Seismic Swarm S20040714.1: Analysis and Regional Geological Context Near Oak Shores, California

Seismic swarm S20040714.1 was recorded with activity commencing at 04:45 on 13 July 2004 and concluding at 05:55 on 7 October 2004. The events were centered 8 km west-southwest of Oak Shores, California. Over 2065 hours and 10 minutes, a total of 888 earthquakes were registered. This swarm represents one of 11 documented swarms in the region since 1 January 2000, with prior activity including 4 events in 2003 and 7 in 2004.

Analysis of the initial 100 events reveals a pattern of low-to-moderate magnitude seismicity concentrated in the first several days. The sequence began with events of magnitude 1.9 at 6 km depth, followed by smaller shocks predominantly between 1.0 and 2.0. A notable peak occurred on 13 July at 23:45:45 with a magnitude 3.9 event at 4 km depth. Subsequent activity on 14 July included magnitudes up to 3.4 at similar shallow depths around 4 km. Depths across these events ranged primarily from 3 km to 7 km, with occasional outliers reaching 16 km or as shallow as 0 km. Magnitudes generally remained below 3.0 after the initial larger shocks, indicating a rapid decay in energy release while maintaining elevated event frequency through mid-July.

The Oak Shores area lies within the central Coast Ranges of California, a region shaped by ongoing transform tectonics along the boundary between the Pacific and North American plates. The San Andreas Fault system exerts primary control on regional deformation, with subsidiary faults such as the Rinconada and Hosgri contributing to distributed seismicity. Historical records document recurrent earthquake swarms in this sector, often linked to fluid migration or aseismic slip transients at shallow crustal levels. The 2003 San Simeon earthquake, located approximately 30 km to the northwest, highlighted the area's potential for larger events, though swarms like S20040714.1 typically produce only minor ground shaking.

Geological mapping indicates that the subsurface consists of Mesozoic Franciscan Complex rocks overlain by Cenozoic sedimentary units, with faulting facilitating the observed shallow seismicity. Updated monitoring by regional networks confirms that such swarms contribute to long-term strain accumulation without immediate foreshock-mainshock-aftershock signatures. The 888 events of this swarm underscore the persistent microseismicity that characterizes the central California coast, informing hazard assessments for nearby communities.

References:
USGS Earthquake Catalog (earthquake.usgs.gov)
California Geological Survey Regional Fault Maps
SeismoSight internal swarm classification records