Seismic Swarm VS20110108.1: Insights into Imperial Valley Seismicity

The seismic swarm VS20110108.1 was recorded southwest of Niland, California, beginning at 19:33 on 7 January 2011 and concluding at 15:54 on 10 January 2011. In 68 hours and 21 minutes, 94 earthquakes occurred within this compact sequence. Magnitudes remained modest, ranging from 0.2 to 2.3, with the largest events clustered on 8 January. Focal depths were consistently shallow, between 0 and 5 km, consistent with activity in the uppermost crust.

This swarm exemplifies the episodic, low-magnitude earthquake clusters typical of the Imperial Valley. Events initiated with isolated micro-earthquakes on 7 January before intensifying into a dense sequence on 8 January, featuring multiple events above magnitude 1.0 within hours. Activity then declined steadily through 9 and 10 January. Such temporal patterns reflect fluid migration or aseismic slip triggering successive failures along small faults rather than a single mainshock-aftershock cascade.

The region lies within the Salton Trough, a tectonically active pull-apart basin formed by right-lateral shear along the boundary between the Pacific and North American plates. The Brawley Seismic Zone and nearby San Andreas and Imperial faults accommodate significant strain, producing both isolated events and swarms. Geothermal fields associated with the Salton Sea further promote swarm behavior through hydrothermal fluid circulation that reduces fault friction. Historical records document 39 swarms in the area since 2000, with notable increases in frequency: one each in 2000–2002, three in 2003, and peaks of 11 in 2009 and 13 in 2010. This progression underscores the persistent seismic productivity of the Imperial Valley.

Geological studies of the Salton Trough highlight its rapid subsidence and thick sedimentary fill overlying thinned continental crust. The shallow seismicity observed in VS20110108.1 aligns with deformation concentrated above approximately 5 km depth, where geothermal reservoirs and fault networks interact. Comparable swarms have repeatedly occurred near the Salton Sea Geothermal Field, illustrating the interplay between tectonic loading and local fluid dynamics.

In summary, swarm VS20110108.1 reinforces the Imperial Valley’s status as one of California’s most seismically active zones outside major fault ruptures. Continued monitoring of such sequences provides valuable data on strain accumulation and potential precursors to larger events along the southern San Andreas system.

References
California Geological Survey. (2023). Imperial Valley seismicity reports.
U.S. Geological Survey Earthquake Hazards Program. (2024). Brawley Seismic Zone overview.
Southern California Earthquake Data Center. Regional swarm catalog 2000–present.