Earth materials may fail and move or deform in several ways (Figure 1). Rotational slides or slumps involve sliding along a curved slip plain, producing slump blocks (Figure 1a, b).
FIGURE 1 – Types of landslides Classification of landslides, based on type of movement, materials, water content, and rate of movement. (Modified
after U.S. Geological Survey 2004. Fact Sheet 2004-3072)
Rotational slides can be very large and deep and are often associated with complex movement. Figure 2 shows why homogeneous, weak materials have failure planes (slip planes) that approximate the arc of a circle.
FIGURE 2 – Curved slip plane. How rotational failure slumps may form a slip plane that approximates the arc of a circle.
Translational sliding is downslope movement of earth materials along a planar slip plane, such as a bedding plane or fracture (Figure 1c, d). Rock fall is the free fall of earth materials from a free face of a cliff (Figure 1e). Flows are the downslope movement of unconsolidated (usually saturated) materials in which particles move about and mix within the mass. Very slow flowage of rock or soil is called creep (Figure 1f ); rapid flowage may be an earthflow, mudflow, or debris flow. An earthflow (Figure 1g and Figure 3) often originates on a slope where soil partially liquefies and runs out.
FIGURE 3 – Earthflow on soft shale near Santa Barbara, California. (Edward A. Keller)
The source area becomes a bowl-shaped depression, and a depositional area spreads out at the toe of the slope, giving the event an hourglass shape. A debris flow or mudflow (Figure 1h) is a mixture of rock, soil, and organic matter that mixes with air and water to flow rapidly down slope. The upper part of the flow is often confined to a channel or valley but may spread out when it is no longer confined. A debris flow has less than 50 percent fines (sand, silt, and clay), whereas a mudflow has more than 50 percent fines by volume.
A debris avalanche (Figure 1i) is a very rapid to extremely rapid debris flow. Large debris avalanches can cause catastrophic damage and loss of life.
Lateral spreads (Figure 1j) are a type of landslide that often occurs on nearly flat slopes or very gentle slopes.
The movement starts with liquefaction of silts, clays, or fine sands during earthquake shaking or other disturbances. The actual movement is lateral extension. If stronger coherent rock or soil is at the surface and is over a soil layer that liquefies, the stronger material may fracture, translate, rotate, or disintegrate and flow. Lateral spreads often start suddenly and then become larger, in a slower, progressive manner.
Subsidence may occur on slopes or on flat ground and involves the sinking of a mass of earth material below the level of the surrounding surface (Figure 1k).
Landslides are commonly complex combinations of sliding and flowage. As an example, Figure 1l and Figure 4 show failures consisting of an upper slump that is transformed to a flow in the lower part of the slide. Such complex landslides may form when water-saturated Earth materials flow from the lower part of the slope, undermining the upper part and causing slumping of blocks of earth materials.
FIGURE 4 – Complex landslide at La Conchita, California (1995). This slide, which had an upper slump block and a lower flow, destroyed the three-story
home in its path. (Edward A. Keller)
Important variables in classifying downslope movements are the type of movement (slide, fall, flow, slump, or complex movement), slope material type, amount of water present, and rate of movement. In general, the movement is considered rapid if it can be discerned with the naked eye; otherwise, it is classified as slow (Figure 1). Actual rates vary from a slow creep of a few millimeters or centimeters per year to very rapid, ranging from 1.5 m (5 ft) per day, to extremely rapid, at 30 m (98 ft) or more per second.