Seismic Swarm S20130731.1: Analysis of Earthquake Activity Near Lemmon Valley, Nevada

Seismic swarm S20130731.1 occurred 2 km west of Lemmon Valley, Nevada, beginning at 10:05 on 31 July 2013 and concluding at 04:59 on 3 August 2013. Over 66 hours and 53 minutes, the swarm produced 57 earthquakes, with magnitudes ranging from -0.7 to 1.8 and focal depths primarily between 4 and 7 km. The sequence featured a high frequency of microearthquakes, including multiple events below magnitude 0.5, consistent with swarm behavior driven by fluid migration or localized stress adjustments rather than a single mainshock-aftershock pattern.

The largest event reached magnitude 1.8 at a depth of 7 km on 31 July at 16:26:40 UTC. Subsequent notable events included magnitudes of 1.7 at 7 km depth and 1.4 at 6–7 km depths. Depths remained shallow throughout, indicating activity within the upper crust. Temporal clustering showed peak activity on 31 July, with diminishing frequency by 2–3 August, culminating in a final event of magnitude -0.3 at 4 km depth.

Regional Geological Context

Lemmon Valley lies within the Basin and Range Province of northern Nevada, characterized by extensional tectonics that produce north-south trending normal faults and horst-graben structures. This tectonic regime results from ongoing crustal stretching between the Sierra Nevada to the west and the stable North American interior to the east. The area forms part of the Walker Lane shear zone, a transitional belt accommodating right-lateral shear and contributing to elevated seismicity in western Nevada.

Historical earthquake records indicate that northern Nevada experiences frequent low-magnitude swarms due to these active fault systems. The regional crust features volcanic and sedimentary rocks overlying Precambrian basement, with Quaternary alluvium filling valley floors. Faults in the vicinity, including segments of the Sierra Nevada frontal fault system and local intrabasin structures, facilitate episodic seismic release at shallow depths.

Swarm History and Patterns

Since 1 January 2000, 14 swarms have been documented in the region. Earlier episodes occurred in 2008 (6 swarms), 2010 (1 swarm), and 2012 (1 swarm), with 2013 recording 6 swarms total. This distribution suggests recurring swarm activity linked to persistent tectonic and hydrothermal influences in the Lemmon Valley area.

Insights from the 2013 Sequence

Analysis of the 57 events reveals predominantly low-energy release, with over half the earthquakes below magnitude 0.5. Depths clustered tightly around 5 km, pointing to a compact source volume. Such characteristics align with fluid-driven swarm mechanisms common in the Basin and Range, where groundwater or magmatic volatiles reduce fault friction. No damage or felt reports were associated with this swarm, underscoring its minor scale relative to larger regional events.

The data illustrate typical swarm evolution: an initial burst followed by gradual decay, without a dominant mainshock. This pattern supports monitoring efforts to distinguish swarms from foreshock sequences preceding larger earthquakes.

References

• Nevada Seismological Laboratory, University of Nevada, Reno
• U.S. Geological Survey Earthquake Catalog
• Nevada Bureau of Mines and Geology, Regional Tectonic Summaries