Seattle's Long-Dormant Fault Line Is a Landslide Powder Keg

For being a west coast city along several major fault lines, Seattle has somehow managed to avoid devastating earthquakes that have rocked cities like San Francisco and Los Angeles. But a new study by the US Geological Survey suggests that, if an earthquake were to hit the area, the city would have to deal with major landslides in addition to the seismic damage.

You rarely hear about landslides in the United States, but the USGS says that, on average, landslides cause about 25 deaths each year and can result in damages of up to $2 billion.

Previously, the USGS has flagged the Seattle area as one in danger of landslides. After heavy storms in the winter of 1996-1997 set off landslides that destroyed hundreds of homes, the USGS set up the Seattle Project Impact, which retrofit some of the area's most vulnerable homes and earthquake-proofed some of the city's highways and roads.

Those landslides were caused by excess rain and storms, but seismically-triggered ones could be worse, and, according to USGS scientist Kate Allstadt, the risk has been underrated in the past.

Allstadt says the Seattle Fault, which runs right underneath the city, is a "locked" fault, meaning it doesn't release seismic energy until there is a major earthquake. The last time that happened was around 900 AD, when huge swaths of the forest were swept into Lake Washington.

"An earthquake is one of lots of scenarios we want to look at, but it's kind of the worst case scenario," Allstadt said. "In Seattle, there's kind of a perfect storm of characteristics because the fault goes right through the city, it's wet, and there are steep slopes all around."

In fact, seismologists aren't even able to pinpoint exactly where the Seattle Fault is because it has been quiet for more than 1,000 years.

According to her study, which simulated earthquakes along where they believe the fault to be, a third of the city that had previously been designated as not being vulnerable to earthquake-induced landslides should be considered at risk. Developed areas in the south part of the city could be the hardest hit, and thousands of buildings could be destroyed.

"If you look at Seattle, there's houses everywhere. They're build on slopes and on hills. We're looking at the possibility of having buildings falling down slopes into Lake Washington or the Puget Sound," she said.

Seattle does get its fair share of earthquakes, but those are caused by a separate fault that runs deep under the Puget Sound. Most recently, a magnitude 6.8 earthquake in 2001 killed one person, injured 407, and caused roughly $2 billion worth of damage.

There hasn't been a massive earthquake in the area in over 1,100 years, true, but Allstadt isn't the first to suggest that an earthquake along that fault could be devastating. In 2010, the Seattle PI did a roundup of what experts had suggested could happen if a major quake hit Seattle. Rob Witter, a coastal geologist with the Oregon Department of Geology and Mineral Industries said that "the amount of devastation is going to be unbelievable." Brian Atwater of the USGS said "it's not a matter of 'if,' but when the next [earthquake] will happen."

There's not much anyone can do to prevent an earthquake along the fault, and there aren't any indications that an earthquake is going to happen there anytime soon, but she said homeowners need to know what they're getting themselves into.

"Know that you can get a house with a pretty view [along a slope], but also know you're living above something pretty powerful," she said.