Electric and magnetic anomalies at the Atotsugawa fault and their implications for fault activity.

Abstract

Observations of the geomagnetic total intensity, electric self-potential and surface resistivity were carried out at the Atotsugawa fault, a 60km long active fault of strike-slip type located in central Japan, in order to investigate the active fault structure and fault activity from electric and magnetic aspects. Notable anomalies were found along a measurement line selected in the central segment of the Atotsugawa fault. The anomalies are summarized as follows. The surface resistivity, as derived from VLF-MT, of the zone bounded on the north by one fault line and on the south by another is lower than that outside the zone. A very local anomaly in self-potential exists at the northern fault line which constitutes the northern boundary of the low resistivity zone. Inside this low resistivity zone exists a somewhat more resistive body. It should be highly magnetized as deduced from a magnetic anomaly amounting to about 600nT which was observed over the resistive body. The low resistivity zone seems to correspond to the fractured zone bounded by the two fault lines, and the northern one would be more active as implied by the self-potential anomaly which can be interpreted in terms of the electrokinetic effect associated with groundwater flow along the fault plane.