Earthquake Risk in Oregon
Like Washington, the greatest threat to Oregon lies in the Cascadia Subduction Zone. A magnitude 9.0 Cascadia event would likely cause damage to 38 percent of the buildings in Portland. In this scenario, tens of thousands could be injured or killed, while more than 250,000 could experience long-term displacement. Building damages could range from $23.5 billion to $36.7 billion as a result of the event (Open-File Report O-18-02, Earthquake regional impact analysis for Clackamas, Multnomah, and Washington counties, Oregon).
But a Cascadia earthquake may not even be the worst-case scenario. Stretching from Oregon City to Scappoose lies the 30-mile-long Portland Hills Fault. Estimates of a large quake striking that fault are less than the Cascadia Subduction Zone, but it would be more devastating. The Portland Hills Fault is estimated to have only had two large seismic events during the last 15,000 years. Still, if a large earthquake were to hit, its devastation could be far worse than a Cascadia rupture.
A magnitude 6.8 event on the Portland Hills Fault could cause more than $83 billion in building damages, which is more than double the high estimate of a 9.0 Cascadia rupture. The building loss ratio could be as high as 32 percent. The number of people displaced from the earthquake and its aftermath might be more than a quarter million of the population. The amount of debris generated from the quake may exceed 33 million tons. Depending on whether the earthquake occurs at night or in the day, total casualties could range from 29,000 to 63,000, respectively. Casualty estimates include everything from minor injuries to fatalities.
Like other places, the time of the year can be a factor in the effects of an earthquake. For example, when the soil is saturated, like during the winter months, the area is more prone to landslides and could result in worse damage than an earthquake in a dry summer. Similarly, earthquakes at night, when most people are home, are less likely to cause a lot of injury when compared to a day-quake where more people are out and about.
Another consideration to account for in gauging the effects of an Oregon earthquake is the high risk for liquefaction or landslides in certain areas, particularly if the soil is saturated. These areas would include a large portion of Portland’s west side, some parts of the city’s inner east side, and long stretches along the Columbia River.
For the central and northern Oregon coast, experts put the probability of a large earthquake between 15 and 20 percent in the next 50 years (When the 'Big One' hits, Portland faces mass casualties, widespread destruction: Study.).
For more information on earthquakes in the Pacific Northwest check out Oregon State University's ebook, Living With Earthquakes In The Pacific Northwest: A Survivor's Guide, by Robert S. Yeats. It can be downloaded from the linked page or viewed online beginning at the copyright page. The book is licensed on a Creative Commons Attribution-NonCommercial-ShareAlike license.