Seismic Swarm S20100828.1 Near Bombay Beach, California

A notable earthquake swarm, designated S20100828.1, occurred 7 km south-southeast of Bombay Beach, California, along the eastern margin of the Salton Sea. The sequence began at 07:58 UTC on 28 August 2010 and concluded at 10:51 UTC on 29 August 2010, spanning 26 hours and 52 minutes. During this interval, 32 earthquakes were recorded.

Event magnitudes ranged from 0.6 to 3.2, with the largest shock occurring at 14:13:27 UTC on 28 August at a focal depth of 3 km. Depths throughout the swarm were predominantly shallow, between 2 km and 13 km, consistent with activity in the upper crust. The temporal distribution showed a rapid onset followed by clustered activity in the evening of 28 August, with magnitudes mostly below 2.5 after the initial peak. Only one event exceeded magnitude 3, underscoring the swarm’s modest energy release.

This swarm forms part of a broader pattern observed in the region. Since 1 January 2000, 30 swarms have been documented near Bombay Beach. Annual counts increased notably after 2008: one swarm each in 2000–2002, two in 2003, one in 2004, three in 2005, five in 2008, ten in 2009, and six in 2010 prior to and including S20100828.1. Such recurrent swarms reflect ongoing tectonic strain accumulation and release.

Geologically, the swarm epicenters lie within the Salton Trough, a tectonically active pull-apart basin formed by right-lateral motion between the Pacific and North American plates along the San Andreas Fault system. The trough experiences crustal extension and is influenced by the Brawley Seismic Zone to the south and the southern San Andreas Fault to the north. High heat flow associated with the underlying magmatic intrusion and geothermal fields promotes brittle failure at shallow depths, favoring swarm-type seismicity rather than large mainshock-aftershock sequences. Historical records indicate that similar swarms have occurred in this locale for decades, often without producing damaging ground shaking.

The 2010 sequence aligns with the characteristic behavior of the Imperial Valley–Salton Sea corridor, where small-magnitude events clustered in time and space serve as indicators of regional stress. Depths concentrated above 10 km suggest activation of shallow fault segments within the sedimentary and metamorphic basement.

Continued monitoring of such swarms contributes to refined seismic hazard assessments for nearby communities, including Bombay Beach and the broader Coachella Valley.

References

USGS Earthquake Catalog
California Geological Survey Fault Activity Map
Southern California Earthquake Data Center Swarm Database