Seismic Swarm VS20120323.1 Near Calipatria, California

The Imperial Valley region of southern California, located approximately 11 km west-northwest of Calipatria, lies within the tectonically active Brawley Seismic Zone. This area forms part of the broader San Andreas Fault system, where the Pacific and North American plates interact through a network of strike-slip faults including the San Andreas, Imperial, and Brawley faults. The zone experiences frequent seismic swarms due to a combination of right-lateral shear, geothermal fluid migration, and minor volcanic influences associated with the nearby Salton Buttes. Shallow crustal depths, typically under 10 km, characterize most events here, reflecting the thin, sediment-filled basin overlying crystalline basement. Seismic swarms in this setting differ from typical mainshock-aftershock sequences by featuring numerous events of similar magnitude without a dominant shock. Historical records since 2000 document 49 such swarms in the region, with notable concentrations in 2009 (11 swarms), 2010 (13 swarms), and 2011–2012 (combined 9 swarms). These episodes often last from hours to several days and are linked to episodic fluid movement along fault planes. Swarm VS20120323.1 began at 08:02 on 22 March 2012 and concluded at 06:47 on 25 March 2012, spanning 70 hours and 45 minutes. During this interval, 73 earthquakes were recorded, with magnitudes ranging from 0.2 to 2.2 and focal depths between 0 and 7 km. Activity initiated with low-magnitude events (mostly 0.3–0.4) at shallow depths on 22 March. Intensity increased markedly on 23 March, including a 2.2 magnitude event at 00:54 and multiple events above magnitude 1.0 clustered between 00:30 and 02:00. Later that day and into 24 March, activity continued with scattered events up to magnitude 2.0, many at depths of 3–6 km. The sequence tapered off with smaller, predominantly shallow events on 25 March. This swarm exhibited classic characteristics of Imperial Valley sequences: tight spatial clustering, rapid succession of events, and a predominance of magnitudes below 2.0. Such patterns suggest triggering by aseismic slip or pore-pressure changes rather than a single large rupture. No damage or felt reports were associated with these microearthquakes, consistent with their low energy release. Ongoing monitoring in the region supports improved understanding of swarm mechanics and their relationship to larger fault behavior. Continued observation aids in distinguishing swarm activity from precursors to significant earthquakes along the Imperial and San Andreas faults.

References

USGS Earthquake Hazards Program reports on Imperial Valley seismicity.
California Geological Survey regional tectonic summaries.
SeismoSight internal swarm classification records.