Seismic Swarm S20150127.1: Analysis of Activity South of Reno, Nevada

The seismic swarm designated S20150127.1 occurred 15 km south of Reno, Nevada, within the tectonically active Walker Lane shear zone. This region forms part of the broader Basin and Range Province, where northwest-directed extension produces normal and strike-slip faulting along systems such as the Sierra Nevada frontal fault and the Carson Range faults. Crustal deformation here accommodates a portion of Pacific–North American plate motion, resulting in elevated background seismicity and recurrent earthquake swarms.

The swarm initiated at 21:29 UTC on 26 January 2015 and concluded at 11:05 UTC on 31 January 2015, spanning 109 hours and 36 minutes. During this interval, 73 earthquakes were recorded. Magnitudes ranged from –0.6 to 2.7, with the largest event (M 2.7) occurring at 21:57 on 26 January at a depth of 7 km. The majority of events clustered between 3 km and 9 km depth, consistent with shallow crustal faulting typical of the Reno basin. A secondary cluster of slightly deeper events (up to 15 km) appeared on 29 January. Activity showed an initial energetic phase on 26–27 January followed by a gradual decline, with sporadic larger events (M ≥ 1.5) distributed through 28–30 January.

Swarm sequences in this area frequently arise from fluid migration along pre-existing faults or from aseismic slip triggering brittle failure. The 2015 sequence exhibited classic swarm characteristics: a rapid onset, absence of a single dominant mainshock, and a high proportion of small-magnitude events. Depths remained predominantly above 10 km, indicating activity within the seismogenic upper crust.

Historical records maintained by SeismoSight document 26 swarms in the same locale since 1 January 2000. Yearly counts include two swarms in 2003, one in 2004, four in 2005, one in 2007, seven in 2008, one in 2010, three in 2012, five in 2013, and two in 2014. These episodes underscore the persistent seismic productivity of the Reno–Carson City corridor.

Regional geology reflects Miocene-to-present extension superimposed on earlier arc-related structures. Quaternary fault scarps and GPS-derived strain rates of several millimeters per year confirm ongoing deformation. The swarm’s location aligns with mapped faults of the North Tahoe–Reno system, where interactions between normal and oblique-slip segments commonly produce clustered seismicity.

References

USGS Earthquake Hazards Program regional tectonic summaries
Nevada Bureau of Mines and Geology geologic maps of the Reno 30×60 quadrangle
SeismoSight internal swarm catalog (S20150127.1 parameters)