Seismic Swarm S20090324.1: Analysis of Activity Near Bombay Beach, California

Seismic swarm S20090324.1 was recorded 3 km south of Bombay Beach, California, beginning at 11:55 on 24 March 2009 and concluding at 09:05 on 31 March 2009. Over 165 hours and 9 minutes, the sequence produced 410 earthquakes. This event occurred in the tectonically active Salton Trough, a pull-apart basin at the southern terminus of the San Andreas Fault where the fault interacts with the Imperial and Brawley fault zones.

The swarm initiated with a magnitude 4.7 earthquake at 6 km depth. Subsequent events were predominantly shallow, with most hypocenters between 3 km and 7 km. Magnitudes in the first 100 events ranged from 0.3 to 4.7, featuring a rapid succession of aftershocks clustered in the initial hours. Depths remained consistent around 4–5 km for the majority of events, with occasional outliers reaching 11–12 km. Notable later events in this subset included a magnitude 3.1 at 5 km depth and a magnitude 2.9 at 4 km depth, illustrating the swarm’s characteristic pattern of moderate mainshocks followed by sustained low-magnitude activity.

The Bombay Beach area lies within the Brawley Seismic Zone, where right-lateral strike-slip motion along the San Andreas Fault transitions into extensional and transtensional regimes associated with the Salton Sea geothermal field. This setting promotes episodic swarm activity driven by fluid migration and aseismic slip rather than classic mainshock-aftershock sequences. Historical records since 2000 indicate 14 prior swarms in the immediate region, distributed across 2000 (1), 2001 (1), 2002 (1), 2003 (2), 2004 (1), 2005 (3), 2008 (4), and 2009 (1). These recurrent swarms reflect ongoing strain accumulation and release along the southern San Andreas Fault, which last produced a major rupture in 1857.

Geological monitoring in this region benefits from dense seismic networks that capture the fine-scale spatiotemporal evolution of swarms. Depths consistently in the upper crust align with the brittle-ductile transition influenced by elevated geothermal gradients near the Salton Sea. Such activity provides critical data for assessing fault interaction and potential triggering across the San Andreas–Imperial fault junction.

References
SeismoSight internal swarm classification records
USGS Earthquake Catalog (regional tectonics)
California Geological Survey fault database