Seismic Swarm Near Dollar Point, California: The April 2021 Event

A seismic swarm designated S20210426.1 occurred 9 km southeast of Dollar Point, California, on the north shore of Lake Tahoe. The sequence began at 15:33 on 25 April 2021 and concluded at 16:42 on 28 April 2021, spanning 73 hours and 9 minutes. During this interval, 45 earthquakes were recorded, with magnitudes ranging from 0.8 to 3.7 and focal depths primarily between 5 and 17 km.

The swarm initiated with a magnitude 3.7 event at 7 km depth, followed rapidly by additional shocks of 2.7 and 2.9. Subsequent activity included a secondary peak on 26 April featuring a magnitude 3.2 event at 6 km depth. Later events remained generally below magnitude 2.5, with the final recorded shock of 1.1 occurring at 8 km depth. Most events clustered at shallow to mid-crustal depths around 5–9 km, consistent with typical swarm behavior in the region.

Dollar Point lies within the northern Sierra Nevada, part of the broader Walker Lane belt that accommodates dextral shear between the Pacific and North American plates. The Tahoe Basin is bounded by active normal faults, including segments of the Tahoe-Sierra frontal fault system and the North Tahoe fault. These structures have produced recurrent seismicity driven by extensional tectonics superimposed on the regional transform regime. The basin fill consists of Quaternary sediments overlying Mesozoic granitic and metamorphic basement, with volcanic rocks from the adjacent Tahoe-Truckee volcanic field contributing to local heterogeneity.

Seismic swarms have been documented in the area since at least 2003. Historical records indicate 24 swarms between 2000 and 2020, with notable clusters in 2003, 2005, 2012, 2018, and isolated episodes in intervening years. These sequences typically exhibit similar magnitude distributions and durations, reflecting fluid migration or aseismic slip along favorably oriented faults rather than mainshock-aftershock sequences.

The 2021 swarm aligns with this established pattern. Its modest maximum magnitude, rapid initial onset, and gradual decay without a dominant aftershock sequence suggest a distributed failure process. Depths remained within the seismogenic zone documented for the Tahoe region, where brittle failure occurs above approximately 15–20 km.

Ongoing monitoring by regional networks continues to track microseismicity in the basin. Such swarms provide valuable data for refining fault models and assessing long-term seismic hazard in this tectonically active portion of the Sierra Nevada.

References

USGS Earthquake Hazards Program – Lake Tahoe region seismicity reports
California Geological Survey – Quaternary fault and fold database
Nevada Seismological Laboratory – Regional earthquake catalog summaries