A low velocity zone beneath the Hida Mountains derived from dense array observation and tomographic method

Seismic waves suffer strong attenuation when propagating beneath the Hida M ountains in Central Honshu, Japan. In order to study this region in detail, we conducted three kinds of dense seismic array observations in and around the Hida Mountains in the summer of 1996. Picking P- and S-wave arrival ti me data from 54 events at 101 stations, 3175 P- and 2335 S-wave arrival tim e data were obtained for our tomographic study. Hypocenter locations and ve locity structure were determined simultaneously. We assessed ray coverage a nd resolution of the velocity structure with checkerboard resolution tests. Ray paths for the model of the obtained velocity structure were examined i n detail. There are two zones of low P-wave velocity, one at a depth of 4 k m and the other at 15 km. The resolution is good at depths of 0-20 km for P -wave velocities and at depths of 0-15 km for S-wave velocities. A high V-P /V-S ratio (2.7) indicates that a partially melting rock exists beneath the Hida Mountains. The deep low velocity zone is located just above the upper /lower crustal boundary. These observations indicate that a magma reservoir exists in the upper crust beneath the Hida Mountains.