SPATIAL-TEMPORAL VARIATIONS OF SEISMICITY AND RESERVOIR PROPERTIES AT THE GEYSERS GEOTHERMAL FIELD, CA, USA

In recent years, earthquakes of M > 3 have caused concern and objection from the public against the use of water injection to increase productivity at geothermal reservoirs worldwide. Production at the EGS site in Basel (Switzerland) was stopped after renewed seismicity caused concern and objection from the public in the city. A planned expansion of the EGS site in Soultz-sous-forêt (France) was suspended after attempts to generate increased permeability through hydrofracturing of the reservoir rock generated an increase in seismicity. As a consequence of these concerns and objections it becomes imperative to understand the relationship between these larger events and the induced stress changes in the medium if sustained generation of geothermal energy is the goal.

In our paper, we will address seismicity trends at the Geysers Geothermal Reservoir, CA USA, and delineate spatial-temporal patterns of seismicity in an attempt to understand the causality of the observed data. Our analysis is based on seismic data recorded by the Lawrence Berkeley National Lab (LBNL) seismic network from 2003 through present. Correlations between water injection/steam production and observed seismicity will be investigated, as well as temporal changes of P- and S-wave velocities throughout the reservoir. Our work is part of a larger project to investigate the relationship between water injection or steam production and the effects on the local and regional stress field including the generation of larger events (M > 3).