Lateral inhomogeneity deduced from 3-D magnetotelluric modeling around the hypocentral area of the 1984 Western Nagano Prefecture earthquake, central Japan

摘要

Earthquake swarms have been observed since 1976 in the southeastern region of Mt. Ontake, where the 1984 Western Nagano Prefecture earthquake (M6.8) occurred. Most earthquake swarm activities show a dense distribution in a narrow area and at shallow depths. We have carried out forward 3-D magnetotelluric modeling for the data set observed around the focal region of the earthquake swarms. An initial 3-D image was constructed based on previous 2-D magnetotelluric analysis results. The resistivity structure shows remarkable lateral inhomogeneity: a low resistivity zone is located at shallow depth around the center of the study area and the low resistivity zone is surrounded by a high resistive structure and its shape is about a 4-km cube. This feature agrees well with the distribution of the degree of water saturation deduced from seismic information. Hypocenters of recent seismicity are distributed around an area between the resistivity boundary and the relatively high-resistance region. This fact strongly suggests that earthquakes in the study area are triggered by fluid (water) in the crust.