Seismic Swarm S20170228.1: Analysis of Activity Near Hawthorne, Nevada

The seismic swarm designated S20170228.1 occurred approximately 28 km west-southwest of Hawthorne, Nevada, in Mineral County. It began at 10:41 UTC on 27 February 2017 and concluded at 19:27 UTC on 30 March 2017, spanning 752 hours and 46 minutes. During this period, 449 earthquakes were recorded, characteristic of swarm behavior where events cluster temporally and spatially without a dominant mainshock.

Analysis of the first 100 events reveals a sequence dominated by low-magnitude earthquakes. Magnitudes ranged primarily from 0.5 to 1.7, with the majority between 0.8 and 1.3. Depths were shallow, concentrated between 5 km and 11 km, consistent with activity along upper crustal faults. The initial events on 27 February showed magnitudes of 0.7–1.2 at depths of 0–10 km. Subsequent activity through early March maintained similar patterns, with occasional peaks such as a 1.7 magnitude event at 11 km depth on 2 March. This distribution indicates diffuse strain release typical of swarm sequences in extensional tectonic settings.

The Hawthorne region lies within the Basin and Range Province of western Nevada, specifically influenced by the Walker Lane belt. This zone accommodates right-lateral shear between the Pacific and North American plates through a network of normal and strike-slip faults. Historical seismicity in the area reflects ongoing extension and fault interactions, with swarms often linked to fluid migration or stress perturbations along pre-existing structures.

Since 1 January 2000, 14 swarms have been documented in the vicinity. These occurred in 2001 (1 swarm), 2006 (2 swarms), 2011 (7 swarms), 2013 (1 swarm), and 2016 (3 swarms). The recurrence underscores persistent seismic potential driven by regional tectonics. Swarm S20170228.1 aligns with this pattern, contributing to the cumulative record of clustered activity without escalation to larger events.

Geological mapping indicates that the swarm epicenters overlie Quaternary alluvium and volcanic units overlying Mesozoic basement rocks. Faults in the area, including segments of the Wassuk Range fault system, exhibit evidence of Holocene movement. Such features facilitate swarm-type seismicity through episodic slip on minor fractures rather than through large, throughgoing ruptures.

Monitoring of this swarm provided valuable data on microseismicity patterns. Event rates peaked in the first several days before gradually declining, a common progression in fluid-influenced sequences. No damage or felt reports beyond instrumental detection were associated with the activity.

Continued surveillance remains essential given the region's history. Integration of swarm statistics with fault models enhances understanding of strain accumulation in the Walker Lane.

References

USGS Earthquake Hazards Program catalog data
Nevada Seismological Laboratory annual reports
Walker Lane tectonic summaries from the Nevada Bureau of Mines and Geology