Seismic Swarm VS20120226.1: Analysis of February 2012 Events near Calipatria, California

The seismic swarm designated VS20120226.1 occurred in a tectonically active portion of the Imperial Valley, approximately 10 km west-northwest of Calipatria, California. This region lies within the Salton Trough, a pull-apart basin formed by the interaction between the San Andreas Fault system to the north and the divergent boundary associated with the East Pacific Rise. The area experiences frequent seismic activity due to right-lateral strike-slip faulting along the Brawley Seismic Zone and related structures, combined with geothermal influences from underlying magmatic intrusions.

The swarm initiated at 09:00 on 26 February 2012 and concluded at 10:14 on 28 February 2012, spanning 49 hours and 13 minutes. During this interval, 88 earthquakes were recorded. Events ranged in magnitude from 0.2 to 3.2, with the majority occurring at shallow depths between 0 and 5 km. Depths extended to a maximum of 9 km in isolated cases. The largest event reached magnitude 3.2 at 11:31 on 26 February, followed closely by additional events of magnitude 2.9 and 2.4 later that day.

Activity concentrated heavily on the first day, with numerous events between magnitudes 1.0 and 2.3 occurring in rapid succession during the afternoon and evening hours. The second day showed a decline in frequency, though several events of magnitude 1.9 to 2.0 were noted around midday on 27 February. The final recorded event was a magnitude 0.2 quake at 10:14 on 28 February. Depths remained predominantly shallow throughout, consistent with fluid migration or stress triggering in the fractured crust of the Imperial Valley.

This swarm aligns with the region's established pattern of episodic seismic clusters. Historical records since 1 January 2000 document 45 such swarms in the vicinity. Yearly occurrences include single events in 2000, 2001, 2002, and 2004; four in 2003 and 2008; three in 2005; eleven in 2009; thirteen in 2010; four in 2011; and two in 2012. These clusters typically involve low-magnitude events and reflect ongoing tectonic adjustment rather than precursors to larger mainshock sequences.

Geological monitoring in the Imperial Valley benefits from dense instrumentation due to the proximity of critical infrastructure, including geothermal facilities and the Salton Sea. Swarms in this setting often correlate with aseismic slip or hydrothermal fluid movement along fault zones. The February 2012 activity contributed to the cumulative understanding of strain release in a transform margin environment where Pacific-North American plate motion is accommodated through distributed faulting.

• SeismoSight internal swarm classification database
• Regional tectonic summaries from the United States Geological Survey
• Imperial Valley seismic network historical catalogs