Absence of Remote Triggering in Geothermal Fields Due to Human Activity

Abstract:

Operational geothermal fields typically have high seismicity rates, which could be caused by both tectonic and anthropogenic activities. Due to the high background seismicity and possible interaction between fluid and seismic waves, geothermal areas have been recognized to be susceptible to large remote earthquakes. However, whether human activity (geothermal production) affects remote earthquake triggering by changing the stress state is unclear. Here we choose two geothermal fields, Coso and Salton Sea in southern California, to study the spatiotemporal distributions of the triggered earthquakes following the 1992 Mw 7.3 Landers and 1999 Mw 7.1 Hector Mine earthquakes. These two geothermal fields have been in operation since 1980s with comparable net capacity, and have long-term geothermal fluid loss. By analyzing the regional catalog recorded by the Southern California Seismic Network, we find that these two operational geothermal areas remain unaffected by the remote mainshocks, whereas the surrounding areas show vigorous triggered responses. We interpret this phenomenon as a result of human activity, which presumably has brought the stress state away from failure by reducing pore pressure. To further understand how much the human activity can affect the stress state, we also conduct a systematic study on Long Valley Caldera in northern California as a comparison site. Long Valley Caldera hosts an active geothermal field with net capacity about one sixth of that in Coso or Salton Sea geothermal field, and the extraction volume is not constantly larger than the injection. We will show comparisons of the triggered response in Long Valley with the two geothermal fields in southern California.