|Title||A comparison of long-term changes in seismicity at The Geysers, Salton Sea, and Coso geothermal fields|
|Publication Type||Journal Article|
|Year of Publication||2016|
|Authors||Trugman D.T, Shearer PM, Borsa A.A, Fialko Y|
|Journal||Journal of Geophysical Research-Solid Earth|
|Type of Article||Article|
|Keywords||fluid; injection; inyo-county; magnitude; model; omori formula; sequence; southern-california; stress changes; w 7.2 earthquake; waste-water injection|
Geothermal energy is an important source of renewable energy, yet its production is known to induce seismicity. Here we analyze seismicity at the three largest geothermal fields in California: The Geysers, Salton Sea, and Coso. We focus on resolving the temporal evolution of seismicity rates, which provides important observational constraints on how geothermal fields respond to natural and anthropogenic loading. We develop an iterative, regularized inversion procedure to partition the observed seismicity rate into two components: (1) the interaction rate due to earthquake-earthquake triggering and (2) the smoothly varying background rate controlled by other time-dependent stresses, including anthropogenic forcing. We apply our methodology to compare long-term changes in seismicity to monthly records of fluid injection and withdrawal. At The Geysers, we find that the background seismicity rate is highly correlated with fluid injection, with the mean rate increasing by approximately 50% and exhibiting strong seasonal fluctuations following construction of the Santa Rosa pipeline in 2003. In contrast, at both Salton Sea and Coso, the background seismicity rate has remained relatively stable since 1990, though both experience short-term rate fluctuations that are not obviously modulated by geothermal plant operation. We also observe significant temporal variations in Gutenberg-Richter b value, earthquake magnitude distribution, and earthquake depth distribution, providing further evidence for the dynamic evolution of stresses within these fields. The differing field-wide responses to fluid injection and withdrawal may reflect differences in in situ reservoir conditions and local tectonics, suggesting that a complex interplay of natural and anthropogenic stressing controls seismicity within California's geothermal fields.
Viewed holistically, our observations indicate that anthropogenic stresses are the primary control on transient changes in seismicity at The Geysers, while they play a less prominent role in modulating seismicity at Salton Sea and Coso. This is not to suggest that induced seismicity is of negligible concern at Salton Sea and Coso. Indeed, the proximity of the Salton Sea and Coso geothermal fields to active faults that are capable of producing large, damaging earthquakes amplifies the importance of even small changes in seismicity due to anthropogenic stresses. Rather, these observations highlight the complicated process of differentiating induced from natural seismicity in California's geothermal fields, and demonstrate that differences in tectonic setting, reservoir conditions, and history of energy production all likely contribute to the differing patterns of seismicity observed within each geothermal field.