“Most bridges in the area have not been seismically retrofitted, creating significant risk to the commuting population from earthquakes.”
Oregon's Regional Natural Hazards Risk Assessment (regarding the southern Willamette Valley)
http://csc.uoregon.edu/pdr_website/projects/state/snhra/snhra.htm

 

If a large earthquake happens in the wintertime, it could trigger landslides on rain-soaked hillsides. If it happens in the summertime, it could trigger forest fires (from downed electric lines and burst natural gas lines). A good depiction of these scenarios is Marc Reisner's book "A Dangerous Place: California's Unsettling Fate" (Penguin Books, 2003). Reisner is also the author of "Cadillac Desert," widely regarded as the definitive book on water conflict in the western US. While "A Dangerous Place" is focused on seismic impacts in the San Francisco and Los Angeles areas, the social impacts of a major earthquake in Cascadia would be comparable.

The 11 earthquakes and tsunamis took place roughly between 300 and 5,400 years ago, with an average occurrence interval of about 510 years (Note: Scientists estimate the chance in the next 50 years of a great subduction zone earthquake is between 10 and 20 percent, assuming that the recurrence is on the order of 400 200 years. The last great subduction zone earthquake occurred about 300 years ago).
Geologic Hazards on the Oregon Coast -
Earthquakes and tsunamis documented at southern Oregon coast
From the Spring, 2002 issue of Oregon Geology
– http://www.oregongeology.com/earthquakes/Coastal/OrGeoEqNTsu.htm

JUNE 1999:

ONE OF CENTRAL OREGON'S DAMS COULD COLLAPSE IN A MODERATE EARTHQUAKE, prompting federal officials to warn people immediately below Wickiup Reservoir to flee to higher ground at the first sign of ground movement. "If people can feel an earthquake in the area, it's probably going to be strong enough to do something to the dam," said Larry Wolf, dam safety expert with the U.S. Bureau of Reclamation in Boise.
It is the first time in the Northwest that the bureau has advised residents to evacuate as standard practice during an earthquake. "Certainly we don't want to create panic, but we want people to be aware," he said. The dam is about 20 miles [32 kilometers] south of Bend and much closer to a number of upscale developments, including Sunriver, which can be packed with 20,000 people on a summer day.
The bureau estimates that floodwaters could endanger roughly 10,000 people. However, Wolf said there would be time for most people to evacuate. The flooded area would roughly follow the channel of the Deschutes River, extending beyond the banks for more than a mile in some places, he said. Because the river channel flattens in some developed areas, floodwaters would take about 14 hours to reach Bend, he said. An earthquake with a magnitude of 5.0 on the Richter scale could cause a catastrophic failure of the 2-mile-long earthen dam. However, he said the chances of such an earthquake are estimated to be about one-tenth of 1 percent each year. Wolf characterized the risk as remote but real. P The Klamath Falls earthquakes in 1993 were pegged at magnitudes of 5.6 and 6.0, and the Scotts Mills earthquake earlier that year in the Willamette Valley hit 5.6.
Wolf said data collected at the dam in recent years indicated that the saturated silt and ash layers of earth beneath the dam could liquefy during an earthquake. The dam was completed in 1949 and holds up to 200,000 acre feet of water, or enough water to cover 200,000 acres to a depth of one foot. The water is used primarily for irrigation in Jefferson County. Wolf said reclamation engineers have suspected for years that Wickiup Dam was at risk. Those fears were confirmed with additional analysis and testing last year. In February, the bureau decided it needed to warn local authorities and the public about the situation and to undertake a $40 million renovation project.
Jim Mumford, who heads the bureau's dam safety division in Boise, said these are far more specific warnings than the bureau has ever issued to Pacific Northwest communities. For example, when the Ochoco Dam near Prineville was at some risk of failing several years ago, the bureau told residents to contact local emergency service officials if there was a problem with the dam. But with Wickiup, he said, there won't be time to await instructions from officials. "This is the first time where we're saying, 'Don't wait for notification. The earthquake is the notification,' " he said. The bureau also has designated escape routes, then posted fliers and sent brochures to area homeowners with maps of those routes. Larry Zakrajsek, who does risk analysis for the bureau, said the agency did not rush to warn people partly because the danger is small and the dam has functioned well for 50 years.
By Gordon Gregory, Correspondent, The Oregonian The Spring Break Quake of 1993, which rattled buildings across western Oregon and caused $30 million in damage, was a harbinger of an 8 or 9 magnitude quake that is in Oregon's future, geologist Donald Hull tells legislators. "It's been 299 years since the last such event," Hull said. "The window of vulnerability is open again." Hull, who is Oregon's chief state geologist, hopes the Legislature will set aside money for better mapping of earthquake hazard zones and for public safety campaigns to let people know what to do when the Big One hits.
The department has been able to retrofit about 60 bridges since the Spring Break quake, but ODOT estimates that at least 1,500 other bridges in western Oregon are in need of at least some earthquake strengthening. Frank Nelson, ODOT's bridge preservation engineer, said eight more bridge projects are planned, and that the department might be able to do an additional four if lawmakers approve a gas tax increase for road repairs. Those projects should at least be enough to keep Interstate 5 -- Oregon's main north-south lifeline -- open in the event of a major earthquake, Nelson said.
Scientific evidence shows that major offshore earthquakes occur off Oregon's coast once every 350 to 500 years. The last one, in 1700, drowned coastal forests and sent tsunami waves across the Pacific so powerful that they destroyed Japanese fishing villages. Such a quake would not only devastate Oregon coastal communities, but inland areas as well. "The Willamette Valley is a big trough full of loose soils, gravel, sands and silts," he said. "When earthquake waves travel through that kind of sediment, they get bigger; they amplify. I'm just praying it won't happen in my lifetime."
Senate President Brady Adams said lawmakers are aware that Oregon is due for another huge earthquake. "It's hard to define in a specific time frame what the risk is. Is it going to happen tomorrow, or 200 years from now?" the Grants Pass Republican said. "There's no question the threat of earthquakes is real, but we also know we have school funding and other needs that are before us today."
Hull said he can't argue with that logic, but still thinks the Legislature should consider increasing at least to a small degree its financial commitment to preparing the state for the Big One. "There's nothing else in our foreseeable future that's going to be as devastating," the state geologist said. "It's not going to do us any good to fund education programs if the school buildings end up falling on kids' heads."

