Seismic Swarm S20110817.1: Earthquake Sequence Southwest of Hawthorne, Nevada

The seismic swarm designated S20110817.1 occurred 12 km southwest of Hawthorne, Nevada, beginning at 01:33 on 17 August 2011 and concluding at 18:40 on 18 August 2011. Over a period of 41 hours and 7 minutes, the sequence comprised 62 earthquakes. All events were shallow, with focal depths ranging from 0 to 5 km, consistent with activity in the upper crust. The sequence initiated with low-magnitude events (0.6–0.9) in the early hours of 17 August. Activity intensified in the evening, culminating in a magnitude 4.0 earthquake at 21:04:38 at a depth of 3 km. This event was followed by numerous aftershocks, including magnitudes of 2.6, 2.1, 2.0, and 1.7. The largest aftershock reached 2.0 at 01:05 on 18 August. Magnitudes remained predominantly below 2.0 thereafter, tapering to a final 1.2 event. Depths stayed shallow throughout, with the majority at 0–3 km, suggesting brittle failure along near-surface faults. This swarm exemplifies typical characteristics of clustered seismicity in the region: a rapid onset, a distinct mainshock, and a prolonged decay of aftershocks without a single dominant rupture. The 62 events reflect distributed strain release rather than a classic mainshock-aftershock pattern. The Hawthorne area lies within the Basin and Range Province of western Nevada, where east-west extension drives normal faulting along range-bounding structures. Historical records indicate recurrent swarm activity, with nine swarms documented since 1 January 2000. Prior episodes occurred in 2001 (one swarm), 2006 (three swarms), and 2011 (five swarms, including S20110817.1). Such sequences are linked to fluid migration or aseismic slip triggering brittle failure on favorably oriented faults. Seismic monitoring in this portion of Mineral County reveals persistent low-level activity driven by regional tectonics. The shallow depths observed in S20110817.1 align with the province’s thin seismogenic crust, where events rarely exceed 10–15 km.

References

SeismoSight internal swarm classification database.
USGS Earthquake Hazards Program regional tectonic summaries for western Nevada.