Seismic Swarm VS20100423.1 Near Niland, California: Geological Context and Event Analysis

The seismic swarm designated VS20100423.1 occurred approximately 13 km west-southwest of Niland, California, in the Imperial Valley region. It began at 05:33 on 22 April 2010 and concluded at 10:32 on 23 April 2010, spanning 28 hours and 58 minutes. During this period, 38 earthquakes were recorded, with magnitudes ranging from 0.3 to 2.4 and focal depths predominantly between 0 and 10 km. The sequence featured several events above magnitude 1.5, including peaks of 2.4 and 2.2, clustered primarily in the early morning hours of 23 April.

This swarm exemplifies the persistent low-magnitude seismic activity typical of the area. The events were shallow, with the majority occurring at depths of 0–3 km, consistent with the region's tectonic and hydrothermal setting. No significant surface rupture or damage was associated with the activity, aligning with the swarm's modest energy release.

The Niland area lies within the Salton Trough, a tectonically active pull-apart basin formed by the interaction of the Pacific and North American plates along the San Andreas Fault system. Specifically, the location falls in the Brawley Seismic Zone, where distributed right-lateral strike-slip faulting and north-south trending normal faults accommodate plate motion. Geothermal activity linked to the nearby Salton Sea geothermal field further influences seismicity through fluid migration and pore-pressure changes that can trigger earthquake swarms.

Geologically, the Imperial Valley features thick sedimentary deposits overlying basement rocks, with ongoing subsidence and high heat flow. Historical records indicate that seismic swarms have been a recurring phenomenon here, driven by both tectonic strain accumulation and anthropogenic factors such as geothermal fluid extraction. Since 1 January 2000, the region has experienced 32 such swarms. Yearly distribution shows variability, with notable concentrations in 2003 (4 swarms), 2008 (5 swarms), 2009 (11 swarms), and 2010 (5 swarms up to the date of this event). Earlier years recorded fewer instances, reflecting either natural fluctuation or improved detection capabilities over time.

These swarms typically involve dozens of events over hours to days, rarely producing magnitudes exceeding 3.0, yet they provide valuable data on fault behavior and stress transfer in the fault network. The 2010 swarm's timing and characteristics fit established patterns observed in the Brawley zone, where swarms often migrate along fault segments without culminating in a mainshock-aftershock sequence.

Continued monitoring by regional seismic networks remains essential for understanding long-term hazard implications in this densely faulted landscape. The interplay of natural tectonics and geothermal operations underscores the need for integrated geophysical studies to refine risk assessments.

References

• United States Geological Survey (USGS) Earthquake Hazards Program: Imperial Valley and Salton Sea seismicity reports.
• California Geological Survey: Regional fault maps and geothermal field assessments.
• Peer-reviewed literature on Brawley Seismic Zone tectonics (e.g., studies in Bulletin of the Seismological Society of America).