Saturday, December 15, 2012

Big quake off Baja continental shelf

A magnitude 6.3 earthquake that occurred early Friday morning about 160 miles offshore of Baja California was the largest event in that region in 40 years, according to a report by the USGS.   The quake was off the edge of the continental shelf, away from the tectonic plate margin where earthquakes are common.  [Right, record of the quake from the AZGS seismic station in Douglas, AZ]

The USGS explained the tectonic setting:

The December 14, 2012 M 6.3 earthquake 250 km southwest of Avalon, California occurred as a result of shallow normal faulting within the oceanic lithosphere of the Pacific plate. This event is located some 400-450 km west-southwest of the plate boundary between the Pacific and North America plates – the San Andreas fault system in southern California – and is not associated with that fault system. Instead, this earthquake represents intraplate faulting along northeast-southwest trending normal faults within the crust of the Pacific plate, just to the west of California’s continental shelf. The causative fault is not known at this time. At the location of this event, the Pacific plate moves to the northwest with respect to the North America plate at a velocity of approximately 54 mm/yr.

While the broad region surrounding the December 14, 2012 event experiences frequent earthquakes along the San Andreas and associated faults in southern California, the area offshore and within 250 km of this earthquake has not hosted any events greater than M 6 over the past 40 years.

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