1995 La Conchita, California, landslide. Photo by R.L. Schuster.
The following is abstracted from U.S. Geological Survey Professional Paper 1183, Landslide Overview Map of the Conterminous United States, by Dorothy H. Radbruch-Hall, Roger B. Colton, William E. Davies, Ivo Lucchitta, Betty A. Skipp, and David J. Varnes; 1982. This has been recently been superceded by U.S. Geological Survey Open-File Report 97-289, Digital Compilation of Landslide Overview Map of the Conterminous United States, compiled by Jonathan W. Godt.
The landslide overview map of the Nation has been constructed to delineate broad areas where landsliding may pose problems in land use, including possible damage to engineering works. In this compilation, landslides are considered to be any downward and outward movement of earth material on a slope. (Turner and Schuster (1996) discuss in detail landslide types and processes). Here we show the southwestern part of the country (figure 2). The complete map will soon be available in digital form (on the WWW: http://geohazards.cr.usgs.gov and http://greenwood.cr.usgs.gov) or in print from the U.S. Geological Survey Information Services (303-202-4210) or from the National Landslide Information Center in Golden, Colorado (800-654-4966).
The landslide map is highly generalized because individual landslides could not be shown on a map of small scale (1:3,750,000) and because detailed landslide information for much of the country is scarce. Therefore, the map is not suitable for local planning or site selection, but it does indicate regions where slope-stability problems should be considered in large-scale planning.
The map displays both the incidence of landslides and susceptibility of the land surface to landslides. Briefly, the map was constructed by evaluating geologic units shown on the geologic map of the United States (King and Beikman, 1974) and classifying them as having high, medium, or low landslide incidence based on number of known landslides, and as having the high, medium, or low susceptibility to landsliding. High incidence was assigned to map units (indicated in red on the map) having more than 15 percent of their area involved in landsliding; medium incidence (in gold) to those having between 15 and 1.5 percent; and low incidence (in tan) to those having less than 1.5 percent. The largely subjective susceptibility indicators were defined as the probable degree of response of the rocks and soils at the surface to natural or artificial cutting or loading of slopes, or to anomalously high precipitation. The same percentages used to delimit landslide incidence were applied to the three categories of susceptibility. For example, a high susceptibility area would exhibit some movement over 15 percent or more of its surface area in response to widespread artificial cutting or high precipitation. The three susceptibility categories classified were: (1) high susceptibility with moderate incidence of landsliding (in pale red); (2) high susceptibility combined with low landslide incidence (in medium brown); and (3) moderate susceptibility combined with low landslide incidence (in green).
Full weight could not be given to the important factors of slope angle and precipitation because no adequate slope or precipitation maps at the appropriate scale existed at the time the map was produced in 1982. A more detailed description about the construction of the map is given in the original U.S Geological Survey Professional Paper 1183.
Areas in the desert Southwest that are presently under arid conditions (MAP less than 10") are especially vulnerable to climatic changes that increase precipitation. There is evidence for numerous relict landslides in the region, related to wetter and cooler climatic conditions during the late Pleistocene Epoch (between about 2 million and 10,000 years ago). Many of these features could be reactivated in a future, wetter climate. Also, warmer sea-surface temperatures in the waters surrounding Baja, California, could lead to a longer, more severe summer monsoon in the Southwest, producing a higher incidence of the intense thunderstorms that produce flash floods and debris flows. Existing, relict landslides may be reactivated or new landslides generated by construction activity, excessive natural loading, or by unusual natural or artificial wetting (such as irrigation), or erosion. Of the four most landslide-prone mountainous regions in the United States, three lie in the Southwest: The Coast Ranges of California, the Colorado Plateau, and the southern Rocky Mountains. Slope-stability characteristics of these physical subdivisions are summarized below. The fourth region is the Appalachian Highlands.
Figure 3. On March 4, 1995, a high-velocity 200,000 cubic yard landslide occurred in marine sediments that form the coastal bluff immediately above the tiny coastal community of La Conchita, California. Nine houses were destroyed or badly damaged within the space of a few seconds in this community, located on U.S. Highway 101 about 15 miles west of Ventura. Photo by R.L. Schuster, USGS.
The high incidence and susceptibility of landslides in the Coast Ranges (as shown by the large number of red and gold areas in figure 2) result from the combination of steep slopes, soft sheared rocks, and periods of heavy precipitation. The rocks of Franciscan assemblage, which have broken by tectonic processes into blocks of various sizes contained in a matrix of sheared material, are especially slide prone. Debris flows are common in southern California during rainstorms, particularly in heavily developed areas where many landslides have been artificially activated. Earthquakes in this area are another common trigger of landslides.
