We love our views. In general, the steeper the hill, the better the view, unless there's a taller hill in the way. Seattle has beautiful scenery, so view properties are in demand. Unfortunately, houses and hills do not always get along because, at least around Seattle, hills fall down. Land is not always a solid asset.
Our big landslides surprise us and hit the world news. Ledgewood on Whidbey and Oso in Snohomish were visited by helicopter news teams as the world wondered what happened. What Mother Nature can move in a minute takes months of dump truck loads. Both neighborhoods are still recovering, in many ways.
Our ground is stable enough for a city, but let's not assume too much. Some cities are built on bedrock, or on old land that settled millions of years ago. Within the last tens of thousands of years, Seattle's been scraped by glaciers, and then covered by sand and gravel after the true earthmovers called glaciers receded. Places within the region are tall piles of rubble.
Gravity works, and it doesn't take a day off. We're not piling more gravel on top of the hills, so eventually the hills will get shorter (unless the tectonic plate beneath us rises, but that can be a bit abrupt). Hills shrink at the edges as slides. Rain speeds the process, adding water as a lubricant. And then, of course, an earthquake shakes the pebbles as if they were ball bearings, and things slide some more. A steep hillside during a wet season that gets hit by a quake has a lot of forces trying to make it a shallower slope. One analysis estimated that thousands of landslides would be triggered. (Geek out on the data in this slideshow, and then dig some more.)
Most influences don't coincide so dramatically, but they still affect us. Small slides are common. We see them as muddy patches along the Burke-Gilman trail. Larger slides knock the Sounder out of service. Over the last century or so, the bluffs from Seattle to Everett have slid about 900 times. Most of the slides don't make the news except as traffic reports. We take it for granted (skip the pun about granite) that a few dump truck loads of rocks can bury train tracks. Those rocks came from someone's backyard, and that backyard just got smaller.
Federal and local governmental authorities have analyzed the region and the risks. The safest approach is to keep all of the houses, roads, and people back from the tops and bottoms of the bluffs; but that's where the views and flat lands exist. Where else would we put the train tracks?
They're looking at the big picture. Each of us sees a smaller and different picture. If you're interested in land that has a lot of tilt to it, see if it is made of silt. You can hire people to analyze and assess specific properties. Find out if your backyard is going to end up in someone else's, or vice versa. Maybe you can do something about it, and then you can truly enjoy the view.
· Whidbey slide [WA DNR]
· Oso slide [wikipedia]
· Seattle slides [Kate Allstadt]
· Seattle Landslides [Kate Allstadt]
· Mitigation Plan [WSDOT]
· Is Seattle Ready To Deal With a Major Earthquake? [CS]
Written by Tom Trimbath.