Seismic Swarm S20210823.1: Analysis of Activity Near Westmorland, California

A notable earthquake swarm, designated S20210823.1, occurred approximately 10 km north of Westmorland in California's Imperial Valley. The sequence initiated at 08:16 UTC on 23 August 2021 and concluded at 19:33 UTC on 25 August 2021, spanning 59 hours and 16 minutes. During this period, 48 earthquakes were recorded, with magnitudes ranging from 0.7 to 4.0 and focal depths primarily between 0 and 9 km.

The swarm featured a prominent mainshock of magnitude 4.0 at 14:57 UTC on 23 August, occurring at a depth of 7 km. Subsequent events clustered in the hours following this shock, including multiple events between magnitudes 1.0 and 2.0 at similar depths around 7 km. Later activity on 24 and 25 August showed a gradual decline in both frequency and magnitude, with the final recorded event at magnitude 1.2 and depth of 3 km. This pattern reflects typical swarm behavior, where energy release occurs through numerous small to moderate events rather than a single dominant rupture.

The Imperial Valley region, situated in the Salton Trough, experiences persistent seismic activity driven by transtensional tectonics at the boundary between the Pacific and North American plates. The area forms part of the Brawley Seismic Zone, an extensional step-over linking the San Andreas and Imperial faults. This setting promotes distributed faulting, fluid migration, and geothermal processes that frequently trigger earthquake swarms. Depths in the observed swarm align with shallow crustal seismicity common to the zone, where brittle failure occurs above approximately 10 km.

Historical records indicate that swarms are recurrent in this locale. Since 1 January 2000, 87 such sequences have been documented, with notable concentrations in 2009 (11 swarms), 2010 (13), 2012 (11), and 2013 (13). Earlier decades showed lower but consistent occurrences, while 2021 alone recorded seven swarms prior to and including S20210823.1. These episodes underscore the zone's role in accommodating plate-boundary strain through episodic, swarm-dominated release rather than infrequent large earthquakes.

The geological framework includes thick sedimentary deposits overlying basement rocks, influenced by the nearby Salton Sea and associated geothermal fields. Ongoing monitoring reveals that swarm activity often correlates with fluid movement along fault networks, contributing to the region's elevated seismicity rates compared to adjacent locked segments of the San Andreas Fault.

References

• USGS Earthquake Catalog (earthquake.usgs.gov)
• California Geological Survey Regional Fault Maps
• SeismoSight internal swarm classification data