Seismic Swarm S20100529.1 Near Ocotillo, California: Geological Context and Initial Event Analysis

The region 12 km east of Ocotillo in Imperial County, California, occupies the western margin of the Salton Trough, a tectonically active pull-apart basin formed by right-lateral shear along the Pacific–North American plate boundary. This setting features northwest-trending strike-slip faults and northeast-trending normal faults that accommodate both transform motion and crustal extension. High heat flow, Quaternary volcanism, and geothermal fields characterize the trough, contributing to frequent microseismicity and swarm behavior.

Seismic swarms in this area arise from fluid migration and aseismic slip rather than mainshock-aftershock sequences. The Imperial Valley and adjacent Brawley Seismic Zone have recorded episodic swarms for decades, reflecting the interplay between the San Andreas, Imperial, and Laguna Salada fault systems. Historical records document elevated activity following the 1940 Imperial Valley and 1979 Imperial earthquakes, underscoring the zone’s persistent strain release.

Swarm S20100529.1 began at 17:21 UTC on 28 May 2010 and continued until 18:21 UTC on 19 August 2010. During 1,993 hours the sequence produced 3,232 earthquakes. The first 100 events, spanning 28–30 May, displayed magnitudes from 0.6 to 3.8 and focal depths predominantly between 3 km and 11 km. Early activity included a 3.8 event at 14:50 UTC on 29 May at 6 km depth, followed within minutes by events of M2.1 and M1.8. Subsequent clusters on 30 May featured repeated M2.3 and M2.8 shocks at depths of 7–10 km. The temporal pattern shows rapid onset, short inter-event times, and a lack of a single dominant mainshock, consistent with swarm dynamics driven by pore-pressure changes in the shallow crust.

These initial events concentrated in a compact volume, with the majority occurring between 4 km and 9 km depth. Magnitude distribution remained below 4.0, and no events exceeded M3.0 after the first day. The shallow focal depths align with the brittle–ductile transition in this high-heat-flow regime, where hydrothermal fluids facilitate distributed failure.

Since 1 January 2000 the same source region has hosted 11 documented swarms. Prior episodes occurred in 2006 (three swarms), 2007 (one swarm), 2009 (three swarms), and 2010 (four swarms including the present sequence). This recurrence highlights the area’s capacity for repeated swarm activity without large-magnitude mainshocks.

The 2010 swarm provides a clear example of swarm-type seismicity in the Salton Trough. Continued monitoring of similar sequences aids in distinguishing fluid-driven swarms from tectonic mainshock sequences, supporting regional hazard assessment.

References

USGS Earthquake Catalog (ANSS Comprehensive Catalog)
California Geological Survey, Fault Activity Map of California
Southern California Earthquake Data Center, Imperial Valley Seismicity Summaries