Seismic Swarm S20110101.1: Analysis of Activity Near Calipatria, California

Earthquake swarm S20110101.1 occurred 12 km west-northwest of Calipatria in California's Imperial Valley. The sequence began at 10:47 on 31 December 2010 and concluded at 01:09 on 2 January 2011, spanning 38 hours and 21 minutes. During this period, 33 events were recorded.

The swarm featured a rapid onset of activity concentrated in the first hours of 1 January 2011. The largest event reached magnitude 3.5 at a depth of 1 km, followed shortly by a magnitude 3.1 shock at similar depth. Additional notable events included magnitudes 2.5 and 2.4, both at 1 km depth. The majority of events registered below magnitude 1.0 and occurred at depths between 0 and 5 km, with most clustered near 1 km or shallower. Activity declined steadily after the initial peak, with only isolated small events continuing into the second day.

This swarm aligns with patterns observed in the Imperial Valley, where clusters of low-to-moderate magnitude earthquakes occur without a single dominant mainshock. Depths remained consistently shallow, consistent with the region's thin crust and active faulting.

The Imperial Valley forms part of the Salton Trough, a tectonically active pull-apart basin at the boundary between the Pacific and North American plates. Right-lateral strike-slip motion along the San Andreas Fault system transitions into the Imperial and Brawley faults, producing frequent seismicity. Geothermal fields and sediment-filled basins further influence event characteristics, often resulting in shallow swarms. Historical records document numerous such sequences in the area, reflecting ongoing strain accommodation.

Since 2000, 39 swarms have been identified in the region under consistent classification criteria. Annual counts show variability, with notable increases in later years: one each in 2000–2002, four in 2003, and progressively higher numbers reaching 12 in 2010. These statistics highlight the recurrent nature of swarm activity along the Imperial fault zone and adjacent structures.

The 2010–2011 swarm contributed to the documented trend of elevated swarm frequency during that interval. No damage or felt reports beyond the immediate area were associated with the sequence, typical for events of this magnitude range.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
California Geological Survey regional fault maps
Southern California Seismic Network bulletins