A Microearthquake Study of Area the Fracture Structure in the Takinoue Geothermal

Abstract

This paper describes promising melhods for analyzing microearthquake data which polentially can resolve underground fraclure structures to accuracies of the order of ten meters. We have been observing microearthquake activity of microearthquakes in the Takinoue geothermal area in the vicinity of the Kakkonda geothermal power plant. Iwate PrefeCture. Norlheast Japan (Figure 1) since December 1982. In this area. high temperature fluid rises through fractures which penetrate a hot hody of intrusive rock (lde, 1985). The shape of the Matsuzawa inlrusive rock formation was recently established primari Iy by core sample analysiS (Doi et al., 1988) (Figure 2).

We delermined hypocenter locations of numerous microearthquakes recorded by eight permanent stations and seven temporal stalions (Figure 2) during a pressure build-up test which took place in June 1985. Many microearthquake hypocenters were located within the Matsuzawa intrusive rock formation (Figure 2). These microearthquakes (hereafter designated "Matsuzawa microearlhquakes"' are presumably associated with fractures within the intrusive rock.