Seismic Swarm VS20091117.1: Earthquake Activity Near Calipatria, California

The seismic swarm designated VS20091117.1 occurred approximately 10 km northwest of Calipatria in California's Imperial Valley. It began at 13:34 on 17 November 2009 and concluded at 09:30 on 19 November 2009, spanning 43 hours and 55 minutes. During this period, 142 earthquakes were recorded. Analysis of the first 100 events reveals predominantly low-magnitude activity. Magnitudes ranged from 0.0 to 3.3, with the largest events measuring 3.3 (at 23:05 and 00:44) and 3.2 (at 00:45). Depths remained shallow, mostly between 0 and 3 km, consistent with near-surface faulting in the region. Temporal clustering shows heightened activity on 17 November evening and into the early hours of 18 November, followed by a gradual decline. Smaller events (magnitudes below 1.0) dominated, accounting for the majority of detections and indicating a typical swarm pattern of numerous minor tremors without a single dominant mainshock. This swarm aligns with the tectonic setting of the Imperial Valley, situated within the Salton Trough. The trough represents a transtensional basin formed by the interaction of the Pacific and North American plates along the San Andreas Fault system. The area lies in the Brawley Seismic Zone, where right-lateral strike-slip faults and associated normal faults facilitate swarm-type seismicity. Historical patterns in the zone demonstrate recurrent swarms driven by fluid migration and stress transfer rather than large locked-fault ruptures. Since 1 January 2000, 25 swarms have been documented in the region. Yearly occurrences include one each in 2000, 2001, 2002, and 2004; four in 2003; three in 2005; five in 2008; and nine in 2009. These events underscore the Imperial Valley's elevated background seismicity, influenced by its position at the southern terminus of the San Andreas Fault and proximity to the Salton Sea geothermal field.

References

• SeismoSight internal swarm classification data (VS20091117.1 parameters and event list)
• USGS Earthquake Hazards Program regional tectonic summaries for the Imperial Valley and Brawley Seismic Zone