15 percent chance the PNW could be hit by a mega-quake, say earthquake experts

The chance of a mega-quake rocking the Puget Sound region in the next 30 years? About 15 percent, according to a state seismologist.

Ahead of the "Great Shakeout" earthquake drill that will include 55 million people worldwide, local earthquake scientists answered questions from Reddit users in an "Ask Me Anything" session Thursday, ranging from details on a potential mega-quake, to predicting when the next disaster will occur.
Reddit user lepetitprance asked after "any update when the big one is supposed to hit Seattle."

They’re of course referencing the idea that Seattle is on the precipice of being hit by an impending mega-quake, known colloquially as “The Big One.”

“The definition of ‘the big one’ isn’t really well-defined,” answered state seismologist Paul Bodin. “Chance of an M9 megathrust? About 15 percent over the next 30 years. Chance of an earthquake on the Seattle Fault? Much less…less than about 5 percent.”

An "M9 megathrust" here refers to a potential offshore earthquake on the Cascadia subduction fault zone off the coast of Washington and British Columbia. Bodin estimated that such a quake would "shake Seattle for a long time, but perhaps not as strongly as a smaller earthquake on the Seattle Fault."
In terms of what kind of warning we'd get if or when the "Big One" hits, Bodin notes that "we have the capability to provide from seconds to a couple minutes of 'early warning' once an earthquake starts, depending on where it starts and where you are."

Such a warning would come thanks to a relatively new system called Shake Alert, operated by the U.S. Geological Survey, and currently in testing while available to the public. The system boasts the ability to detect “significant earthquakes so quickly that alerts can reach many people before shaking arrives.”

What to do when the “Big One” hits

A handful of questions were about what to do during a major earthquake.

“If you are in a car, slow down, put on your emergency flashers to warn other drivers, pull over to a safe spot (away from overpasses, trees, telephone poles and anything that could fall on your car), put on your emergency brake, cover your head and neck and wait for the shaking to stop,” said Maximilian Dixon, the geologic hazards manager with the WA Emergency Management Division.

For anyone outdoors, Director of Engineering for Simpson Strong-Tie Emory Montague advises you “drop, cover, and hold on.”

“If you’re outside a building, find the closest protection from falling debris. I wouldn’t consider this expert advice, but if it were me, I’d get under the closest SUV or park bench or something sturdy that I could hide under. I’d say the same hazards exist for both glass/steel and brick buildings. In both cases you can expect dangerous falling debris,” he adds.

If you’re indoors, Montague warns that you “won’t have enough time to get out [of] a building, so it’s better to have a plan for a safe spot.”

“You want to find a sturdy table that you can get out from easily after an earthquake,” he continues. “One of the reasons it’s not recommended to run out of a building is because debris will likely be falling from the building and may strike you.”

You can read the rest of the AMA on Reddit here, where plenty of other questions are addressed.

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