Seismic Swarm S20101218.1: Analysis of Activity Near Ocotillo, California

The seismic swarm designated S20101218.1 occurred in the tectonically active Imperial Valley region of Southern California, centered 17 km east-southeast of Ocotillo. This sequence began at 23:57 on 17 December 2010 and concluded at 01:16 on 20 December 2010, spanning 49 hours and 18 minutes. During this period, 36 earthquakes were recorded, with magnitudes ranging from 0.6 to 4.1 and focal depths between 3 km and 10 km.

The Imperial Valley lies within a complex zone of interaction between the Pacific and North American plates. Right-lateral strike-slip motion along regional faults, including segments of the San Andreas system and subsidiary structures such as the Imperial and Brawley faults, drives the area's seismicity. The region experiences both mainshock-aftershock sequences and swarm-type activity, the latter characterized by clustered events without a dominant mainshock. Depths in this swarm remained shallow, consistent with the brittle upper crust in the Salton Trough, where high heat flow and sedimentary basin fill influence rupture characteristics.

The largest event reached magnitude 4.1 at 07:04 on 18 December at a depth of 9 km. Subsequent notable shocks included three events above magnitude 3.0 within a two-hour window later that morning, peaking at magnitude 3.5. Smaller events dominated the later stages, with the final recorded shock of magnitude 2.0 occurring at 01:16 on 20 December at a depth of 3 km. Temporal clustering shows peak activity during the first 24 hours, followed by a gradual decline, typical of swarm behavior driven by fluid migration or aseismic slip rather than static stress transfer alone.

Historical records indicate elevated swarm frequency in the area since 2000. Seventeen swarms have been documented, with annual counts of three in 2006, one in 2007, three in 2009, and ten in 2010. This pattern reflects the Imperial Valley's persistent background seismicity, where swarms often occur near fault intersections or volcanic-adjacent zones without producing surface rupture.

Geological context underscores the area's long-term hazard. The Salton Trough represents a pull-apart basin formed by plate-boundary transtension, with ongoing sedimentation and geothermal activity. Paleoseismic studies reveal recurrent moderate-to-large earthquakes along nearby faults, contributing to regional strain accumulation. Updated monitoring by regional networks continues to track microseismicity, aiding in refined hazard models for Imperial County communities.

References

United States Geological Survey Earthquake Hazards Program reports on Imperial Valley seismicity.
California Geological Survey regional fault and basin summaries.
SeismoSight internal swarm classification database.