A new configuration and an aseismic slab of the descending Philippine Sea plate revealed by seismic tomography

We obtained the three-dimensional seismic velocity structure of the uppermo st mantle beneath the Kanto, Tokai and Chubu regions, central Japan, where the Philippine Sea plate descends, by means of travel time tomography using 18 years of P phase data recorded by a regional seismic network. A distinc t low velocity zone, which was the oceanic crust of the descending Philippi ne Sea plate, was found beneath the Kanto region. The low velocity zone rev ealed a new configuration of the Philippine Sea plate different from any mo dels obtained through the analysis of hypocenter distribution and focal mec hanisms. We also found a high velocity area in the deep aseismic region bel ow the volcanic front between the Kanto and Chubu regions. It extends to th e Philippine Sea plate in both regions. It was likely to be an aseismic ext ended section of the Philippine Sea plate. The existence of the aseismic Ph ilippine Sea plate beneath the Chubu region was confirmed. There was anothe r low velocity area beneath the active volcanoes. High Poisson's ratio area s were found both beneath the volcanoes and in the low velocity zone of the Philippine Sea plate near the volcanic front under the Kanto region. (C) 2 001 Elsevier Science B.V. All rights reserved.