Seismic Tomography Research In Kyushu, Japan

In recent years, seismic tomography method, as a powerful tool for studying the Earth interior structure, has been applied extensively. By using the seismic tomography method developed by Zhao et al. [Zhao et al., 1992], 3-D velocity structure under the Kyushu area was investigated. We determine detailed 3-D Vp and Vs structures of the crust and uppermost mantle beneath the Kyushu Island, southwest Japan, using a large number of arrival times from local earthquakes. From the obtained Vp and Vs models, we further calculate Poisson's ratio images beneath the study area. By using this large data set, we successfully image the 3-D seismic velocity and Poisson's ratio structures beneath Kyushu down to a depth of 150 km with a higher spatial resolution than the previous studies. Our tomography results show very clearly high-velocity anomalies corresponding to the subducting Philippine Sea slab and low-velocity anomalies in the mantle wedge. Strong low-velocity anomalies are distributed extensively along the volcanic front and extend to the back-arc side in the crust and upper mantle. There are two causes of the Unzen volcanism: the first is fluids supplied by the dehydration processes of the descending Philippine Sea slab; the second is the hot upwelling materials related to the opening of the Okinawa Trough. The crustal seismicity is distributed in both the high- and low-velocity zones, but most distinctly in the low Poisson's ratio zone. Under the forearc region of the Kyushu subduction zone, prominent low-velocity anomalies are visible along the upper boundary of the subducting Philippine Sea slab and large interplate earthquakes are located around the low-velocity zones, which is similar to the features revealed in the Northeast Japan subduction zone.