Seismic Swarm S20100209.1: February 2010 Activity Near Borrego Springs, California

An earthquake swarm designated S20100209.1 occurred approximately 22 km north-northwest of Borrego Springs in San Diego County, California. The sequence began at 05:13 on 8 February 2010 and concluded at 03:40 on 12 February 2010, spanning 94 hours and 26 minutes. During this interval, 44 earthquakes were recorded, with magnitudes ranging from 0.2 to 2.5 and focal depths between 2 km and 16 km.

The swarm initiated with a cluster of low-magnitude events on 8 February, including several between 1.4 and 2.3 that occurred within the first hour. Activity continued at a moderate rate through 9 February, featuring events such as a magnitude 2.1 at 11:56 and a magnitude 1.4 shortly afterward. Peak magnitude reached 2.5 on 10 February at 03:48, followed by diminishing activity that ended with a magnitude 0.4 event on 12 February. Depths remained predominantly in the upper to mid-crust, consistent with typical shallow seismicity in the region.

This swarm represents one of eight documented sequences in the area since 2000. Earlier swarms occurred in 2001 (one event), 2002 (two events), 2003 (one event), 2005 (two events), and 2009 (two events). Such episodic clusters reflect the fault system's tendency toward distributed, low-magnitude activity rather than isolated large ruptures.

The Borrego Springs region lies within the Anza-Borrego Desert, part of the broader San Andreas Fault system in southern California. Seismicity here is primarily driven by the San Jacinto Fault Zone, a major right-lateral strike-slip structure that accommodates a significant portion of the Pacific-North American plate boundary motion. The zone exhibits high strain rates and frequent small-to-moderate earthquakes, with historical records indicating recurrent swarm-like behavior along subsidiary faults such as the Coyote Creek and Clark strands. Depths of 2–16 km align with the seismogenic crust in this transform boundary setting, where brittle failure occurs above the ductile transition zone.

Tectonic studies confirm that the San Jacinto Fault Zone has produced multiple surface-rupturing events in the Holocene, underscoring its role in regional seismic hazard. The 2010 swarm occurred in an area of known microseismicity, contributing to ongoing monitoring efforts that track strain accumulation and fault interaction.

References

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