Earthquake Swarm S20130828.1: Seismic Activity Near Niland, California

The earthquake swarm designated S20130828.1 occurred 5 km south of Niland, California, in the Imperial Valley. It began at 01:18 on 27 August 2013 and concluded at 18:18 on 28 August 2013, spanning 41 hours and producing 37 recorded events.

Magnitudes ranged from 0.4 to 1.5, with the majority below 1.0. The largest shocks reached 1.5 and clustered during the afternoon and evening of 27 August as well as the early hours of 28 August. Depths remained shallow, primarily between 0 and 8 km, consistent with activity along near-surface fault strands in the region.

Seismic swarms of this type reflect the tectonic setting of the Imperial Valley. The area lies within the Brawley Seismic Zone, where the San Andreas Fault system interacts with the Imperial Fault and associated cross-faults. High heat flow from underlying geothermal sources facilitates fluid migration that can trigger episodic, low-magnitude sequences rather than single large ruptures.

Since 1 January 2000, sixty swarms have been documented in the immediate vicinity. Annual counts show notable variability: one swarm each in 2000–2002, four in 2003, one in 2004, three in 2005, four in 2008, eleven in 2009, thirteen in 2010, four in 2011, eleven in 2012, and six in 2013. This pattern underscores the recurrent nature of swarm activity driven by the local fault network and hydrothermal regime.

The 2013 sequence fits within this established behavior, contributing to the cumulative record of swarm occurrence without exceeding historical magnitude thresholds for the locale. Continued monitoring supports improved characterization of swarm recurrence and potential links to regional strain accumulation.

References

• SeismoSight internal swarm classification records
• USGS earthquake catalog for Imperial Valley, California (events 2000–2013)