Microseismicity from a hydrothermal reservoir has been monitored by a geothermal operation company, Okuaizu Geothermal Co., Ltd. (OAG) at Yanaizu-Nishiyama geothermal area, where one of the largest geothermal plants in Japan has been under operation since 1995. A number of microseismic events, including four felt earthquakes with a magnitude M JMA larger than 3.0 (estimated by Japan Meteorological Agency: JMA), have been detected and recorded by the network.
Authors in Tohoku University have analyzed a total number of 14,000 microseismic events which occurred in this field in a period of 1996 to 2012 to investigate relation between production/injection of geothermal fluids and microseismicity using analysis techniques of microseismicity which include high resolution hypocenter determination by multiplet clustering and double difference (DD). We have found that there is no clear relation between the activity of microseismicity and the change in the production rate, although increased seismic activity associated with injection around at the bottom of an injection well was observed. It has been also revealed that the four felt earthquakes had closely similar fault plane solutions (FPSs), which have inclinations around 45° to NE, even the orientations of the multiplet seismic structures showed various azimuths and inclinations, suggesting that most of the accumulated strain energy within the seismically active zone was released from particular weak structure as felt earthquakes.