The Big-Shake, Big-Wave Theory

If a megaquake like the one that hit Japan last March were to strike the U.S., the Pacific Northwest coast would be the likeliest spot. Geologists have their eyes on the Cascadia subduction zone, a 740-mile seam where the Juan de Fuca and North American plates meet. The CSZ has been building up tension for more than 300 years, say some seismologists. If that te

Bruce Barcott

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A. MINUTE 0:00. The North American plate slides 57 feet west over the heavier Juan de Fuca plate, which moves east about an inch per year. The quake’s initial pressure wave travels through the Earth’s crust at 13,000 miles per hour and reaches the Northwest coast in ten seconds.

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B. MINUTE 0:00. Displaced water creates a low standing wave, which peels off in two directions. One wave heads toward Japan at 450 miles per hour, reaching Honshu in ten hours. The other will reach the Northwest coast in 20 minutes.

C. MINUTE 3:00. The North­west coast drops five feet
in elevation.

D. MINUTE 5:00. The earth­quake stops. The tsunami is now 35 miles offshore.

E. MINUTE 10:00. The wave is 15 miles offshore. Water at the coast has begun to recede.

F. MINUTE 20:00. The edge of the tsunami hits the coast at flash-flood speed, about 30 mph, and then slows to 11 mph on dry land. The water will rise to 30 feet in places and proceed as far as six miles inland.

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