Detection of Aseismic Slip and Poroelastic Reservoir Deformation at the North Brawley Geothermal Field From 2009 to 2019

The North Brawley Geothermal Field, located within the Brawley Seismic Zone of Southern California, presents a case study for understanding seismic hazards linked to fluid injection and geothermal energy extraction. An earthquake swarm near the geothermal field in 2012 included two earthquakes with magnitudes greater than 5 and was potentially preceded by a years-long aseismic slip transient. To better understand ground deformation around the geothermal field, including its evolution with time and its physical mechanisms, we analyze deformation before, during, and after the swarm using ground- and satellite-based geodetic techniques between 2009 and 2019. We integrate observations from GNSS, Sentinel-1, TerraSAR-X, UAVSAR, and leveling surveys into a single deformation history. Modeling of this new collection of observations at the North Brawley Geothermal Field provides evidence for 80% more pre-swarm aseismic slip than previously recognized from 2009 to 2012. During the 2012 Brawley swarm, our geodetic slip inversions closely match the results of seismic waveform inversions from the swarm events. After the 2012 swarm, surface deformation is dominated by poroelastic deformation of a shallow fluid reservoir at <1 km depth rather than fault slip. The deformation history and seismicity catalogs at North Brawley suggest a cessation of fault-related slip during the similar to 7 years after the 2012 earthquake swarm.

Automated summary

The North Brawley Geothermal Field in Southern California provides a case study for understanding seismic hazards associated with fluid injection and geothermal energy extraction. This study analyzes ground deformation before, during, and after the 2012 earthquake swarm in the area using geodetic techniques. The results show a significant amount of pre-swarm aseismic slip and highlight the dominance of poroelastic deformation after the swarm. It suggests a cessation of fault-related slip in the North Brawley region after the 2012 earthquake swarm.