Seismic Swarm S20150520.1: Analysis of Activity Northwest of Indian Springs, Nevada

Seismic swarm S20150520.1 occurred approximately 28 km northwest of Indian Springs, Nevada, from 05:19 on 19 May 2015 to 12:49 on 21 May 2015. Over 55 hours and 30 minutes, the sequence produced 77 earthquakes. Magnitudes ranged from -0.8 to 1.8, with the majority below 0.5 and focal depths clustered between 8 km and 13 km. The largest events reached magnitude 1.8 at 03:30:31 on 20 May and magnitude 1.3 at both 02:53:42 and 08:52:24 on the same day.

The swarm exhibited typical characteristics of clustered microseismicity, with events concentrated in two main pulses on 20 May between 02:00–04:00 and 08:00–10:00 UTC. Depths remained consistent, indicating a compact source volume at mid-crustal levels. No events exceeded magnitude 2.0, consistent with low-energy release in this portion of the Basin and Range.

The Indian Springs region lies within the southern Basin and Range Province of Nevada, an area undergoing active east-west crustal extension. This tectonic setting produces distributed normal faulting along north-south trending structures. The local geology includes Quaternary alluvium overlying Paleozoic carbonate and siliciclastic rocks, with several mapped faults capable of generating small to moderate earthquakes. Historical seismicity in the area reflects both tectonic strain accumulation and occasional swarm-like sequences driven by fluid migration or minor stress perturbations.

Since 1 January 2000, six swarms have been recorded in the vicinity. These include two events in 2000, two in 2002, one in 2005, and the 2015 sequence documented here. Such recurrent swarm activity suggests the presence of favorably oriented faults that respond episodically to regional extension rather than through large mainshock-aftershock sequences.

The 2015 swarm did not produce reported damage or felt shaking beyond instrumental detection. Depths averaging near 11 km place the activity below the shallow sedimentary cover, within crystalline basement where brittle failure can occur at low stress levels. Continued monitoring of similar sequences aids in refining seismic hazard models for southern Nevada, where proximity to population centers such as Las Vegas underscores the value of microseismicity catalogs.

References
USGS Earthquake Catalog (earthquake.usgs.gov)
Nevada Seismological Laboratory regional reports
Basin and Range Province tectonic summaries, USGS Professional Papers