Seismic Swarm S20221231.1 Near Borrego Springs, California

A seismic swarm designated S20221231.1 occurred 16 km north of Borrego Springs in San Diego County, California. The sequence began at 12:12 on 31 December 2022 and concluded at 08:38 on 3 January 2023, spanning 68 hours and 26 minutes. During this period, 86 earthquakes were recorded, with magnitudes ranging from 0.3 to 4.1. The largest event, magnitude 4.1, struck at a depth of 3 km shortly after the swarm initiated.

The region lies within the Peninsular Ranges province, part of the broader Pacific-North American plate boundary. Tectonic deformation is accommodated primarily along the San Jacinto Fault Zone, a major right-lateral strike-slip system that branches from the San Andreas Fault. This zone exhibits high slip rates and frequent moderate seismicity. The Anza-Borrego area experiences distributed faulting, including segments of the Coyote Creek and Clark faults, which contribute to swarm activity through fluid migration and stress triggering in fractured rock.

Earthquake swarms in this setting typically feature clustered events without a dominant mainshock-aftershock pattern. The S20221231.1 sequence followed this behavior, with the magnitude 4.1 event accompanied by numerous smaller shocks at depths mostly between 1 and 6 km. A secondary cluster of events reached depths of 12–14 km later in the sequence, suggesting possible downward migration of activity.

Historical records since 2000 document 42 swarms in the immediate vicinity. Annual counts include single swarms in 2001, 2003, 2009, 2014, and 2018; two each in 2002, 2005, and 2015; three each in 2011 and 2020; four each in 2013 and 2016; and six each in 2010, 2012, and 2022. This recurrence underscores the persistent seismic productivity of the San Jacinto Fault Zone.

The swarm's temporal distribution showed peak activity within the first 24 hours, followed by a gradual decline. Most events remained below magnitude 2.0, consistent with background swarm characteristics observed along the fault zone. No damage or felt reports beyond minor shaking were associated with the sequence.

References

• USGS Earthquake Catalog (earthquake.usgs.gov)
• Southern California Earthquake Data Center (scedc.caltech.edu)
• USGS San Jacinto Fault Zone overview (usgs.gov)