Earthquake Swarm S20100405.7: Seismic Activity Near Borrego Springs, California

An earthquake swarm designated S20100405.7 occurred in Southern California, centered 19 kilometers north-northwest of Borrego Springs. The sequence began at 00:34 on 5 April 2010 and concluded at 18:39 on 10 April 2010, spanning 138 hours and four minutes. During this period, 62 earthquakes were recorded, with magnitudes ranging from 0.0 to 2.8 and focal depths between 4 and 14 kilometers.

The swarm exhibited typical characteristics of clustered seismic activity without a dominant mainshock. Events were distributed across multiple days, with the highest magnitudes (2.7 and 2.8) occurring early on 5 April at depths of 13 and 10 kilometers, respectively. Subsequent activity included numerous events below magnitude 2.0, reflecting a diffuse release of strain rather than a classic foreshock-aftershock pattern. Depths remained consistently shallow, averaging around 10 kilometers, consistent with activity in the upper crust of this tectonically active region.

The location lies within the San Jacinto Fault Zone, a major component of the broader San Andreas Fault system in Southern California. This zone accommodates significant right-lateral strike-slip motion as part of the Pacific-North American plate boundary. The Anza-Borrego Desert area experiences frequent seismicity due to its position along multiple fault strands, including the Coyote Creek Fault and related structures. Historical records indicate elevated swarm activity in this corridor, driven by fluid migration and stress interactions along the fault network.

Since 2000, eight swarms have been documented in the region, occurring in 2001, 2002 (two events), 2003, 2005 (two events), 2009, and 2010. This pattern underscores the recurrent nature of swarm-type sequences in the area, often linked to the complex fault geometry and hydrothermal influences present in the Salton Trough transition zone.

Such swarms provide valuable data for understanding fault behavior and improving seismic hazard assessments in Southern California. Monitoring continues through regional networks to track potential changes in activity.

References

SeismoSight internal classification records for swarm S20100405.7.
USGS Earthquake Hazards Program regional fault maps and historical seismicity data.