Seismic Swarm S20031003.1: Analysis of Activity Near Carnelian Bay, California

Seismic swarm S20031003.1 occurred 2 km southeast of Carnelian Bay, California, on the north shore of Lake Tahoe. The sequence began at 04:34 on 3 October 2003 and concluded at 01:25 on 5 October 2003, spanning 44 hours and 50 minutes. During this interval, 34 earthquakes were recorded, with magnitudes ranging from 0.3 to 1.9 and focal depths predominantly between 18 and 26 km.

The temporal distribution shows the highest event frequency during the first 12 hours, followed by a gradual decline. Early events clustered at depths near 24 km, while later activity included slightly shallower hypocenters. Magnitudes remained modest throughout, with no event exceeding 1.9, consistent with swarm behavior rather than a mainshock-aftershock sequence.

The Lake Tahoe basin lies within the Sierra Nevada physiographic province, where extensional tectonics associated with the Walker Lane belt accommodate right-lateral shear between the Pacific and North American plates. The region features active normal and strike-slip faults that have shaped the basin over the past several million years. Historical seismicity includes moderate events such as the 1966 Truckee earthquake, underscoring ongoing tectonic strain accumulation.

Depths around 24 km place the swarm within the lower crust, where brittle failure can occur under elevated pore-fluid pressures or thermal conditions. Such swarms in the Tahoe region are often linked to fluid migration along pre-existing fault networks rather than sudden stress transfer from a single large rupture. No surface rupture or damage was reported for this sequence, aligning with its low-magnitude character.

Monitoring by regional seismic networks provides continuous data that refine fault models and improve probabilistic hazard assessments for the northern Sierra Nevada. Continued observation of similar swarms contributes to understanding strain release mechanisms in this tectonically active corridor.

References

SeismoSight internal swarm classification records.
USGS Earthquake Catalog (pre-2023 updates for Lake Tahoe region).
California Geological Survey fault activity database.