M10: The really big one will probably happen along the Cascadia subduction zone between Cape Mendocino in northern California and Vancouver Island in southern British Columbia

M10: The really big one will probably happen along the Cascadia subduction zone between Cape Mendocino in northern California and Vancouver Island in southern British Columbia
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The post from the The Economist provides details about how the next big one will happen somewhere in North California. The recent Japanese disaster should serve as lesson to prepare this inevitable disaster.

“while the San Andreas fault is capable of inflicting untold damage on built-up areas, it is not the monster portrayed in popular culture. If it ruptured from end to end, it would unleash an earthquake with a magnitude of 8.3—less than a tenth the size of the megaquake that wreaked havoc on Japan last week. Besides, it is most unlikely ever to do so. There is a chunk in the middle where the two sides of the fault creep gently passed one another, relieving the stress as they do. In short, the San Andreas is effectively two separate, shorter faults, each capable of giving birth to a quake only a third the size of the whole. That makes them tiddlers compared with the leviathans stalking the depths off Japan.

That is not to say that a diminished San Andreas could not devastate a major conurbation like Los Angeles. Indeed, a forecast prepared by the United States Geological Survey in 2008 reckoned that if a magnitude 7.8 quake ruptured the San Andreas fault in the desert 100 miles east of Los Angeles, it would topple 1,500 buildings in the metropolitan area and leave 2,000 people dead, 50,000 injured and 250,000 homeless. As grim as that may sound, it would be modest compared with the destruction wrought by the smaller magnitude 6.9 shock that erupted a stone’s throw from Kobe in 1995. Like real estate, the damage earthquakes inflict has more to do with location than size.

The most likely megaquake on the West Coast would be much further north—in fact, 50 miles off the coast between Cape Mendocino in northern California and Vancouver Island in southern British Columbia. This 680-mile strip of seabed is home to the Cascadia subduction zone, where oceanic crust known as the Juan de Fuca plate is forced under the ancient North American plate that forms the continent. For much of its length, the two sides of this huge subduction zone are locked together, accumulating stresses that are capable of triggering megaquakes in excess of magnitude 9.0 when they eventually slip. As such, Cascadia is more than a match for anything off the coast of Japan.

What makes Cascadia such a monster is not just its length, but also the shallowness of the angle with which the encroaching tectonic plate dives under the continental mass. The descending plate has to travel 40 miles down the incline before it softens enough from the Earth’s internal heat to slide without accumulating further frictional stresses. Could the fault unzip from end to end and trigger a megaquake—along with the mother of all tsunamis? You bet. By one account, it has done so at least seven times over the past 3,500 years. Another study suggests there have been around 20 such events over the past 10,000 years. Whatever, the “return time” would seem to be within 200 to 600 years.

And the last time Cascadia let go? Just 311 years ago.”

Source: The Economist – http://www.economist.com/blogs/babbage/2011/03/megaquakes

Photo Credit: The Cascadia Subduction Zone – The USGS – http://earthquake.usgs.gov