Aftershock activity of the 2011 off the Pacific coast of Tohoku earthquake from ocean bottom seismometer network observation

The 2011 off the Pacific coast of Tohoku earthquake (M=9.0) occurred offshore of northeast Japan region on March 11, 2011. The source region of this earthquake is considered to spread over a large region with a width of 500 km. Judging from its source mechanism, the mainshock is estimated to be located at the plate boundary between the subducting Pacific plate and landward plate. To obtain a precise aftershock distribution is important for understanding of mechanism of the earthquake generation. This kind of information is also useful for studies of earthquake prediction in other trench areas. In order to study the aftershock activity of this event, we totally deployed more than seventy ocean bottom seismometers (OBSs) in the source region of the main shock. The observation area is 500 km x 200 km and has a high aftershock activity. The spatial interval of OBS is approximately 25 km to cover the whole source region. We use various types of the OBS system. Most of OBSs has three-component seismometer with natural frequency of 4.5 Hz and others use broadband seismic sensor or accelerometer. Four days after the mainshock, deployment of OBSs was started, and had been completed in end of March using two research ships. The deployed OBSs were recovered from the mid-April to May, and new OBSs were deployed at the same position to continue the observation. The second deployed OBSs have been recovered after approximately one-month observation. In south of the source region, thirty-four long-term OBSs (LT-OBSs) had been deployed before the occurrence of the mainshock, and we recovered three LT-OBSs to clarify the depth distribution of aftershocks. Using the data of the OBSs, 99 aftershocks were located. Most of the aftershocks were located in a depth range of 5 - 30 km and concentrate in the plate boundary region. In addition, aftershocks occurred within the subducting oceanic crust and the 6.2-km/s layer of the landward plate. No aftershocks were found in the mantle of the subducting plate. From the results of a previous seismic survey using OBSs and controlled sources, the subducting Philippine Sea plate is estimated to be in contact with the subducting Pacific plate. The southern end of the seismic activity region of the aftershocks corresponds to the contact region of two subducting plate. We infer that the rupture of the mainshock sequence was terminated at the oceanic plate contact region. From the first-term OBS data, we obtained aftershock distribution in the northern source region. One hundred and seventy two aftershocks from March 17 to April 15, 2011 were located. Most of the aftershocks have a depth shallower than 60 km and the aftershocks generally form a plane dipping to the land. However, a high aftershock activity is also observed in the subducting oceanic crust and the overriding landward plate. Compared to the slip distribution of the mainshock, there is a low seismicity along the plate boundary where large slip is estimated.