Seismic Swarm S20240721.2 Near Dollar Point, California

A seismic swarm designated S20240721.2 occurred 8 km southeast of Dollar Point, California, beginning at 14:19 on 20 July 2024 and concluding at 03:52 on 22 July 2024. Over 37 hours and 32 minutes, the swarm produced 51 earthquakes. Magnitudes ranged from -0.2 to 3.2, with the largest event recorded at 03:02 on 21 July. Depths clustered primarily between 5 km and 11 km, consistent with shallow crustal activity in the region.

The sequence featured an initial cluster of events on 20 July, including a magnitude 2.7 shock at 14:19:55 followed by numerous microearthquakes. Activity intensified overnight, culminating in the peak magnitude 3.2 event and several magnitude 2+ aftershocks on 21 July. Smaller events continued at a declining rate until the swarm terminated early on 22 July. This pattern reflects typical swarm behavior, where energy releases through numerous small events rather than a single mainshock-aftershock sequence.

Dollar Point lies along the western margin of Lake Tahoe in the northern Sierra Nevada. The area forms part of an active extensional regime within the Walker Lane belt, a zone of distributed deformation east of the San Andreas Fault system. Local faulting includes normal faults that accommodate Basin and Range-style extension, contributing to the formation of the Lake Tahoe basin as a structural graben. Seismicity in this setting commonly occurs at depths of 5–12 km, aligning with the observed swarm depths.

Historical records indicate recurrent swarm activity in the vicinity. Since 1 January 2000, 27 swarms have been documented near Dollar Point. These events occurred in the following years with the noted counts: 2003 (3), 2004 (1), 2005 (3), 2007 (1), 2008 (1), 2012 (3), 2013 (2), 2014 (2), 2015 (1), 2016 (1), 2017 (1), 2018 (3), 2019 (1), 2020 (1), 2021 (2), and 2023 (1). Such swarms typically involve low-magnitude events and short durations, mirroring the characteristics of S20240721.2.

Swarm events in this tectonic environment provide insight into fluid migration and stress transfer along fault networks. The July 2024 sequence, with its rapid onset and gradual decay, suggests episodic release of stored strain without exceeding the threshold for a larger mainshock. Continued monitoring supports refined understanding of regional seismic hazards in the northern Sierra Nevada.

References

USGS Earthquake Hazards Program – Lake Tahoe region seismicity reports
California Geological Survey – Sierra Nevada fault maps and historical data
SeismoSight internal swarm classification records