Earthquake experts lay out latest outlook for the ‘Really Big One’ that’ll hit Seattle

Alan Boyle
A color-coded computer simulation from 2016 shows how researchers think tsunami waves propagated from a magnitude-9 Cascadia subduction zone earthquake in the year 1700. Scientists believe such quakes occur every 500 years or so on average. (NOAA / Pacific Tsunami Warning Center)

Earthquake experts say current building codes don’t reflect the riskiest features of the Seattle area’s geology — but the outlook for survivability looks a lot better if the Really Big One can just hold off for a few more years.

That’s the bottom line from a session focusing on Seattle’s seismic hazards, presented at ground zero today during the American Association for the Advancement of Science’s annual meeting. The session — titled “Is the Coast Toast?” — followed up on a 2015 New Yorker article that painted a grim picture of the possibilities, based on studies of the Pacific Northwest’s Cascadia subduction zone.

The Cascadia subduction zone, centered along a submarine fault just off the West Coast, is known to be capable of generating magnitude-9 quakes, based on the geological and historical evidence for a massive tsunami that reached Japan in 1700.

Seismologists estimate that such quakes and tsunami waves occur roughly every 500 years on average. “We say that there’s approximately a 14% chance of another approximately magnitude-9 earthquake occurring in the next 50 years,” said Erin Wirth, a geophysicist at the University of Washington and the U.S. Geological Survey.

To assess the potential effects of a Cascadia mega-quake, Wirth and other researchers are conducting a six-year, $3 million study known as the M9 Project.

One of the project’s experiments involved running 50 simulations of Cascadia quakes under a variety of conditions. The researchers found wide variation in the effects, depending on whether the undersea fault ruptured in a direction pointing away from Seattle (which would be good) or toward the city (which would be bad).

Another issue has to do with the fact that Seattle is built atop a sedimentary basin with relatively soft soil, which would amplify the strength of a seismic shock.

“A good analogy is, this is like a bowl of Jell-O,” Wirth said. “If you have a bowl of Jell-O, or a Jell-O mold on top of a plate, and you shake that plate, the Jell-O is so weak it’s going to move much more than the plate that you’re shaking. That’s kind of what’s happening here.”

And as if that weren’t bad enough, the Cascadia fault is so extended that the resulting quake is expected to last for about 100 seconds. That’s significantly longer than the duration of a typical California earthquake, and that adds to the bad news, said UW engineering professor Jeffrey Berman.

“Our building code is all built on the California experience, because that’s where we’ve had a lot of earthquakes and a lot of building and infrastructure damage,” Berman said. “So we haven’t really incorporated long-duration effects in building codes that are in use in the U.S., because we haven’t really had long-duration earthquakes. … We’re hoping that our work will actually go to change that.”

Earlier analyses of earthquake effects pegged the risk of collapse for buildings up to 20 stories tall at less than 10%. The M9 Project’s updated analysis, which takes account of longer-duration quakes as well as the Seattle area’s sedimentary basin amplification effect, would raise that projected risk to somewhere between 20% and 50% depending on building height and the standards followed for construction.

The good news is that building codes are due to be strengthened nationwide.

“They’re national codes,” Berman explained. “They have to go through a pretty intense vetting process, and they get adopted by local jurisdictions and modified. That process just takes time. This research will appear, but it won’t appear until 2023, likely. Now the city is holding some discussions on what to do, given our findings. It should be doing things ahead of that time, but that would be outside the national building code process.”

In reply to questions about where he’d rather be during a mega-quake, Berman noted that low-rise buildings in Seattle — say, up to three stories tall — would “do relatively well” in the event of a magnitude-9 quake. And when it comes to higher-rise buildings, the risk is lower for structures built after the mid-1980s.

“There was a really big transition that followed a couple of bigger earthquakes in California, and there were really big changes in the building code,” Berman explained. “So I think that’s what I would look at first: to stay out of buildings that were built before 1984 or so.”

That being said, Berman advised against obsessing over the Really Big One.

“You know, life is full of risks,” he said. “The risk of dying in a building in a earthquake in Seattle is less than [the risk of] dying in a car, if you get in a car and drive on I-5 today, right? It’s about learning and doing better as we move on, not necessarily being paralyzed.”

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