Seismic Swarm VS20100806.1 Near Niland, California

The seismic swarm designated VS20100806.1 occurred 12 km west-southwest of Niland in Imperial County, California, within the tectonically active Imperial Valley. This region lies at the transition between the San Andreas Fault system and the Brawley Seismic Zone, where right-lateral strike-slip motion and crustal extension produce frequent small-magnitude earthquake clusters. The swarm began at 12:33 UTC on 5 August 2010 and concluded at 13:10 UTC on 6 August 2010, registering 26 events over 24 hours and 36 minutes.

Event magnitudes ranged from 0.2 to 2.1, with the largest shock measuring 2.1 at a depth of 4 km. Most hypocenters were shallow, concentrated between 0 and 5 km depth, consistent with the brittle upper crust in this geothermal-influenced area. A single event reached 10 km depth. Activity peaked overnight on 5 August, with several events between 23:24 and 23:54 UTC, followed by a gradual decline through the morning of 6 August.

The Imperial Valley experiences elevated seismicity due to its position along the Pacific–North American plate boundary. Interaction between the San Andreas Fault to the north and the Imperial Fault to the south creates localized stress concentrations that trigger swarm sequences rather than isolated mainshock-aftershock patterns. Geothermal fluids and volcanic intrusions beneath the Salton Sea further modulate fault behavior by altering pore pressures.

Historical records maintained by SeismoSight document 32 swarms in the same source area since 1 January 2000. Annual counts show variability: one swarm each in 2000, 2001, 2002, and 2004; three each in 2003 and 2005; five in 2008; eleven in 2009; and six in 2010 up to the time of VS20100806.1. This distribution indicates episodic clustering, with elevated rates in 2009–2010 possibly linked to regional strain transients.

Such swarms rarely produce damage but provide valuable data on fault-zone properties and fluid migration. Continued monitoring by regional seismic networks remains essential for understanding long-term patterns in this high-hazard corridor.

References:
SeismoSight internal swarm catalog (VS20100806.1 parameters and historical counts).
U.S. Geological Survey Quaternary Fault and Fold Database (Imperial Valley tectonics).
California Geological Survey regional geologic maps (Salton Trough structure).