Seismic Swarm VS20101112.1: Analysis of Activity Near Calipatria, California
A notable earthquake swarm designated VS20101112.1 was recorded in the Imperial Valley region of southern California. The sequence initiated at 19:32 UTC on 11 November 2010 and concluded at 06:11 UTC on 13 November 2010, spanning 34 hours and 39 minutes. Centered 12 km west-northwest of Calipatria, the swarm produced 28 earthquakes, with magnitudes ranging from 0.4 to 2.7 and focal depths mostly between 0 and 6 km.
This event occurred within the Brawley Seismic Zone, a tectonically active area at the southern terminus of the San Andreas Fault system. The Imperial Valley lies in the Salton Trough, a pull-apart basin formed by dextral shear between the Pacific and North American plates. High heat flow from underlying magmatic intrusions and active transform faulting contribute to frequent microseismicity and occasional swarms. The zone experiences distributed deformation through numerous short fault segments rather than a single master fault, resulting in clustered earthquake sequences without a dominant mainshock-aftershock pattern.
The temporal distribution of the VS20101112.1 swarm showed an initial cluster of low-magnitude events on 11 November, followed by a peak of activity between 11:24 and 11:38 UTC on 12 November. During this intense phase, 15 events occurred within 14 minutes, including the largest shock of magnitude 2.7 at 3 km depth. Subsequent events tapered off, with isolated shocks continuing until early 13 November. Depths remained shallow throughout, consistent with brittle failure in the upper crust influenced by geothermal gradients in the region.
Historical records indicate elevated swarm activity in the Imperial Valley since 2000. A total of 33 swarms have been documented through 2010, with annual counts as follows: one each in 2000, 2001, 2002, and 2004; three in both 2003 and 2005; five in 2008; eleven in 2009; and seven in 2010. This pattern reflects the persistent tectonic loading and fluid migration processes characteristic of the Salton Trough.
Seismic swarms in this setting typically arise from aseismic slip or pore-pressure changes rather than sudden stress drops associated with larger tectonic events. The shallow depths and small magnitudes observed in VS20101112.1 align with these mechanisms. Ongoing monitoring by regional networks continues to track such activity, providing data that refine models of fault interaction and strain accumulation along the plate boundary.
The Imperial Valley's geology features thick sedimentary sequences overlying crystalline basement, with Quaternary alluvium masking many active faults. Geothermal fields nearby further modulate local seismicity through hydrothermal circulation. Continued observation of swarms like VS20101112.1 contributes to improved understanding of short-term seismic hazards in this densely faulted landscape.