Seismic Swarm S20040202.1: Analysis of Activity Near Oak Shores, California

Seismic swarm S20040202.1 occurred 10 km west-southwest of Oak Shores in San Luis Obispo County, California. The sequence began at 21:18 UTC on 1 February 2004 and concluded at 10:03 UTC on 3 February 2004, spanning 36 hours and 44 minutes. During this interval, 34 earthquakes were recorded, with magnitudes ranging from 1.1 to 2.2 and focal depths between 2 km and 9 km.

The events clustered tightly in both space and time, exhibiting the characteristic pattern of a swarm in which no single mainshock dominates. The largest event reached magnitude 2.2 at 08:12 UTC on 2 February at a depth of 5 km. Other notable shocks included a magnitude 2.1 at 10:02 UTC on 3 February (depth 4 km) and multiple magnitude 1.9 events distributed across the first two days. Depths remained shallow throughout, consistent with activity within the upper crust.

This swarm fits within the broader seismic environment of central coastal California. The region lies along the transform boundary between the Pacific and North American plates, where right-lateral strike-slip motion is accommodated primarily by the San Andreas Fault system and associated secondary structures. Oak Shores sits within the Coast Ranges geologic province, underlain by Franciscan Complex rocks and younger sedimentary units that record long-term deformation. Historical seismicity in the area reflects both the main San Andreas trace to the east and distributed faulting nearer the coast.

Since 1 January 2000, six swarms have been identified in the locale according to SeismoSight internal classification. Prior activity includes five swarms in 2003 and one additional swarm in 2004. Such episodic clustering is typical of the region, where fluid migration or aseismic slip on minor faults can trigger sequences without producing a dominant mainshock.

The temporal distribution of the 34 events shows an initial peak within the first 12 hours, followed by a gradual decline. Magnitudes stayed below 2.3, indicating low energy release and minimal potential for damage. Depths averaged approximately 5 km, placing the activity above the brittle-ductile transition and within seismogenic layers commonly observed in central California.

In summary, swarm S20040202.1 represents a short-lived, low-magnitude sequence embedded in the ongoing tectonic regime of the central California coast. Continued monitoring of similar clusters contributes to refined understanding of fault interactions and strain distribution along the plate boundary.

References

USGS Earthquake Catalog (historical seismicity data for central California)
California Geological Survey, Regional Geologic Maps (Coast Ranges province)
SeismoSight internal swarm classification records