Abstract

A huge plate boundary earthquake eventuated between the Pacific plate and the landward plate in 2011 and was named the 2011 off the Pacific coast of Tohoku earthquake. Following the mainshock, many aftershocks were generated. It is essential to obtain spatiotemporal variation of aftershock activity for understanding of mechanism of the earthquake generation. Because extensive seafloor aftershock observations using many ocean bottom seismometers (OBSs) were performed following the mainshock until September 2011, a precise aftershock distribution just after the mainshock had been revealed. After the urgent OBS aftershock observation, we started seismic monitoring using long-term OBSs (LTOBSs) in the source area. Thirty-nine LTOBSs were deployed in September 2011, and the observation had been continued for approximately ten months. In September 2013, thirty LTOBSs were deployed in the northern source region and recovered one year later. From long-term observations, spatiotemporal variation of the aftershock activity in the northern source region of the mainshock was revealed. Because we had carried out a LTOBS observation in the study area before the mainshock, the seismicity before the mainshock and aftershock activities was compared. Source positions of the events were determined by P and S-wave arrival times, and focal mechanisms of them were obtained using polarities of vertical components of the first arrivals. Large slip areas have little seismicity during the whole period of the observations. It is deduced from a low seismicity area that the stress in the large slip region had not been recovered 3 years after the mainshock. In contrast, a seismic activity become high just after the mainshock comparing the seismicity before the mainshock in the region off Iwate prefecture. Although many earthquakes in the inside of the plates had normal and strike-slip focal mechanisms from 2011 to 2012, some thrust focal mechanism events also occurred. A ratio of earthquakes occurring near the plate boundary with thrust focal mechanism seems to increase in 2013 and 2014 compared to 2011 and 2012. We interpret the increase in thrust-type earthquakes as indicative of stress change starting at the edge of the large slip region of the mainshock.