Seismic Swarm S20100405.5: Analysis of Activity Near Ocotillo, California

The seismic swarm designated S20100405.5 began at 16:03 on 4 April 2010 and concluded at 09:01 on 28 April 2010. Centered 9 km northwest of Ocotillo in Imperial County, California, the sequence produced 571 earthquakes over 568 hours and 58 minutes. This event occurred within the tectonically active Salton Trough, a pull-apart basin formed by right-lateral strike-slip motion along the Pacific–North American plate boundary. The region features the intersection of the San Andreas, San Jacinto, and Elsinore fault systems, resulting in frequent distributed seismicity and occasional earthquake swarms.

Examination of the first 100 events reveals predominantly low-magnitude activity. Magnitudes ranged from 0.5 to 3.7, with the majority falling between 1.0 and 2.5. Only four events exceeded magnitude 3.0, including peaks of 3.7 on 5 April at 15:08:17 and 3.6 on 7 April at 09:25:51. Depths were shallow, concentrated between 0 and 10 km, consistent with brittle failure in the upper crust of this extensional regime. Temporal clustering showed elevated rates during the initial 48 hours, followed by a gradual decline, typical of swarm behavior driven by fluid migration or aseismic slip rather than a single mainshock-aftershock cascade.

The Ocotillo area has recorded multiple swarms since 2000, with five episodes documented through 2010. Earlier swarms occurred in 2006 (three episodes) and 2007 (one episode), underscoring recurrent strain release along local fault segments. Such sequences contribute to long-term seismic hazard assessment by illuminating zones of elevated crustal permeability and stress heterogeneity.

Geological mapping indicates that the swarm epicenters align with northwest-trending faults subsidiary to the main plate-boundary structures. Historical records show that similar swarms in the Imperial Valley have occasionally preceded or accompanied larger regional events, although each sequence remains independent in its immediate triggering mechanics.

This analysis of S20100405.5 provides a quantitative baseline for understanding swarm dynamics in southern California. Continued monitoring of microseismicity in the Salton Trough remains essential for refining probabilistic forecasts and mitigating risks to nearby infrastructure.

References

United States Geological Survey Earthquake Catalog
California Geological Survey Fault Activity Map
SeismoSight Internal Swarm Classification Records