The correlation of earthquake swarms and local velocity heterogeneities in the Brawley seismic zone, southern California

Earthquake swarms with the possible largest magnitude greater than 5.0 frequently strike the Brawley seismic zone in southern California. The latest swarm of earthquakes occurred on 5 June 2021, including a magnitude 5.2 quake. The Salton Sea Geothermal Field lies at the northern end of the Brawley seismic zone. A good understanding of the detailed subsurface structure is necessary for seismic hazard assessment and the monitoring of geothermal resources exploration in this region. We apply the novel adjoint-state traveltime tomography method to image the crustal P- and S-wave velocity structures of the Brawley seismic zone and the adjacent area to the west. This new method requires no ray tracing but maintains a high level of accuracy in complex media. Highresolution P- and S-wave velocity models are constructed by inverting 85,321 first P-wave arrival times and 43,912 first S-wave arrival times, respectively. The velocity models reveal strong crustal heterogeneities. At 5-12 km depth the Brawley seismic zone is occupied by earthquake-prone high-velocity rocks; while the near surface is dominated by low-velocity sedimentary rocks. The source zone of the 1987 Superstition Hills earthquake sequence is filled with low-velocity rocks, possibly suggesting the presence of fluids. Fluids may have also played a critical role in the generation of earthquake swarms in the Brawley seismic zone.