Seismic Swarm S20031116.1 Near Mina, Nevada

Seismic swarm S20031116.1 was recorded 27 km southeast of Mina, Nevada, from 20:11 on 15 November 2003 to 21:33 on 17 November 2003. Over 49 hours and 21 minutes, the sequence produced 52 earthquakes. The events clustered tightly in time and space, characteristic of swarm behavior where no single mainshock dominates.

The largest event reached magnitude 4.3 at 21:19 on 15 November at a depth of 7 km. Other notable shocks included magnitudes 4.2 at 20:11 on 15 November (8 km depth) and 3.5 at 06:08 on 16 November (10 km depth). The majority of events ranged between magnitude 1.0 and 2.5, with depths predominantly between 0 km and 10 km. Activity began with an immediate cluster of moderate events on the evening of 15 November, continued at a steady rate through 16 November, and tapered off by the afternoon of 17 November.

The Mina region lies within the central Walker Lane belt of western Nevada, part of the broader Basin and Range province. This tectonic domain accommodates northwest-directed dextral shear and extension between the Sierra Nevada block and the stable North American interior. Active normal and strike-slip faults cut through Miocene volcanic and sedimentary rocks, creating a network of Quaternary scarps that record repeated late Pleistocene to Holocene surface rupture. Regional strain rates derived from GPS networks indicate 8–12 mm per year of right-lateral motion across the Walker Lane, sufficient to generate frequent moderate earthquakes.

Historical seismicity in Mineral County includes the 1932 Cedar Mountain earthquake (M 7.2) and multiple M 6+ events during the 1954 Rainbow Mountain–Fairview Peak sequence. These larger earthquakes occurred on northwest-striking faults that share structural similarities with the fault system hosting swarm S20031116.1. Since 2000, only one swarm has been documented in the immediate area, underscoring the episodic nature of clustered seismicity in this portion of the Walker Lane.

Depth distribution within the swarm shows most hypocenters between 4 km and 8 km, consistent with the brittle–ductile transition in the local crust. Shallower events (0–3 km) occurred early in the sequence, while the deepest shock (14 km) was recorded late on 17 November. This pattern suggests initial failure on shallower fault segments followed by limited downward migration.

Swarm S20031116.1 illustrates how distributed shear in the Walker Lane can produce prolonged, low-to-moderate magnitude sequences without a clear triggering mainshock. Such episodes contribute to the cumulative seismic moment release that defines the long-term hazard of the Mina region.

References

• U.S. Geological Survey Earthquake Catalog
• Nevada Bureau of Mines and Geology, Quaternary Fault Database
• Nevada Seismological Laboratory, University of Nevada, Reno
• SeismoSight internal swarm classification records