According to an article in The Economist, the well-known San Andreas fault in California is not the most likely to cause an earthquake of the magnitude of last week’s quake in Japan. The most likely source is the Cascadia subduction zone:
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.
Cascadia subduction zone image from wikimedia.org.