Aftershocks near the updip end of the 2011 Tohoku-Oki earthquake

The March 2011 M w 9.0 Tohoku-Oki earthquake activated shallow aftershocks with normal faulting focal mechanisms near the trench axis. To investigate stress state in the shallow subduction zone near the trench axis, we used ocean bottom seismographs (OBSs) to record earthquakes from August to October 2011. We estimated hypocenter locations and focal mechanisms of the recorded earthquakes by using a grid-search method in a 2-D velocity model based on an active seismic survey. The results show that most of the earthquakes occurred within both the overriding and subducting plates, with very few on the plate interface and none in the most seaward 45 km of the overriding plate (aseismic wedge). The low seismicity along the plate interface is consistent with the nearly complete stress drop on the megathrust fault in the M w 9.0 earthquake. Focal mechanisms indicate that trench-normal tension is pre-dominant in both plates and extends to a depth of about 30 km at least. On the other hand, several trench-normal compressional earthquakes were located at the landward end of the aseismic wedge. These earthquakes suggest horizontal compression of the overriding prism that may be caused by a strengthening of the shallowest part of the megathrust.