Seismic Swarm S20050916.1: A Detailed Examination of the 2005 Earthquake Sequence near Johnson Lane, Nevada

Seismic swarm S20050916.1 was recorded by the SeismoSight network beginning at 15:06 on 16 September 2005 and concluding at 03:20 on 18 September 2005. The events occurred approximately 8 km east-northeast of Johnson Lane in western Nevada. Over a period of 36 hours and 13 minutes, the swarm comprised 81 earthquakes.

The sequence exhibited classic swarm characteristics, featuring numerous events of comparable magnitude without a single dominant mainshock. The largest event reached magnitude 4.2 at a depth of 10 km shortly after initiation. Subsequent activity included multiple events between magnitude 2.0 and 3.2, followed by a prolonged tail of smaller shocks predominantly below magnitude 2.0. Depths clustered tightly between 7 km and 12 km, indicating a shallow crustal source consistent with regional faulting patterns.

Western Nevada lies within the Basin and Range Province, where ongoing crustal extension produces abundant normal faults. The Johnson Lane area sits near the transition between the Sierra Nevada block and the Walker Lane belt, a zone of distributed shear and elevated seismicity. This tectonic setting accounts for the frequent occurrence of earthquake swarms driven by fluid migration or aseismic slip along fault networks rather than classic aftershock sequences.

Historical records document recurrent low-to-moderate seismicity in the Carson Valley region. Instrumental monitoring since the mid-twentieth century has captured numerous swarms and isolated events, underscoring the persistent strain accumulation along range-bounding faults. Updated geophysical studies continue to refine fault models in the area, confirming active extension rates of several millimeters per year.

Analysis of the 81 events reveals a rapid initial rate of occurrence that decayed over the following day. Magnitudes followed a typical Gutenberg-Richter distribution for swarm activity, with the majority of energy release concentrated in the first few hours. Depths remained stable, suggesting a confined seismogenic volume.

Such swarms provide valuable data for understanding local stress regimes and improving real-time monitoring capabilities. Continued observation in this portion of the Walker Lane belt remains essential for assessing long-term seismic hazard.

References
SeismoSight internal swarm classification records for S20050916.1.
USGS Earthquake Hazards Program regional tectonic summaries for western Nevada.
Nevada Seismological Laboratory historical earthquake catalog.