Seismic Swarm S20221220.1: Analysis of Activity Near Ferndale, California

The seismic swarm designated S20221220.1 began at 10:34 UTC on 20 December 2022 and concluded at 13:58 UTC on 26 December 2022. Centered 7 km southwest of Ferndale in Humboldt County, California, the sequence produced 165 earthquakes over 147 hours and 24 minutes. This event provides a focused view of regional tectonic behavior in one of North America’s most active seismic zones.

Ferndale lies near the Mendocino Triple Junction, where the Pacific, North American, and Juan de Fuca plates converge. The area experiences frequent seismicity driven by right-lateral strike-slip motion along the Mendocino Fracture Zone and convergence associated with the Cascadia subduction zone. Crustal depths in the recorded sequence ranged primarily between 15 km and 29 km, consistent with the transition zone between the shallow accretionary prism and deeper basement rock.

The swarm initiated with a magnitude 6.4 earthquake at 17 km depth. Subsequent events included a magnitude 4.9 shock at 26 km depth within five minutes, followed by a steady sequence of smaller events. Magnitudes in the first 100 recorded shocks clustered between 1.6 and 3.5, with the majority falling between 2.0 and 2.7. Depths remained stable around 17–19 km for most aftershocks, indicating a consistent rupture volume. Notable later events within the initial 100 included a magnitude 4.3 earthquake at 19 km depth on 20 December at 22:06 UTC and several magnitude 3+ events distributed throughout the first day.

Temporal analysis of the first 100 events shows the highest rate of occurrence in the first six hours, with activity tapering but persisting through the following days. The distribution of depths and magnitudes suggests a diffuse release of strain rather than a single dominant fault plane, typical of swarm behavior in this tectonic setting.

Historical records indicate that swarms of comparable scale have been infrequent since 2000. Only three prior swarms were documented in the region: one each in 2002, 2015, and 2021. This low recurrence rate underscores the episodic nature of clustered seismicity near the triple junction and highlights the value of detailed swarm monitoring for understanding long-term strain accumulation.

The 2022 sequence adds to the catalog of observations that help refine models of fault interaction and stress transfer in northern California. Continued analysis of depth and magnitude patterns from such events supports improved characterization of seismic hazard in the vicinity of the Mendocino Triple Junction.

References

USGS Earthquake Catalog
California Geological Survey Regional Reports
SeismoSight Internal Swarm Classification S20221220.1