3-DIMENSIONAL Q STRUCTURE BENEATH THE KANTO-TOKAI DISTRICT, JAPAN

The Q structure of the upper mantle beneath the Kanto-Tokai district, central Japan, where the Eurasian, Pacific and Philippine Sea plates are converging, is investigated from seismic P- and S-wave spectral ratios by means of a three-dimensional (3-D) inversion method. It is found that the subducting Philippine Sea and Pacific plates are characterized by high-Q zones. A high-Q zone also exists between the Philippine Sea plate and the Pacific plate, to the east of the volcanic front. On the other hand, a low-Q zone in the uppermost mantle extends westward horizontally from the volcanic front under the Philippine Sea plate, and appears to exist over the wedge mantle beneath the western Tokai district. A very low Q value obtained here is roughly equal to the average Q value in the low-velocity layer derived from surface wave and free oscillation data. A low-Q region that suggests a magma chamber under active volcanoes is not detected in the crust.