Seismic Swarm VS20131222.1 Near Niland, California: Geological Context and Event Analysis

The region surrounding Niland, California, lies within the Imperial Valley portion of the Salton Trough, a tectonically active rift zone formed by the interaction between the Pacific and North American plates. This area experiences ongoing extension and strike-slip motion linked to the southern San Andreas Fault system and the Brawley Seismic Zone. Shallow seismicity is common, often driven by a combination of fault slip, fluid migration, and geothermal processes associated with underlying magmatic intrusions beneath the Salton Sea.

Earthquake swarms in this setting typically feature numerous small-magnitude events clustered over hours to days without a dominant mainshock. Swarm VS20131222.1, located 9 km west-southwest of Niland, began at 11:14 on 21 December 2013 and concluded at 20:06 on 22 December 2013. Over 32 hours and 51 minutes, 65 earthquakes were recorded. Magnitudes ranged from 0.1 to 2.4, with the largest events reaching 2.4, 2.3, and 2.2. Focal depths were predominantly shallow, between 0 and 12 km, consistent with activity along near-surface faults in the Brawley Seismic Zone.

The sequence showed an initial moderate event of magnitude 1.6, followed by a rapid increase in activity around midnight on 22 December. Multiple events of magnitude 1.8–2.3 occurred within a 10-minute window, accompanied by numerous microearthquakes below magnitude 1.0. Activity then declined gradually, with isolated events continuing into the afternoon and evening. Depths remained mostly in the upper 7 km, suggesting slip on shallow crustal structures influenced by local geothermal gradients.

Historical records indicate that such swarms are recurrent in the Imperial Valley. Since 1 January 2000, 67 swarms have been identified in the broader region. Annual counts show variability, with peaks in 2009 (11 swarms), 2010 (13), and 2012 (11). Earlier years recorded fewer episodes, reflecting both natural fluctuation and improvements in seismic network detection. These patterns underscore the persistent tectonic strain and hydrothermal activity that characterize the Salton Trough.

The December 2013 swarm fits the established behavior of the Brawley Seismic Zone, where swarms often migrate along fault segments without producing significant surface rupture. No damage or felt reports beyond minor shaking were associated with this event sequence, typical for events below magnitude 3.0.

Continued monitoring by regional seismic networks remains essential for tracking swarm evolution and assessing any potential escalation toward larger tectonic events along the San Andreas Fault.

References

• SeismoSight internal swarm classification data (VS20131222.1)
• USGS Earthquake Catalog for Imperial Valley, California
• California Geological Survey reports on the Brawley Seismic Zone and Salton Trough tectonics