Monday 22 October 2012

Central California shaken by Earthquake.

On Saturday 20 October at 11.55 pm, local time (6.55 am on Sunday 21 October, GMT), the United States Geological Survey recorded a Magnitude 5.3 Earthquake at a depth of 9.4 km in Monterey County, central California, roughly 27 km east of King City, or 147 km southeast of San Hose. Earthquakes this large and this shallow are potentially dangerous, even in an area well prepared for seismic activity such as California, and the USGS estimates that a quake of this size in central California has a 24% chance of causing fatalities. Fortunately on this occasion no serious damage or injuries were reported, but people reported feeling the quake from San Francisco to Bakersfield.

Map showing the location of the 20 October Earthquake. Google Maps.

The quake took place directly on the San Andreas Fault, which bisects California from north to south and which separates the Pacific Plate to the west from the North American Plate to the east. The San Andreas is a slip-strike (or transform) fault, the two plates are moving past one another horizontally, the Pacific Plate moving northward and the North American Plate to the south.

Map showing the major fault zones of California. California Geological Survey.

The fault runs off the coast of California at Cape Mendocino, continuing under the sea until it meets the Mendocino Fracture Zone. To the north of this the fault becomes a subduction zone, with the Gorda Plate fragment, a chunk of the ancient and now largely subducted Juan de Fuca Plate.

Map showing the progress of the San Andreas Fault to the north, where it becomes the Gorda Plate subduction zone. Humbolt State University.

To the south the San Andreas Fault grades into the Imperial Fault Zone, a more complex area of slip-strike faulting, which in turn grades into the Gulf of California Rift Zone, a developing divergeant plate margin that develops southward into the East Pacific Rise, separating the Rivera, Cocos and Nazca Plates to the west from the North American, Caribbean and South American Plates to the east. 

The San Andreas Fault develops into the East Pacific Rise to the south. Imperial College London.

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