Portland Conference Declares West Coast Unprepared for Inevitable Cascadia Subduction Zone Quake
June 27, 2011 • 8:38AM

At the American Institute of Architects conference held at Portland State University in Portland, Oregon, scientists argued June 24 that within the next 50 years, Washington state and northern Oregon face a 10 to 15 percent probability of an offshore quake powerful enough to kill thousands and launch a tsunami that would level coastal cities. Off southern Oregon, the probability of an 8-or-higher magnitude earthquake is greater — 37 percent.

Oregon State University's Chris Goldfinger, one of the world's top experts on subduction-zone quakes, and other authorities, said the Northwest is dangerously unprepared for a massive quake they consider inevitable. Oregon is not nearly as prepared as Japan for a major earthquake, let alone a tsunami. At least 300,000 Oregon children attend school in buildings vulnerable to collapse when the Big One comes. Oregon stores much of its liquid fuel, for example, in tanks on soil prone to liquefaction along the Willamette River north of Portland.

Goldfinger and his colleagues have advanced the field of paleoseismology, examining sediments and rocks for signs of ancient earthquakes. They took core samples recently in Japan's Sendai plain, confirming that the Jogan tsunami in the year 869 swept farther than the March 11 wave, stopped only by hills 3 miles inland in what is now Sendai city.

Goldfinger's team has used similar techniques to date and fingerprint 19 undersea ruptures that have occurred during the last 10,000 years along the entire tectonic-plate margin from California to British Columbia.

Twenty-two more earthquakes have occurred along the southern parts of the Cascadia zone off Oregon. Cascadia quakes also seem to trigger activity along the San Andreas fault line to the south.

The largest Cascadia quake, known as T11, hit 5,900 years ago. The most recent one, called T1, occurred around 9 p.m. Jan. 26, 1700. The roughly 9-magnitude quake sent a tsunami to Japan. That was 311 years ago. The farther south along the Northwest coast, the more frequently quakes occur.

An 8.5-magnitude earthquake would kill an estimated 5,000 Oregon residents at minimum, said Yumei Wang, geohazards team leader at the Oregon Department of Geology and Mineral Industries. Many would probably die in unreinforced masonry structures prone to collapse, she said, including school buildings. State grants to upgrade schools and emergency buildings have lapsed.

Many Oregon bridges are also vulnerable, said Peter Dusicka, a PSU civil engineering professor. His modeling projects Oregon bridge damage exceeding $1 billion from a 9-magnitude Cascadia quake. A less powerful 6.5-magnitude earthquake in the Portland hills would cause more than $1.5 billion in bridge damage, Dusicka expects.

"It wasn't really until the 1990s that a worthwhile earthquake load was considered for bridges" in Oregon, Dusicka said. "Yet most of the building was in the '60s and '70s."

Engineers have learned, Dusicka said, that seismic retrofitting can reduce bridge damage. Portland's Marquam Bridge, built in 1966, had a basic retrofit in 1995 to prevent a superstructure failure. But its vulnerable columns have not been addressed.

Other spans, such as the Fremont Bridge, built in 1973, remain vulnerable. "In Oregon we have done next to nothing," Dusicka said. The state has no current funding for seismic bridge retrofits, he said.

Likewise, Oregon has no seismic requirements for dams, and no mandate to evaluate dams and mitigate risks, Wang said.

Engineers doing seismic retrofits often concentrate on preventing structures from collapsing and killing people, but not on enabling them to survive shaking well enough to be used after an earthquake, Miyamoto said. The result is that buildings that survive the quake may need to be demolished.

In Christchurch, New Zealand, for example, there are downtown high-rises that avoided collapse, but are now leaning dangerously, and must be demolished. Miyamoto said downtown Portland would have similar problems.