Seismic Swarm S20100724.2: Analysis of Activity Near Ocotillo, California

Seismic swarm S20100724.2 was recorded in the area 9 km east of Ocotillo, California, beginning at 10:42 on 23 July 2010 and concluding at 23:13 on 27 July 2010. Over the 108-hour-and-30-minute period, a total of 62 earthquakes were detected. This sequence represents one of several swarms documented in the region since 2000, highlighting the persistent seismic character of the Imperial Valley.

The swarm unfolded with a series of low-magnitude events that increased in frequency during the early hours of 24 July. The largest shock reached magnitude 3.9 at a depth of 3 km on 24 July at 02:29:41. Subsequent activity included multiple events between magnitudes 1.0 and 2.4, with depths ranging from 1 km to 10 km. Notable later shocks comprised a magnitude 2.4 event at 2 km depth on 25 July at 17:10:35 and a magnitude 2.3 shock at 4 km on 27 July at 23:13:20. The temporal distribution showed clustering on 24 and 25 July, followed by a gradual decline toward the end of the sequence.

Geologically, the Ocotillo area lies within the Salton Trough, a tectonically active pull-apart basin formed by the interaction of the Pacific and North American plates along the San Andreas fault system. The region experiences distributed deformation through a network of strike-slip and normal faults, including segments of the Elsinore and Coyote Creek faults. Shallow seismicity is common due to the thin crust and high heat flow associated with the underlying spreading center beneath the Salton Sea. Historical records indicate that the Imperial Valley has produced both swarm-type sequences and larger mainshock-aftershock events, consistent with its role in accommodating right-lateral shear between the two plates.

Since 1 January 2000, fourteen swarms have occurred in the vicinity. Earlier episodes were noted in 2006 (three swarms), 2007 (one swarm), and 2009 (two swarms), with eight additional swarms recorded in 2010 alone. These recurrent clusters underscore the area’s tendency toward episodic, distributed microseismicity rather than isolated large earthquakes.

The 2010 swarm provides further evidence of the Imperial Valley’s ongoing tectonic adjustment. Depths predominantly between 3 km and 6 km align with the brittle-ductile transition zone in this high-temperature setting. No damage or felt reports beyond instrumental detection were associated with the sequence, typical for events of this magnitude range.

Continued monitoring of such swarms contributes to refined understanding of fault interactions and strain accumulation in southern California’s complex plate-boundary zone.