Seismic Swarm S20110629.1: Analysis of Activity Near Hawthorne, Nevada

A notable earthquake swarm, designated S20110629.1, occurred 12 km southwest of Hawthorne, Nevada, beginning at 09:03 on 28 June 2011 and concluding at 12:48 on 8 July 2011. Over 243 hours and 45 minutes, the sequence produced 300 earthquakes. This event aligns with the region's established pattern of episodic seismic swarms within the Basin and Range province.

The swarm unfolded in the Walker Lane belt, a zone of distributed dextral shear accommodating roughly 20 percent of Pacific-North America plate motion. Hawthorne lies near the intersection of northwest-trending strike-slip faults and north-south normal faults that bound the Wassuk Range and adjacent basins. Crustal extension rates in this portion of western Nevada average 1–2 mm per year, with seismic release occurring through both mainshock-aftershock sequences and swarm-like clusters.

Examination of the first 100 events reveals predominantly shallow hypocenters. Depths ranged from 0 to 6 km, with the majority occurring between 0 and 3 km. Magnitudes remained modest; the largest event reached M 3.4 on 29 June 2011 at 13:24:53 UTC. Subsequent peaks included M 2.8 and M 2.7 events on 28 June. The sequence displayed typical swarm characteristics: rapid onset, fluctuating rates without a single dominant mainshock, and gradual decay over days.

Time-series inspection shows highest productivity during the first 48 hours, followed by episodic bursts through early July. Most events clustered tightly in both space and time, consistent with fluid-driven or aseismic-slip triggering mechanisms documented in similar Walker Lane swarms.

Since 1 January 2000, six swarms have been recorded in the immediate area. Earlier episodes occurred in 2001 (one swarm) and 2006 (three swarms). The 2011 sequence represents the second swarm that year, underscoring recurrent strain release along the same fault network.

Such swarms contribute to long-term seismic hazard assessment for Mineral County. Although individual events rarely exceed M 4, clustered activity can elevate ground-motion probabilities over short intervals and informs models of fault interaction in the central Walker Lane.

References

• U.S. Geological Survey Earthquake Catalog (ANSS Comprehensive Catalog)
• Nevada Seismological Laboratory regional bulletins
• SeismoSight internal swarm classification records