In just the past 30 years, we in Oregon have gone from thinking we never have major earthquakes here to knowing that we get whoppers - force 9.0+ Cascadia Subduction Zone 'Megathrust' earthquakes. This understanding is so recent that Oregon had no seismic construction codes until 1988, and few structures built before 1994 have seismic construction.
Contrast that with the construction and planning for earthquakes in Japan and Chile - the best in the world, and far better than ours have been - yet both Japan and Chile have seen major death and destruction from such earthquakes just in the past year. If a 9.0 Megathrust quake happened here right now, the death and destruction would be far worse.
If there is a bright spot, it was revealed recently in Eastmoreland by P.S.U. Geologist Scott Burns - who has been teaching at the local university for 21 years, got much camera time on local TV last March when Japan was struck with its Megathrust earthquake, and who recently received an award as the 'top engineering geologist in the United States'.
Burns said that current calculations are that northern Oregon gets a huge quake every 480 years, and 'full margin' earthquakes covering vast areas offshore every 600 years. The bright spot is that the odds currently suggest that the Portland area's next Megathrust quake may yet be over 100 years in the future. (But, southern Oregon and northern California are overdue - these occur every 287 years there.)
When one of these monsters strikes, recovery is VERY slow, warned Burns - preparations should be made not just for the days after such a monster earthquake, or even for weeks - but for years afterward. He commented that he recently visited Christchurch, New Zealand, the site of a destructive earthquake a year ago. Over 200 homes had been bulldozed because they were knocked off their foundations, and still, a year later, many of the remaining homes in the town still have no sewer or water service.
The four destructive forces these earthquakes unleash, he explained, in the ground-shaking, are 'amplification' (resonances in the soil that amplify the earthquake groundwaves), 'liquefaction' (sand and saturated soil turn to liquid and structures built upon them collapse), landslides, and tsunamis.
At the present time, offshore tsunamis are not thought to be likely to follow the turns of the Columbia River to reach Portland, although that is still being researched. But Portland is very vulnerable, in sections, to the other three forces.
For historic perspective, Burns quoted the work of Chris Goldfinger at O.S.U. in 2007 which determined that there had been 34 Megathrust earthquakes in Oregon in the past 10,000 years, of which 19 were 'full margin' events affecting coastal areas from California to Canada; 5 were only focused on Northern Oregon.
The largest earthquake in the Northwest since records have been kept was the magnitude 7.1 quake in Olympia, Washington, which despite its distance accounted for several deaths in Oregon. Burns pointed out that each order of magnitude on this scale means a 30-times increase in strength - so the difference between Olympia's 7.1 event, and the eventual 9.1 Megathrust quake, is that the latter will be 900 times stronger.
We generally do not feel any earthquake under magnitude 2.5, but there is generally damage when an earthquake tops magnitude 5.5. The widely-felt 1993 'Spring Break Quake' centered near Molalla was a magnitude 5.6.
Burns is also an expert on urban landslides in this area, as well as on the ice-age 'Missoula Floods' which deluged our part of the country again and again over 10,000 years ago, stripping the soil from eastern Washington and depositing much of it in the Willamette Valley.
Scott Burns spoke at the Eastmoreland Golf Course Grill on October 31st, at the Monday noon meeting of the Southeast Portland Rotary Club. Burns himself is a Rotarian, and a member of the large Downtown Portland club, of which he has been President.
The Southeast Portland Rotary Club welcomes guests to its Monday noon meetings; a schedule of future speakers, and information about the club and of Rotary are found online at: www.SoutheastPortlandRotary.com .