Seismic Swarm VS20110328.1 Near Niland, California: Geological Context and Event Analysis

The region surrounding Niland, California, lies within the Salton Trough, a tectonically active pull-apart basin formed by the interaction of the San Andreas Fault system and the Imperial Fault. This area experiences frequent seismic swarms due to right-lateral strike-slip faulting, geothermal fluid migration, and crustal extension associated with the transition between the Pacific and North American plates. The Brawley Seismic Zone, located immediately west of Niland, is a well-documented source of clustered microseismicity, often linked to the underlying geothermal fields and the Salton Sea geothermal anomaly.

Swarm VS20110328.1 was recorded 14 km west-southwest of Niland. The sequence initiated at 22:44 on 27 March 2011 and concluded at 16:53 on 29 March 2011, spanning 42 hours and 9 minutes. During this interval, 42 earthquakes were detected, with magnitudes ranging from 0.2 to 3.4 and focal depths between 0 and 5 km. The largest event, magnitude 3.4, occurred at 04:56 on 28 March at a depth of 3 km. Most activity remained below magnitude 1.5, consistent with the swarm-like character typical of the Brawley Seismic Zone, where events cluster without a dominant mainshock-aftershock pattern.

Temporal distribution showed peak activity during the early morning hours of 28 March, followed by a gradual decline. Shallow depths (predominantly 1–3 km) suggest involvement of near-surface faults and hydrothermal processes rather than deeper crustal rupture. Such shallow swarms are common in this geothermal setting, where fluid pressure changes can trigger brittle failure along pre-existing fractures.

Historical records indicate elevated swarm frequency in the region since 2000. A total of 40 swarms have been identified through 2011, with annual counts increasing notably after 2008: five in 2008, eleven in 2009, thirteen in 2010, and one recorded in 2011 prior to VS20110328.1. Earlier years showed lower incidence, with one to three events per year between 2000 and 2005. This trend may reflect improved detection capabilities or changes in local stress and fluid conditions.

The geological setting of the Imperial Valley amplifies swarm potential. High heat flow from the underlying magma body beneath the Salton Sea promotes hydrothermal circulation that reduces effective normal stress on faults. Combined with ongoing dextral shear from the San Andreas system, this environment produces episodic bursts of small-magnitude events rather than infrequent large earthquakes.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
California Geological Survey, Imperial Valley Fault Maps
SCEC Community Fault Model documentation