Seismic Swarm S20130219.1: Analysis of Activity Near Westmorland, California

The seismic swarm designated S20130219.1 occurred 8 km north of Westmorland in California's Imperial Valley. It began at 23:37 UTC on 18 February 2013 and concluded at 13:46 UTC on 23 February 2013, spanning 110 hours and 9 minutes. During this interval, 57 earthquakes were recorded, with magnitudes ranging from 0.2 to 1.8 and focal depths between 0 and 10 km. The majority of events clustered at depths of 3–8 km, consistent with shallow crustal deformation in the region. This swarm exemplifies typical behavior in the Imperial Valley, where earthquake clusters often lack a dominant mainshock and instead feature numerous events of similar size. The temporal distribution showed peak activity on 19–21 February, including several events of magnitude 1.3–1.8 on 21 February. Such patterns align with fluid migration or aseismic slip along local fault segments rather than a single rupture. The Imperial Valley lies within the Salton Trough, a tectonically active pull-apart basin formed by the interaction of the San Andreas Fault system and the divergent boundary between the Pacific and North American plates. The Brawley Seismic Zone, immediately north of the swarm epicenter, is a well-documented area of distributed right-lateral shear and frequent swarm activity. Shallow seismicity here reflects ongoing extension and strike-slip motion, with the crust thinned by rifting processes linked to the Gulf of California. Historical records indicate persistent swarm activity in this setting. Since 1 January 2000, 55 swarms have been identified in the immediate region, with notable concentrations in 2009 (11 swarms), 2010 (13 swarms), and 2012 (11 swarms). Earlier years recorded fewer events, such as single swarms in 2000–2002 and 2004. This long-term pattern underscores the Imperial Valley's role as a locus of episodic, low-magnitude seismic release that accommodates plate-boundary strain without producing large individual earthquakes. The 2013 swarm's characteristics—short duration, modest magnitudes, and shallow depths—mirror prior episodes and highlight the value of dense seismic monitoring for tracking strain transients in this high-hazard corridor.

References

SeismoSight internal swarm classification database (S20130219.1 parameters and historical counts).
United States Geological Survey Earthquake Hazards Program regional tectonic summaries for the Imperial Valley and Brawley Seismic Zone.