It’s Temblor Time in Pacific Northwest, Scientists Warn

Every 300 to 600 years, a shift in the massive crustal plates off the Oregon Coast unleashes powerful quakes that roil the ground, swallow the coast and reshape the landscape.

With the help of Indian legends, tales of tsunamis that hit Japan and telltale forests buried in the surf, geologists have been able to piece together that the last big quake shook the state around 1700. That means another one is due.

“The next one could occur, who knows, tomorrow,” said Mark Darienzo, earthquake program coordinator with Oregon’s Office of Emergency Management.

Geologists say the next magnitude 8 or 9 quake likely will begin from a fault offshore, and as they map out their disaster scenarios, it’s not a question of if it will happen--but when.

In the picture they paint, the violent quake quickly reduces soil in some areas to mush and crumbles the structures sitting atop it.

People in towns and cities are crushed or buried as unreinforced brick buildings collapse. Wood-frame houses twist and shake, and some older homes are wrenched from their foundations. Many bridges fail. Roads and highways throughout the region are cut by shifting earth and landslides.

Sandy soil along the Columbia and Willamette rivers turns to soup in a process called liquefaction. The supple clay beneath Portland’s west suburbs and the Eola Hills northwest of Salem shivers like thick gelatin, multiplying the power of every rumble. Some homes and streets may slide down Portland’s West Hills.

Within a half hour, giant waves --called tsunamis--hit the Oregon coast, swallowing the beach, seaside motels, vacation homes and shops.

In the aftermath, towns are battling fires and coping with utility outages. In the days ahead, Oregonians will count their dead and begin the cleanup.

The site where two massive plates slide past each other 32 to 70 miles off the coast of Oregon and Washington is known as the Cascadia subduction zone.

Until the last decade, scientists thought the deep crustal faults off Oregon’s coast were aseismic and unable to produce large earthquakes.

But paleoseismologists--who study the history of earthquakes from layers in the earth’s crust--found evidence in the 1980s that the entire Northwest coast has been repeatedly subject to quakes of magnitudes 8 and 9.

Since then, scientific knowledge about earthquake hazards in Oregon has grown exponentially.

“Because they figured out there’s this big oceanic plate offshore that could produce these big faults, the research in this area started to really take a big increase,” said Yumei Wang, director of earthquake programs at the Oregon Department of Geology and Mineral Industries.

In addition to the danger of an offshore quake, researchers also warn of surface quakes--those beginning in smaller crustal faults found in Oregon.

Scientists believe many of these faults could generate a quake that, although smaller than a subduction zone quake, could cause significant damage depending on proximity and soil conditions.

“So we started to increase our awareness that there is a plate boundary fault, and then all the other faults like these that are faults within plates also became a much bigger deal,” Wang said.

The Scotts Mills earthquake of March 25, 1993 (magnitude 5.6) and the twin Klamath Falls temblors of Sept. 20, 1993 (magnitudes 5.9 and 6.0) were crustal events. Both caused millions of dollars in damage.

In the Portland area alone, geologists have identified five significant surface faults. They also have mapped faults in Salem and are working on maps of the Eugene area.

“There’s a bunch of different techniques for finding faults,” Wang said. “One is looking at the landscape . . . another one is going out to verify and trenching, actually taking a backhoe and trenching through and seeing if there’s evidence of very shallow surface displacement.”

In addition to observation of land features and test digging, geologists are using sophisticated computer programs to analyze magnetic readings of the earth’s surface and other data, creating models and maps.

“Basically, there are high and low magnetic signatures in the earth, and some rocks have a higher magnetic signature and some rocks have a lower magnetic signature,” Wang said. “If you had a fault that displaced that basalt . . . you would see a difference in the magnetic signature.”

Aeromagnetic imaging has helped produce maps indicating where possible surface faults exist in the Willamette Valley. Soil and engineering tests have helped produce information about which buildings might fail.

Wang’s agency worked with the state Building Codes Division to upgrade construction standards on the Oregon coast south of Newport to the same level used in California.

State geologists also have mapped tsunami hazard zones along the coast. New warning signs are being posted and evacuation routes are being planned, Darienzo said.

“There’s been a lot of press, a lot of activity about earthquake risks and hazards,” Darienzo said. “But people aren’t going to move away. They’re still going to build in the fault zone, in the risk areas.

“There’s always a need for more education.”