Seismic Swarm S20110927.1: Geological Context and Event Analysis Near Hawthorne, Nevada

The seismic swarm designated S20110927.1 occurred 15 km southwest of Hawthorne, Nevada, within the tectonically active Walker Lane belt. This region forms part of the broader Basin and Range Province, where northwest-directed dextral shear between the Pacific and North American plates produces distributed normal and strike-slip faulting. Shallow crustal extension here has generated recurrent earthquake swarms since at least the late Quaternary, with mapped faults including segments of the Wassuk Range fault system and related structures that accommodate regional strain rates of several millimeters per year.

The swarm initiated at 09:16 on 26 September 2011 and concluded at 02:30 on 28 September 2011, spanning 41 hours and 13 minutes. During this interval, 35 earthquakes were recorded. Event magnitudes ranged from −0.1 to 1.7, with the majority below 1.0; focal depths remained very shallow, predominantly at 0–3 km and reaching a maximum of 5 km. The sequence exhibited typical swarm characteristics: a rapid onset, fluctuating rates without a single dominant mainshock, and gradual decay.

Temporal distribution showed peak activity during the first 24 hours, including multiple events above magnitude 1.0 clustered between 00:13 and 00:23 on 27 September. Later phases produced isolated events of comparable size before activity ceased. All events were consistent with the low-magnitude, shallow-depth regime expected for fluid-assisted or aseismic-slip-triggered swarms in this portion of the Walker Lane.

Historical records indicate nine comparable swarms in the same locale since 1 January 2000. Earlier episodes occurred in 2001 (one swarm) and 2006 (two swarms), while 2011 alone accounted for six distinct swarms. This clustering underscores the persistent seismic productivity of the area, driven by ongoing transtensional deformation.

The combination of geological setting and swarm statistics highlights the value of dense local monitoring for distinguishing swarm behavior from foreshock sequences that might precede larger events.

References
SeismoSight internal swarm classification database
USGS Earthquake Hazards Program regional tectonic summaries for the Walker Lane belt