Seismic Swarm S20130820.1 Near Westmorland, California
An earthquake swarm designated S20130820.1 was recorded 5 km east of Westmorland in Imperial County, California. The sequence began at 14:27 on 19 August 2013 and concluded at 11:33 on 23 August 2013, spanning 93 hours and 6 minutes. During this interval, 47 earthquakes were detected, with magnitudes ranging from 0.4 to 2.9 and focal depths between 0 and 13 km.
The swarm exhibited typical characteristics of clustered seismicity in the region, featuring numerous small events without a single dominant mainshock. Early activity on 19 August included events of magnitude 1.7 and 1.4. Peak intensity occurred on 20 August, when the largest event of magnitude 2.9 was recorded at a depth of 8 km, accompanied by multiple shocks between magnitudes 1.1 and 2.5. Activity gradually declined over the following days, with the final events on 23 August registering magnitudes below 1.0 at shallow depths.
This swarm occurred within the Salton Trough, a tectonically active pull-apart basin formed by the interaction of the Pacific and North American plates. The area lies along the Brawley Seismic Zone, where right-lateral strike-slip faulting and crustal extension produce frequent earthquake swarms. Geothermal fields and volcanic features in the Imperial Valley further contribute to elevated seismicity through fluid migration that can trigger clustered events.
Historical records indicate that 60 swarms have occurred in the vicinity since 1 January 2000. Annual counts show variability, with notable increases in 2009 (11 swarms), 2010 (13 swarms), and 2012 (11 swarms). Earlier years such as 2003, 2008, and 2011 recorded between four and five swarms each, underscoring the persistent nature of swarm activity in this portion of the Imperial Valley.
Such sequences provide insight into the mechanics of swarm events, which often reflect episodic slip on fault networks influenced by both tectonic stress and hydrothermal processes. Depths predominantly between 3 and 11 km align with the brittle-ductile transition zone in the region, where small-magnitude events can occur in rapid succession.
References
SeismoSight internal classification for swarm S20130820.1 data.
United States Geological Survey regional tectonic summaries for the Imperial Valley and Salton Trough.