Seismic Swarm VS20040316.1: Analysis of Activity Near Niland, California
The seismic swarm designated VS20040316.1 occurred 7 km west-southwest of Niland, California, in the Imperial Valley region of the Salton Trough. This area lies within a tectonically active rift zone formed by the interaction of the Pacific and North American plates along the southern San Andreas Fault system. The Salton Trough experiences frequent earthquake swarms due to extensional faulting, geothermal fluid movement, and volcanic processes associated with the underlying crustal spreading center.
The swarm initiated at 02:40 on 15 March 2004 and concluded at 23:04 on 17 March 2004, spanning 68 hours and 23 minutes. During this period, 64 earthquakes were recorded. Event magnitudes ranged from 1.1 to 3.6, with the largest events reaching 3.6 and 3.5. Depths were predominantly shallow, concentrated between 0 and 5 km, though a few events extended to 11 km. Activity showed temporal clustering, with the highest rate of events occurring in the early hours of 16 March 2004, including multiple magnitude 2.0–2.5 shocks within minutes of each other.
Analysis of the sequence reveals characteristics typical of swarm behavior rather than a classic mainshock-aftershock pattern. No single dominant event triggered the majority of subsequent activity; instead, events occurred in episodic bursts. Depths remained consistently shallow for most shocks, suggesting involvement of near-surface fault structures and possible hydrothermal influences common in the geothermal fields surrounding the Salton Sea. The swarm's compact spatial footprint and lack of clear migration align with fluid-driven triggering mechanisms observed in this rift environment.
The Imperial Valley has a documented history of recurrent swarms. Since 1 January 2000, six such swarms have been identified in the broader region, with prior episodes occurring in 2000, 2001, 2002, and three separate swarms in 2003. This pattern underscores the area's elevated background seismicity, driven by ongoing plate-boundary deformation and local geothermal systems.
Overall, VS20040316.1 exemplifies the swarm-type activity that defines seismic hazard in the Niland vicinity. Continued monitoring of similar sequences provides critical data for understanding stress transfer and fluid dynamics along the plate boundary.
References USGS Earthquake Hazards Program catalog data for Imperial Valley events California Geological Survey reports on Salton Trough tectonics and geothermal seismicity Southern California Seismic Network bulletins on Brawley Seismic Zone activity