Structure of the upper part of the Philippine Sea plate estimated by later phases of upper mantle earthquakes in and around Shikoku, Japan

In the Chugoku, Kinki and Shikoku districts, southwest japan, a distinct pair of later P and S phases is frequently observed after P and S phases For earthquakes at depths less than 60 km occurring in the vicinity of the west Seto Inland Sea, caused by the subduction of the Philippine Sea (PHS) plate. The observed phases are dominant in all three components with an apparent velocity almost equal to the P and S velocity of the lower crust. If the upper mantle earthquakes take place within a low-velocity oceanic crust overlying the high-velocity pi-IS plate, and if this oceanic crust is in contact with the continental lower crust between the sources and the receivers, waves traveling through the oceanic and the lower crust can be observed as later phases. Travel times and synthetic waveforms were calculated using a three-dimensional ray tracing method and a three-dimensional Gaussian-beam method. The calculated travel times and amplitudes of the later phases are in good accordance with the observed values. These indicate that in the subducting oceanic crust, a gabbroic phase remains without transformation to eclogitic rocks to depths of about 60 km. Similar later phases are observed in the Chugoku district from the upper mantle earthquakes beneath the Shikoku district. These indicate that the continental lower crust is in contact with the subducting oceanic crust beneath Shikoku and that no mantle wedge exists. (C) 2000 Elsevier Science B.V. All rights reserved.