The Rattle in Seattle: How It Happened

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Cars sit under a blanket of fallen bricks in Seattle's Pioneer Square District

Most Seattle-area residents expect their only jolt of the day to come from Starbucks, but Wednesday morning they got a second, most unwelcome kick from a moderate earthquake centered 35 miles south of the city. The quake, which measured 6.8 on the Richter scale, left an estimated $2 billion in damages. While the shake-up was undeniably scary, northwesterners are counting their blessings Thursday — 272 injuries were reported (all mild), and no deaths were directly linked to the seismic shift.

Did anyone expect this earthquake? Can we expect more in the future? Roger Bilham, professor of geology at the University of Colorado at Boulder, spoke with Thursday morning. What happened Wednesday in the Northwest?

Roger Bilham: What happened was a very important earthquake. Important in that it is the largest that's happened since geologists recognized that the Seattle region is one of the few places in the U.S. that could have a large thrust earthquake. Wednesday, a very small patch slipped in the collisional boundary south of Seattle.

The northwestern U.S. is neglected in terms of earthquake exploration. For a long time, just because of the way the area's tectonic plates are arranged, we knew we could see gigantic quakes there — but we didn't know anything else about what to expect. But in the past 20 years, scientists on geological digs have found submerged forests along the coastline. Geologists use these discoveries to date past quakes, and because of this new evidence, we now know that there have been major northwestern earthquakes over the past couple thousand years. And that means, of course, that there will probably be another. So it's only lately that we've realized we've got a whopper on our doorstep.

How was Wednesday's quake different from what we saw in 1989 in San Francisco? That quake measured 7.1 on the Richter scale and caused a lot more damage.

In California, earthquakes are caused by tectonic plates sliding past one another. We're familiar with these plates, and we know where the fault lines are, and so we can prepare for the quake, at least in part, by enforcing strict building codes. The Seattle area is defined by a plate sliding toward North America — and when that plate hits the coastline, it pushes underneath the continental shelf. Wednesday's quake originated more than 30 miles under the earth, and so by the time the shock got to the surface, we didn't feel it as much as we would have a shallower quake [like those generally seen in California].

Is there some larger geological significance to Wednesday's quake?

Bilham: It's a wake-up call. A moderate quake like this can be a very helpful reminder that there's worse to come.

The best thing that can come out of this quake will be improved building codes. Moderate quakes test the building codes in the affected area — they can shake loose a few buildings that might have otherwise fallen down altogether in a larger earthquake. And buildings are what cause the bulk of earthquake damage; if a quake happened in the middle of a cowfield, there'd be no damage at all.

So is there any way to predict when the bigger earthquake(s) will actually happen?

Bilham: Not exactly. There are people working very hard on figuring out when the big one will happen. We can dig down and figure out when things were last disturbed in quake activity, and then develop probabilities based on those findings. But these are just probabilities — earthquake prediction, as you can probably guess, is not a precise art.