A magnitude 5.1 earthquake struck off the Northern California coast late Saturday.
According to the U.S. Geological Survey, the earthquake just before midnight was about 12 kilometers (7.5 miles) west of Petrolia in Humboldt County and 56 kilometers (35 miles) southwest of Eureka.
The depth was just over 30 kilometers (18.5 miles).
The shaking could be felt as far south as San Francisco and as far east as beyond Yuba City, according to reports submitted to USGS.
There were no immediate reports of damage. Below you will find some ‘I felt it’ reports:
Why is this California earthquake interesting?
The M5.1 earthquake hit near the Mendocino Triple Junction, where three tectonic plates (Gorda plate, the North American plate, and the Pacific plate) and several fault lines meet (among others the Cascadia Subduction Zone and the San Andreas Fault).
In oder to better visualize what happening at this spot, imagine a busy intersection with a three-way stop sign at rush hour. It takes only a few irresponsible drivers who don’t want to wait their turn and traffic jams and accidents are likely to occur.
Something very similar is happening every day in the Earth’s crust under Cape Mendocino. There upheaval is caused not by impatient drivers but by tectonic plates, which are crashing into each other with unstoppable momentum.
This tectonic three-way intersection is called the Mendocino Triple Junction. It is one of the few places in the world where three of the gigantic plates which continuously drift on the Earth’s mantle meet.
Along the length of most of California, the San Andreas Fault defines the boundary between the Pacific Plate to the west and the North American Plate to the east. Along the fault line, the Pacific Plate slides horizontally in a northnorthwesterly direction with respect to North America.
Under Cape Mendocino, this trip comes to an abrupt halt, because another plate gets in the way. North of the Cape the off-shore Gorda Plate, a small cousin of the Pacific Plate, glides under the North American Plate in what is called a subduction zone.
The boundary line between the Pacific Plate and the Gorda Plate is a fault similar to the San Andreas, called variably the Mendocino Transform or the Mendocino Fracture Zone. It goes from Cape Mendocino for several hundred miles due west into the Pacific Ocean.
Along this fault, the Gorda Plate slides horizontally to the east with respect to the Pacific Plate with a speed of about 2 inches per year.
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