Comparative Study of Earthquake Clustering in Relation to Hydraulic Activities at Geothermal Fields in California

Earthquake clustering properties are investigated in relation to fluid balance H ( t ) (the difference of fluid injection and production rates) using about nine years of data from The Geysers (both the entire field and a local subset), Coso, and Salton Sea geothermal fields in California. Individual earthquake clusters are identified and classified using the nearest‐neighbor approach of Zaliapin and Ben‐Zion (2013a, https://doi.org/10.1785/0120150211 , 2013b, https://doi.org/10.1093/gji/ggw300 ). These are used to calculate nine complementary cluster statistics as time series with a step of about one month. Three alternative techniques (moving window correlation, analysis of variance, and regression) are employed to assess the relations between (possibly nonstationary) time series of cluster statistics and H ( t ). A total of 108 pairwise relations between cluster statistics and H ( t ) are analyzed to clarify effects of fluid activities on seismicity in different places. The seismic clustering response to the fluid balance differs among the examined fields. The Geysers and Salton Sea areas display the highest and lowest clustering responses, respectively. The proportion of clusters consisting of a single event with no offspring (singles) is correlated significantly with H ( t ) at all examined data sets, with a lower proportion of singles during periods of high fluid balance. This may reflect increased susceptibility to earthquake triggering in time intervals with high injection rates. The background seismicity rates significantly increase with H ( t ) at the Geysers and Coso, while an opposite relation holds at the Salton Sea. This could be related to the high structural and tectonic complexity at the Salton Sea compared to the other two geothermal fields.