Seismic Swarm S20210201.1 Near Ocotillo, California: Event Analysis and Regional Context

Seismic swarm S20210201.1 occurred approximately 15 km east of Ocotillo in Imperial County, California. The sequence initiated at 08:41 UTC on 31 January 2021 and concluded at 05:54 UTC on 4 February 2021, spanning 93 hours and 12 minutes. During this interval, 68 earthquakes were recorded.

Event magnitudes ranged from 0.6 to 3.7, with the majority falling between 1.0 and 2.0. Depths were predominantly shallow, concentrated between 2 km and 11 km, though isolated events reached 16 km. Notable activity included a magnitude 3.5 event on 1 February at 04:18 UTC (depth 11 km) and a magnitude 3.7 event on 3 February at 09:23 UTC (depth 11 km). The swarm exhibited typical characteristics of clustered microseismicity, with multiple events occurring within minutes of one another during peak periods on 1 and 3 February.

The Ocotillo region lies within the Salton Trough, a tectonically active pull-apart basin formed by the interaction of the Pacific and North American plates. This area forms part of the broader San Andreas Fault system, where right-lateral strike-slip motion transitions into extensional regimes. Local faulting includes segments of the Elsinore Fault zone and subsidiary structures associated with the Imperial Fault, contributing to elevated background seismicity and occasional geothermal manifestations.

Historical records indicate recurrent swarm activity in the vicinity. Since 1 January 2000, 22 swarms have been documented in the same locale. These episodes cluster in specific years: three in 2006, one in 2007, three in 2009, thirteen in 2010, one in 2011, and one in 2019. Such patterns reflect the region's persistent strain accumulation and release along distributed fault networks.

Analysis of swarm S20210201.1 reveals a compact temporal footprint with no single dominant mainshock, consistent with fluid-driven or aseismic-slip-triggered sequences common in the Salton Trough. Depths align with the brittle-ductile transition zone in this high-heat-flow environment, where seismicity rarely exceeds 15–20 km.

References

• SeismoSight internal swarm catalog (S20210201.1 parameters and event list)
• USGS Earthquake Catalog (regional historical swarm statistics and fault mapping)
• California Geological Survey (Salton Trough tectonic framework)