Seismic Swarm VS20120304.1: Analysis of Activity Near Calipatria, California

A seismic swarm designated VS20120304.1 was recorded 10 km northwest of Calipatria, California, from 10:33 on 3 March 2012 to 22:28 on 6 March 2012. The sequence lasted 83 hours and 54 minutes and comprised 72 earthquakes. Magnitudes ranged from 0.3 to 2.3, with the largest event occurring at the onset. Depths were predominantly shallow, between 0 and 9 km, consistent with activity in the upper crust of the Imperial Valley.

The swarm exhibited typical characteristics of clustered microseismicity, with an initial stronger shock followed by numerous smaller events. Activity peaked in the early hours of 4 March, when multiple events clustered within minutes. Subsequent days showed declining frequency, though isolated events persisted until the swarm concluded on 6 March. No single mainshock dominated; instead, the pattern reflected distributed stress release along local fault strands.

The Calipatria area lies within the Salton Trough, a tectonically active pull-apart basin formed by oblique extension between the Pacific and North American plates. This region forms part of the Brawley Seismic Zone, where right-lateral shear is accommodated by north-northwest-striking faults and geothermal fields. High heat flow from underlying magmatic intrusions promotes fluid-driven seismicity, a recognized mechanism for swarm generation in the Imperial Valley.

Geological mapping and geophysical studies indicate that the local crust consists of sedimentary fill overlying crystalline basement, with active faults including segments of the Imperial and San Andreas systems. Historical records document repeated swarm episodes linked to fluid migration and aseismic slip transients. The 2012 swarm fits within this framework, occurring in a zone of elevated background seismicity.

Since 2000, 47 swarms have been identified in the region. Yearly counts show variability: one each in 2000, 2001, 2002 and 2004; three in 2003 and 2005; five in 2008; eleven in 2009; thirteen in 2010; four in 2011; and four in 2012. This distribution underscores the recurrent nature of swarm activity driven by the interplay of tectonic loading and hydrothermal processes.

Such sequences provide insight into strain accommodation without large-magnitude rupture. Continued monitoring supports hazard assessment in this densely faulted corridor.

References

USGS Earthquake Catalog
California Geological Survey, Imperial Valley Fault Maps
SCEC Community Fault Model Documentation