From the Associated Press MAY 1999:

SEATTLE -- New research indicates that a massive earthquake could occur directly underneath the Oregon Coast Range and the western portion of the Willamette Valley. For nearly 15 years, scientists have warned that a magnitude 8 or 9 earthquake could strike about 30 miles [48 kilometers] offshore and rock the coast, causing severe shaking and huge tsunamis. However, recent data gathered from satellites by scientists at Oregon State University and three other institutions show that the colossal quake could hit much farther inland and cause more severe damage to a larger area -- including the more populated cities of the Willamette Valley such as Portland, Salem and Eugene.
No one knows when such an earthquake might strike the Northwest, but the geologic evidence suggests that such quakes occur about every 400 years, plus or minus 200 years. The last major earthquake on the Oregon coast -- believed to be a magnitude 9 -- occurred 300 years ago, previous studies showed.
The research team found that the locked portion of the Cascadia Subduction Zone -- where the eastward-moving Juan de Fuca Plate plunges under the western-moving North American Plate -- extends beneath the Coast Range and as far as the western side of the Willamette Valley. The locked zone probably is wider than previously thought, although the new data give less information about the width.
The researchers expected to find little movement because of the lack of earthquakes and previous data that showed little uplift in central-western Oregon, something commonly associated with a locked subduction fault. Instead, they found that the ground is moving nearly half an inch a year toward the northeast. The rapid velocity worries earthquake researchers and indicates that the underlying plates are locking up rather than sliding by each other, resulting in incredible strain.
As the Juan de Fuca Plate presses forward to the northeast in the locked zone, it causes the piggybacking North American Plate to bulge upward and inland toward the northeast. The pressure continues to build for years until an earthquake unleashes the stress in one powerful jerk, causing the bulge to collapse and forcing the area to drop instantly.
"We were very surprised by the results we got," said Goldfinger, an OSU assistant professor of oceanography. "It was quite different from what we expected. We thought this would be an area that would show little, if any, movement." The half-inch of movement each year is imperceptible, but the accumulated pressure that has been stored since the last major earthquake in 1700 can only be unleashed in an earthquake.
"That means there's been 300 years of strain that will be released," said John L. Nabelek, a seismologist and OSU associate professor of oceanography who participated in the study. "And it's just not the proximity of the strain to larger cities that is a concern, but we've found that the surface area of the entire locked zone is much larger than previously thought. That means a larger quake."
Goldfinger said the data suggest that the two plates are "essentially bolted together -- they're 100 percent coupled.
"In addition, the Coast Range is an extremely strong, rigid block of rock that is more than capable of accumulating the sort of energy you need for a large earthquake."
The new findings have made Goldfinger, who in previous years argued that the largest subduction-zone quake was more likely to be a magnitude 8 than a 9, rethink his theory. "This changes my views 180 degrees," he said. "The whole argument for an 8 rather than a 9 disappears." Although quakes of either size would be devastating, shaking from a magnitude 9 event would last two to three minutes -- about twice as long as the shaking a magnitude 8 quake would produce.
Researchers elsewhere in the Northwest have come up with similar results using the satellite-based Global Positioning System. The locked zone between the plates extends farther landward beneath Washington and Southern Oregon as well, and a little farther under Vancouver Island than previously thought. A larger research effort, planned next year, will examine an area from Northern California to Canada, including Portland.
The Cascadia Subduction Zone is a 750-mile [1207 kilometers] long fault that runs 60 to 150 miles [96 to 241 kilometers] offshore from British Columbia to Northern California. Similar subduction zones have produced the two largest recorded earthquakes in the world -- a magnitude 9.5 quake on the coast of Chile in 1960 and a magnitude 9.2 quake in southern Alaska in 1964.
No quakes of that size have been measured in Oregon's brief recorded history, but evidence from buried marshes along the coast indicate that such events occurred at least seven times in the past 3,000 years. The last one hit the coast in January 1700, and large quakes appear to have struck about 1,100 years ago, 1,300 years ago and 1,700 years ago.
Curt D. Peterson, a professor of geology at Portland State University who has uncovered many of the buried marshes along the Northwest coast, said the new research supported his decade-old theory that the locked zone might be twice as wide as thought and capable of generating a huge quake.
"I hope this new evidence is going to help planners and government agencies get back on track about the seriousness of the hazard. The metro areas such as Seattle and Portland need to examine what a magnitude 9 means in terms of the whole region going all at once," Peterson said.
Mark Darienzo, earthquake and tsunami program coordinator for the Oregon Office of Emergency Management, said the study supported concerns that a huge subduction-zone earthquake "is not just a coastal problem, but could be an inland problem as well."
"More research is needed," Darienzo said, "but these new findings show that the potential for such a quake can't be overlooked -- it shouldn't be just tossed aside."
By Richard L. Hill of The Oregonian staff

