Seismic Swarm S20220825.1: Analysis of Activity Near Borrego Springs, California

Seismic swarm S20220825.1 was recorded 8 km north-northeast of Borrego Springs, California, beginning at 07:32 on 25 August 2022 and concluding at 10:51 on 26 August 2022. Over this 27-hour period, 27 earthquakes were detected, with magnitudes ranging from 0.2 to 3.2 and focal depths between 9 and 12 km. The sequence initiated with a magnitude 3.2 event at 11 km depth, followed by numerous smaller aftershocks clustered tightly in both time and space. Subsequent events included a magnitude 1.8 shock at 9 km depth and several magnitude 1.0–1.6 tremors, all maintaining shallow crustal depths consistent with local fault structures.

The temporal distribution showed the highest rate of activity in the first several hours, with events occurring at intervals of minutes, before tapering to isolated occurrences on 26 August. This pattern reflects typical swarm behavior driven by fluid migration or aseismic slip along fault segments rather than a single mainshock-aftershock sequence. Depths remained remarkably consistent, indicating a confined source volume within the upper crust.

Borrego Springs lies within the tectonically active Salton Trough region of southern California, part of the broader Pacific–North American plate boundary. The area is influenced by the San Jacinto Fault Zone and associated secondary structures such as the Coyote Creek and Clark faults, which accommodate right-lateral strike-slip motion. These faults exhibit complex segmentation and have produced both large historical earthquakes and frequent microseismicity. Crustal deformation here results from the transition between the San Andreas Fault system to the north and the Imperial Fault to the south, creating a setting prone to episodic swarm activity.

Since 1 January 2000, 41 swarms have been documented in the region. Yearly counts include one swarm each in 2001, 2003, 2014, and 2018; two each in 2002, 2005, 2015, and 2017; three each in 2009, 2011, 2012, 2020, and 2022; four each in 2013 and 2016; and six in 2010. This long-term record underscores the recurrent nature of swarm sequences along these fault networks.

The 2022 swarm fits within this established pattern of low-to-moderate magnitude events at shallow depths, contributing to ongoing monitoring of strain accumulation in the area. Such activity provides valuable data for refining models of fault interaction and seismic hazard assessment in the Anza-Borrego Desert.

References

• SeismoSight internal swarm classification records (S20220825.1 parameters and historical swarm statistics since 2000).
• United States Geological Survey (USGS) Quaternary Fault and Fold Database for southern California fault mapping.