Cracking the Coastal Translational Landslides Code

Have you ever thought about buying the perfect dream home on a grassy hill, overlooking a nice river or an ocean? Well, I would suggest thinking again. Every year many hillsides are eroded or made smaller by a landslide, especially in coastal areas like Vashon Island. It’s good to know what kind of warning signs you need to be aware of, how landslides happen, and resources you can use in the future to help you.

Vashon Island is a small island just off the coast of Gig Harbor and Renton and just so happens to be located on a fault line. This particular fault line is called the Cascadia Subduction zone, which is 700 miles long. Along this fault line earthquakes happen about every 500 years (Goldfinger et al., 2012) in the northern zone off Washington. These large-magnitude (~ 9 M), or smaller earthquakes, can trigger landslides all along the northern half of the western US coast. The landslides at Vashon Island are a type of mass-wasting called, translational landslides. A translational landslide is created when a force, like an earthquake, loosens the top layer of the soil causing that thin layer to slide down across the stronger layer beneath it. Because the first layer isn’t as strong as the second layer it can’t withhold this force so it slides down along the bottom layer. The force doesn’t have to be an earthquake, landslides can also occur from adding a building or road to the top of a slope, or by cutting the base of the slope during construction, or by wave action. Other ways to trigger would be suturing a slope with water, which can decrease the strength of the sediment, especially at the contact of two different rock types on the layers colliding (Mickelson). As the layer slides down it can collect small loose sediments and/or large slabs of bedrock and deposit them along the way, if your home is on the top of that thin layer or in front of the landslide's path there could be serious damage done to the house; such as disrupting septic, sewer, and water lines and cracking the foundation (WA DNR/ DOGAMI).

Homeowners need to be aware of danger signs before a landslide happens to protect themselves and their property. On Vashon Island, there are hundreds of houses located along the coast. These houses are mostly located on a hillside or in front of one; both areas are in danger of landslides, and houses in both areas can be dangerously impacted. Some of the biggest warning signs that a landslide could occur in an area are large or growing cracks that move in a downslope motion and tilted or curved trees (Highland and Bobrowsky, 2008). Both of these characteristics are caused by the way the water is infiltrating the ground and how the water is discharged. If you see these features near a home you’re wanting to move into I would strongly suggest otherwise. You could also do research on the area you live to find out if there is any history of landslides in the area or if there is a high potential for them.