Seismic Swarm S20060914.1 Near Ocotillo, California

Seismic swarm S20060914.1 occurred 6 km west-southwest of Ocotillo in Imperial County, California. The sequence began at 00:11 UTC on 14 September 2006 and concluded at 19:28 UTC on 15 September 2006, spanning 43 hours and 17 minutes. During this interval, 38 earthquakes were recorded.

The swarm took place within the tectonically active Salton Trough, a pull-apart basin formed by right-lateral shear along the Pacific–North American plate boundary. This region lies at the transition between the San Andreas Fault system to the north and the Imperial Fault to the south. Shallow seismicity is common here because of ongoing crustal extension and the presence of geothermal fluids that can facilitate slip on faults.

The recorded events ranged in magnitude from 0.3 to 4.0, with the largest shock (M4.0) occurring at 00:11:06 UTC on 14 September at a depth of 9 km. A secondary peak reached M3.6 at 01:25:14 UTC the same day, also at 9 km depth. Most events clustered between 3 km and 7 km depth, consistent with the brittle–ductile transition zone in this high-heat-flow area. Depths rarely exceeded 9 km, reflecting the limited thickness of the seismogenic crust in the Imperial Valley.

No single mainshock dominated the sequence; instead, energy release was distributed across numerous small events, a hallmark of swarm behavior. Such swarms in the Salton Trough have been linked to aseismic creep, fluid migration, or minor magmatic intrusions associated with the underlying spreading center. Historical swarms in the same general vicinity occurred in 1975, 1981, and 2005, underscoring the recurrent nature of this activity.

The 2006 swarm produced no reported damage or felt shaking beyond light tremors in nearby communities. Its modest magnitudes and shallow depths limited potential impact. Continued monitoring by regional networks remains essential, as the Imperial Valley hosts critical infrastructure and lies near the U.S.–Mexico border.

References
USGS Earthquake Catalog (events 2006)
Southern California Seismic Network bulletins
USGS Professional Paper 1515, The San Andreas Fault System
California Geological Survey Fault Activity Map (2023 update)