Seismic Swarm Near Ocotillo, California: November 2006 Event Analysis

A notable earthquake swarm designated S20061129.1 was recorded 10 km north of Ocotillo, California, beginning at 09:06 on 29 November 2006 and concluding at 13:34 on 30 November 2006. Over this 28-hour and 28-minute period, 34 earthquakes were detected, providing insight into localized seismic behavior in the region.

The swarm exhibited a typical pattern of clustered, low-to-moderate magnitude events without a dominant mainshock. Magnitudes ranged from 0.6 to 4.1, with the largest event occurring at 21:10:55 on 29 November at a depth of 3 km. Other notable shocks included a magnitude 3.9 event at 12:17:35 (depth 3 km), a magnitude 3.7 at 21:12:52 (depth 2 km), a magnitude 3.8 at 22:15:39 (depth 1 km), and a magnitude 3.1 at 21:26:17 (depth 1 km). Depths were predominantly shallow, between 0 and 3 km, consistent with activity along near-surface fault structures.

This swarm represents the first of two such sequences recorded in the area since 1 January 2000. The events were tightly grouped in time, with the majority occurring within the first 24 hours, followed by a rapid decline in activity.

The Ocotillo region lies within the tectonically active southern California desert, part of the broader Pacific-North American plate boundary zone. This area features a network of strike-slip faults that accommodate regional shear strain. Historical seismicity includes both isolated events and occasional swarms, reflecting the complex interaction of faults in the Imperial Valley and adjacent mountain ranges. Shallow focal depths observed during the 2006 swarm align with known patterns of brittle failure in the upper crust of this desert setting.

Geological mapping indicates that the local subsurface consists of sedimentary basins overlying crystalline basement rocks, with faulting influenced by both regional plate motion and local structural features. Such conditions can facilitate episodic swarm activity driven by fluid migration or stress transfer along minor faults.

Analysis of the temporal distribution shows peak activity between 12:00 and 22:00 on 29 November, after which events became sporadic. Depths remained consistently shallow throughout, suggesting a confined source volume. No damage or felt reports beyond instrumental detection were associated with the sequence, typical for events below magnitude 5 in sparsely populated desert terrain.

Continued monitoring of this sector remains important given its position within a historically active portion of the plate boundary. The 2006 swarm underscores the value of dense seismic networks in capturing subtle patterns that may precede or accompany larger regional events.

References

United States Geological Survey Earthquake Catalog
California Geological Survey Regional Fault Maps
SeismoSight Internal Swarm Classification Database