Could a catastrophic earthquake really destroy Seattle? Risk warning raises new fears

July 2015 – SEATTLE – The last earthquake of roughly magnitude 9 in the Pacific Northwest happened 75 years before the United States existed. One day, possibly in the not-too-distant future, the Earth’s crust will again convulse in a megaquake. What will happen then will dwarf any natural disaster our country has ever experienced. This week in the New Yorker, Kathryn Schulz, who lives in Oregon, scared the living bejeezus out of us by describing the aftermath of the coming Cascadia megathrust earthquake in gut-wrenching detail. Think of the 2011 Japan earthquake and tsunami that sparked the Fukushima disaster—only the Northwest is nowhere nearly as prepared as Japan was. A word of caution if you read the article: If you live in Seattle, you’ll probably find yourself wanting to sleep outside tonight.

Here’s a telling excerpt: By the time the shaking has ceased and the tsunami has receded, the region will be unrecognizable. Kenneth Murphy, who directs FEMA’s Region X, the division responsible for Oregon, Washington, Idaho, and Alaska, says, “Our operating assumption is that everything west of Interstate 5 will be toast.”

Seattle’s excellent alternative weekly, the Stranger, emphasizes the main takeaways from the story: The earthquake will be really bad, the tsunami will be worse for many people, this event is now overdue, and the Northwest isn’t prepared for it. So, how accurate is this doomsday scenario? Did Schulz exaggerate any bits? Should you start hoarding Tillamook cheese and Black Butte Porter? Should we start a petition to the NFL to relocate the Seahawks to Omaha? Thankfully, this is why we have Reddit. On Tuesday, a group of Northwest earthquake experts, including Washington state seismologist John Vidale, answered reader questions on Reddit’s “IAmA” channel. The discussion generated more than 800 comments.

One main theme that emerged from the discussion was that yes, an earthquake of the magnitude Schulz describes could happen at any moment, but that she took a bit of poetic license in describing the aftermath, especially the tsunami threat to Seattle and Portland. There was a fairly desperate question asking the experts to highlight inaccuracies in the Schulz piece (another commenter described Facebook posts from worried parents considering home schooling their kids for fear of them being crushed or drowned by a tsunami). Vidale said: “Communications may black out, transportation may grind to a halt, stores conceivably could run out of goods for a while, but that doesn’t constitute ‘toast’ in one’s mind.” In a separate interview, Vidale said the article was “a little Hollywood” but otherwise pretty accurate.

The experts repeatedly emphasized that the risk to Seattle from a megaquake-triggered tsunami was “insignificant,” in part because Seattle is protected by the Olympic Peninsula and Puget Sound. In Portland, which is 100 miles from the coast but still relatively close to sea level, there’s little chance the tsunami would travel up the Columbia River with enough force to do significant damage. But directly on the coast, it’s a vastly different story. People will be fully exposed to any tsunami. And there, based on where the major fault lines run, the earthquake will be so strong it will temporarily overcome the force of gravity, flinging houses (and people) into the air. In Seattle, the shaking will likely be strong enough to topple at least one skyscraper (but not the Space Needle). Portland would be even worse off, the experts confirmed. The most frightening thing I learned from the Reddit discussion was that, counterintuitively, a smaller quake would probably only increase the pressure on the fault—boosting the chances of a megaquake. And there’s no way around the fact that a very, very big earthquake will hit the Northwest eventually.

But when? Vidale said that the chances of the worst-case scenario happening in your lifetime, if you’re planning on living another 50 years or so, is about 15 percent. That’s probably a better way of looking at the recurrence statistics than on an annual basis. Historically, the frequency of major earthquakes in the region is about one every 300 years, which means we’re overdue for a megaquake if you average the past 10,000 years of Northwest geology. But the spacing between past magnitude-9 quakes was between 200 and 900 years. If the fault system maintains that pattern, the next big one could happen again tomorrow or in the year 2600. There’s no way to know. –Slate