Northwest US at risk for huge earthquake

March 3, 2010 at 12:00a.m.

Associated Press

LOS ANGELES

The disaster in Chile has brought new attention to an undersea fault along the Pacific Northwest capable of producing the same type of mega earthquake and inflicting heavy damage on bustling cities such as Seattle, Portland and Vancouver.

The fault has been dormant for more than 300 years, but when it awakens — tomorrow or decades from now — the consequences could be devastating.

The last rupture unleashed the largest known quake to hit the Lower 48 — a magnitude-9 that sent tsunami waves crashing into Japanese coastal villages.

Recent computer simulations of a hypothetical magnitude-9 quake found that shaking could last two to five minutes — strong enough potentially to cause poorly constructed buildings from British Columbia to Northern California to collapse and severely damaging highways and bridges.

Such a quake would also send powerful waves rushing to shore in minutes. Though big cities such as Portland and Seattle would be protected from severe flooding, low-lying seaside communities may not be as lucky.

The Pacific Northwest “has a long geological history of doing exactly what happened in Chile,” said Brian Atwater, a geologist with the U.S. Geological Survey and University of Washington. “It’s not a matter of if but when the next one will happen.”

The Pacific Northwest fault behaves much like the one that broke offshore in Chile that triggered a magnitude-8.8 quake. Shaking lasted 21‚Ñ2 minutes, and the temblor destroyed or badly damaged 500,000 homes.

Located just 50 miles off the coast, the 680-mile-long Casacadia fault is part of several seismic hot spots around the globe where plates of the Earth’s crust grind and dive.

There’s an 80 percent chance the portion of the fault off southern Oregon and Northern California would break in the next 50 years and produce a megaquake.