Seismic Swarm S20131014.1: Analysis of Activity Near Niland, California

A notable earthquake swarm, designated S20131014.1, occurred 16 km west-southwest of Niland, California, from 08:45 on 13 October 2013 to 19:06 on 15 October 2013. The sequence lasted 58 hours and 20 minutes and comprised 83 events. Magnitudes ranged from 0.3 to 3.6, with the largest shock recorded at 3.6 at a depth of 1 km on 14 October at 12:38:23 UTC. Most events clustered at shallow depths between 0 and 5 km, consistent with activity in the upper crust.

The swarm exhibited typical characteristics of clustered seismicity without a dominant mainshock-aftershock pattern. Early events on 13 October remained below magnitude 1.5, followed by a peak in frequency and intensity on 14 October when multiple events exceeded magnitude 2.0, including a 2.9 at 06:43 and a 2.8 at 07:16. Activity tapered on 15 October with only a few events above magnitude 1.0. Depths showed minor variation, with the majority occurring at 1–2 km, suggesting fluid migration or localized stress changes within the shallow fault network.

This region lies within the Imperial Valley of southern California, part of the broader San Andreas Fault system where the Pacific and North American plates interact. The area overlaps the Brawley Seismic Zone and lies adjacent to the Salton Sea geothermal field, where high heat flow and hydrothermal circulation promote swarm-type seismicity. Geological formations include thick sedimentary sequences overlying basement rock, with active normal and strike-slip faults accommodating extensional and shear strain. Historical records indicate persistent seismic productivity driven by both tectonic loading and geothermal processes.

Swarm activity has been recurrent since at least 2000. Between 1 January 2000 and the end of 2013, 63 swarms were documented in the vicinity. Annual counts include one each in 2000–2002, four in 2003, and elevated rates in later years, peaking at 13 swarms in 2010 and 11 in both 2009 and 2012. These episodes reflect the area’s structural complexity and ongoing geothermal influence rather than large tectonic release.

The 2013 swarm fits established patterns of moderate-magnitude, shallow events typical of the Imperial Valley. No damage or felt reports beyond the immediate region were associated with the sequence, underscoring its microseismic character. Continued monitoring remains essential given the proximity to critical infrastructure and the Salton Sea geothermal operations.

References
SeismoSight internal swarm catalog (S20131014.1 parameters and event list).
USGS Earthquake Catalog (regional seismicity and fault data for Imperial Valley).
California Geological Survey (geothermal and fault zone descriptions).