Seismic Swarm S20070117.1 Near Ocotillo Wells, California

Seismic swarm S20070117.1 was recorded southwest of Ocotillo Wells in San Diego County, California. The sequence began at 16:07 on 17 January 2007 and concluded at 12:46 on 21 January 2007, spanning 92 hours and 38 minutes. During this period, 64 earthquakes were registered. The events clustered tightly in space and time, with magnitudes ranging from 0.8 to 3.2 and focal depths predominantly between 3 and 6 km.

The swarm initiated with a magnitude 3.8 event at 3 km depth, followed rapidly by smaller aftershocks. Subsequent activity included several events above magnitude 2.0, such as a 2.9 quake at 5 km depth on 17 January and a 3.1 event at 4 km depth on 18 January. The largest shock reached magnitude 3.2 at 3 km depth on 19 January. Most events occurred at shallow crustal levels, consistent with brittle failure in the upper 10 km of the crust. No single dominant mainshock was evident; instead, the sequence displayed the characteristic swarm pattern of numerous similar-sized events without clear foreshock-aftershock decay.

The region lies within the Salton Trough, a tectonically active pull-apart basin formed by right-lateral shear along the Pacific–North American plate boundary. This area is influenced by the southern San Andreas Fault system and the San Jacinto Fault Zone, which accommodate a significant portion of the relative plate motion. The local geology consists of Quaternary alluvial sediments overlying older crystalline basement rocks of the Peninsular Ranges. Historical seismicity in the vicinity includes numerous moderate events linked to the Coyote Creek and Elsinore fault strands, with notable activity documented in the 1950s and 1960s.

Swarm sequences like this one are common in the Imperial Valley and Anza-Borrego region, often associated with fluid migration or aseismic slip transients that promote distributed failure. Depths recorded during the swarm align with the typical seismogenic zone in this extensional step-over, where high heat flow and sedimentary cover promote shallow earthquake nucleation.

The 2007 swarm did not produce reported surface rupture or significant damage, reflecting its modest energy release. Such episodes contribute to long-term strain accommodation along the plate boundary and are monitored closely by regional seismic networks for potential escalation into larger events.

References

USGS Earthquake Catalog
California Geological Survey Fault Database
Southern California Seismic Network reports