Seismic Swarm S20120201.1 Near Sierraville, California

Seismic swarm S20120201.1 was recorded 1 km SSW of Sierraville, California, commencing at 23:13 on 31 January 2012 and concluding at 04:47 on 4 February 2012. Over 77 hours and 34 minutes, the sequence comprised 62 earthquakes. Event magnitudes ranged from -0.4 to 1.3, with the majority registering below 0.5. Focal depths clustered predominantly between 32 and 35 km, though several shallower events occurred at 4–11 km and isolated deeper readings reached 28–29 km.

The temporal distribution showed peak activity on 1 February, with 18 events concentrated between 04:57 and 17:57. Subsequent days exhibited declining frequency, tapering to single events by early 4 February. This pattern is characteristic of swarm sequences, where numerous small-magnitude events occur without a dominant mainshock. Depths near 33 km suggest activity within the mid-crust, consistent with regional extensional regimes.

Sierraville lies in Sierra County within the northern Sierra Nevada, at the transition between the Sierra Nevada block and the Basin and Range province. The area experiences distributed deformation along normal and strike-slip faults associated with the Walker Lane belt, a zone of right-lateral shear accommodating approximately 20–25% of Pacific–North America plate motion. Local structures include the Mohawk Valley fault and segments of the Honey Lake fault zone, which have produced historical moderate earthquakes.

Seismic swarms in this setting often reflect fluid migration along pre-existing fractures or aseismic slip transients rather than magmatic processes. The 2012 sequence aligns with this tectonic framework, where small events cluster without exceeding magnitude 2.0.

Historical records indicate 11 swarms in the region since 1 January 2000. Earlier episodes occurred in 2004 (one swarm) and 2005 (one swarm), followed by heightened activity with seven swarms in 2011 and two in 2012. Such recurrent clustering underscores persistent low-level strain release along regional faults.

Overall, swarm S20120201.1 exemplifies background seismicity in the northern Sierra Nevada transition zone, providing data for refining models of crustal deformation and fault interaction in this portion of California.

References

USGS Earthquake Catalog
California Geological Survey fault database
UNAVCO geodetic monitoring reports