M 6.2 - 7km N of Petrolia, CA

Tectonic Summary

The December 20, 2021, M 6.2 earthquake offshore of northern California near Petrolia, California, and was located in the vicinity of the Mendocino triple junction – the region where the Pacific, North America, and Juan de Fuca plates meet. The M6.2 was preceded by an M5.7 earthquake 30 km to the west, which occurred on either an east-west striking right-lateral fault or on a north-south striking left-lateral fault. The location and faulting mechanism indicate that the M5.7 earthquake likely occurred on or near the east-west trending Mendocino transform fault that demarks the plate boundary between the Pacific plate and southern Juan de Fuca plate (alternatively termed the Gorda plate).

The M6.2 event occurred onshore, near Petrolia, and preliminary analysis suggests that the M6.2 event occurred in the Gorda plate, based on the depth and the focal mechanism. Most of its aftershocks appear to be in the Gorda plate.

Earthquakes are common in the region around the Mendocino triple junction. Oblique motion between the southern Juan de Fuca/Gorda plate and Pacific plate causes north-south compression within the Gorda plate and right-lateral translation along the boundary between the plates. In the past century, there were 40 other earthquakes of M6 or larger, including six earthquakes M7 or larger, within 250 km of the December 20 earthquakes. These prior earthquakes primarily occurred along the Mendocino transform fault, in the Cascadia subduction zone, or within the Juan de Fuca/Gorda plate. The 12/20/21 M6.2 occurred approximately 8 km NW of the April 25, 1992, M7.2 earthquake, which caused significant shaking across northern California, multiple landslides, a rockfall and a tsunami reaching 1.1 meters at Crescent City, California.