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Following the 7.0 magnitude earthquake recorded Thursday morning in Cape Mendocino, California, 193 aftershocks of lesser intensity have been recorded in the area, according to the United States Geological Survey (USGS).
Since the strongest tremor struck Thursday morning, aftershocks have ranged from magnitude 2.1 to magnitude 4.7.
The December 5 magnitude 7.0 Cape Mendocino earthquake occurred approximately 100 km southwest of Ferndale, California, off the coast of Northern California, in the vicinity of the Mendocino Fracture Zone.
According to the USGS, this earthquake occurred near the Mendocino triple junction, the region where the Pacific, North American and Juan de Fuca/Gorda plates meet.
The quake occurred at 10:44 a.m. off Cape Mendocino in Humboldt County and the National Weather Service issued a tsunami warning that was later canceled shortly before noon.
The Humboldt County Sheriff's Office said in an update Thursday night that damage assessments are ongoing throughout the county, but no major earthquake-related damage has been reported so far.
Earlier in the day, Gov. Gavin Newsom proclaimed a state of emergency in Mendocino, Humboldt and Del Norte counties that authorizes state agencies to enter into contracts to quickly assist with any emergency response in the counties and allows fairgrounds and state property to be used for shelters and other efforts.
According to the USGS, there is a 53 percent chance of one or more aftershocks greater than magnitude 5 occurring during the next week.
"The number of aftershocks will decrease over time, but a large earthquake may temporarily increase the number of aftershocks," the agency said.
The focal mechanism solutions, he said, indicate that the rupture occurred as a result of a tear fault on a steeply inclined fault that trends east-southeast or north-northeast.
The location, depth and fault mechanism indicate that this event likely occurred at or near the Mendocino Fracture Zone, an east-southeast-trending fault zone that forms the boundary between the Pacific Plate to the south and the subducting Gorda Plate to the north, he added.
The agency noted that earthquakes are common in the region around the Mendocino triple junction because oblique motion between the southern Juan de Fuca/Gorda plate and the Pacific plate causes north-south compression within the Gorda plate and right-lateral translation along the plate boundary.
In the past century, there have been at least 40 other earthquakes of magnitude 6 or greater, including five earthquakes of magnitude 7 or greater, within 250 km of the December 5, 2024, earthquake. These previous earthquakes occurred primarily along the Mendocino transform fault, in the Cascadia subduction zone, or within the Juan de Fuca/Gorda plate.
With information from Bay City News.
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