AN EARTHQUAKE THIS SIZE THREATENS A TERRIBLE TOLL -

SEATTLE -- The figures jolt the imagination: more than 5,000 deaths, nearly 8,000 people injured, 30,000 buildings destroyed, $12 billion in economic damage. Those are the extensive losses Oregon might face in a magnitude 8.5 earthquake from the Cascadia Subduction Zone off the coast, according to a study by the Oregon Department of Geology and Mineral Industries.
In addition, a subduction-zone quake along with earthquakes on many faults throughout Oregon -- those that have a 10 percent chance of producing a quake in the next 50 years -- would result in at least 25,000 injuries, 80,000 buildings destroyed and more than $31 billion in economic damage.
"As large as those figures are, they're conservative estimates," said Yumei Wang, a geotechnical engineer with the Geology Department who co-wrote the report with Lou Clark, an agency geologist. "But they give the public a better idea of what the effects of destructive earthquakes could be in Oregon."
Wang discussed the recently released report this week in Seattle at the 94th annual meeting of the Seismological Society of America. The study provides the first estimates of the quake-related losses that each of Oregon's 36 counties could experience.
The estimates were produced using sophisticated computer software developed to determine earthquake risks. It combines information about a variety of factors, such as geography, buildings, demographics, economics, ground shaking and soils. The estimates were given for two models: one that looks at the impact of a magnitude 8.5 subduction-zone quake and the other that looks at total earthquakes on many faults in Oregon in a 500-year period -- or those faults considered to have a 10 percent chance of producing an earthquake in the next 50 years. The report was done before new findings were announced that show a subduction zone quake could hit much farther inland, possibly making damage and death totals even higher.
Scientists worry that a quake from the subduction zone, where the North American and Juan De Fuca plates converge, could be as powerful as a magnitude 8 or 9 quake. Oceanside communities would be especially at risk because of their proximity to the zone and because they could be struck by a quake-generated tsunami.
But Oregon also is at risk from earthquakes along crustal faults, such as the magnitude 5.6 Scotts Mills quake in March 1993 that caused $30 million in damage and the magnitude 5.9 and 6.0 quakes near Klamath Falls in September 1993 that caused $10 million in damage.
Wang and Clark said the 60-page report, "Earthquake Damage in Oregon: Preliminary Estimates of Future Earthquake Losses," underestimates the property damage, injuries and fatalities because the computer model did not take into account tsunamis from an offshore quake or unreinforced masonry buildings, the most hazardous type of structure in an earthquake. The study indicated that 100 people would die in a subduction-zone quake and 500 people would die in all quakes.
However, Wang calculates that unreinforced masonry buildings could result in 2,000 fatalities, while a tsunami would kill more than 3,000 people. "The tsunami estimate is low because it's based on the populations of low-lying coastal communities," she said. "That number could greatly increase during the tourist season when there are thousands of more people on the coast."
In addition to loss of life and property, the study also estimates these effects from a magnitude 8.5 subduction quake:
• One-third of schools would be unusable.
• Nearly one third of broadcasting stations would be shut down.
• About one-third of essential facilities such as police stations, fire stations and emergency-operation centers could not function.
• About 17,300 households would be displaced.
• About 18 percent of highway bridges would be knocked out.
"There's a large margin of error in the numbers," Clark said, "and we're working to refine those. But what this study does is enable people to look at the impact of a big earthquake on their communities. With these numbers, people can start to understand why we need to pay attention to this hazard and local communities can start figuring out how much they want to devote to planning for this."
By Richard L. Hill of The Oregonian staff Wednesday May 5, 1999