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Location:
10km ESE of Willits, CA
Period:
4 Oct 2020 04:12:12 - 8 Oct 2020 13:02:13 (4 days 8 hours 50 minutes)
Volcanoes in 100km radius:
Clear Lake Volcanic Field(60km)
Earthquakes:
58
4 swarms found nearby.
2000
S20000518.1(16.9km)
17 May
4 days 4 hours
56 earthquakes
2007
S20070418.2(15.7km)
17 Apr
8 days 1 hours
107 earthquakes
2020
19 Aug
4 days 10 hours
99 earthquakes
2026
24 Jun
3 days 7 hours
83 earthquakes
AI-generated article — for informational and entertainment purposes only. May contain inaccuracies. Full disclaimerFound an error?

Earthquake Swarm Analysis: Willits, California, October 2020

An earthquake swarm designated S20201004.1 occurred 10 km east-southeast of Willits, California, from 04:12 UTC on 4 October 2020 to 13:02 UTC on 8 October 2020. Over 104 hours and 50 minutes, the sequence produced 58 events. This activity took place in northern California’s Coast Ranges, a region shaped by the interaction of the San Andreas Fault system and the Mendocino Triple Junction.

The Mendocino Triple Junction marks the meeting point of the Pacific, North American, and Juan de Fuca plates. Right-lateral strike-slip motion along the Maacama Fault and related structures dominates local tectonics. Crustal deformation here produces frequent small earthquakes, with occasional swarms reflecting fluid migration or slow slip along fault zones rather than a single large rupture.

The October 2020 swarm began with a magnitude 3.4 event at 6 km depth. Subsequent activity remained modest, with magnitudes mostly between 1.0 and 2.0. The largest later shock reached magnitude 2.9 on 7 October at 5 km depth. Depths clustered between 4 and 7 km, indicating shallow crustal sources consistent with the Maacama Fault zone. Only four events exceeded magnitude 2.5, underscoring the swarm’s low-energy character.

Temporal distribution showed peak rates during the first 24 hours, followed by a gradual decline. By 8 October, activity had tapered to isolated micro-earthquakes. No damage or felt reports above moderate intensity were associated with the sequence.

Historical records indicate such swarms are uncommon in this precise area. Since 1 January 2000, only three swarms have been documented: one each in 2000, 2007, and 2020. This low recurrence suggests episodic rather than continuous swarm behavior along the local fault network.

Geodetic and seismic monitoring by regional networks continues to track strain accumulation near the Mendocino Triple Junction. Understanding swarm statistics helps refine probabilistic models for northern California seismicity and supports improved hazard assessment for communities near Willits.

References
USGS Earthquake Catalog (earthquake.usgs.gov)
California Geological Survey, Fault Activity Map of California
USGS, Tectonic Setting of the Mendocino Triple Junction