Seismic tomography of the uppermost mantle beneath southwestern Japan: Seismological constraints on modelling subduction and magmatism for the Philippine Sea slab

摘要

Many studies have been made on the subduction of the Pacific slab and the magmatism in northeastern Japan, but not on the subduction of the Philippine Sea slab and the magmatism in southwestern Japan. Primary reasons may be that seismological networks in southwestern Japan were sparse as compared with those in northeastern Japan and that geology including volcanism of southwestern Japan is more complicated than that of northeastern Japan. However, recent instrumental development of dense seismological networks in the Japanese Islands has provided us with high quality data not only for northeastern Japan but for southwestern Japan. One of the outcomes from the development is the increase of accuracy of arrival time readings of P- and S-waves and resultant hypocenter determination. We attempt to elucidate fine image of the uppermost mantle structure beneath the Japanese Islands and to find evidence for the relation between the magmatism and subduction process. We apply travel time tomography to 216,247 P- and 98,207 S-wave arrival times observed at 1,328 seismic stations from 5,242 earthquakes in and around the Japanese Islands, and obtain three-dimensional variations of P- and S-wave velocity structure. In Chubu and Kyushu, the subducting Philippine Sea slab bends downward in the depth range of 50 to 70 km. In some nonvolcanic regions, remarkable anomalies of high Poisson's ratio (and low S-wave velocity) are seen in the depth range of 25 to 40 km near the upper boundary of the Philippine Sea slab or the Moho discontinuity, and approximately coincide with the hypocenter distribution of deep low-frequency earthquakes. The anomalies of high Poisson's ratio are also seen near the upper boundary of the Philippine Sea slab or the overlying mantle wedge down to a depth of about 60 km, but are not seen after the downward bending of the slab, in the forearc region. The anomalies are probably caused by separated fluid or hydrous minerals. These characteristics should be taken into account in numerical modelling of the subduction of young slabs (e.g., Philippine Sea slab) and associated phenomena (e.g., magmatism).