Petrolia, CA…From the USGS…The December 20, 2021, M 6.2 earthquake offshore of northern California near Petrolia, California, occurred as the result of strike-slip faulting at relatively shallow depth. The earthquake 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 earthquake likely occurred on or near the east-west trending Mendocino fracture zone that demarks the plate boundary between the Pacific plate and southern Juan de Fuca plate (alternatively termed the Gorda plate).

The earthquake was located in the vicinity of the Mendocino triple junction – the region where the Pacific, North America, and Juan de Fuca plates meet. The faulting mechanism of the December 20 earthquake is most consistent with the earthquake having occurred between the Juan de Fuca/Gorda and Pacific plates, as opposed to on the San Andreas fault (the boundary between the North America and Pacific plates) or on the Cascadia subduction zone interface (the boundary between the North America and Juan de Fuca plates). However, the preliminary location of the earthquake east of the Mendocino triple junction indicates that the earthquake may have occurred on a portion of the Mendocino fracture zone that was previously subducted beneath North America. At the location of the earthquake, the Juan de Fuca/Gorda plate moves southeastward with respect to the Pacific plate at a rate of 47 mm/yr.

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 earthquake. These prior earthquakes primarily occurred along the Mendocino fracture zone, in the Cascadia subduction zone, or within the Juan de Fuca/Gorda plate. On June 15, 2005, an M7.2 earthquake occurred northwest of the December 20 earthquake within the Juan de Fuca/Gorda plate on a northeast-striking left-lateral strike slip fault. The 2005 earthquake likely resulted from compression within the Juan de Fuca/Gorda plate. As is common for strike slip faults, the 2005 earthquake did not produce a tsunami, and preliminary analysis of the December 20 earthquake suggests that a tsunami was not generated.