Seismic Swarm S20000523.1: Analysis of Activity Near Indian Springs, Nevada

Seismic swarm S20000523.1 occurred approximately 36 km northwest of Indian Springs, Nevada, from 07:11 on 22 May 2000 to 17:42 on 23 May 2000. Over this 34-hour and 30-minute period, 40 earthquakes were recorded, with events distributed across shallow depths ranging from 0 to 12 km. Magnitudes remained low throughout, peaking at 2.5, consistent with typical swarm behavior involving clustered microseismicity rather than a single dominant mainshock.

The sequence began with an initial event of magnitude 0.0 at 12 km depth on 22 May at 07:11:35. Subsequent activity included a magnitude 1.5 event at 10 km depth roughly one hour later, followed by a magnitude 2.5 shock at 8 km depth by 08:40:46. Many events registered magnitudes at or below 0.0, including several negative values, indicating very small energy releases. Depths showed variation, with numerous events clustered between 4 and 9 km, though some occurred at or near the surface (0 km). The swarm concluded with a magnitude -0.1 event at 4 km depth on 23 May at 17:42:20.

This swarm represents the sole recorded instance in the region since 1 January 2000, highlighting the infrequent nature of such clustered seismic episodes in the immediate area. The tight temporal grouping and lack of a clear aftershock decay pattern distinguish it from typical mainshock-aftershock sequences.

The region lies within the Basin and Range Province of southern Nevada, characterized by extensional tectonics that produce north-south trending normal faults and horst-graben topography. Indian Springs sits near the transition zone between the Spring Mountains and the Nevada National Security Site, where Cenozoic volcanic and sedimentary rocks overlie older Paleozoic carbonates and clastics. Active faulting in the area, driven by roughly east-west extension at rates of several millimeters per year, contributes to background seismicity. Historical monitoring in southern Nevada has documented both natural tectonic events and induced seismicity linked to underground nuclear testing conducted through 1992 at the Nevada National Security Site. Post-testing, residual stress adjustments and fluid migration along faults may influence low-level activity, though the 2000 swarm aligns with natural swarm patterns observed elsewhere in the province.

Geological mapping indicates proximity to structures such as the Rock Valley fault system, which accommodates part of the regional strain. Depths of 0–12 km place the events within the seismogenic upper crust, where brittle failure predominates. The low magnitudes and shallow foci suggest minimal surface rupture potential, consistent with the swarm's limited energy release.

Overall, swarm S20000523.1 provides a snapshot of microseismic processes in an extensional setting, underscoring the value of dense seismic networks for detecting subtle activity that informs regional hazard assessment. Continued monitoring supports understanding of long-term strain accumulation in this tectonically active portion of the western United States.