Seismic Swarm S20031123.1: Analysis of Activity Near Carnelian Bay, California

Seismic swarm S20031123.1 was recorded in the Lake Tahoe region of California, centered 0 km east-southeast of Carnelian Bay. The sequence began at 13:48 on 22 November 2003 and concluded at 13:35 on 25 November 2003, spanning 71 hours and 47 minutes. A total of 36 earthquakes were detected during this interval.

The events exhibited low to moderate magnitudes, with the largest reaching 1.9. Depths ranged from 0 km to 28 km, though the majority clustered between 22 km and 27 km. Early activity on 22 November included an initial 0.5-magnitude event at 17 km depth, followed by several 1.4–1.6 magnitude shocks at approximately 24 km. Subsequent events maintained a similar depth profile, with occasional shallower occurrences such as a 0.3-magnitude quake at 14 km on 23 November. The sequence tapered with smaller events, ending with a 0.5-magnitude shock at 8 km depth.

This swarm represents the first of two documented swarms in the area since 1 January 2000. The clustered timing and consistent focal depths suggest a localized release of strain along pre-existing faults rather than a mainshock-aftershock sequence. Magnitudes remained below levels typically associated with surface damage, indicating primarily microseismic behavior.

Carnelian Bay lies on the northern shore of Lake Tahoe within the northern Sierra Nevada. The basin formed through extensional tectonics associated with the Walker Lane belt, a zone of distributed right-lateral shear that accommodates a portion of the Pacific-North American plate boundary motion. Active normal and strike-slip faults transect the region, including structures linked to the Tahoe fault system. Seismicity in this setting often occurs at mid-crustal depths due to the interaction of regional extension and local volcanic or hydrothermal influences beneath the lake.

Historical records show recurrent low-level earthquake activity around Lake Tahoe, consistent with the ongoing tectonic deformation of the Sierra Nevada. Swarms such as S20031123.1 contribute to the understanding of episodic strain accumulation and release along these faults. Depths exceeding 20 km align with the brittle-ductile transition zone in this part of the crust, where fluids or aseismic slip may trigger clustered events.

Continued monitoring of the Carnelian Bay vicinity supports regional hazard assessment, as even small swarms provide data on fault behavior and stress conditions in the Walker Lane transition zone.

References

USGS Earthquake Catalog
SeismoSight Internal Swarm Classification Records