Seismic Swarm S20130920.1 Near Borrego Springs, California

The seismic swarm designated S20130920.1 occurred approximately 10 km north of Borrego Springs in San Diego County, California. It initiated at 11:36 on 19 September 2013 and concluded at 21:41 on 20 September 2013, spanning 34 hours and 4 minutes. During this interval, 31 earthquakes were recorded, with magnitudes ranging from 0.3 to 3.8 and focal depths between 10 and 14 km.

The sequence featured a notable mainshock of magnitude 3.8 at 06:10 on 20 September, followed by an immediate aftershock cluster. Smaller events predominated before and after this peak, illustrating the clustered, non-mainshock-driven nature typical of swarms. Depths remained consistent in the shallow crustal range, consistent with activity along strike-slip structures in the region.

Borrego Springs lies within the Peninsular Ranges province, adjacent to the San Jacinto Fault Zone—one of southern California’s most active fault systems. This zone accommodates a significant portion of the Pacific–North American plate boundary motion through right-lateral strike-slip faulting. The local geology comprises Mesozoic granitic and metamorphic rocks overlain by Quaternary alluvial and lacustrine sediments within the Borrego Badlands and surrounding basins. Historical seismicity in the area includes the 1968 magnitude 6.6 Borrego Mountain earthquake on the Coyote Creek Fault, which produced surface rupture and demonstrated the capacity for moderate-to-large events nearby.

Since 2000, 21 swarms have been documented in the immediate vicinity, with annual counts varying: one each in 2001, 2003, and 2013; two each in 2002, 2005, and 2009; three each in 2011 and 2012; and six in 2010. These recurrent swarms reflect ongoing strain accumulation and release along subsidiary faults and fault segments within the broader San Jacinto system.

The 2013 swarm provides insight into swarm dynamics in this tectonically active corridor. The rapid onset of low-magnitude events, followed by the largest shock and subsequent decay, suggests fluid-assisted or aseismic slip triggering mechanisms rather than a classic foreshock–mainshock–aftershock progression. Such patterns aid in distinguishing swarm behavior from isolated mainshock sequences and support refined probabilistic forecasts for the San Jacinto Fault Zone.

References

United States Geological Survey Earthquake Hazards Program
Southern California Earthquake Data Center
California Geological Survey Fault Activity Map