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2 November 2012
The San Andreas Fault line cuts through the state of California for more than 600 miles; from near El Centro in the south to north of the city of San Francisco.
One of the places where scientists have intensively studied the fault is the little town of Parkfield, in southern Monterey County.
It was decided in the 1980's that Parkfield was the best place to try and "capture" an earthquake. That is, to have varieties of instruments in place throughout the valley to measure the before, during and after of a slippage on the San Andreas fault.
The reason for choosing Parkfield for this grand seismology experiment was that moderately large earthquakes (magnitude 6.0) had taken place on average every 11 years since settlers began living there in the mid nineteenth century.
That regularity was very unusual among seismic zones around the world, and for that reason, Parkfield was given (or took) the title of "Earthquake Capitol of the World". Regular earthquakes; very high density of earthquake instruments.
If you are unfamiliar with earthquake faults, like the San Andreas, they aren't always easily spotted. There aren't generally great chasms marking out the trace of the fault. Trained geologists can pick out the signs of a fault zone, but for the untrained, it is easier to see the faults effects on manmade or natural features.
This bridge is famous for showing the effects of the San Andreas fault, since the fault passes directly beneath it. The land mass we can see at the far end of the bridge is the beginning of the North American plate which extends all the way to the mid-Atlantic Ocean. The portion where the photographer was standing is the eastern boundary of the Pacific plate. The opposite plate seems to be moving to the right no matter which side on which you are standing.
At the Parkfield location, as with many other earthquake faults, most of the actual movement takes place during earthquake events. The stress of the plates moving against each other is released with the movement as seen in the bridge and depicted in the monument. The energy released "moves the earth" and is transmitted through the ground which results in shaking.
- the road visible in the center is Vineyard Canyon Road and the San Andreas Fault line runs approximately 100 feet to the left of the road. The white bridge shown above is at the far end of the road. The road bends to the lower right of the photo while the fault line continues straight and exits the bottom left of the photo.