Seismic Swarm S20110701.1: Analysis of Activity Near Westmorland, California

Seismic swarm S20110701.1 occurred 15 km NNW of Westmorland in Imperial County, California. The sequence began at 17:11 on 1 July 2011 and concluded at 19:58 on 2 July 2011, spanning 26 hours and 46 minutes. During this period, 36 earthquakes were recorded.

Event parameters show predominantly shallow foci, with depths ranging from 0 km to 9 km. Magnitudes remained low, peaking at 2.3. The initial event measured 1.5 at 1 km depth, followed by numerous micro-earthquakes below magnitude 1.0 clustered in the evening of 1 July. A notable 2.3 event occurred at 20:50:53 on 1 July at 2 km depth. Activity tapered overnight, with only four events recorded on 2 July.

The Imperial Valley lies within the Salton Trough, a pull-apart basin formed by dextral shear between the Pacific and North American plates. This tectonic setting produces frequent earthquake swarms along the Brawley Seismic Zone and adjacent right-lateral strike-slip faults. High geothermal gradients and sedimentary infill contribute to shallow seismicity. The region experiences both swarm-type sequences and occasional larger mainshock-aftershock events linked to the San Andreas and Imperial fault systems.

Historical records indicate 43 swarms in the area since 1 January 2000. Yearly counts include single swarms in 2000, 2001, 2002, and 2004; four in 2003; three in 2005; five in 2008; eleven in 2009; thirteen in 2010; and three in 2011. These episodes reflect ongoing strain accommodation in the transtensional regime.

The 2011 swarm fits established patterns of short-duration, low-magnitude clusters typical of the Imperial Valley. Such activity provides data on local stress conditions without producing significant surface rupture or damage.

References

USGS Earthquake Hazards Program regional tectonic summaries
California Geological Survey Imperial Valley fault maps
SeismoSight internal swarm classification records