SEISMIC-WAVES GUIDED BY UNTRANSFORMED OCEANIC-CRUST SUBDUCTING INTO THE MANTLE - THE CASE OF THE KANTO DISTRICT, CENTRAL JAPAN

A model of the subduction of oceanic crust is established through an analysis of the upper mantle earthquakes occurring beneath the Kanto district, central Japan. A distinct pair of later P and S phases (Xp and Xs commonly observed at specific stations for these events in a depth range of 40-60 km, and is considered to take place in association with the subducting motion of the Philippine Sea plate. The Xp phase is found, from its particle motions, to be a longitudinal seismic wave. A principal component analysis of wavelets including the initial P and the Xp phases shows that the latter arrives at the Earth's surface with a slightly shallower angle relative to the former. Close agreement between the relative arrival time ratio of (Xs - S)/(Xp - P) and the Vp /Vs ratio of crustal or mantle rocks suggests that both Xp and Xs travel along the same path as longitudinal and tangential waves, respectively. One can clearly observe these later phases at stations located where the upper surface of the Philippine Sea plate is apparently in direct contact with the bottom of the crust of the Honshu island. Apparent velocities of the Xp and the Xs; phases are comparable to the seismic velocities of the main layer of the oceanic crust or the lower continental crust, which are never expected for an upper mantle earthquake if the velocity structure is laterally homogeneous. Travel-time curves for several earthquakes indicate that these apparent velocities can be directly related to the seismic velocity in the source region. The hypocentral distribution of the events related to the subducting motion of the Philippine Sea plate does not form such a thin seismic zone as observed in southwest Japan but extends over the entire thickness of the Philippine Sea plate. The earthquakes from which the later phases are observed occur only in a shallower portion of this thick and contorted seismic zone. The deeper portion does not generate such events. These characteristics are best explained if these later phases are regarded as the seismic waves guided by the subducted portion of the gabbroic oceanic crust at the top of the Philippine Sea plate. Taking the result of our earlier work into account, we conclude that the subducting oceanic crust remains in a gabbroic phase without transformation to eclogitic rocks at depths down to about 60 km along the entire northern boundary of the Philippine Sea plate. © 1990.