Bridges of the twenty-first centuryOregon's bridges to be readied for the Big One

Published 18 November 2009

There are 2,671 bridges in Oregon’s highway system; researchers develop a computer model which, for the first time, gives state authorities bridge-by-bridge estimates of damage, repair cost, and traffic delay costs associated with a shattering western Oregon quake; the new tool would allow engineers to prioritize which of the state’s bridges should get seismic upgrades

An earthquake such as the big one that hit the Pacific Northwest in 1700 would wipe out many of Oregon’s bridges, cause billions of dollars of damage, make much of U.S. 101 and Interstate 5 impassable, and sever all road connections between the coast and the Willamette Valley.

The Oregonian’s Eric Mortenson writes that this is a dire scenario that the most sober of scientists see as inevitable.

These scientists, though, are also excited about a computer model that, for the first time, gives them bridge-by-bridge estimates of damage, repair cost, and traffic delay costs associated with a shattering western Oregon quake.The model could, they say, help Oregon prepare itself and keep the region moving in the event of a disaster.

Using information gained from an April simulation of quakes ranging up to 9.0 magnitude, engineers can now prioritize which of the 2,671 bridges in Oregon’s highway system should get seismic upgrades. This is a $3 billion job, Oregon Department of Transportation spokesman Dave Thompson said.

There are lots of details, engineering-wise, that will help us plan better which bridges to triage and in what order,” he said.

An earthquake of 8.0 to 9.0 magnitude — extremely powerful, from the well-known, deep-set Cascadia Subduction Zone off the coast — would destroy 399 bridges and heavily damage 621, according to the joint study by ODOT and Portland State University. The remaining 1,651 would remain serviceable. Bridges built before 1975, when seismic standards came into vogue, are highly vulnerable to damage or collapse, the study found. About 64 percent of the bridges on the state highway system were built before the 1970s.

The computer program estimated the ground motion, surface fault rupture, and degree to which the supporting ground around bridges would liquefy and slosh away. Major damage would occur throughout western Oregon, including in the state’s most heavily populated areas of Portland, Salem and Eugene.

The Oregon coast would be entirely cut off, Thompson said. All connecting highways from the coast to the Willamette Valley go through mountainous terrain that would be susceptible to landslides in a major earthquake. In addition, dozens of older bridges that might collapse fill coastal highways.

Mortenson notes that even a smaller, closer-to-home earthquake limited to fissures at the Earth’s crust would do tremendous damage in the Portland area alone, according to the study. A 7.0 magnitude quake would collapse five bridges and cause extensive damage to 48 others. Damage would reach an estimated $1.6 billion for bridge repair and replacement and $68 million for the costs of increased travel times.

Oregon has done some seismic work. Bridges built after 1995 were constructed to tougher seismic standards and are considered to be much less vulnerable to damage or collapse, according to the study. On I-5, the Boone Bridge at Wilsonville and the double-decker Marquam Bridge in Portland have received earthquake retrofits.

Most of the quakes that jolt Oregon are small; the largest in recent years were a 5.6 magnitude quake in Scotts Mills in 1993 and 5.9 and 6.0 quakes in Klamath Falls that same year. Researchers, however, have long speculated that Oregon is due — perhaps within the next fifty years — for a repeat of the estimated 9.0 earthquake that crushed the region in January 1700

That quake, about 75 miles offshore in the Cascadia Subduction Zone, ripped a 600-mile fault from British Columbia to Northern California. Native American oral traditions tell of a violent shaking and huge flood; the quake also produced an “orphan tsunami” that struck Japan. The quake was confirmed by studying the growth rings of trees killed at the mouth of the Columbia River and by comparing it to the oral traditions and to records kept in Japanese villages. The tsunami was called an “orphan” because it came unexpectedly: there was no accompanying storm and villagers did not feel the Cascadia quake.