Seismic Swarm VS20210104.1: Analysis of Activity Near Niland, California

A seismic swarm designated VS20210104.1 occurred 8 km southwest of Niland, California, beginning at 18:04 on 4 January 2021 and concluding at 15:24 on 5 January 2021. Over 21 hours and 19 minutes, the sequence produced 43 earthquakes. Magnitudes ranged from 0.9 to 3.9, with the two largest events—a 3.9 and a 3.4—recorded within the first 15 minutes. Depths were predominantly shallow, clustered between 1 km and 4 km, consistent with activity in the upper crust of the region.

The sequence began with rapid-fire events, including multiple shocks above magnitude 3.0 in the initial hour, followed by a gradual decline in both frequency and magnitude. Later events remained small, mostly below magnitude 2.0, with the final recorded shock at magnitude 1.3. This pattern reflects typical swarm behavior, where energy release occurs through numerous moderate events rather than a single mainshock-aftershock sequence.

Niland lies within the Imperial Valley portion of the Salton Trough, a tectonically active rift zone formed by the interaction of the Pacific and North American plates. The area experiences distributed deformation along the San Andreas Fault system, the Imperial Fault, and the Brawley Seismic Zone. Geothermal fields and volcanic features associated with the Salton Sea further influence local seismicity through fluid migration and heat flow.

Since 2000, the region has recorded 80 seismic swarms. Activity peaked in 2010 and 2013 with 13 swarms each, followed by notable clusters in 2009 and 2012. Earlier decades showed lower but persistent swarm occurrence, indicating ongoing tectonic strain accumulation and release.

Such swarms are common in the Salton Trough due to its transtensional setting, where strike-slip and extensional faulting coexist. Shallow depths observed in VS20210104.1 align with historical patterns linked to geothermal reservoirs and fault intersections. Continued monitoring remains essential given the proximity to critical infrastructure and populated areas.

References

SeismoSight internal swarm classification records.
USGS Earthquake Catalog for Imperial Valley, California.
California Geological Survey reports on Salton Trough tectonics.