Earthquake Swarm S20090321.1: Seismic Activity Near Bombay Beach, California

Earthquake swarm S20090321.1 occurred 4 km south of Bombay Beach, California, beginning at 08:54 on 21 March 2009 and concluding at 12:45 on 23 March 2009. Over 51 hours and 51 minutes, the sequence produced 50 earthquakes, providing a clear example of clustered seismic activity in the Imperial Valley region.

The events ranged in magnitude from 0.3 to 3.3, with the largest shock recorded at magnitude 3.3 on 21 March at 20:17:03. Depths varied between 0 and 11 km, though most clustered between 4 and 7 km. Activity showed distinct pulses, including a notable concentration of events between 20:12 and 20:52 on 21 March, followed by lower-frequency occurrences through 22 March and a final series on 23 March.

Regional Geological Context

Bombay Beach lies along the eastern margin of the Salton Sea within the Salton Trough, a pull-apart basin formed by the interaction of the San Andreas Fault to the north and the Imperial Fault to the south. This tectonic setting produces frequent small-magnitude earthquakes and occasional swarms driven by right-lateral strike-slip motion and associated subsidiary faulting. The shallow crust in this area is further influenced by geothermal gradients and fluid migration linked to the underlying magmatic system beneath the Salton Sea geothermal field.

Historical Swarm Patterns

Seismic swarms have been recurrent in the region. Since 1 January 2000, thirteen swarms have been documented, distributed across the following years: one each in 2000, 2001, 2002, and 2004; two in 2003; three in 2005; and four in 2008. These episodes typically feature dozens of events over periods of one to three days, with magnitudes rarely exceeding 4.0. Swarm S20090321.1 fits this established pattern both in duration and event count.

Insights from Event Distribution

Analysis of the 50 recorded events reveals temporal clustering and a predominance of shallow focal depths. The initial 24 hours accounted for the majority of higher-magnitude shocks, after which activity declined sharply. Depths near 4 km were especially common during peak periods, consistent with slip on shallow fault segments within the Brawley Seismic Zone. Such distributions aid in characterizing fluid-driven or aseismic-slip-triggered sequences typical of the Imperial Valley.

References

USGS Earthquake Catalog (historical seismicity data for Imperial Valley)
California Geological Survey, Regional Fault and Seismic Hazard Reports
Southern California Earthquake Data Center, Salton Trough Tectonic Summaries