Seismic Swarm VS20100111.1: Analysis of Activity Near Calipatria, California

A notable earthquake swarm, designated VS20100111.1, occurred in the Imperial Valley region of southern California. The sequence began at 13:22 on 10 January 2010 and concluded at 15:53 on 20 January 2010, centered 9 km west-northwest of Calipatria. Over 242 hours and 31 minutes, 233 earthquakes were recorded.

The Imperial Valley lies within the Salton Trough, a tectonically active pull-apart basin formed by the interaction between the San Andreas Fault system and the Imperial Fault. This area experiences frequent seismic activity due to right-lateral strike-slip faulting and associated geothermal processes linked to the underlying magma chamber beneath the Salton Sea. Historical records show that swarms in this zone often cluster along the Brawley Seismic Zone, reflecting episodic strain release rather than mainshock-aftershock sequences.

Examination of the first 100 events reveals predominantly low-magnitude activity. Magnitudes ranged from 0.2 to 3.5, with the majority below 1.0. Depths were consistently shallow, mostly between 0 and 5 km, indicating brittle failure in the upper crust. Early events on 10–11 January were microseismic (0.2–0.7), transitioning to slightly larger shocks by 13 January. A notable escalation occurred on 15 January, featuring events of 3.0, 2.2, 3.4, and a peak of 3.5, all at depths of 1–2 km. These larger events were interspersed with numerous smaller tremors, consistent with swarm behavior driven by fluid migration or aseismic slip.

Regional statistics underscore the area's swarm-prone nature. Since 1 January 2000, 26 swarms have been documented in the vicinity, with notable concentrations in 2008 (5 swarms) and 2009 (11 swarms). Earlier years recorded fewer episodes: one each in 2000–2002, three in 2003, one in 2004, three in 2005. Such patterns align with the region's ongoing tectonic loading and hydrothermal influences.

This swarm exemplifies the Imperial Valley's characteristic low-magnitude, high-frequency seismicity. Continued monitoring remains essential given proximity to critical infrastructure and population centers.

References

• USGS Earthquake Catalog (historical swarm data for Imperial Valley)
• California Geological Survey reports on Salton Trough tectonics