Landslides exist throughout Washington State and can be triggered in many ways.

• Rainfall—Prolonged or intense rainfall and rain-on-snow events can saturate soils and trigger landslides.
• Earthquakes—Intense shaking during earthquakes can cause the ground to fail. The earthquakes of 1949, 1965, and the Nisqually earthquake of 2001 produced numerous landslides throughout the Puget Sound region.
• Water-level changes—Rapid lowering of water levels can trigger landslides, especially along dams, coastlines, reservoirs, and rivers.
• Human activities—Vegetation removal, mining, loading on a slope, excavation at the base of a slope, and leakage from pipes can all trigger landslides.
• Geology—Easily weathered rock types and sandy or clay-rich soils are especially susceptible to landslides.

Union Pacific branch rail line collapse during the 1965 Puget Sound earthquake near Tumwater, Washington. Photo by G.W. Thorsen.

The Role of Gravity

Landslides nearly always move down a slope. This is because the force of gravity—which acts to move material downhill—is usually counteracted by two things: (1) the internal strength of the material, and (2) the friction of the material on the slope. A landslide usually occurs because the internal strength of the rock, soil, or sediment becomes less than the force of gravity.

Steeper slopes have a greater proportion of downhill force.

The Role of Water

The addition of water to material on a slope can make landslides more common. This is because water adds significant weight to the slope as it seeps into the ground and becomes groundwater. This extra weight adds to the gravitational force. Water also lowers the strength of the material which can make it less able to withstand the force of gravity. Water also reduces friction and increases pore fluid pressure.

The addition of water can decrease friction, decrease strength, and increase the weight of the material. All of these properties are important for landslides.

Pore fluid pressure is the pressure that water exerts on all of the nearby grains. When there is little water and low pore fluid pressure, most grains are touching and the material is relatively strong. When there is a lot of water and the pore fluid pressure is high, the water buoys up the grains and forces them apart. This buoyant force weakens the material and is also why your feet sink into beach sand after a wave washes over and adds water. These processes help to explain why landslides are much more common during the rainy season, and especially common during or right after large storms.

Water weighs a lot (about 8 pounds per gallon). If 1 acre of land soaks up 1 inch of rain, the land gains about 234,000 pounds. The addition of water can increase pore fluid pressure and separate grains. This and other processes decrease strength and can promote landslides.

The Role of Friction

The amount of friction between a deposit of rock or soil and the slope that it rests on plays a large role in when landslides happen. Imagine trying to slide a large rock along a flat surface—it's very difficult because of the friction between the rock and the surface. Pushing the rock is easier if the surface slopes downhill or is slippery. The same is true for landslides—for the same kind of material, steeper slopes have less friction, making landslides more common. Any change to the Earth's surface that increases the slope (for example, river incision or the removal of material at the base of a slope by humans) or that reduces the friction of a slope (such as the addition of water) can increase the likelihood of a landslide.

The addition of water increases weight and reduces friction.

In general, landslides can be categorized as shallow or deep-seated and this difference can determine their speed and size. Shallow landslides typically occur during the winter months in western Washington and during the summer months in eastern Washington, but are possible at any time. Deep-seated landslides can also occur at any time. Many of the landslide areas in Washington are a mixture of different slide types.

Shallow Landslides

Shallow landslides are rooted in the soil layer and often form slumps along roadways or fast-moving debris flows down valleys. These types of landslides are often called 'mudslides' by the news media. Shallow landslides also occur as flows, slides, or rockfalls and topples.

Deep-seated Landslides

Deep-seated landslides are those that fail below the rooting depth of trees and vegetation. They are often slow moving, but can also move rapidly. Deep-seated landslides can cover large areas and devastate infrastructure and housing developments. These landslides usually occur as translational slides, rotational slides, or large block slides. Deep-seated landslides are typically much larger than shallow landslides, in terms of both surface area and volume.

A deep-seated landslide may appear stable for years, decades, or even centuries. These long-lived features can be partially or entirely reactivated for a variety of reasons.

Flows

Flows are generally a slurry mixture of water, soil, rock and (or) debris that moves rapidly downslope. Flows may or may not be confined to a channel.

Earthflows may have a characteristic 'hourglass' shape. The slope material loses strength and runs out, often forming a bowl or depression at the head. Flows usually occur in fine-grained material on moderate, water-saturated slopes.

Debris flows usually occur in steep gullies, move very rapidly, and can travel for many miles. They may contain more coarse material than a mudflow when channelized. Slopes where vegetation has been removed by fire or humans are at greater risk for debris flows and many other types of landslides.

Debris avalanches are unchannelized debris flows that move very rapidly. They typically do not mobilize far and sometimes move like a snow avalanche.

Lahars are debris flows that originate on volcanoes. A volcanic eruption can rapidly melt snow and ice, causing a deluge of rock, soil, ash, and water that accelerates down the slopes of a volcano, devastating anything in its path. They can travel great distances and damage structures in flat areas far from their source. Communities near rivers draining Mount Rainier and Glacier Peak are at greatest risk.

Lahar on Mount St Helens. Image courtesy of USGS.

Lateral spreads occur on very low-angle slopes toward a free face such as a cliff or embankment. Movement is accompanied by cracking of the ground. Failure is often caused by liquefaction (when soil is transformed from a solid to a liquid), usually because of an earthquake.

Soil creep is the very slow (inches/year), steady, downward movement of soil or rock. Creep is indicated by curved tree trunks, bent fences or retaining walls, tilted poles or fences, and small soil ripples or ridges.

Slides

Slides are the downslope movements of soil or rock along a discrete or narrow failure surface and can be deep-seated or shallow. The initiation of slides, like flows or rockfalls, is sensitive to steep slopes, the additional weight of water or other loads, and friction along their base.

Translational slides usually fail along geologic discontinuities such as faults, joints, bedding surfaces, or the contact between two rock types. They move out or down along a planar surface with little tilting, and can travel great distances. Translational slides can contain loose sediments or large slabs of bedrock.

Block slides are a type of translational slide that occur when large and relatively intact slabs of rock or earth are rapidly transported downslope. These type of landslides can be large and damaging and occur where alternating layers of strong and weak rock slope downhill.

Rotational slides (slumps) are landslides that occur along a curved or spoon-shaped surface. Back-tilting may occur near the scarp of the landslide and there is often a toe of displaced material. Rotational slides often occur because the internal strength of the material is overcome by its own weight. They are usually composed of relatively loose or unconsolidated material.

Falls and topples

Falls and topples are usually rapid, downward movement of large pieces of rock or debris. Sometimes this is enough rock to cover a road or block a stream or river. Rockfalls and topples are common in Washington’s mountain highways.

Landslide figures in the "Types of Landslides" section are modified from U.S. Geological Survey Fact Sheet 2004-3072 and Cruden, D. M.; Varnes, D. J., 1996, Landslide types and processes, in Turner, A. K.; Schuster, R. L., editors, Landslides—Investigation and mitigation: National Academy Press; National Research Council Transportation Research Board Special Report 247, p. 36-75.