Seismic Swarm S20230324.1 Near Borrego Springs, California

The March 2023 seismic swarm designated S20230324.1 occurred 9 km south-southwest of Borrego Springs in San Diego County, California. It began at 17:29 UTC on 23 March 2023 and concluded at 19:53 UTC on 25 March 2023, spanning 50 hours and 24 minutes. During this interval, 50 earthquakes were recorded, with magnitudes ranging from 0.6 to 3.4 and focal depths predominantly between 9 km and 13 km. The largest event, a magnitude 3.4 earthquake, occurred at 13:45 UTC on 24 March at 11 km depth, followed by an immediate sequence of aftershocks clustered at similar depths. The majority of events exhibited depths around 11–12 km, consistent with activity along mid-crustal fault segments in the region. This swarm fits within a broader pattern of seismic activity in the area. Since 1 January 2000, 31 swarms have been documented near Borrego Springs. Their annual distribution includes single swarms in 2001, 2002, 2003, 2007, 2011, 2012, 2015, 2016, and 2018; three in 2009, 2010, and 2020; two in 2013 and 2017; and seven in 2022. These recurrent episodes reflect ongoing strain release along local fault networks. Borrego Springs lies within the Anza-Borrego Desert, part of the Salton Trough at the southern terminus of the San Andreas Fault system. The region is transected by the San Jacinto Fault Zone and subsidiary structures such as the Coyote Creek Fault. These northwest-striking right-lateral strike-slip faults accommodate a significant portion of the relative motion between the Pacific and North American plates. Seismicity here is typically shallow to mid-crustal, and earthquake swarms often occur without a single dominant mainshock, instead featuring numerous events of similar magnitude distributed over hours to days. The 2023 swarm’s depth distribution and temporal clustering align with this tectonic regime, where fluid migration or aseismic slip may trigger successive failures on closely spaced fault strands. No damage or injuries were reported from the March 2023 sequence, consistent with the modest magnitudes involved. Continued monitoring by regional seismic networks remains essential for characterizing swarm evolution and assessing potential links to larger events on the San Jacinto Fault Zone.

References

• USGS Earthquake Catalog (ANSS Comprehensive Catalog)
• Southern California Seismic Network (SCSN) reports
• California Geological Survey fault and seismicity maps