Seismic Swarm VS20110402.1: Analysis of Activity Near Niland, California
Seismic swarm VS20110402.1 was recorded in the Imperial Valley region of southern California, centered 10 km west-southwest of Niland. The sequence began at 02:32 on 2 April 2011 and concluded at 14:53 on 3 April 2011, encompassing 34 earthquakes over 36 hours and 20 minutes. Magnitudes ranged from 0.3 to 2.4, with the majority occurring at shallow depths between 0 and 7 km. The largest event reached magnitude 2.4 at 02:32:51 on 2 April, followed closely by additional activity including events of magnitude 1.9 and 1.7 later that day.
This swarm exemplifies typical clustered seismicity in the area, where events occur in rapid succession without a dominant mainshock. Depths remained predominantly crustal, consistent with shallow faulting and fluid-related processes. The sequence tapered off by mid-afternoon on 3 April, with the final recorded magnitude 1.4 event at 14:53:27.
The Imperial Valley lies within the Salton Trough, a tectonically active pull-apart basin formed by the transition between the San Andreas Fault system and the Imperial Fault. Right-lateral strike-slip motion along the boundary between the Pacific and North American plates drives frequent small-magnitude earthquakes. Geothermal fields near the Salton Sea further contribute to swarm activity through hydrothermal fluid circulation that alters stress on nearby faults.
Historical records indicate 41 swarms in the region since 2000. Activity increased notably after 2008, with 5 swarms in 2008, 11 in 2009, 13 in 2010, and 2 in 2011. Earlier years showed lower frequency, including single swarms in 2000, 2001, 2002, and 2004, and three each in 2003 and 2005. This pattern reflects the persistent tectonic strain and geothermal influence characteristic of the Brawley Seismic Zone.
Swarm events such as VS20110402.1 provide insight into episodic strain release. The tight clustering of low-magnitude quakes at depths of 1–3 km suggests involvement of shallow crustal structures, potentially linked to local fault segments or geothermal reservoirs. Such sequences help monitor evolving stress conditions without producing significant surface rupture or damage.
References
- United States Geological Survey Earthquake Catalog
- California Geological Survey Regional Fault Maps
- Southern California Earthquake Data Center Historical Seismicity Records