Seismic Analysis of Swarm S20111019.1 Near Sierraville, California

The earthquake swarm designated S20111019.1 occurred 1 km west-northwest of Sierraville in Sierra County, California. It began at 06:36 on 19 October 2011 and concluded at 11:46 on 23 October 2011, spanning 101 hours and 9 minutes during which 106 earthquakes were recorded. Sierraville lies within the northern Sierra Nevada, near the transition to the Basin and Range province. This region experiences seismic activity influenced by extensional tectonics along the Sierra Nevada frontal fault system and related structures. Historical earthquake swarms in the area reflect periodic strain release in the deep crust, consistent with the broader tectonic regime of the western United States. According to internal SeismoSight classification data, four swarms have occurred in the region since 1 January 2000. These took place in 2004 (one swarm), 2005 (one swarm), and 2011 (two swarms). Analysis of the first 100 events reveals predominantly microseismic activity. Magnitudes ranged from -0.6 to 1.6, with the majority falling between -0.3 and 0.5. Depths clustered tightly between 27 km and 33 km, suggesting a deep crustal source. Notable exceptions included a 1.6 magnitude event at 30 km depth on 19 October at 22:18:41 and a shallower 0.8 magnitude event at 8 km on 21 October at 17:46:14. The sequence began with low-magnitude events and included several magnitude 1.2–1.3 shocks distributed over the first two days, followed by a gradual decline in frequency and intensity. This pattern indicates a typical swarm behavior driven by fluid migration or slow slip rather than a single mainshock-aftershock sequence. Depths around 30 km align with known lower-crustal seismicity in the northern Sierra Nevada, where temperatures and pressures permit brittle failure at greater depths than in many other continental settings.

References

USGS Earthquake Catalog California Geological Survey Regional Fault Maps SeismoSight Internal Swarm Database