Low electrical resistivity along an active fault, the Yamasaki fault.

Abstract

The Yamasaki fault is a transcurrent fault extending over a distance of 80km in the western part of Japan along which microearthquakes occur densely. Electromagnetic investigation has been conducted in this area. The results of electrical resistivity measurements conducted during the period from 1975 to 1978 are presented in this paper.Resistivity measurements were made by several different methods, such as direct current methods and electromagnetic induction techniques. It has been revealed by the direct current methods that there is a belt along the fault in which the electrical resistivity is low, ranging from 100 to 400ohm-m, whereas the resistivity is higher than 10, 000ohm-m far outside. This belt is about 1km wide at some places. Inside this low resistivity belt, there run streaks of extremely low resistivity, such as 20ohm-m. The locations of these streaks correspond to the surface traces of the fault plane. Magnetotelluric investigations for a frequency range of 8 to 20Hz have indicated that the low resistive belt is circumscribed by a zone with intermediate resistivity of 1, 000ohm-m. Outside the area, on the other hand, the resistivity is higher than 10, 000ohm-m. From the investigations so far conducted, it may be said that the low resistive zone has a three-fold structure consisting of an inner low resistive belt of 100 to 400ohm-m, the outer zone of 1, 000ohm-m, and narrow streaks of very low (20ohm-m) resistivity inside the inner belt.The low resistive zone is suggestive of existence of a fractured zone with high water content along the fault. The difference between the low and the relatively low resistivity zone might arise from difference in degree of fracture and degree of water content. On the other hand, extremely low resistivity streaks perhaps indicate the surface traces of the fault plane along which clay mineral saturated with water has been developed.