Seismic Swarm S20200109.2: Analysis of Activity Near Little Lake, California

Seismic swarm S20200109.2 was recorded 22 km east-southeast of Little Lake, California, from 15:55 UTC on 8 January 2020 to 15:15 UTC on 9 January 2020. The sequence lasted 23 hours and 19 minutes and included 27 earthquakes. This event occurred within the tectonically active Eastern California Shear Zone, a region characterized by distributed right-lateral strike-slip faulting and frequent small-magnitude seismicity. The area lies near the Coso Volcanic Field, where geothermal activity and fluid movement along faults contribute to episodic earthquake swarms rather than mainshock-aftershock sequences.

The swarm displayed typical characteristics of low-magnitude events clustered in time and space. Magnitudes ranged from -0.6 to 2.4, with the largest event (magnitude 2.4) occurring at 02:50 UTC on 9 January at a depth of 3 km. Most events were shallower than 10 km, consistent with brittle failure in the upper crust. Depths varied between 2 km and 13 km, suggesting activity along multiple fault strands or within a fluid-rich zone. Negative magnitudes indicate microseismicity detectable only by dense local networks.

The sequence began with events around magnitude 0.5–0.7 at depths of 6–12 km. Activity intensified overnight, including several events near magnitude 0.3–0.5 at 8–9 km depth. The peak magnitude event was followed by smaller after-events at shallower depths (2–6 km). Later events on 9 January included magnitudes up to 1.2 at depths of 4–9 km, tapering off by mid-afternoon. Such temporal clustering and depth variation are common in Coso-region swarms, often linked to pore-pressure changes or magmatic fluid migration.

The Little Lake area has a documented history of swarm activity. Since 1 January 2000, 81 swarms have been identified in the region. Annual counts show variability, with notable years including 2019 (29 swarms), 2010 (7 swarms), and 2004 (7 swarms). The elevated rate in 2019 aligns with regional stress perturbations following the Ridgecrest earthquake sequence of July 2019, which occurred approximately 50 km to the southeast. Earlier swarms in 2000–2018 averaged 1–5 events per year, reflecting background tectonic loading in the shear zone.

Geologically, the Coso Volcanic Field hosts Quaternary rhyolite domes and basaltic flows overlying a shallow magmatic heat source. This setting promotes hydrothermal circulation that can trigger swarms through increased pore pressure. Faults in the area strike northwest-southeast, accommodating Pacific-North America plate motion at rates of several millimeters per year. Historical seismicity catalogs confirm persistent low-level activity, with swarms serving as indicators of subsurface fluid dynamics rather than precursors to larger earthquakes.

Overall, swarm S20200109.2 exemplifies the recurrent, low-magnitude seismicity typical of this portion of eastern California. Continued monitoring supports improved understanding of fault interactions and geothermal processes in the region.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
California Geological Survey regional fault maps
Coso Volcanic Field geothermal studies (peer-reviewed literature)