Seismic Swarm VS20100222.1: Analysis of Activity Near Niland, California

The region surrounding Niland, California, lies within the Imperial Valley portion of the Salton Trough, a tectonically active rift zone formed by the interaction between the Pacific and North American plates. This area experiences frequent microseismicity due to its position along the southern extension of the San Andreas Fault system and the Brawley Seismic Zone. Geothermal gradients are elevated because of shallow magmatic intrusions and fault-controlled fluid circulation, contributing to the area's history of earthquake swarms rather than isolated large-magnitude events.

Seismic swarm VS20100222.1 began at 13:19 on 21 February 2010 and concluded at 09:21 on 23 February 2010. Over 44 hours and 2 minutes, the sequence produced 28 earthquakes with magnitudes ranging from 0.2 to 1.5 and focal depths between 0 and 14 km. The events clustered primarily on 22 February, with a notable concentration between 12:21 and 12:47, during which 17 earthquakes occurred. Depths were predominantly shallow, with most events located at 0–6 km, consistent with fluid-driven processes in the geothermal environment.

Time-series analysis of the swarm reveals a rapid onset followed by a peak in activity within the first 24 hours and a quick decay. The largest events reached magnitude 1.5, and no event exceeded magnitude 2.0, underscoring the low-energy character typical of swarm sequences in this locale. Depths showed minor variation, with the initial event at 14 km and subsequent activity migrating to shallower levels.

Historical records maintained by SeismoSight indicate that 28 swarms have occurred in the region since 1 January 2000. Annual counts include one swarm each in 2000, 2001, 2002, and 2004; three each in 2003 and 2005; five in 2008; eleven in 2009; and two in 2010. This pattern demonstrates persistent swarm activity over the two-decade interval, with a marked increase in frequency after 2008.

The geological setting explains the recurrence of such sequences. Crustal extension and strike-slip faulting facilitate episodic fluid migration that triggers numerous small events without producing a single dominant mainshock. Monitoring data from the region confirm that swarm durations commonly span one to three days, aligning with the 44-hour span observed in VS20100222.1.

References

SeismoSight internal swarm classification database (VS20100222.1 parameters and historical statistics since 2000).
United States Geological Survey Earthquake Catalog (regional tectonics and geothermal context for Imperial Valley).