Seismic Swarm S20150722.1 Near Pahrump, Nevada: Geological Context and Event Analysis

Seismic swarm S20150722.1 occurred approximately 50 km NNW of Pahrump, Nevada, in the Basin and Range province. This tectonically active region experiences ongoing crustal extension along normal faults, producing frequent low-magnitude earthquake sequences. The swarm began at 15:22 on 21 July 2015 and concluded at 10:57 on 25 July 2015, spanning 91 hours and 34 minutes. During this interval, 218 earthquakes were recorded.

The first 100 events displayed predominantly negative magnitudes, with depths ranging from 2 km to 9 km. The sequence featured a notable mainshock of magnitude 3.0 at 15:51:54 on 22 July 2015, occurring at 7 km depth. Subsequent events remained small, with the largest after this peak reaching magnitude 1.4. Depths clustered tightly around 4–7 km, consistent with shallow crustal faulting typical of the area.

Pahrump lies within a zone of distributed deformation influenced by the Walker Lane shear zone to the west and the broader extensional regime of the Great Basin. Historical records since 1 January 2000 document 34 prior swarms in the vicinity, with annual counts as follows: 2000 (5), 2002 (7), 2003 (2), 2005 (2), 2006 (1), 2007 (2), 2008 (6), 2009 (4), 2010 (3), 2012 (1), and 2014 (1). These recurrent swarms reflect episodic strain release along segmented normal faults without significant surface rupture.

Analysis of the initial events indicates a classic swarm pattern: rapid onset of microseismicity, a single moderate event, and gradual decay. No events exceeded magnitude 3.0 in the examined subset, underscoring the low-energy character of the sequence. Depths remained stable, suggesting activity confined to the upper seismogenic crust.

Such swarms contribute to long-term seismic hazard assessment in southern Nevada by delineating active fault segments. Continued monitoring supports refined models of regional extension rates, which average 1–2 mm per year across the Basin and Range. The 2015 swarm aligns with established patterns of clustered, low-magnitude activity that characterize the tectonic setting.

References

• SeismoSight internal swarm classification database
• USGS Earthquake Catalog (historical swarm verification)