Seismic Swarm East of Topaz, California: November 2005 Analysis

A seismic swarm designated S20051119.1 was recorded 3 km east of Topaz, California, beginning at 01:59 on 19 November 2005 and concluding at 21:58 on 22 November 2005. Over the 91-hour 59-minute interval, 62 earthquakes were registered. This event exemplifies swarm-type seismicity, characterized by a concentrated sequence of small-magnitude events without a dominant mainshock.

The Topaz area lies within the Walker Lane shear zone, a tectonically active belt of distributed deformation east of the Sierra Nevada. This region accommodates northwest-directed right-lateral shear and north-south extension linked to Basin and Range province dynamics. Shallow crustal faulting predominates, with many events occurring at depths of 0–10 km. Negative depth values in some recordings reflect standard catalog conventions for surface or near-surface sources rather than subsurface anomalies.

Historical records since 1 January 2000 document two prior swarms in the immediate vicinity, one in 2000 and another in 2003. These recurrent clusters indicate persistent low-level strain release along minor faults, often influenced by fluid migration or stress perturbations within the fractured crust.

Magnitudes during the 2005 swarm ranged from 0.1 to 2.1. The largest events reached 2.1 and 2.0 on 20 November, both at very shallow focal depths. The majority of events clustered below magnitude 1.0, with 19 November showing the highest rate of activity. Depths remained predominantly shallow, consistent with the regional fault architecture. No damage or felt reports were associated with the swarm, underscoring its microseismic character.

Swarm sequences such as this provide insight into episodic strain accommodation. Unlike typical mainshock-aftershock sequences, energy release occurs through numerous similar-sized events, suggesting distributed slip or hydraulic triggering. The temporal progression—from an initial burst on 19 November, through sustained activity on 20–21 November, to final events on 22 November—illustrates the characteristic waxing and waning pattern of swarm behavior in the Walker Lane.

References

SeismoSight internal swarm classification S20051119.1
USGS Earthquake Catalog and Walker Lane tectonic summaries
California Geological Survey regional fault database