Seismic Insights from Swarm S20021121.1 near Silver Springs, Nevada

An earthquake swarm designated S20021121.1 was recorded southeast of Silver Springs in western Nevada. The sequence began at 00:49 UTC on 21 November 2002 and concluded at 15:25 UTC on 22 November 2002, encompassing 30 events over 38 hours and 35 minutes. All events were shallow, with depths ranging from 7 km to 16 km and magnitudes between 1.1 and 3.5. The largest shock reached magnitude 3.5 at a depth of 8 km.

The swarm exhibited a typical pattern of clustered, low-to-moderate energy release without a dominant mainshock. Early events on 21 November included several magnitude-2+ shocks within the first few hours, followed by a peak event at 07:51 UTC. Activity then tapered, with smaller events persisting into the afternoon of 22 November. Depths remained consistent around 11–14 km for most shocks, suggesting a compact source volume within the brittle upper crust.

Western Nevada lies within the Basin and Range Province, where east-west extension has produced a landscape of north-trending mountain ranges and intervening valleys bounded by normal faults. The Silver Springs area sits near the transition between the Sierra Nevada block and the Walker Lane belt, a zone of distributed shear that accommodates a portion of Pacific-North America plate motion. This tectonic setting produces frequent small earthquakes and occasional swarms as strain is released along segmented fault systems.

Historical records document numerous seismic episodes in the region. The 1954 Rainbow Mountain–Stillwater sequence and the 1932 Cedar Mountain earthquake both occurred within 100 km, illustrating the persistent activity along range-front faults. Modern instrumental monitoring by the Nevada Seismological Laboratory and the USGS Advanced National Seismic System has catalogued hundreds of similar low-magnitude swarms since the late twentieth century, many linked to fluid migration or aseismic slip transients.

Analysis of S20021121.1 indicates that the events likely occurred on a minor normal fault subsidiary to the main range-bounding structures. The narrow depth range and rapid onset are consistent with swarm behavior driven by pore-pressure changes rather than tectonic loading alone. No damage or felt reports beyond the immediate vicinity were associated with the sequence.

Continued seismic monitoring remains essential for understanding strain accumulation in this portion of the Walker Lane. Although individual swarms such as S20021121.1 release limited energy, they contribute to the long-term seismic budget of the Basin and Range and help refine fault models used for hazard assessment.

References

USGS Earthquake Catalog
Nevada Seismological Laboratory Annual Reports
USGS National Seismic Hazard Maps (updated 2018)
Basin and Range Province tectonic summaries, USGS Professional Papers