Seismicity in the incoming/subducting Philippine Sea plate off the Kii Peninsula, central Nankai trough

摘要

Along the Nankai trough, southwestern Japan, the Philippine Sea plate (PHS) is subducting beneath the Eurasian plate, and large interplate earthquakes have occurred repeatedly. The latest large thrust earthquake at the central Nankai trough off the Kii Peninsula was the 1944 Tonankai earthquake (M w = 8.2). Although a previous ocean bottom seismograph (OBS) experiment from November 2001 to February 2002 showed seismicity around the rupture area of the 1944 Tonankai earthquake, there were large uncertainties regarding hypocenter locations, especially with respect to depth. We conducted another OBS experiment to obtain accurate locations of earthquakes and their focal mechanisms. The new results show earthquake clusters at the trough axis within the oceanic crust. The clusters were located near faults cutting the oceanic crust, and thus the earthquakes may have occurred along preexisting faults in the incoming PHS. Seismicity in the subducting PHS is characterized by seismic quiescence in the oceanic crust of the coseismic rupture area of the 1944 Tonankai earthquake. The change in seismicity in the oceanic crust can be explained by a change in the stress in the incoming, subducting PHS. The stress in the oceanic crust of the PHS changes from trough-normal tension at the trough axis, caused by the bending of the PHS, to downdip compression beneath the Kii Peninsula. The seismicity in the subduction zone likely relates to the balance of stress in the incoming and the subducting plate as well as to coupling between the subducting and overriding plates.