The occurrence of induced seismicity in the form of felt earthquakes has been recognized as one of the critical environmental burdens associated with recent geothermal development. For this reason, research into enabling technologies for risk assessment of larger magnitude induced events has taken on an increased priority. In this study, we have applied a seismostatistical modeling method to microseismic data collected at two geothermal fields and have investigated its feasibility for risk assessment. Here we have applied the Epidemic Type Aftershock Sequence (ETAS) model (Ogata, 1988), which has been widely used in the area of natural seismology, as a first step in this geothermal earthquake risk assessment study. The microseismicity observed at Yanaizu-Nishiyama, one of the largest hydrothermal geothermal fields in Japan, has been successfully modeled with ETAS. Induced seismicity at the Basel EGS site in Switzerland was also modeled with ETAS, in this case using short moving time windows since modeling for the whole seismically active period was unsuccessful. Our study suggests that one of the parameters in the ETAS model, which has previously been interpreted to represent the occurrence rate of events triggered by external forcing (Hainzl and Ogata, 2005), can in both our cases be correlated with the treatment/stimulation injection rate into each reservoir. Our preliminary results demonstrate the feasibility of seismostatistical modeling in assessing the risks of felt earthquakes associated with various human operations in geothermal reservoirs, although further investigation and modeling of behavior of induced seismicity is required.

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