Atlantic Cities

Seattle's Hilly Neighborhoods Could Slide Into the Water During the Next Earthquake

Seattle's Hilly Neighborhoods Could Slide Into the Water During the Next Earthquake
Reuters

Seattle is full of slope-side real estate with gorgeous views of Lake Washington and Puget Sound. It's also primed for possible disaster, thanks to these very same hilly areas that could hurtle into the water during the next big earthquake.

When a quake strikes, it's easy to focus on the major structural damage that directly results from the shaking. But a secondary hazard, landslides, can be just as problematic in terms of property damage and the hampering of rescue efforts. With all its hills, Seattle is practically begging for a quake-triggered landslide-fest: The subsequent battering of the city's infrastructure would be "extensive and potentially devastating," with possibly more than 8,000 buildings in landslide-danger zones, according to new study coauthored by the University of Washington's Kate Allstadt.

Allstadt, a doctoral student and duty seismologist with the Pacific Northwest Seismic Network, is part of what's likely the most extensive exploration of Seattle's incipient landslides. She got involved in the research after moving to town and noticing the sheer abundance of steep inclines – not a great thing considering the city sits right atop the Seattle Fault, thought to be responsible for an estimated 7.5 magnitude quake that occurred around 900 A.D.

"I thought, I wonder what would happen to these slopes if there's an earthquake on this fault or any nearby?" she says. "Nobody's really studied in a quantitative way just how much landsliding there would be. I thought that would be important, because we need to prepare."

The 900 A.D. quake delivered such a tremendous wallop created a deadly tsunami and sent great tracts of woodland falling into Lake Washington. (Grab some scuba gear, and you can see the sunken forests hiding in the murk.) That was back when the area was undeveloped. Nowadays, the slopes around the water are densely populated, and the impact of a similar event would be enormous. "There would be a lot of houses on these chunks of soil, entire hillsides" that could suddenly dislodge and tumble downhill, Allstadt says.

Using computer simulations and a seismological tool called the Newmark Method, Allstadt and colleagues John Vidale and Art Frankel created several maps showing where the threat is greatest. As the Seattle Fault runs just south of downtown, neighborhoods in a line from West Seattle to Beacon Hill to Mount Baker are right in the cross hairs (although they're far from the only 'hoods at risk):

There are a few things that could minimize or maximize the landslide destruction. If a quake happened during a wet part of the year, such as early spring, the soil might be moist and much more conducive to catastrophic landslides. And the size of the temblor obviously matters: While the city experiences magnitude 1 and 2 earthquakes daily, it's much less common for it to encounter a seismic buffalo like the 6.8 magnitude Nisqually quake in 2001. (For their simulations, Allstadt and company used a 7.0 shallow-ground quake originating from the Seattle Fault.)

Here are a few zooms of the neighborhoods showing both a dry-soil scenario (left) and the worst-case saturated-soil one (right). First, the key:

This is south of downtown along Interstate 5:

North Seattle:

Delridge and South Park:

South in Arbor Heights:

So how likely is it that headlines might soon be screaming about massive landslides in Seattle? That's uncertain: Earthquakes are hard to predict, and Seattle's recorded human history doesn't date back as far as many other quake-prone lands, such as Japan, so it's hard to measure patterns in seismic activity. One estimate puts the fault zone's "recurrence intervals" at between 200 to 12,000 years, meaning a sizable quake could already be looming in the pipeline.

 That's why it's crucial to start prepping, says Allstadt – finding out what microregions are especially vulnerable, planning rapid responses in and out of these zones, predicting what sewerage and electric infrastructure could be knocked out, educating home-buyers on the risks of living on uncertain slopes. "It could be now or a couple thousand years," she says. "We just don't know."

Top image: A car lies crushed by falling debris in Seattle after the 2001 Nisqually earthquake. (Reuters)

John Metcalfe is a staff writer at The Atlantic Cities. All posts »

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