Seismic Swarm S20091101.1: Analysis of Activity Near Westmorland, California

On November 1, 2009, a seismic swarm designated S20091101.1 occurred approximately 18 km NNW of Westmorland, California. The sequence began at 10:11 and concluded at 23:04, encompassing 26 earthquakes over 12 hours and 52 minutes. Event magnitudes ranged from 0.3 to 2.3, with the majority below 2.0, and focal depths remained shallow, predominantly between 0 and 7 km. Notable events included a magnitude 2.3 quake at 19:28:36 (1 km depth) and another at 20:46:52 (1 km depth), alongside a magnitude 2.2 event at 21:23:52 (1 km depth). These characteristics align with typical swarm behavior in the region, featuring clustered low-magnitude activity without a dominant mainshock.

The swarm unfolded in two primary phases: an initial cluster from 10:11 to 11:25 and a later intensification from 19:26 onward. Depths clustered near the surface, with 15 events at 0–2 km, suggesting shallow crustal processes. Such patterns reflect fluid migration or stress adjustments along local fault networks rather than large-scale tectonic rupture.

Westmorland lies within California's Imperial Valley, part of the Salton Trough—a tectonically active pull-apart basin formed by the interaction of the Pacific and North American plates. The area experiences elevated seismicity due to its position along the Brawley Seismic Zone, a right-stepping offset in the San Andreas Fault system. This zone accommodates dextral shear through distributed faulting and frequent earthquake swarms. Historical records indicate the Imperial Valley has hosted multiple swarms, driven by both tectonic loading and geothermal influences from the underlying Salton Sea geothermal field.

Seismic swarms have been documented in the region since at least 2000, with 22 total events recorded through 2009. Yearly distribution shows variability: single swarms in 2000, 2001, 2002, 2004, and 2006; two in 2003; three in 2005; five in 2008; and seven in 2009. This frequency underscores the persistent microseismicity of the Imperial Valley, where small events often occur in clusters without producing significant surface rupture.

Geological monitoring in the Salton Trough benefits from dense seismic networks that capture these shallow sequences effectively. The 2009 activity near Westmorland fits established patterns of swarm recurrence, contributing to ongoing assessments of regional fault mechanics and hazard potential.

References

USGS Earthquake Hazards Program – Imperial Valley seismicity reports
California Geological Survey – Salton Trough tectonic summaries
SeismoSight internal swarm classification database