Tidal modulation of seismicity at the Coso geothermal field

Abstract

Studying the seismicity triggering response of fault systems to periodic stress fluctuations can improve our understanding of earthquake nucleation, rupture failure processes, and local stress states. Geothermal fields are well known to be susceptible to triggering, as the injection and extraction activities change the local stress and fluid flow conditions. Here, we examine the modulation of earthquakes by Earth tides within California's Coso geothermal field (CGF) and its vicinity. To maximize our resolution to detect modulation of small earthquakes, we take advantage of the new Quake Template Matching catalog in southern California, which has nearly twice as many events in the Coso region as the standard catalog and is complete down to about magnitude 0.3. We observe strong tidal triggering of earthquakes within the CGF, even though the fluctuations of tidal stresses are small (∼2 kPa). The tidally-triggered earthquakes tend to occur near the time of maximum tensile tidal stress. The signal is strongest near the edges of the zone of new production wells, suggesting fluid pressure gradients encourages triggering at tidal periods.