Seismic Swarm S20051207.2 Near Carnelian Bay, California
A seismic swarm designated S20051207.2 was recorded in the vicinity of Carnelian Bay, California, on the north shore of Lake Tahoe. The sequence began at 04:00 UTC on 7 December 2005 and concluded at 00:45 UTC on 10 December 2005, spanning 68 hours and 44 minutes. During this interval, 40 earthquakes were registered, with magnitudes ranging from 0.1 to 1.7 and focal depths primarily between 6 and 15 km.
The events clustered tightly in both space and time, exhibiting typical swarm characteristics of gradual onset, fluctuating rates, and absence of a single dominant mainshock. Shallow foci (mostly 7–11 km) indicate activity within the upper crust, consistent with the tectonic regime of the northern Sierra Nevada. Magnitudes remained low throughout, with only a few events exceeding magnitude 1.0, the largest reaching 1.7 on 9 December at 15:22 UTC.
This swarm fits within a broader pattern of episodic seismic activity in the Lake Tahoe basin. Since 1 January 2000, eight swarms have occurred in the region, including three in 2003, two in 2004, and three in 2005. Such recurrent clustering reflects ongoing strain accommodation along distributed faults associated with the transtensional Walker Lane belt east of the Sierra Nevada crest.
Geologically, Carnelian Bay lies within the Tahoe-Truckee graben, where normal and strike-slip faults accommodate extension and right-lateral shear. The basin is underlain by Mesozoic granitic and metamorphic rocks of the Sierra Nevada batholith, overlain by Quaternary volcanic and sedimentary deposits. Regional seismicity is driven by Pacific–North American plate interaction, with the Tahoe fault system capable of producing moderate events. Historical records document M 6+ earthquakes in the broader Tahoe area during the late 19th and early 20th centuries, underscoring the potential for larger releases of accumulated strain.
The December 2005 swarm did not produce felt shaking or damage, consistent with its low-magnitude energy release. Continued monitoring by regional networks remains essential for distinguishing background swarm behavior from possible precursors to larger events.
References
USGS Earthquake Catalog (queried for 2005 Lake Tahoe region events)
California Geological Survey, Fault Activity Map of California
Nevada Seismological Laboratory, University of Nevada, Reno – Lake Tahoe Basin seismicity reports