structures obtained by active seismic studies (e.g. Miura et al., 2003). Many events occurred along the plate boundary. We also compared the hypocenter locations with aftershock distribution of the seismic observation conducted immediately after 2011 Tohoku Earthquake (Shinohara et al., 2012). Shinohara et al., (2012) reported that the low seismicity region has seen at the shallow part of the plate interface in the off-Fukushima. On the other hand, our results showed the seismicity is not low at the same region. This difference may reflect the change of stress fields at a ruptured plate boundary.