Utilization of scattered surface waves to image lateral discontinuities around Japan

Abstract

Japan has both a dense coverage of seismic instruments and a rich distribution of large earthquakes. Its islands have been covered completely by Hi-net. Core-reflected phases are detected all over Japan, which are overlain by scattered surface waves. By applying muting and wavenumber filtering to isolate the scattered surface waves, potential scatterer areas become visible in south Japanese arc system. By applying Kirchhoff migration method, the imaging of a more focused scatterer model is obtained on the south edge of Okinawa Island. Previous studies of Okinawa region show positive contrast in velocity, magnetic and gravity anomalies in between the Keram Gap and Okinawa Island. The outcome of calculated reflection coefficient shows such agreement with the positive contrast. The formation of the scatterer might be due to three different ex- tension fields: the extension of the backarc in northeast Okinawa Trough because of the horizontal mantel flow, the arc-parallel extension all over the Trough except for the northwest part, and the arc perpendicular extension in southeast Ryukyu Arc area.