Seismic Swarm S20221220.2 Near Hydesville, California

A seismic swarm designated S20221220.2 was recorded 6 km north-northeast of Hydesville in Humboldt County, California. The sequence began at 10:47 UTC on 20 December 2022 and ended at 05:24 UTC on 22 December 2022, lasting 42 hours and 37 minutes. During this interval, 56 earthquakes were detected.

The events clustered at focal depths between 15 km and 27 km, with the majority occurring between 18 km and 23 km. Magnitudes ranged from 0.9 to 3.9. The largest event, magnitude 3.9, occurred at 15:09 UTC on 20 December at 22 km depth. Other notable shocks included two magnitude-3.0 and 3.3 events later the same day and on 21 December. Activity was most intense during the first 12 hours, after which the rate declined steadily until the final recorded event.

Hydesville lies within the southern Cascadia subduction zone near the Mendocino Triple Junction, where the Pacific, North American, and Juan de Fuca plates interact. This tectonic setting produces frequent moderate seismicity and occasional larger events. The regional crust accommodates both strike-slip motion along the Mendocino fracture zone and thrust faulting associated with subduction. Historical records document damaging earthquakes in 1992 (Mw 7.2 Cape Mendocino) and 2010 (Mw 6.5), underscoring the area’s persistent seismic hazard.

According to internal classification records, only one swarm has been identified in the Hydesville vicinity since 1 January 2000. The present sequence therefore represents the sole documented swarm in more than two decades.

The swarm produced no reported damage or felt intensities above moderate levels in nearby communities. Its occurrence at mid-crustal depths is consistent with background seismicity patterns observed throughout the Eel River basin and adjacent offshore regions.

References

USGS Earthquake Catalog (ANSS Comprehensive Catalog)
California Geological Survey Quaternary Fault and Fold Database
Pacific Northwest Seismic Network regional bulletins