Seismic Swarm S20130728.1: Analysis of Microearthquake Activity Near Lemmon Valley, Nevada

Seismic swarm S20130728.1 was recorded 2 km west-southwest of Lemmon Valley, Nevada, beginning at 10:06 on 27 July 2013 and concluding at 10:08 on 28 July 2013. Over this approximately 24-hour period, 25 earthquakes were detected, with magnitudes ranging from -0.6 to 0.9 and focal depths between 0 and 6 km. The events clustered tightly in both time and space, characteristic of swarm behavior where numerous small quakes occur without a distinct mainshock.

The sequence initiated with a magnitude 0.3 event at 4 km depth. Subsequent activity included multiple events below magnitude 0.5, interspersed with slightly larger shocks such as the 0.9 magnitude event recorded at 5 km depth on 27 July at 14:59. Activity remained low-magnitude throughout, with the final event registering magnitude 0.6 at 3 km depth. Such microseismicity often reflects fluid migration or minor stress adjustments along local fault structures rather than significant tectonic release.

Lemmon Valley lies within the Basin and Range Province of western Nevada, a region dominated by extensional tectonics and numerous normal faults. The area forms part of the Walker Lane belt, a zone of distributed shear accommodating Pacific-North American plate motion. This setting produces frequent small earthquakes, with historical records showing episodic swarms linked to fault segments near the Truckee Meadows and Pyramid Lake regions. Depths in this swarm align with typical shallow crustal seismicity in the province, where brittle failure occurs above approximately 10-15 km.

Since 2000, 17 swarms have been documented in the vicinity. These occurred in 2003 (1 swarm), 2004 (1), 2005 (1), 2008 (7), 2010 (1), 2012 (1), and 2013 (5). The 2013 cluster, including S20130728.1, underscores recurring low-level activity that contributes to long-term strain accommodation without producing damaging events.

Monitoring of such swarms aids in refining local seismic hazard models. Although individual events remain imperceptible to residents, their patterns help delineate active structures and inform preparedness in this tectonically dynamic portion of Nevada.

References

• Nevada Seismological Laboratory historical earthquake catalog (2000-2013).
• USGS Earthquake Hazards Program regional tectonic summaries for the Basin and Range Province.