Seismic Swarm S20120910.1 Near Bombay Beach, California

Seismic swarm S20120910.1 was recorded in the Salton Trough region of southern California, centered 8 km south-southeast of Bombay Beach. The sequence began at 17:58 UTC on 9 September 2012 and concluded at 19:37 UTC on 10 September 2012, spanning 25 hours and 39 minutes. During this interval, 28 earthquakes were detected, with magnitudes ranging from 0.9 to 3.2 and focal depths primarily between 1 km and 8 km.

The largest event reached magnitude 3.2 at 15:44 UTC on 10 September at a depth of 3 km. This was followed within minutes by two magnitude 2.6 events at depths of 2 km and 3 km. Most activity clustered between 15:00 and 17:00 UTC on 10 September, with events occurring at intervals of seconds to minutes. Shallower events (1–3 km) dominated the later phase, while a few deeper shocks (7–8 km) occurred toward the end of the sequence.

The swarm occurred within the Brawley Seismic Zone, a tectonically active area at the southern terminus of the San Andreas Fault. Here, the Pacific and North American plates interact across a complex network of strike-slip and normal faults within the Salton Trough pull-apart basin. Ongoing right-lateral shear and localized extension produce frequent earthquake swarms, often without a single dominant mainshock. Historical records since 2000 document 48 such swarms in the region, with notable increases in 2009 (10 swarms) and 2010 (11 swarms).

This swarm's temporal distribution shows a rapid onset after the initial magnitude 1.4 event, followed by a peak in frequency and magnitude roughly 21 hours later. Depths remained consistently shallow, consistent with fluid migration or aseismic slip triggering brittle failure in the upper crust. No damage or felt reports beyond the immediate area were associated with these low-magnitude events.

Such sequences provide insight into fault behavior along the San Andreas system and contribute to ongoing monitoring of strain accumulation near the Salton Sea geothermal field.

References

SeismoSight internal swarm catalog S20120910.1
USGS Earthquake Catalog (Salton Trough region)
California Geological Survey, Imperial Valley seismicity reports