Characteristics Of Earthquakes Observed At Yanaizu-Nishiyama Geothermal Field, Japan

A 65,000 kW geothermal power plant is under operation at Yanaizu-Nishiyama, Fukushima, Japan since 1995. Seismic activity around the site was observed in the exploration and construction phase of this plant, and, hence, seismic monitoring network which consists of 5 near-surface stations was deployed in 1988. Most of the earthquakes had hypocenters around a fault, which has been considered as a production zone, at a depth of 2 km, although the hypocenters and production zone are not fully overlapped. An earthquake with a moment magnitude of 4.9 occurred in November, 2009. The hypocenter of the earthquake was around the center of the cloud of hypocenters of previously occurred microearthquakes and the estimated fault plane solution by JMA showed that a fault with a NW-SE azimuth and a dip around 45 degree had a slip in normal fault mode. The seismic activity was low prior to the mainshock, and the hypocenters of the aftershocks showed a re-activation of branching seismic structure extending to southeast side of the cloud of the hypocenters. We have investigated relationship between production/injection rate, spatio-temporal distribution of the hypocenters, and magnitude of the earthquakes, and have found that seismic activity and magnitude is not clearly correlated to the human operation to the reservoir in the history of the operation of the power plant. Multiplets, which are clusters of events with close similarity because of common source mechanism and earth transfer function, were identified, and the time series of occurrence, hypocentral location, and orientation of the multiplet seismic structure were investigated. The origin time of the multiplet events distributed randomly with a little correlation to the production rate from the reservoir. The seismic structure of the multiplet showed that there are three trends in the orientation of the structures, suggesting that they are correlating to the orientation of the fracture system inside the seismically active zone.