Takanobu Yokokura

Electromagnetic heterogeneity of the seismogenic region of 1962 M6.5 Northern Miyagi Earthquake, northeastern Japan

Magnetotelluric surveys were carried out to in- vestigate the electrical resistivity structure of the seismo- genic region of the 1962 Northern Miyagi Earthquake (M6.5) in Miyagi Prefecture, northeastern Japan. A 2-D inversion reveals the existence of a deep conductive zone and overly- ing re sistivezonein theuppe r crust. A positiveae romag- netic anomaly is also recognized above the resistive zone, which is interpreted as a buried Kitakami granitoid pluton. By comparison with these ismicity, wefound that themi- croearthquakes occur just above the deep conductor and in the resistive zone, and that several S-wave reflectors are just above the deep conductor. Since the S-wave reflectors sug- gest the existence of fluid beneath them, the deep conduc- tor can be interpreted as a fluid-filled zone. Moreover, we suggest that the seepage of the fluid from the conductive fluid-filled zone to the resistive granitoid pluton can become a trigger of the earthquakes.

Deep structure of the Nagamachi-Rifu fault deduced from small aperture seismic array observations

A seismic reflection survey using both explosives and vibrators was conducted in June 2001 around the Nagamachi-Rifu fault, northeastern Japan. We carried out observations of four small aperture seismic arrays in the area to reveal detailed structures of the fault. Array analysis was applied to waveform data from 15 explosives to obtain P-wave scatterer distributions in the area. The obtained P-wave scatterer distribution correlates in space with microearthquake activities and heterogeneous structures such as S-wave reflectors, a structure of caldera, and Mohorovicic discontinuity. We could also image that a sub-horizontal layer with a length of about 10 km exists in the deep extension of the Nagamachi-Rifu fault beneath the seismogenic layer.