Seismic Swarm VS20210201.1: Analysis of Activity Near Niland, California

Seismic swarm VS20210201.1 was recorded in the Imperial Valley of southern California, centered 14 km west-southwest of Niland. The sequence began at 20:36 UTC on 31 January 2021 and concluded at 19:51 UTC on 1 February 2021, spanning 23 hours and 14 minutes. During this interval, 38 earthquakes were registered, with magnitudes ranging from 0.4 to 2.5 and focal depths predominantly between 3 km and 6 km.

The temporal distribution showed two main clusters. The initial phase on 31 January featured eight events within the first 15 minutes, including a magnitude 2.5 earthquake at 20:42. Activity then declined before resuming with additional events through the evening. On 1 February, a second cluster occurred between 00:12 and 00:32, again including a magnitude 2.5 event, followed by sporadic smaller shocks until the final recorded event at 19:51. Depths remained shallow throughout, consistent with the region’s typical seismogenic zone.

This swarm exemplifies the short-duration, clustered seismicity common in the area. Most events clustered tightly in both time and space, with no single mainshock dominating the sequence. Such patterns reflect fluid migration or aseismic slip along fault networks rather than classic foreshock-mainshock-aftershock behavior.

The Niland region lies within the Salton Trough, a transtensional basin formed by the interaction of the San Andreas Fault system and the divergent boundary between the Pacific and North American plates. The Brawley Seismic Zone, which extends through the Imperial Valley, hosts frequent earthquake swarms driven by right-lateral strike-slip faulting and geothermal processes linked to the underlying magma chamber beneath the Salton Sea. Historical records since 2000 document 85 swarms in the immediate vicinity, with notable increases in 2009 (11 swarms), 2010 (13), 2012 (11), and 2013 (13). These episodes underscore the persistent tectonic strain accumulation and release characteristic of the southernmost San Andreas Fault system.

Geothermal gradients in the Salton Trough exceed 200 °C/km in places, promoting hydrothermal circulation that can trigger swarm activity through pore-pressure changes. The shallow depths observed in VS20210201.1 align with this environment, where brittle failure occurs above the brittle-ductile transition at approximately 6–8 km.

Continued monitoring by regional seismic networks remains essential for distinguishing background swarm activity from potential precursors to larger events along the San Andreas or Imperial faults. The 2021 sequence fits within the established recurrence pattern of low-magnitude, high-frequency swarms that dominate the local seismic catalog.

References

SeismoSight internal swarm classification VS20210201.1
USGS Earthquake Catalog (regional tectonics of the Salton Trough)
Southern California Seismic Network annual reports on Imperial Valley seismicity