It’s no secret that Western Washington is prone to landslides. The combination of glacial soils, steep slopes and water creates a risk that’s greater than in other parts of the U.S.
And it’s not just river valleys like the one near Oso: The region’s coastal bluffs are also danger zones that have experienced large landslides in recent decades.
But that still doesn’t deter people from living in those slide-prone areas.
The Poster Child For Landslide Risk
Ruth Trail has lived in the Magnolia neighborhood of Seattle for 23 years.
She said she loves it for the views. Standing in a park that sits atop a bluff overlooking Elliot Bay, Trail can see downtown Seattle, Mt. Rainier, West Seattle and the Olympic mountains.
It was here, 18 years ago, that a deep-seated landslide hit. During an especially wet winter, a big chunk of the bluff collapsed, wiping out five houses below.
Trail remembers coming to see it. “They kind of quarantined this area,” she said, looking down the hill. “It wasn’t until you came up to it that you saw the devastation of it.”
Evidence of that landslide can still be seen today: It looks like someone has carved a bowl out the side of the hill. A fence surrounds the area, and it’s covered in grass and little trees.
Geologists consider Perkins Lane Seattle’s poster child for landslide risk.
“The Perkins Lane area has been failing back since the 1930s; and probably long before that,” said David Montgomery, a geomorphology professor at the University of Washington.
Coastal bluffs in general are at particular risk for slides, Montgomery said. The geology of the bluffs is unstable — glacial till on top of sand on top of clay — and Puget Sound is eroding them perhaps a centimeter a year on average.
“If you have something going back a centimeter a year, it takes 100 years to go back a meter. The problem, of course, is [a landslide] happens all at once and goes back many meters,” Montgomery said.
When the slope at Perkins Lane failed in the winter of 1996, residents were lucky in a sense: The slide moved slowly, and they were able to evacuate their homes. No one was killed.
Now, the portion of the street where the big slide hit is closed off, and no new homes have been built in the slide zone. But many homes remain nearby, in an established and rather pricey community, perched between the steep bluff above and the water below.
Montgomery wouldn’t venture a guess as to whether a catastrophic landslide could occur here. He said scientists just don’t know enough. Earthquakes and tsunamis have been well studied, but landslides, not so much.
Modern imagining technology called lidar — a portmanteau of light and radar — shows that big landslides have occurred in the past all along the coast. Scientists know that once a big landslide happens, subsequent landslides are more likely because the slopes become weakened.
“There is an awful a lot of ruggy, disrupted old landslide terrain along the bluffs. I couldn’t give you my assurance that there aren’t other places that could fail like the end of Perkins Lane did,” Montgomery said. “My hope is obviously that they don’t.”
Montgomery’s worst-case scenario is an earthquake during a very wet winter. “That could surprise everybody,” he said.
‘Out Of Sight, Out Of Mind’
Kyle Cavens answered the door at a home not far from the slide area.
He’s a full-time student, and even before his family moved here, he said he knew about the risk. His geology class had taken a field trip to the neighborhood to learn about the history of landslides there.
“I know that this entire area is at risk for falling into the ocean,” he said.
These houses were built before the city had steep slope regulations, but Cavens said many of the homes in the area have been extensively retrofitted with steel pylons anchoring them to the hillside.
City workers are around a lot also, keeping tabs on what’s happening with the slope above. They’ve built extensive drainage systems to keep water off the slopes.
Seattle’s Department of Planning and Development has four geotechnical engineers on staff who review any new development plans in slide-prone areas.
Cavens said residents on Perkins Lane don’t really worry about the danger.
“I know people know about it, but not really talk about it much. We just go day to day and we don’t really think about it,” he said. “We know it’s going to happen, but not today, not tomorrow. So, out of sight, out of mind.”