Earthquake Swarm S20041014.1 Near San Simeon, California: Analysis and Regional Context

An earthquake swarm designated S20041014.1 occurred approximately 6 km east-northeast of San Simeon, California, between 13 October and 21 October 2004. The sequence lasted 199 hours and 52 minutes and included 97 events, all of low magnitude. This swarm exemplifies the clustered seismicity common in Central California, where tectonic stresses along the Pacific-North American plate boundary produce frequent small earthquakes without a dominant mainshock.

The San Simeon region lies within the San Andreas Fault system, specifically influenced by the Hosgri Fault and nearby offshore structures. These faults accommodate right-lateral strike-slip motion as part of the broader transform boundary. Historical records indicate elevated activity following the 22 December 2003 magnitude 6.5 San Simeon earthquake, which ruptured a blind thrust fault and triggered aftershocks that transitioned into swarm-like patterns. Depths in the 2004 swarm ranged primarily from 0 to 12 km, consistent with brittle failure in the shallow crust of this tectonically active zone.

The temporal distribution showed peak activity on 13–14 October, with the largest event reaching magnitude 2.7 at a depth of 6 km. Subsequent days featured smaller events, many below magnitude 1.5, distributed across shallow to mid-crustal levels. Such swarms often reflect fluid migration or aseismic slip along fault segments, relieving stress gradually rather than through a single large rupture. In the San Simeon area, this behavior aligns with post-2003 stress adjustments, where the crust continues to accommodate regional shear.

Since 1 January 2000, sixteen swarms have been recorded in the vicinity. The years 2003 and 2004 were particularly active, hosting five and eleven swarms respectively. This clustering suggests episodic strain release influenced by both the 2003 mainshock and ongoing plate motion. Depths and magnitudes remained modest throughout, indicating that the swarm did not involve deeper mantle processes or significant fault segment failure.

Geologically, the Central California coast features complex fault interactions within the Santa Lucia Range and adjacent marine terraces. The Franciscan Complex and overlying sedimentary units provide the crustal framework, with seismicity concentrated along pre-existing weaknesses. Updated regional models from the U.S. Geological Survey emphasize that swarms in this setting rarely exceed magnitude 3.0 yet contribute to long-term hazard assessment by delineating active fault strands.

The 2004 swarm adds to the understanding of swarm recurrence in the post-2003 environment. Its 97 events, spread over nearly eight days, illustrate the diffuse nature of energy release typical in this segment of the fault system. Continued monitoring remains essential, as similar sequences may precede or follow larger events along the central coast.

References

U.S. Geological Survey Earthquake Hazards Program – Central California seismicity reports.
SeismoSight internal swarm classification database.