Seismic Swarm S20031224.1 Near Cayucos, California

Seismic swarm S20031224.1 was recorded 10 km north-northeast of Cayucos in San Luis Obispo County, California. The sequence began at 21:47 on 23 December 2003 and concluded at 16:55 on 26 December 2003, spanning 67 hours and 8 minutes. During this interval, 170 earthquakes were registered.

Analysis of the first 100 events reveals a compact temporal distribution with magnitudes predominantly between 1.2 and 3.0. Depths clustered between 0 km and 13 km, indicating shallow crustal activity consistent with regional faulting. The initial event reached magnitude 2.8 at 13 km depth, followed by a rapid succession of smaller shocks. Peak magnitudes of 3.0 and 2.8 occurred within the first 24 hours, after which activity gradually declined while maintaining a steady rate of low-magnitude events. Depths showed no systematic migration, remaining distributed across the upper crust throughout the swarm.

The Cayucos region lies within the central California coastal zone, where the Pacific and North American plates interact along a complex network of strike-slip and thrust faults. The area is influenced by the Hosgri Fault Zone offshore and the San Simeon Fault on land, both accommodating right-lateral shear associated with the broader San Andreas system. Historical seismicity includes the magnitude 6.5 San Simeon earthquake of 22 December 2003, located approximately 50 km northwest, which occurred one day prior to swarm onset. This proximity suggests possible stress triggering, although the swarm itself remained a distinct, lower-magnitude sequence.

Since 1 January 2000, three seismic swarms have been documented in the immediate vicinity, with S20031224.1 representing the earliest recorded event in this modern catalog. Such swarms are characteristic of the region’s tectonics, often occurring without a single dominant mainshock and reflecting distributed strain release along secondary fault structures.

Ongoing monitoring by regional seismic networks continues to refine understanding of fault interactions along California’s central coast. The 2003 swarm provides a valuable baseline for comparing subsequent activity and assessing long-term seismic patterns in this tectonically active margin.