Seismic Swarm VS20131009.1 Near Niland, California: Event Analysis and Regional Context

The seismic swarm designated VS20131009.1 occurred 12 km west-southwest of Niland, California, in the Imperial Valley. It began at 10:41 on 8 October 2013 and concluded at 10:26 on 10 October 2013, spanning 47 hours and 44 minutes. During this period, 83 earthquakes were recorded. This sequence exemplifies the recurrent swarm activity characteristic of the region’s tectonically complex setting.

All events remained small, with magnitudes ranging from 0.2 to 2.2. The largest shocks reached magnitude 2.2 at depths of 0 km and 1 km on 9 October. Most events clustered between 0.3 and 0.8 magnitude, occurring at shallow depths of 0–9 km. Depth distribution showed a concentration in the upper 5 km, consistent with fluid-driven or geothermal influences common in the area. Temporal patterns indicated an initial sparse phase on 8 October, followed by a pronounced increase in frequency and a minor peak on 9 October afternoon, before tapering on 10 October.

Regional Geological Setting

Niland lies within the Imperial Valley, part of the Salton Trough, a tectonically active pull-apart basin formed by the interaction of the San Andreas Fault system and the Imperial Fault. The Brawley Seismic Zone, immediately north of the Imperial Fault, hosts frequent earthquake swarms due to right-lateral shear and localized extension. Shallow seismicity here is often linked to geothermal fluids and volcanic processes associated with the underlying spreading center beneath the Salton Sea. The region experiences ongoing subsidence and high heat flow, contributing to the shallow focal depths observed in swarm events.

Historical Swarm Activity

Since 1 January 2000, 62 swarms have been documented in the same locale. Annual counts reveal episodic clustering: 2000–2005 averaged fewer than four events per year, while 2009–2012 showed elevated rates, peaking at 13 swarms in 2010 and 11 in both 2009 and 2012. In 2013 alone, seven swarms occurred prior to VS20131009.1. This pattern underscores the area’s persistent swarm-prone behavior, driven by its position at the transition between transform and extensional regimes.

Insights from Swarm Characteristics

The 2013 sequence displayed typical swarm traits: rapid onset, lack of a dominant mainshock, and migration of activity across a compact volume. Shallow depths and low magnitudes align with fluid migration or aseismic slip triggering brittle failure on small faults. Such events rarely produce surface rupture but can influence local geothermal reservoirs and groundwater systems.

Continued monitoring of the Imperial Valley remains essential for understanding how swarm sequences relate to larger fault behavior along the San Andreas system.

References
SeismoSight internal classification records for swarm VS20131009.1
United States Geological Survey regional tectonic summaries for the Imperial Valley and Salton Trough