Spatio-temporal variations of shallow seismic velocity changes in Salton Sea Geothermal Field, California in response to large regional earthquakes and long-term geothermal activities

Abstract

We measure spatio-temporal variations of seismic velocity changes in Salton Sea Geothermal Field, California based on cross correlations of daily seismic traces recorded by a borehole seismic network from December 2007 to January 2014. We find clear co-seismic velocity reductions during the 2010 M 7.2 ​El Mayor–Cucapah, Mexico earthquake at ∼100 ​km further south, followed by long-term recoveries. The co-seismic reductions are larger with longer post-seismic recoveries in higher frequency bands, indicating that material damage and healing process mostly occurred in the shallow depth. In addition, the co-seismic velocity reductions are larger for ray paths outside the active fluid injection/extraction regions. The ray paths inside injection/extraction regions are associated with smaller co-seismic reductions, but subtle long-term velocity increases. We also build 3D transient water flow models based on monthly injection/extraction rates, and find correlations between several water flow parameters and co-seismic velocity reductions. We interpret the relative lack of co-seismic velocity changes within the geothermal region as unclogging of fracture network due to persistent fluid flows of geothermal production. The long-term velocity increase is likely associated with the ground water depletion and subsidence due to net production.