Seismic Swarm S20120103.1 Near Calpine, California

Earthquake swarm S20120103.1 was recorded 5 km west of Calpine in Sierra County, California. The sequence began at 07:25 on 3 January 2012 and concluded at 10:35 on 6 January 2012, spanning 75 hours and 9 minutes. During this interval, 69 earthquakes were registered.

The swarm occurred in a region of the northern Sierra Nevada characterized by active faulting along the Sierra Nevada Fault system and associated extensional structures. This tectonic setting accommodates regional strain from Pacific-North American plate interactions, occasionally producing clustered seismicity rather than isolated mainshock-aftershock sequences. Depths of recorded events clustered between 29 km and 33 km, with isolated shallower occurrences near 8–13 km, consistent with activity within the mid-crustal brittle-ductile transition zone of the Sierra Nevada block.

Magnitudes remained low throughout, ranging from –0.6 to 1.7. The largest events reached 1.7 and occurred on 3 January between 22:25 and 23:34. Subsequent activity on 4 and 5 January included several events of magnitude 1.2–1.6, after which the sequence tapered. The absence of a dominant mainshock and the tight spatial-temporal clustering are typical of swarm behavior, often linked to transient fluid pressure changes or slow slip within fractured crust.

Historical records indicate nine swarms have occurred in the same area since 1 January 2000. Earlier episodes were documented in 2004 (one swarm) and 2005 (one swarm), followed by elevated activity in 2011 (seven swarms). This pattern suggests recurring localized stress release within a structurally favorable volume west of Calpine.

The January 2012 swarm adds to the documented record of non-stationary seismicity in the northern Sierra Nevada. Continued monitoring of such sequences provides insight into the interplay between deep crustal processes and surface fault systems in this portion of California.

References
SeismoSight internal swarm classification S20120103.1
USGS Earthquake Catalog (regional Sierra County events)