Seismic Swarm S20131016.1: Analysis of Activity Near Calipatria, California

Seismic swarm S20131016.1 occurred 8 km west of Calipatria in California's Imperial Valley. The sequence began at 03:45 on 16 October 2013 and concluded at 01:15 on 18 October 2013, spanning 45 hours and 29 minutes. During this period, 50 earthquakes were recorded.

The Imperial Valley lies within the Salton Trough, a tectonically active pull-apart basin formed by the interaction of the Pacific and North American plates along the San Andreas Fault system. This region experiences frequent seismic swarms due to right-lateral strike-slip faulting and associated geothermal activity. The Brawley Seismic Zone, immediately south of the swarm location, is a well-documented area of concentrated microseismicity linked to both tectonic loading and fluid migration in the subsurface.

Magnitudes in the swarm ranged from 0.1 to 1.6. Depths were predominantly shallow, between 0 and 7 km, with a single event at 11 km. Activity clustered in distinct pulses, with elevated rates on the first day followed by a gradual decline. The largest event, magnitude 1.6, occurred on 17 October at a depth of 1 km.

Historical records indicate 64 swarms in the region since 1 January 2000. Annual counts show variability: one swarm each in 2000, 2001, 2002, and 2004; four in 2003; three in 2005; five in 2008; eleven in 2009; thirteen in 2010; four in 2011; eleven in 2012; and nine in 2013. This pattern reflects the persistent tectonic stress regime and episodic fluid-driven triggering characteristic of the Imperial Valley.

Such swarms typically do not produce significant surface rupture but contribute to the overall strain release along the plate boundary. Continued monitoring remains essential for understanding long-term seismic hazards in this densely faulted landscape.

References

SeismoSight internal swarm classification data.
USGS Earthquake Hazards Program regional tectonic summaries.