Seismic Swarm S20000926.1: Analysis of Activity Near Topaz, California

Seismic swarm S20000926.1 occurred approximately 4 km northwest of Topaz, California, in Mono County. The sequence began at 06:19 on 26 September 2000 and concluded at 01:13 on 29 September 2000, spanning 66 hours and 53 minutes. During this interval, 103 earthquakes were recorded.

Analysis of the first 100 events reveals a typical swarm pattern characterized by numerous small-magnitude shocks without a single dominant mainshock. Magnitudes ranged from 0.2 to 4.4, with the largest event occurring at 07:20:29 on 26 September at a depth of 5 km. Depths clustered between 0 and 14 km, indicating shallow crustal activity consistent with regional extensional tectonics. Early events on 26 September included several above magnitude 3.0, such as a 3.4 at 07:27:29 and a 3.3 at 07:35:57, followed by a gradual decline in both frequency and size through 28 September.

The Topaz area lies within the Walker Lane belt, a zone of distributed right-lateral shear east of the Sierra Nevada. This belt accommodates a portion of the Pacific–North American plate boundary motion through a combination of strike-slip and normal faulting. The local geology features Quaternary alluvium overlying Mesozoic granitic rocks of the Sierra Nevada batholith, with nearby north-northwest-trending faults that have produced historical seismicity. Updated regional studies confirm ongoing deformation rates of several millimeters per year, driven by transtensional stresses.

Seismic swarms in this part of eastern California have been documented since the early twentieth century, often linked to fluid migration or magmatic processes associated with the broader Long Valley volcanic system to the south. Although the 2000 swarm produced no reported damage, its timing and location align with known episodes of elevated microseismicity along the California–Nevada border. Depths recorded in the swarm fall within the brittle upper crust, where most regional earthquakes nucleate.

Continued monitoring by regional seismic networks remains essential for distinguishing swarm behavior from potential foreshock sequences in this tectonically active corridor.

References

USGS Earthquake Hazards Program regional reports on Walker Lane seismicity.
California Geological Survey bulletins on Mono County fault systems.
SeismoSight internal swarm classification S20000926.1.