Seismic Activities and Tectonics in and around Kyushu and the Northern Half of Nansei Shoto

Abstract

We reviewd characteristics of seismic activities in and around Kyushu and the northern half of Nansei Shoto (Ryukyu Islands), mainly on the basis of hypocenter distributions and focal mechanisms. Interplate and intraplate earthquakes, inclusive of intermediate-depth events, considerably differ in seismicity from region to region along the arc. Most of the boundaries between the seismic regions correspond to left-lateral tectonic lines geologically inferred and/ or narrow zones of crustal earthquakes of left-lateral strike slip types across the arc. Thus we surmise that the Philippine sea plate spiitted into several segments subducts with low coupling under the Kyushu-Ryukyu arc.
Not only rifting along the Okinawa trough but also southeastward spreading from the Shanshi graben with left-lateral movements is probably important for crustal earthquakes of normal and strike-slip fault types dominant in an extensional stress field in the marginal sea and inland areas.