Earthquake Swarm VS20231003.1: Seismic Activity Near Calipatria, California
An earthquake swarm designated VS20231003.1 occurred 13 km west-northwest of Calipatria, California, beginning at 09:50 UTC on 3 October 2023 and concluding at 21:09 UTC on 4 October 2023. Over 35 hours and 18 minutes, the sequence produced 29 earthquakes, all of small magnitude and shallow depth. The events clustered tightly in time, with the majority occurring within the first 12 hours on 3 October.
The swarm exhibited typical characteristics of low-magnitude, high-frequency seismic sequences common in the region. Magnitudes ranged from 0.8 to 2.1, with peak activity including events of 2.1 and 2.0. Focal depths remained predominantly between 2 km and 5 km, indicating shallow crustal sources. Later events on 4 October reached slightly greater depths of 6–7 km but maintained similar magnitudes below 2.0. No single mainshock dominated the sequence; instead, energy released through numerous comparable events.
This activity aligns with the tectonic framework of the Imperial Valley within the Salton Trough. The region forms part of the Pacific–North American plate boundary, where right-lateral transform motion along the San Andreas system transitions into extensional spreading. The Imperial and Brawley faults, along with associated cross-faults, accommodate both strike-slip and normal faulting. Elevated heat flow and fluid migration linked to the nearby Salton Sea geothermal field further promote swarm-type seismicity rather than classic mainshock–aftershock patterns.
Historical records document persistent swarm behavior in this zone. Since 1 January 2000, 95 swarms have been identified in the immediate area. Annual counts show notable concentrations in 2010 and 2013 (13 swarms each), 2012 (11 swarms), and 2009 (11 swarms). Earlier years such as 2003, 2008, and 2021 also recorded multiple episodes, underscoring the recurrent nature of clustered microseismicity driven by the local fault network and fluid dynamics.
The October 2023 swarm fits within this established pattern. Its short duration, limited magnitude range, and shallow depths mirror previous episodes, suggesting ongoing adjustment along minor fault segments without escalation to larger ruptures. Such swarms contribute to the gradual release of tectonic strain in the Imperial Valley and provide valuable data for monitoring crustal stress and fluid involvement.
References
- SeismoSight internal swarm classification records for VS20231003.1 and historical swarm statistics (2000–2023).
- United States Geological Survey (USGS) regional tectonic summaries for the Imperial Valley and Salton Trough.