Washington State Department of Ecology
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large slides
Large slide at Possession Beach
Occasionally, large, catastrophic landslides occur on Puget Sound.

Very large, rapid landslides periodically strike Puget Sound's shoreline. These large slumps or slides can cut 50 or more feet into the upland and involve tens of thousands of tons of earth. Fortunately, such slides are relatively rare on Puget Sound, but the potential consequences along a developed shoreline would be catastrophic.
 
How large slides work
A large slide cuts deep into the slope, depositing tons of soil and debris at the base  
more about large slides

Catastrophic impacts
Unlike more common shallow landslides, which often remove only a few feet of material from a slope, a larger slide may cut back many tens of feet into the upland, possibly threatening even those structures set well back from the bluff's edge. Obviously, were a large slide to occur along a bluff lined with homes or above a beach community built at the base of the slope, the consequences would be severe.

Recent large slides
A recent example of such a large slide was the Woodway slide south of Edmonds in early 1997. Older examples include a landslide that occurred in 1979 between Glendale and Possession on South Whidbey Island and another landslide north of Brownsville in 1983.
Woodway
Woodway slide

Persistent evidence
Large slides can deposit many tens of thousands of cubic yards of soil and debris onto the beach. This material may take decades to erode. Large historic slides can sometimes be recognized by large stands of evenly aged alder on the slope, blocks of clay and debris eroding from the beach itself, or small eroding promontories along otherwise straight stretches of shoreline.
vegetation on large old slide

Hard to predict
Geologists understand little about these large landslides and therefore cannot easily predict when or where future slides might happen. This makes it difficult to prepare for or to prevent such slides. Studies of these landslides indicate that they often cut deep into stable, underlying sediments, possibly where groundwater has seeped into fractures.

Slides and earthquakes
A dramatic landslide occurred south of Point Defiance following the 1949 South Puget Sound earthquake. The event emphasizes the potential risk of landsliding associated with large earthquakes. The slide occurred immediately north of Salmon Beach, a small residential community built on piles over the beach at the base of steep slopes in Tacoma.
 
links

Landslide Hazards of Seattle, WA, and vicinity, USGS
Includes a link to landslide monitoring data.
 
Department of Natural Resources
Report on Ledgewood Beach landslide
 
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