A large earthquake of magnitude 6.3 (M6.3) struck off the Californian coast around 185 miles south west of San Diego in the early hours of Friday, December 14, 2012.
Although the epicentre of the quake was underwater, the quake was not sufficiently large to generate a tsunami and no tsunami alert has been issued.
The tremor, which occurred at a depth of around six miles, was followed by at least one aftershock of M4.3 but at the time of writing no injuries or damage to property had been reported.
California Earthquake: Tectonic Setting
The Pacific coast of North America is notoriously vulnerable to seismic activity. Much of the area represents the boundary between the Pacific and North American tectonic plates (it is part of the seismically active zone known as the Pacific Ring of Fire) while further south, off the Mexican coast, the situation becomes more complex as the Pacific, Cocos and North American plates are juxtaposed.
The epicenter of California’s 14 December quake was located within the Pacific Plate approximately 300 miles from the plate boundary itself, at a point where the boundary (which varies in nature along its length) changes from a constructive margin at which new ocean crust is formed, to a transform margin, at which crust is neither generated nor lost.
Movement along the transform section, which forms the notorious San Andres Fault Zone, is lateral, or strike-slip. Latest information from the USGS, however, suggests that the earthquake resulted from faulting within the Pacific Plate, rather than movement at the plate boundary.
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