P-wave velocity structure in the southernmost source region of the 2011 Tohoku earthquakes, off the Boso Peninsula deduced by an ocean bottom seismometer survey

We conducted a seismic experiment using ocean bottom seismometers and controlled sources in the region off Ibaraki and Boso-peninsula in 2008. This region is the southern edge of the rupture zone of the 2011 off the Pacific coast of Tohoku Earthquake. We modeled P-wave seismic velocity structure beneath the profile using 2-D ray-tracing method. The characteristics of the seismic velocity structure as follows. (1) The crustal structure in southern part is more heterogeneous than that in the northern part. (2) The northward dipping structure presents the subduction of the Philippine Sea plate in the southern part. (3) The subducting oceanic crust of the Pacific plate is in contact with the island arc crust at the depth of about 17 km depth in northern region and the depth of 20 km in a distance of 170km from the southern end of the profile. The geometry of the top of PAC changes at a distance of about 200km of the profile. (4) The contact zone of Philippine Sea plate and Pacific plate is not imaged clearly. It seems that the contact zone is characterized by strong heterogeneity result from large deformation. The southern end of the seismic active region of the aftershocks and the co-seismic slip of the 2011 Tohoku earthquake correspond to the contact region of the Philippine Sea plate and the Pacific plate. The large afterslip region coincides with the collision region of the two plates. The termination of the mainshock rupture and the large afterslip in the collision region are thought to be caused by the collision of two oceanic plates.