Seismic Swarm S20021204.1: Analysis of Activity Near Beatty, Nevada

Seismic swarm S20021204.1 was detected 46 km east-southeast of Beatty, Nevada, beginning at 19:57 on 3 December 2002 and concluding at 11:07 on 11 December 2002. Over 183 hours and 9 minutes, the sequence produced 89 earthquakes. Magnitudes ranged from -0.3 to 1.1, with the majority of events falling between 0.0 and 0.5. Focal depths clustered between 5 km and 12 km, indicating shallow crustal activity consistent with regional faulting. The temporal distribution showed an initial cluster of low-magnitude events on 3 December, followed by sustained low-level seismicity through 4–6 December. Activity intensified slightly on 5 December with the largest event (magnitude 1.1 at 11 km depth). Subsequent days featured sporadic bursts, including additional magnitude-1.0 and 1.1 events on 10 and 11 December. Depths remained stable, suggesting a consistent source volume rather than progressive migration. This swarm aligns with patterns observed in the Basin and Range Province, where the area experiences east-west extension accommodated by normal faults. Beatty lies within the Walker Lane tectonic belt, a zone of distributed shear and faulting east of the Sierra Nevada. The local geology includes Miocene volcanic rocks, basin-fill sediments, and Quaternary alluvium overlying Paleozoic carbonates. Historical seismicity in the region reflects both tectonic strain accumulation and possible contributions from hydrothermal or magmatic processes associated with nearby volcanic fields. Since 1 January 2000, eleven swarms have occurred in the vicinity. Earlier sequences include five events in 2000 and six in 2002, underscoring recurrent swarm behavior. Such episodes typically involve hundreds of microearthquakes that release strain without producing a dominant mainshock, distinguishing them from classical aftershock sequences. The 2002 swarm’s characteristics—tight spatial clustering, low magnitudes, and short overall duration—mirror other documented swarms in the southern Great Basin. These events often occur along favorably oriented faults under the influence of regional extension. Depths of 5–12 km place the activity within the seismogenic zone where brittle failure predominates. Ongoing monitoring by regional networks continues to track microseismicity in this area, contributing to refined hazard assessments for central Nevada. The recurrence of swarms highlights the importance of dense seismic coverage for distinguishing background activity from potential precursors to larger events.

References

USGS Earthquake Catalog
Nevada Seismological Laboratory, University of Nevada, Reno
SeismoSight internal swarm classification database