Abstract
Large earthquakes occur along the Nankai trough, SW Japan, where the Philippine Sea plate is subducting beneath southwestern Japan arc. To understand tectonics related to the occurrence of these large earthquakes, we need to elucidate crustal and upper mantle structures of the subducting Philippine Sea plate and the overlying southwestern Japan arc. In the summer of 1999, we conducted a highly dense onshore-offshore integrated seismic experiment in the eastern part of Shikoku Island and the adjacent Nankai trough, SW Japan. Controlled seismic signals both from land explosives and air-gun sources were recorded by the land seismic stations. We obtained high signal-to-noise ratio data along the entire length of the land profile.
Analyzing these explosive and air-gun data recorded by the land seismic stations, we could obtain a detailed geometry of the subducting Philippine Sea plate and the crustal and upper mantle structure beneath eastern Shikoku Island. The signals from the air-gun source are especially useful to reveal a structure of an ocean-continental transition zone, which is not obtained using only land explosive data. The uppermost crust beneath eastern Shikoku Island is covered with a surface layer with velocities of 4.0-5.0km/s. The surface velocity shows a remarkable lateral change at the geological boundary between granitic rocks and an accretionary belt. The velocity of the uppermost crust is obtained as 5.7km/s. The island arc Moho is about 33km deep beneath northeastern Shikoku Island and the crustal thickness seems to thin toward the north. Beneath the southern edge of the land profile, the top of the subducting Philippine Sea plate is located at a depth of about 18km with a dip angle of approximately 12 degrees. The subduction angle steepens beneath this point. The top of the subducting plate can be traced to a depth of about 30km, almost parallel to the Wadati-Benioff seismic plane. This subduction angle is shallower than that beneath the Kii peninsula. Hypocenter distribution associated with the underthrusting of the Philippine Sea plate beneath eastern Shikoku Island is located beneath the subducting oceanic Moho. The seismicity underlies a contact zone between the nreanic and island-arc crusts.
