Earthquake Swarm S20200819.1 Near Willits, California

An earthquake swarm designated S20200819.1 occurred 8 km east-southeast of Willits in Mendocino County, California. The sequence began at 00:46 UTC on 19 August 2020 and concluded at 11:42 UTC on 23 August 2020, spanning 106 hours and 56 minutes. During this period, 99 earthquakes were recorded.

The swarm initiated with a magnitude 2.9 event at 7 km depth. Within the first hour, activity escalated to a peak magnitude of 4.2 at 00:55 UTC, followed closely by a magnitude 3.2 at 01:43 UTC and a magnitude 3.9 at 01:59 UTC. Subsequent events remained predominantly below magnitude 2.5, with depths concentrated between 5 km and 9 km. Notable later shocks included magnitude 2.4 events on 21 and 22 August. The final recorded event reached magnitude 1.5 at 7 km depth.

This swarm unfolded in a tectonically complex region near the Mendocino Triple Junction, where the Pacific, North American, and Juan de Fuca plates interact. The area lies adjacent to the northern termination of the San Andreas Fault and experiences distributed deformation through the Maacama and Bartlett Springs fault zones. Shallow crustal seismicity here often manifests as swarms driven by fluid migration or aseismic slip rather than mainshock-aftershock sequences.

Historical records indicate limited swarm activity in the vicinity since 2000. Only two prior swarms have been documented: one in 2000 comprising a single event and another in 2007 also limited to one event. The 2020 sequence therefore represents the most substantial swarm activity recorded in the area during the past two decades.

Such episodic clusters contribute to ongoing strain release along secondary faults east of the main San Andreas trace. Depths consistently under 10 km align with the brittle-ductile transition zone typical of the northern California Coast Ranges. No damage or felt reports beyond light shaking were associated with the larger events in this swarm.

SeismoSight internal classification
USGS Earthquake Catalog
California Geological Survey regional fault maps