Seismic Analysis of the S20161228.1 Earthquake Swarm near Hawthorne, Nevada

The S20161228.1 earthquake swarm was recorded 28 km west-southwest of Hawthorne, Nevada, in the western Basin and Range province. This swarm began at 08:18 UTC on 28 December 2016 and concluded at 20:23 UTC on 24 February 2017, spanning 1404 hours and 5 minutes. During this interval, 4521 earthquakes were detected, highlighting a prolonged period of elevated seismicity typical of fluid-influenced or tectonically stressed zones in the region.

The initial phase featured two magnitude 5.6 events occurring within four minutes of each other at depths of 11 km and 12 km. Subsequent activity rapidly declined in magnitude, with events predominantly ranging between 1.2 and 3.8. Depths for the first 100 events clustered between 6 km and 12 km, averaging near 9–10 km, consistent with brittle failure in the upper crust. Notable early aftershocks included a magnitude 5.5 event at 09:13:47 UTC on the same day at 8 km depth, alongside numerous magnitude 2.0–3.0 shocks distributed across similar focal depths. This pattern reflects a classic swarm sequence where mainshocks are not followed by a standard aftershock decay but instead by sustained, diffuse activity.

The Hawthorne area lies within the Walker Lane shear zone, a northwest-trending belt of distributed deformation that accommodates approximately 20% of the relative motion between the Pacific and North American plates. Normal and strike-slip faulting dominate due to ongoing extension and right-lateral shear. Historical seismicity in Mineral County shows recurrent swarm behavior linked to both tectonic loading and possible hydrothermal fluid migration along fault networks.

Since 1 January 2000, 17 swarms have occurred in the immediate region. These include two swarms each in 2001, 2006, 2015, and 2016; one each in 2004 and 2013; seven in 2011; and two in 2016 prior to the S20161228.1 sequence. Such clustering indicates persistent strain accumulation and release along local structures, with swarms often lasting weeks to months and producing thousands of events without a single dominant mainshock.

Geological mapping and geophysical studies confirm that the crust here consists of Mesozoic to Cenozoic volcanic and sedimentary rocks overlying Paleozoic basement, cut by numerous normal faults. Seismicity depths align with the seismogenic zone above the brittle-ductile transition, typically 10–15 km in this extensional setting. No surface rupture was associated with the 2016–2017 swarm, consistent with its moderate magnitudes and distributed nature.

This sequence underscores the value of dense seismic monitoring for understanding swarm dynamics in tectonically active areas of Nevada. Continued observation helps refine hazard assessments for nearby communities and infrastructure.

References

Nevada Seismological Laboratory, University of Nevada, Reno.
U.S. Geological Survey Earthquake Hazards Program.
Walker Lane Geodetic and Seismic Studies, Nevada Bureau of Mines and Geology.