The Big-Shake, Big-Wave Theory
See how a megaquake would shake out in the Pacific Northwest
Officials in each town are aware of the CSZ megaquake-and-tsunami risk. They’ve all printed up evacuation-route maps. In Cannon Beach, they’ve even talked about building a new city hall that would double as a tsunami safe house. But nobody has ever had the money to build anything, other than installing tsunami-warning sirens.
Not that there will be much warning. Even today, when it comes to earthquake prediction, the earth remains a poker player without a tell. “The best we can do is let people know how the shaking will spread once an earthquake starts,” says University of Washington geologist John Vidale, director of the Seattle-based Pacific Northwest Seismic Network. “Japan’s system is the best in the world. Within 30 seconds of the start of the March 11 earthquake, they broadcast a warning that it would be at least a magnitude 8.0.” No such system yet exists in the United States, though Vidale’s and other teams are working on one.
“Let me tell you this,” Patrick Corcoran says as we stroll down Broadway, Seaside’s main drag. “There’s a shop a couple blocks up the street that sells T-shirts that say TSUNAMI EVACUATION PLAN: (1) GRAB BEER. (2) RUN LIKE HELL.”
“And honestly,” he says, “that’s not a bad strategy.”
After 312 years, the Cascadia subduction zone can no longer contain the strain. It ruptures at a spot 55 miles west of Cannon Beach and quickly spreads along 700 miles of its 740-mile length. The North American plate slips anywhere from 45 to 57 feet to the southwest, sliding over the Juan de Fuca plate. It doesn’t happen instantly. A mass that large—remember, we’re talking about crust more than 50 miles deep—takes time to move. But upon its first lunge, the CSZ sends out a pressure wave, or P-wave, that travels through the earth’s crust at 13,000 miles per hour. It reaches the West Coast within ten seconds. That first P-wave, the earthquake’s leading edge, hits Ocean Shores, Cannon Beach, and Seaside. Thirty seconds later it reaches Portland; in 50 seconds, it hits Seattle. At the University of Washington’s Seismology Lab in Seattle, the seismometers jump. Geologists read the data and declare the earthquake a 9.1. It’s the full rip.
The first few seconds feel like any other strong earthquake: jarring. “The pressure wave is like a jackhammer, rat-tat-tat-tat-tat,” explains Goldfinger, who happened to be outside Tokyo—at a geology conference to discuss the Sumatra earthquake—during the March 11 Sendai quake.
The sound is majestic and awesome. In his book A Dangerous Place, author Marc Reisner wrote of his experience in San Francisco during the 1989 Loma Prieta earthquake: “What I remember most vividly is the grinding, the unearthly noise of great surfaces and structures grating together.” Chris Goldfinger recalled the sound of leaves rattling on trees. In Japanese houses, the sound was an unrelenting clatter of metal and glass.
In the offices, apartments, and high-rise condos of Seattle and Portland, uncertainty creeps into half a million heads: Freeze or flee? In videos shot during the Japanese megaquake, the overwhelming emotion on display isn’t panic or raw fear. It’s focused anxiety and strategic calculation. They are trying to figure out what to do.