Seismic Insights into Swarm S20200810.1: Bombay Beach, California

An earthquake swarm designated S20200810.1 was recorded 12 km south-southeast of Bombay Beach, California, from 21:03 UTC on 9 August 2020 to 09:13 UTC on 14 August 2020. Over 108 hours and 9 minutes, the sequence produced 319 events, with detailed parameters available for the first 100. This activity occurred within the Brawley Seismic Zone, a tectonically active region at the southern terminus of the San Andreas Fault system.

The Imperial Valley lies at the boundary between the Pacific and North American plates, where right-lateral strike-slip motion dominates. The Salton Sea occupies a pull-apart basin formed by offset segments of the San Andreas and Imperial faults. Crustal extension and geothermal heat flow in this area facilitate swarm-type seismicity rather than large mainshock-aftershock sequences. Depths of the recorded events ranged from the surface to 12 km, consistent with the shallow brittle crust in the region.

The first 100 events exhibited magnitudes between 0.6 and 4.5, with the majority below 2.0. A notable cluster occurred on 10 August between 15:55 and 16:04 UTC, including a 4.5 event at 2 km depth followed by several events above magnitude 3.0 at similarly shallow depths. Later events in the initial hundred showed a gradual return to lower magnitudes and slightly greater average depths. Temporal spacing varied from seconds to hours, typical of fluid-driven or aseismic-slip-triggered swarms.

Since 2000, 77 swarms have been documented in the same locale. Annual counts peaked in 2010 (13) and 2013 (13), with lower activity in intervening years. The 2020 swarm represents the sole episode recorded that year under the current classification. These recurrent swarms underscore persistent strain accumulation along the southern San Andreas and subsidiary faults.

Regional monitoring networks operated by the U.S. Geological Survey and the California Institute of Technology provide continuous coverage. Data from these instruments confirm that swarm activity in the Salton Trough often correlates with aseismic creep and minor changes in pore-fluid pressure. No surface rupture or significant damage was associated with S20200810.1, aligning with the modest magnitude distribution.

Continued observation of such sequences contributes to refined probabilistic forecasts for larger events on the San Andreas Fault. The geological setting remains one of the most closely watched seismic zones in North America due to its proximity to major population centers and critical infrastructure.

References
U.S. Geological Survey Earthquake Catalog
Southern California Seismic Network (SCSN)
California Geological Survey fault maps
SeismoSight internal swarm classification records