Seismic Swarm S20030713.1 Near Topaz Ranch Estates, Nevada

Seismic swarm S20030713.1 was recorded in western Nevada beginning at 18:25 on 12 July 2003 and concluding at 08:54 on 14 July 2003. The sequence lasted 38 hours and 29 minutes and produced 105 earthquakes centered 2 km west-northwest of Topaz Ranch Estates. This event marks the second swarm documented in the area since 1 January 2000, following earlier activity in 2000.

The region lies within the Basin and Range Province, a tectonically active extensional domain characterized by numerous north-trending normal faults. Ongoing crustal stretching produces frequent small-magnitude earthquakes and occasional swarms. Topaz Ranch Estates sits near the California-Nevada border, where fault systems accommodate right-lateral shear and extension. Shallow focal depths in this setting commonly range from surface levels to approximately 10 km, consistent with the brittle upper crust in the province.

Analysis of the first 100 events shows magnitudes between 0.7 and 2.8, with the majority falling between 1.2 and 1.9. Depths varied from -1 km to 9 km, indicating activity throughout the shallow crust. The largest event reached magnitude 2.8 at 10:42 on 13 July at a depth of 4 km. Several magnitude-2.0 to 2.7 shocks occurred during the peak period on 13 July, while later events remained below magnitude 2.2. Timing indicates a rapid onset followed by sustained low-level activity through the night of 12–13 July, with a secondary increase around midday on 13 July.

The swarm’s compact spatial footprint and short duration align with typical swarm behavior in the Basin and Range, where fluid migration or aseismic slip can trigger clustered seismicity without a single dominant mainshock. No damage or felt reports beyond instrumental detection were associated with this sequence.

References SeismoSight internal swarm classification S20030713.1 USGS Earthquake Hazards Program regional seismicity summaries for western Nevada