1999 年 44 巻 2 号 p. 83-91
We have detected a clear reflected S-wave (SxS) in seismograms of an earthquake swarm in the crust beneath Osoresan volcano in northern Tohoku Disthct. The location of S-wave reflector is estimated from travel time differences between SxS and S-wave. It is located within a previously known low-velocity zone of P-wave in the uppermost mantle beneath the volcano. The reflector has a size of at least 6 km×8 km and is distributed in the depth range of 44-48 km. It is inclined to the south-southwest, with a steep dip angle of about 40°. The large reflection coefficient of the reflector and its close proximity to the P-wave low-velocity zone beneath Osoresan volcano strongly suggest that the body of reflector is filled with molten materials. We infer that the low-velocity zone beneath the volcano, i.e., the root of the volcano, consists of magma bodies of this sort, one of which we have detected presently.