DEEP, LOW-FREQUENCY MICROEARTHQUAKES IN OR AROUND SEISMIC LOW-VELOCITY ZONES BENEATH ACTIVE VOLCANOS IN NORTHEASTERN JAPAN

Precise hypocenter relocations have been made for shallow intraplate microearthquakes beneath northeastern Japan by applying source-region station corrections. The relocated hypocenter distribution shows that most of the events are confined to the upper 15 km of the crust which forms the brittle seismogenic zone in this volcanic arc. It also shows that exceptionally deep microearthquakes, well below the base of the seismic-aseismic boundary, occur at twelve locations of northeastern Japan. They occur in the lowermost crust or in the uppermost mantle, where rheological properties of rocks are presumed to be in the ductile regime. All the 153 events presently detected in the lowermost crust or in the uppermost mantle have anomalously low predominant frequencies (1-5.5 Hz) both for P and S waves, in contrast to those (8-20 Hz) -of normal shallow events in the brittle seismogenic zone. The magnitudes of these events are small (M less-than-or-equal-to 2.2). Moreover, all of these events are located beneath active volcanoes and/or near P wave low-velocity zones in the uppermost mantle. The proximity of these events' to the active volcanoes and other anomalous features suggest that these deep low-frequency events are generated by magmatic activity of mantle diapirs in the uppermost mantle.