Seismic Swarm S20040528.1 Near the Central California Coast

A notable earthquake swarm, designated S20040528.1, occurred near the coast of Central California between 05:15 on 27 May 2004 and 17:04 on 29 May 2004. Over 59 hours and 48 minutes, 49 earthquakes were recorded. The events clustered in a compact area, with magnitudes ranging from 0.5 to 3.4 and focal depths between 0 and 9 km. The largest shock reached magnitude 3.4 at 18:15 on 27 May.

This swarm exemplifies typical low-magnitude, high-frequency seismic sequences along the tectonically active margin. Most events remained below magnitude 2.0, with only four exceeding 2.5. Depths stayed consistently shallow, indicating activity within the upper crust where brittle failure predominates.

Central California lies at the boundary between the Pacific and North American plates. The dominant structure is the San Andreas Fault system, a right-lateral strike-slip transform boundary accommodating approximately 35–40 mm per year of relative plate motion. Secondary faults, including the San Gregorio–Hosgri and Sur–Nacimiento systems, contribute to distributed deformation along the coast. Historical records document recurrent moderate earthquakes, including the 1925 Santa Barbara event and the 2003 San Simeon earthquake (magnitude 6.5), which occurred roughly 50 km south of the swarm location.

Swarm activity in the region has shown episodic clustering. Since 1 January 2000, twelve swarms have been identified. Four occurred in 2003 and eight in 2004, suggesting elevated background seismicity during that interval. Such swarms often reflect fluid migration or aseismic slip transients rather than mainshock–aftershock sequences driven by a single large rupture.

The May 2004 swarm displayed classic swarm characteristics: a gradual onset, multiple comparable-magnitude events without a clear dominant mainshock, and rapid decay within three days. Magnitudes followed a Gutenberg–Richter distribution typical of the area, and the absence of events above magnitude 4.0 kept impacts minimal. No surface rupture or significant damage was reported.

Ongoing monitoring by regional seismic networks continues to refine understanding of fault interactions along the central coast. These data contribute to probabilistic hazard models that incorporate both characteristic large earthquakes on the San Andreas Fault and the more frequent swarm-type activity observed offshore and nearshore.

References

USGS Earthquake Hazards Program – Central California Seismicity
California Geological Survey – Fault Activity Map of California
ANSS Comprehensive Earthquake Catalog (ComCat)