Deeply eroded sequences of sedimentary rocks, commonly interbedded soft shales and massive, competent, jointed sandstones, make the Colorado Plateau an area of numerous rock falls, slumps, complex block slides, and debris flows. Many flat-topped plateaus and mesas, where resistant rocks cap weaker ones, are rimmed by large landslide blocks. This characteristic feature of the Colorado Plateau is indicated on the map (figure 2) by the areas shown in red and gold, denoting high or moderate landslide incidence.
Figure 4. West end of the Grand Mesa, east of Grand Junction, Colorado. Landslide scar exposes bedrock of Grand Mesa basalt (top), unnamed claystone, and Green River Formation (white).
The Southern Rocky Mountains (ranges in southern Wyoming, Colorado, and northern New Mexico) are complex in rock type and climate, and so the landslides there are also complex. Nearly all kinds of slope-failure processes have been or are currently active in the region. Landslides are most abundant in areas underlain by marine shale (layers of mud deposited on ancient sea floor); by fine-grained sediments deposited in ancient lakes and river systems; or by volcanic sequences, especially those that have been hydrothermally altered, or which include weak, permeable beds of volcanic debris, or are comprised of massive volcanic rocks underlain by weaker sedimentary rocks, such as siltstone and shale. On the map (figure 2), this complexity is shown by the large areas colored red or gold (indicating landslides have been observed with high or moderate incidence), and also by the large number of areas interpreted to be highly or moderately susceptible to landsliding (shown by the pale red, medium brown, and green colors).
Figure 5. McClure Pass, south of Aspen, Colorado, is an area of the Rocky Mountains that has chronic landslide problems along roadcuts. This particular car plunged into the landslide in the middle of the night, after the landslide occurred. No one was injured. Photo by Terry Taylor, Colorado State Patrol.
Figure 6. Landslide occurring on the road to the Zion Park
Lodge, Zion National Park, Utah, in the spring of 1995. One hundred people
were stranded in the Lodge for two days since this was the only route into
and out of the Lodge area. The landslide also damaged the sewer line adjacent
to the road.
Figure 7. Debris flow west of Glenwood Springs, Colorado, September 1994. The slide temporarily blocked traffic on westbound Interstate 70 and threatened to dam the adjacent Colorado River, which would have caused devastating flooding. The debris flow was caused by intense rains that saturated the hillside, already denuded of vegetation from summer wildfires on adjacent Storm King Mountain.
Campbell, R.H., 1975, Soil slips, debris flows, and rainstorms in the Santa Monica Mountains and vicinity, southern California: U.S. Geological Survey Professional Paper 851, 51 p.
Cleveland, G.B., 1971, Analysis of mudslide risk in southern Ventura County, California--part 2, regional landslide prediction: California Division of Mines and Geology Open-File Report 72-23, 33 pp.
Colton, R.B., Holligan, J.A., Anderson, L.W., and Patterson, P.E., 1976, Preliminary map of landslide deposits in Colorado: U.S. Geological Survey Miscellaneous Investigations Series Map I-964, scale 1:500,000.
Godt, J.W., 1997, Digital Compilation of Landslide Overview Map of the Conterminous United States: U.S. Geological Survey Open-File Report 97-289, scale 1:3,750,000.
Grater, R.K., 1945, Landslide in Zion Canyon, Zion National Park, Utah: Journal of Geology, v. 53, pp. 116-124.
King, P.B., and Beikman, H.M., 1974, Geologic Map of the United States (exclusive of Alaska and Hawaii): Reston, Va., U.S. Geological Survey, scale 1:2,500,000, 2 sheets.
National Oceanic and Atmospheric Administration, 1974, Climates of the States: Port Washington, N.Y., Water Information Center, Inc., 2 volumes.
Radbruch-Hall, D.H., Colton, R.B., Davies, W.E., Lucchitta, I., Skipp, B.A., and Varnes, D.J., 1982, Landslide Overview Map of the Conterminous United States: U.S. Geological Survey Professional Paper 1183, 25 pp., scale 1:7,500,000.
Robinson, E.S., Mechanical disintegration of the Navajo Sandstone in Zion Canyon, Utah: Geological Society of America Bulletin, v. 81, pp. 2799-2806.
Turner, A.K., and Schuster, R.L., eds., 1996, Landslides: Investigation and Mitigation: Washington, D.C., National Research Council, Transportation Research Board Special Report 247, 675 pp.