Washington state Department of Ecology
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deep-seated slides

Puget Sound's shoreline contains many large, deep-seated dormant landslides.

Numerous deep-seated landslides are found along the slopes that surround Puget Sound. Often referred to as ancient landslides, these features may have existed for several millennia. Every few years or decades, particularly wet conditions may cause these slides to reactivate. Typically large, these landslides range in size from less than an acre to several that extend over a mile of shoreline. The larger ones often consist of several smaller blocks that may move independently.
 
how deep-seated slides work
large blocks of earth shift when groundwater levels rise  
More about deep-seated slides

Damage can be severe
Even when movement on a large landslide is limited to only a few feet, damages to structures and property may be severe. Minor movement on deep slides may precipitate lethal shallow slides on adjoining steep slopes.
home damaged by deep-seated slide

Hard to predict
Movement on many deep-seated landslides is both incremental and infrequent. As a result, evidence of past slide movement may be obscured by heavy vegetation or land development activity. Even where slides are recognized, geologists may be unable to accurately predict the frequency or magnitude of potential future movement.

Repair costs can be high
Large slides can be extremely expensive and difficult to stabilize. Community scale solutions may be necessary, since actions on individual properties may be of little effect. Reducing groundwater levels may help, but provides few guarantees. Structural measures -- buttresses, retaining walls, or even complete excavation and regrading of the slide -- are likely to be cost-prohibitive on all but the highest value property. Such measures may be difficult to construct in an already developed area, and can result in serious environmental impacts.
Carlyon beach slide
Damaged house in Carlyon beach area

Also called rotational movements
Deep-seated landslides are often referred to as rotational earth movements, since the mass of the slide rotates backward as gravity pulls it downward. The rotation can lead to groups of trees being uniformly tilted toward the back of the slide and ponding of water that can no longer drain downslope. The base of a rotational slide may be pushed upward, creating a diagnostic toe bulge. Where the base of the slide is at beach level, the resulting uplifted beach is quite distinctive, although it will often erode away within several years.

 
links

Kelso Landslide Photos, nw.geoscience.com. Photos of homes damaged by a widespread slide in Kelso, Washington.
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