Earthquakes come and go. Everybody knows of some that are more likely such as the San Andreas fault in California. What about the quieter places in the world? An earthquake of similar magnitude is in store for Oregon at some point, and scientists at Oregon State University have raised the warning flag again -- they predict it could be soon -- in a new report. In fact, the probability of a major quake in the next 50 years could range as high as 40 percent. The group also studied the historic intervals between quakes over the last 10,000 years. Given that the last known major quake was in the year 1700, a quake in the next 50 years fits the pattern.
The last Cascadian earthquake set off a tsunami that not only struck Cascadia's Pacific coast, but also crossed the Pacific Ocean to Japan, where it damaged coastal villages. Written records of the damage in Japan pinpoint the earthquake to the evening of January 26, 1700.
That earthquake collapsed houses of the Cowichan people on Vancouver Island and caused numerous landslides. The shaking was so violent that people could not stand and so prolonged that it made them sick. On the west coast of Vancouver Island, the tsunami completely destroyed the winter village of the Pachena Bay people with no survivors. These events are recorded in the oral traditions of the First Nations people on Vancouver Island.
There are a lot more people in the Cascade area nowadays; the potential for harm is much more than then.
"Right now, we have already exceeded 75 percent of the known recurrence intervals over the last 10,000 years. By the year 2060, we will have exceeded 85 percent of them, if we don't have an earthquake by then," said Goldfinger, an author. "If the Cascadia fault had a warranty against failure, it would have expired many years ago."
The Southern Oregon coast faces the greatest risk. Between Florence and Cape Mendocino, Calif., the report predicts an earthquake between 8.1 and 8.3 magnitude in the next 50 years at about 40 percent.
A larger earthquake like the 9.0 magnitude one in Japan has only a 10 percent chance, but could affect the entire Oregon coast.
The threat isn't just the quake's power, however. The Cascadia Subduction Zone will probably cause shaking for 3-5 minutes, said Scott Ashford of OSU, not involved in the study.
A lot of the seismic design standards have been based on quakes of 30 seconds, common in California, and don't account for the extended shaking.
"Oregon is not ready. We have a bunch of legacy infrastructure," said Ashford, "but it's never been tested and was never designed for Cascadia Subduction Zone earthquakes."
Wang says that the greatest risk to the Northwest is liquefaction, where wet, loose soil behaves like a liquid. Buried tanks and pipes can float out of the ground, and buildings can sink. She said that all of the port terminals along Oregon riverbanks are on easily-liquefied soil and that critical infrastructure like electricity and fuel could easily be disrupted.
An Oregon Department of Transportation report from 2009 states that many major roads, including Interstate 5, will be completely impassable following a megaquake because of damage like falling overpasses. As of 2009, 178 of Oregon's 2,567 bridges had received a retrofit for seismic stability.
In an extreme earthquake, Ashford said, "electricity will be down, roads will be down and you can't go to pharmacy to refill something. Everybody needs to take responsibility to be prepared."
"After every earthquake, people always wish they had done something," said Wang.
On the coast, people could have less than 15 minutes to reach high ground if there's a tsunami.
"I go to the coast, and I enjoy staying in a hotel with my family," said Ashford. But, "I always look: where's my evacuation route?"
The lack of previous seismic events in Oregon explains some of the state's unpreparedness. "It's enigmatic," says Goldfinger. "Of all the subduction zones in world, the Cascadia Subduction Zone is the quietest that we know of."
For further information see Earthquakes.
Subduction Zone image via Oregon State University