Seismic Swarm S20040317.1: Analysis of Activity Near San Simeon, California

Seismic swarm S20040317.1 was recorded in the region 6 km east of San Simeon, California. The sequence began at 07:04 on 16 March 2004 and concluded at 02:01 on 24 March 2004, spanning 186 hours and 56 minutes. During this interval, 281 earthquakes were detected.

The swarm occurred within the tectonically complex central California coastal zone, where the San Andreas Fault system interacts with the Hosgri Fault and other subsidiary structures. This area lies near the boundary between the Pacific and North American plates, resulting in frequent seismic activity driven by right-lateral strike-slip motion and associated thrust faulting. The 2003 magnitude 6.5 San Simeon earthquake, centered approximately 11 km northeast of the town, had already heightened regional strain, and subsequent sequences often reflect continued adjustment along these faults.

Analysis of the first 100 events reveals a predominance of low-magnitude earthquakes. Magnitudes ranged from 0.7 to 4.5, with the majority falling between 1.0 and 2.0. Depths were shallow, concentrated between 2 km and 7 km, consistent with activity in the upper crust of this coastal setting. Notable events included a magnitude 3.5 quake at 07:50 on 17 March, followed later that day by the sequence peak of magnitude 4.5 at 23:53. Multiple magnitude 2.5–2.8 events clustered in the hours immediately after the largest shock, indicating a rapid release of accumulated stress.

Temporal distribution showed an initial gradual increase in activity on 16 March, accelerating sharply on 17 March with the main events. Depths remained consistent across the early phase, suggesting a compact source volume rather than migration along a single fault plane. The data indicate swarm-like behavior typical of fluid-influenced or triggered sequences in this part of the San Andreas system, where small events often occur without a single dominant mainshock.

Historical records maintained by SeismoSight document nine swarms in the San Simeon area since 1 January 2000. Prior episodes occurred in 2003 (four swarms) and 2004 (five swarms), underscoring the recurrent nature of clustered seismicity following the 2003 mainshock. Such patterns align with the broader geological framework of the central California coast, where aftershock sequences and swarms can persist for months due to the region's heterogeneous fault network.

Overall, swarm S20040317.1 exemplifies the ongoing seismic productivity east of San Simeon. Continued monitoring remains essential given the proximity to populated coastal communities and critical infrastructure.

References

SeismoSight internal swarm classification records.
USGS Earthquake Hazards Program regional fault database.
California Geological Survey tectonic summaries for central coast.