Earthquake Swarm S20160926.1: Seismic Activity Near Bombay Beach, California

The region surrounding Bombay Beach, California, lies within the tectonically complex Imperial Valley, where the southern San Andreas Fault transitions into the Brawley Seismic Zone. This area experiences frequent earthquake swarms due to right-lateral strike-slip faulting combined with geothermal and volcanic influences from the nearby Salton Sea. Depths of seismic events typically range from 1 to 10 kilometers, reflecting shallow crustal processes in this extensional regime. Swarm S20160926.1 began at 11:03 on 26 September 2016 and concluded at 18:35 on 2 October 2016. Over 151 hours and 31 minutes, the sequence produced 322 earthquakes centered 6 kilometers southeast of Bombay Beach. Analysis of the first 100 events reveals predominantly low-magnitude activity, with events clustered at shallow depths between 2 and 5 kilometers. Magnitudes ranged from 1.0 to a peak of 4.3, recorded at 14:31 on 26 September at a depth of 2 kilometers. Subsequent notable shocks included a 3.2-magnitude event at 13:14 and a 3.0-magnitude quake at 15:47, both at shallow depths. The sequence showed rapid succession of events in the initial hours, with many tremors between 1.5 and 2.6 in magnitude occurring within minutes of one another. Depth distribution among these events remained consistent, averaging around 3 kilometers, indicating brittle failure in the upper crust. Temporal patterns suggest episodic energy release, with bursts of activity separated by brief lulls. No events exceeded magnitude 4.3 in the initial segment, underscoring the swarm's characteristic of numerous small shocks rather than a dominant mainshock-aftershock sequence. Historical records since 1 January 2000 document 67 swarms in the vicinity. Annual counts include single swarms in 2000, 2001, 2002, 2004, and 2014 through 2016. Elevated activity occurred in 2009 with 11 swarms, 2010 with 12, 2012 with 10, and 2013 with 13. This pattern highlights the persistent seismic productivity of the Brawley Seismic Zone, driven by ongoing plate boundary deformation. Such swarms contribute to understanding fault interactions and stress transfer along the San Andreas system. Continued monitoring supports hazard assessment in this populated and infrastructurally sensitive region.

References

USGS Earthquake Catalog SeismoSight Internal Swarm Database