Seismic Swarm S20161204.1: Analysis of Activity Near Hawthorne, Nevada

A seismic swarm designated S20161204.1 occurred 22 km southeast of Hawthorne, Nevada, between 09:37 on 3 December 2016 and 05:55 on 5 December 2016. Over 44 hours and 18 minutes, the sequence produced 47 earthquakes. Magnitudes ranged from -0.7 to 0.9, with the majority falling below 0.5. Focal depths varied between 0 km and 13 km, indicating a mix of very shallow and mid-crustal events typical of the local extensional regime.

The sequence began with a magnitude 0.5 event at 1 km depth on 3 December. Subsequent activity on 4 December included numerous events clustered between 01:13 and 04:22, with peaks in rate around 01:18–02:35 and 03:00–03:39. Later events on 4 December and the final two on 5 December showed continued low-magnitude release at depths of 3–11 km. The tight spatial and temporal clustering, combined with the absence of a dominant mainshock, confirms swarm-like behavior rather than a classic foreshock–mainshock–aftershock pattern.

This swarm fits within the broader seismic character of western Nevada. The region lies in the Basin and Range Province, where northwest-directed extension occurs along north- to northeast-striking normal faults. Hawthorne sits near the southern end of the Wassuk Range and adjacent to Walker Lake, an area influenced by the Walker Lane belt—a zone of distributed shear that accommodates a significant portion of Pacific–North America plate motion. Historical and instrumental records document frequent microseismicity and occasional moderate events (M 5–6) along range-front faults in Mineral County.

Since 1 January 2000, sixteen swarms have been identified in the same source area. Earlier episodes occurred in 2006 (two swarms), 2011 (one), 2012 (one), 2014 (four), 2015 (four), and 2016 (four, including the present sequence). These recurrent swarms suggest persistent fluid involvement or slow slip on favorably oriented structures within the same fault network.

The low magnitudes and shallow-to-moderate depths observed in S20161204.1 are consistent with the ongoing low-level strain accumulation across the central Walker Lane. No damage or felt reports were associated with this swarm, underscoring the predominantly aseismic or microseismic nature of deformation in this portion of the Basin and Range.

References
USGS Earthquake Catalog (2000–2023)
Nevada Seismological Laboratory regional seismicity reports
USGS Quaternary Fault and Fold Database of the United States