Q: When is an earthquake not an earthquake? A: When it's two earthquakes! The M6.2 Petrolia, CA earthquake on Dec 20, 2021, was really two earthquakes. Learn more at
Two overlapping earthquakes on a seismic record look like one big messy event. On December 20, 2021, after Petrolia residents felt shaking, the USGS-UC Berkeley Northern California Seismic System processed the data and announced that a magnitude 6.2 had occurred offshore on the Mendocino transform fault.Map showing the revised location of the M6.2 earthquake that occurred on 12/20/2021. The magnitude 5.7 and 4.0 foreshocks are located offshore along the Mendocino fault.
In the subsequent weeks since the event, USGS and UC Berkeley staff have carefully analyzed the data in collaboration with local experts at Humboldt State University. They have concluded that the Petrolia earthquake was really two Petrolia earthquakes 10 seconds apart in two different locations. The first event was a magnitude 5.7 offshore 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 .
Because of the logarithmic nature of earthquake magnitudes, M5.7 + M6.2=M6.24, which is why the magnitude of the initial single event didn’t change when it was split into two events. Seismologists are still working to disentangle the two events more precisely, and the magnitudes of the two events may adjust slightly in the future.
Earthquakes are common in the region around the Mendocino triple junction, which is the meeting place of the Pacific, Gorda, and North American plates and marks the transition between the right-lateral strike-slip motion dominated by the San Andreas Fault System to the south and the oblique-collisional motion of the Cascadia Subduction zone to the north.
The December 20, 2021, 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. The 1992 event, which is generally considered to have occurred on a shallow thrust fault in the North American plate, was followed by two large aftershocks of M6.5 and M6.
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