Seismic Swarm S20200321.1 Near Indian Hills, Nevada: Analysis and Regional Context

The seismic swarm designated S20200321.1 occurred 5 km east-northeast of Indian Hills, Nevada, beginning at 01:33 UTC on 21 March 2020 and concluding at 06:23 UTC on 4 April 2020. Over 340 hours and 49 minutes, the sequence produced 402 earthquakes. This event cluster unfolded in the western Basin and Range province, a region defined by active crustal extension along normal and strike-slip faults within the Walker Lane belt.

The initial event reached magnitude 4.5 at a depth of 8 km. Subsequent activity consisted predominantly of events below magnitude 2.0, with depths concentrated between 6 km and 11 km. The first 100 recorded events exhibited a rapid onset of aftershocks following the mainshock, with magnitudes declining sharply and most remaining under 1.5. Depths showed limited variation, clustering around 7–9 km, consistent with shallow crustal faulting typical of the area. Activity rates peaked in the first hours and gradually decreased, illustrating classic swarm behavior without a single dominant mainshock-aftershock sequence.

Indian Hills lies along the eastern margin of the Carson Range, where northwest-trending faults accommodate both dextral shear and extension. These structures form part of the broader Walker Lane, a transitional zone between the Sierra Nevada and the extending Basin and Range. Historical seismicity in the region reflects ongoing tectonic strain accumulation and release, with swarms often linked to fluid migration or aseismic slip on fault segments.

Since 1 January 2000, eight swarms have been documented in this locale. Prior episodes occurred in 2003 (1 swarm), 2005 (1), 2013 (2), 2017 (1), 2018 (2), and 2019 (1). Such recurrent clustering underscores the area's propensity for episodic, diffuse seismicity rather than isolated large-magnitude ruptures.

The 2020 swarm's spatiotemporal pattern aligns with previous activity, reinforcing the importance of continuous monitoring for hazard assessment in this tectonically active corridor.

References

SeismoSight internal swarm classification records.
USGS Earthquake Hazards Program regional tectonic summaries.
Nevada Seismological Laboratory fault and seismicity databases.