Seismic Swarm S20040301.1 Near Cayucos, California: March 2004 Sequence Analysis

The seismic swarm designated S20040301.1 was recorded 15 km north of Cayucos in San Luis Obispo County, California. Activity began at 00:40 on 1 March 2004 and concluded at 07:39 on 25 March 2004, spanning 582 hours and 59 minutes during which 484 earthquakes were registered. This event cluster occurred within the tectonically active Central California coastal margin, where distributed faulting accommodates right-lateral shear between the Pacific and North American plates. The local geology features Mesozoic Franciscan Complex rocks overlain by Cenozoic sedimentary and volcanic units. Offshore, the Hosgri Fault Zone forms a major strand of the San Gregorio–San Simeon–Hosgri system, capable of producing significant earthquakes. Onshore structures include the Cambria and Oceanic faults, which contribute to background seismicity. Historical records show recurrent moderate events along these faults, with the 2003 San Simeon earthquake (Mw 6.5) occurring roughly 30 km northwest and highlighting the region’s potential for both mainshock-aftershock sequences and swarm-like behavior. Analysis of the first 100 events reveals a typical swarm signature: magnitudes ranging from 0.7 to 2.8, with the majority between 1.0 and 2.0. Depths were shallow, concentrated between 3 and 8 km, consistent with brittle failure in the upper crust. The largest event reached M 2.8 on 3 March at 4 km depth. Temporal clustering was pronounced on 1–4 March, after which rates declined steadily. No single dominant mainshock was evident; instead, energy release occurred through numerous similar-sized events, a pattern often linked to fluid migration or aseismic slip transients in this geologic setting. Since 1 January 2000, nine swarms have been identified in the immediate area. Prior activity included five swarms in 2003 and four in 2004, underscoring episodic swarm recurrence along the local fault network. Such sequences typically produce negligible surface effects but provide valuable data for refining seismic hazard models in the Central Coast region.

References

USGS Earthquake Hazards Program – Quaternary Fault and Fold Database
California Geological Survey – Fault Activity Map of California
SeismoSight internal swarm classification records