Seismic Swarm S20040205.1 Near Templeton, California

A seismic swarm designated S20040205.1 was recorded 9 km west-northwest of Templeton in San Luis Obispo County, California. The sequence began at 17:27 UTC on 4 February 2004 and concluded at 16:36 UTC on 7 February 2004, encompassing 57 earthquakes over 71 hours and 9 minutes. All events were of low magnitude, with the largest reaching 2.9, and hypocentral depths remained shallow between 1 km and 9 km.

The swarm initiated with a magnitude 1.3 event at 5 km depth. Early activity on 4 February featured events up to magnitude 1.8. On 5 February the sequence intensified, including a magnitude 2.9 shock at only 1 km depth and several magnitude 2.0–2.5 events clustered between 3 km and 6 km. Activity on 6 February produced the second-highest magnitude of 2.6 at 2 km depth, accompanied by multiple magnitude 2.0–2.2 events. The final day, 7 February, recorded several magnitude 2.0–2.5 shocks before the sequence tapered off with smaller events at greater average depths.

Magnitudes throughout the swarm ranged from 0.7 to 2.9, with the majority falling between 1.0 and 2.0. Depths showed no systematic migration, remaining consistent with shallow crustal faulting typical of the region. The tight temporal clustering without a single dominant mainshock is characteristic of swarm behavior driven by fluid migration or aseismic slip along minor faults.

Templeton lies within the central California Coast Ranges, a tectonically active zone marking the transform boundary between the Pacific and North American plates. The area is transected by the northwest-trending Rinconada Fault and subsidiary strands of the San Andreas system. Quaternary alluvial deposits overlie Mesozoic Franciscan Complex basement and Tertiary sedimentary units, providing a structural setting conducive to distributed microseismicity. Historical records document recurrent earthquake swarms in this portion of the Coast Ranges, often linked to right-lateral strike-slip motion and local stress perturbations.

Since 1 January 2000 the same 9 km radius around Templeton has hosted eight documented swarms. Five occurred in 2003 and three in 2004, indicating elevated swarm frequency during this interval compared with surrounding years. Such episodic clustering is consistent with the region’s background rate of small earthquakes driven by Pacific–North American plate motion of approximately 35–40 mm per year.

The February 2004 swarm produced no reported damage or felt intensities above weak levels, reflecting the limited energy release of its largest events. Continued seismic monitoring remains essential for distinguishing isolated swarms from potential precursors to larger earthquakes along nearby faults.

References

USGS Earthquake Catalog (queried for central California, 2000–2004)
California Geological Survey, Fault Activity Map of California
SeismoSight internal swarm classification records