Seismic Swarm S20211219.1: Analysis of Activity Near Little Lake, California

Seismic swarm S20211219.1 occurred 8 km north-northeast of Little Lake in Kern County, California. The sequence began at 16:30 on 18 December 2021 and concluded at 16:35 on 31 December 2021, spanning 312 hours and 5 minutes. During this period, 143 earthquakes were recorded. This swarm aligns with the region's established pattern of clustered seismic events driven by tectonic stresses in the Eastern California Shear Zone.

The Little Lake area lies within a complex fault network that includes the Little Lake Fault and influences from the nearby Garlock Fault. These structures accommodate right-lateral shear between the Pacific and North American plates. The Coso Volcanic Field, located immediately to the north, adds geothermal and magmatic components that can modulate local seismicity through fluid migration and crustal heating. Depths of recorded events typically range from 0 to 4 km, consistent with shallow brittle failure in this transtensional regime.

Historical records since 2000 document 109 swarms in the vicinity. Annual counts show variability, with peaks of 24 swarms in 2019 and 20 in 2020. Earlier decades recorded lower frequencies, such as nine in 2004 and seven in 2010. These episodes reflect ongoing strain accumulation and release along the shear zone, occasionally linked to larger regional events.

Analysis of the first events reveals predominantly low-magnitude activity. The initial event measured magnitude 1.1 at 3 km depth. Subsequent shocks remained below magnitude 3.0, with the largest reaching 3.0 on 19 December at 3 km depth. Depths clustered between 1 and 4 km, while a few negative values appear in the catalog as internal classification markers. Magnitudes showed a typical swarm distribution: numerous events between 0.5 and 1.5, interspersed with occasional spikes above 2.0 later in the sequence, such as 2.6 on 25 December.

Time progression indicates an initial burst on 18–19 December followed by sustained low-level activity through 26 December. Events on 22 December included several above magnitude 2.0 within minutes of each other, illustrating the clustered nature of swarm behavior. Overall, the sequence produced no events exceeding magnitude 3.0 in the examined subset, underscoring the swarm's modest energy release compared with mainshock-aftershock sequences.

This activity fits the long-term tectonic framework of eastern California, where distributed faulting and volcanic influences sustain recurrent swarms. Continued monitoring remains essential for distinguishing background rates from potential precursors to larger ruptures along major regional faults.

References

SeismoSight internal swarm classification S20211219.1
USGS Earthquake Catalog (regional tectonics of the Eastern California Shear Zone)
California Geological Survey fault database (Little Lake and Garlock Fault characteristics)