Earthquake Swarm S20040727.1 Near Oak Shores, California

Seismic swarm S20040727.1 occurred in a tectonically active region of central California, centered 8 km southwest of Oak Shores in San Luis Obispo County. This area lies within the Coast Ranges, where ongoing deformation results from the transform boundary between the Pacific and North American plates. The swarm began at 07:21 on 26 July 2004 and concluded at 08:29 on 5 August 2004, spanning 241 hours and 7 minutes. During this interval, 140 earthquakes were recorded.

Analysis of the first 100 events reveals predominantly low-magnitude activity, with most events registering between 0.9 and 2.3. A notable outlier reached magnitude 3.7 on 27 July at 03:38:17, accompanied by a magnitude 3.0 event later that hour. Depths clustered between 3 km and 9 km, though occasional events extended to 11 km and 12 km, consistent with shallow crustal faulting typical of the region. Temporal distribution showed peak frequency in the initial 48 hours, followed by a gradual decline, with events distributed across multiple minor faults rather than a single structure.

The broader geological setting features complex fault networks linked to the San Andreas system and subsidiary structures such as the Rinconada and Hosgri faults. These accommodate right-lateral shear and produce both mainshock-aftershock sequences and swarm-type activity driven by fluid migration or aseismic slip. Historical records indicate elevated swarm frequency in this sector of the Coast Ranges, reflecting distributed strain accumulation.

Since 1 January 2000, 16 swarms have been documented in the vicinity. Prior activity includes five swarms in 2003 and eleven in 2004, underscoring recurrent episodic seismicity. Such patterns align with the area's long-term seismic character, where small-magnitude clusters serve as indicators of stress redistribution without producing significant surface rupture.

This swarm exemplifies the background seismic behavior expected in central coastal California, where monitoring supports hazard assessment and fault characterization. Continued observation remains essential for distinguishing swarm behavior from foreshock sequences preceding larger events.

References

USGS Earthquake Hazards Program catalog data
SeismoSight internal swarm classification records