Seismic Swarm S20171207.1 Near Borrego Springs, California

An earthquake swarm designated S20171207.1 occurred 14 km southwest of Borrego Springs in San Diego County, California. The sequence began at 21:53 on 6 December 2017 and concluded at 10:28 on 11 December 2017, spanning 108 hours and 34 minutes during which 120 earthquakes were recorded.

The Borrego Springs region lies within the Peninsular Ranges province of southern California, where the crust is deformed by the interaction between the Pacific and North American plates. The area is crossed by the San Jacinto Fault Zone, a major right-lateral strike-slip system that accommodates a significant portion of the plate-boundary slip. This fault zone is one of the most seismically active structures in the state and has produced numerous moderate to large earthquakes in the historical and instrumental record. Depths of events in the swarm were predominantly between 10 and 13 km, consistent with the typical seismogenic depth range along the San Jacinto Fault.

Analysis of the first 100 events shows a rapid onset followed by a decaying rate of activity. The initial event at 21:53 on 6 December registered magnitude 1.0 at 12 km depth. Within the first hour, several events reached magnitude 2.6 and 3.9, both at approximately 11 km depth. Subsequent activity included additional events of magnitude 3.0, 3.5, and two magnitude 2.2 shocks, with the majority of events falling between magnitude 0.5 and 1.9. Depths remained stable, rarely deviating outside the 10–13 km interval. The sequence lacked a single dominant mainshock and instead exhibited the characteristic swarm behavior of numerous events of similar size distributed over several days.

Since 1 January 2000, ten swarms have been documented in the immediate vicinity. These occurred in 2003 (one swarm), 2007 (one), 2009 (three), 2010 (three), 2013 (one), and 2015 (one). The 2017 swarm fits the established pattern of episodic clustered seismicity along this segment of the fault zone.

The Anza-Borrego region continues to be monitored closely because of its proximity to major population centers and critical infrastructure. Ongoing seismic surveillance provides data that improve understanding of fault mechanics and help refine regional hazard assessments.