Rapidly growing populations in the semiarid to arid regions of the southwestern United States require water. The water is used not only for basic needs (drinking, sewage systems, etc.), but also to maintain lifestyles transferred from wetter areas of the country (suburban lawns, golf, backyard swimming pools). These uses can be met either by purchases of surface water in an already over-allocated water-supply system (e.g., the Colorado River) or, most readily, by tapping ground water. However, ground-water withdrawal can adversely affect the sedimentary deposits that fill typical ground-water basins in the desert. These effects include differential compaction, reactivation of old faults, and surface fissuring, and can have considerable impact on human infrastructures (Figs. 1 and 2).
Figure 1. Photograph of Las Vegas Valley Water district Well No. 5 showing well-head protrusion caused by subsidence. Photo by John W. Bell, 1989.
Land in the Las Vegas Valley has been subsiding due to ground- water withdrawal (Figs. 1 and 2) since about 1935. This subsidence and its relations to specific wells and faults have been very well documented by leveling studies conducted along survey lines, some of which were initially established in 1935 during studies related to the construction of Hoover Dam (Lake Mead). This report briefly sketches the magnitude of the subsidence problem and summarizes the ongoing studies, chiefly conducted by the Nevada Bureau of Mines and Geology. Studies by Bell and others (1992) and Bell (1981) are the chief sources of information and are freely quoted and paraphrased here without direct attribution.
Figure 2. Photograph of house in Windsor Park subdivision in North Las Vegas showing differential settlement to a combination of fault scarp movement and fissuring. Note fissure crossing road toward house. Photo by John W. Bell.
Las Vegas Valley is the most rapidly growing metropolitan area in the U.S. (1995 Census Bureau report) and now has well over one million people. About 20% of the present water supply (~375,000 acre-feet/year) is from ground water. Annual ground-water withdrawals began to exceed estimated annual recharge in 1946 (Maxey and Jameson, 1948). Since 1968, annual withdrawals have been gradually reduced (in 1991 the water district began re-injecting water into the subsurface) but have consistently exceeded natural recharge levels by factors of two to three. As a result of continued long-term overdraft, water levels have declined more than 90 m in some portions of the valley.
Land subsides due to fluid withdrawal as a result of a decrease in
sediment volume (Poland and Davis, 1969). This consolidation is related to
an increase in effective stress within the deposit and to the grain size. As
the effective stress increases, fine-grained deposits (silt and clay) undergo
plastic deformation accompanied by a permanent rearrangement of
sediment particles. For a given stress increase, the amount of deformation
will be highest in fine-grained deposits because they have higher porosity
and are more compressible than coarse-grained deposits. The fine-grained
Tertiary and Quaternary basin-fill deposits that fill Las Vegas Valley have
geologic and hydrologic properties that are very conducive to consolidation
upon fluid extraction.
Land subsidence in the Las Vegas area is primarily related to ground-water withdrawal. Although the rate of subsidence has remained relatively constant for the last decade, recent urban development has intensified the occurrence of fissuring and structural damage. Areas within the valley that have been heavily pumped and show large water-level declines have also been the sites of major elevation change, surface deformation, and damage.
Subsidence due to ground-water withdrawal is superimposed on this broad regional depression. Maxey and Jameson (1948) first noted that the valley was subsiding in apparent relation to water withdrawals. By 1963, the center of the valley had subsided as much as 1 m and by 1980, about 1.5 m (Bell, 1981; Fig. 1). Subsidence still continues; a new compilation by Bell and Ramelli (1991) using 1986-87 data showed that the location and rates of subsidence have remained relatively constant at least since 1963 (fig. 3). A broad regional subsidence bowl occupies the central portion of Las Vegas Valley. Three localized subsidence bowls are superimposed on the broad pattern and are located in the central (downtown), southern (Las Vegas Strip), and northwestern parts of the valley. These localized bowls have had at least 75 cm of subsidence since 1963; the northwestern bowl has subsided more than 1.5 m since 1963.
Figure 3. Quaternary faults and fissure zones in the Las Vegas area. Contours show subsidence measured only from 1963 to 1986-87. Short lines are level lines across faults (see fig. 3 for leveling history of line 1).
Bell and others (1992) reported that the leveling history across the
faults indicates they are preferred sites for localized, subsidence-induced,
vertical differential movement. Data for four of the lines across three
separate fault zones (Fig. 3) indicate that the faults moved at
relatively constant rates for the entire 1978-1991 period. Line 1 (Fig.
4) crossed the large, northeast-trending, Eglington scarp. Relative
elevation changes were measured along this line between 1978 and 1985,
when the line was destroyed by suburban development. These changes
indicate that the northwest (upthrown) side of the fault was subsiding at a
rate of about 5 cm/yr. Along the four lines reported by Bell and others
(1992), either the sharpest differential displacements closely coincide with
the central portion of the fault scarp (Fig. 4), or the differential offsets are
initiated at the scarp. In all four cases, the displacements are antithetic,
opposite to the original geologic sense of displacement. The antithetic
movement is attributed to the presence of adjacent localized subsidence
bowls (Fig. 3).
Figure 4. Level line 1 across the Eglington scarp in the northwestern part of Las Vegas Valley (fig. 3). Top shows benchmarks (small squares), topography, fault scarp, and zone of fissuring. Bottom shows relative change in elevation on benchmarks for the period 1978-1985.
Faults and fissures are spatially associated within Las Vegas Valley
(Fig. 3). The eight principal zones of fissuring and many of the
minor zones are closely coincident with known or inferred geologic faults,
confirming that the faults may be preferred sites for the tensile strains
required to induce fissuring. A small percentage of the known fissures
occur as concentric features encircling high-yield water wells.
In 1989, the U.S. Department of Housing and Urban Development began requiring special subsidence hazard assessments for property located in close proximity to known subsidence features. This requirement was primarily a consequence of the structural damage caused by fissuring in the Windsor Park subdivision of North Las Vegas (Fig. 2); estimated total costs for repair or replacement of more than 240 damaged or threatened homes in this area were $12-13 million. This subdivision lies within fissure zone E (Fig. 3) at the junction of two major faults. Other areas exhibiting incipient signs of structural distress in homes in 1991 include fissure zones B, along the Eglington scarp, and H, in the southern part of the valley (Bell and others, 1992). The guidelines specified detailed subsidence studies and specialized construction design for all new developments within 150 m of a mapped fault. A statistical analysis of fissures throughout the valley, however, indicates that the specified zone would include only about 45% of all mapped fissures, and that a zone 610 m wide around mapped faults is required to include about 90% of the fissures.
Bell, J.W., 1981, Subsidence in Las Vegas Valley: Nevada Bureau of Mines and Geology Bulletin 95, 84 p.
Bell, J.W., Price, J.G., and Mifflin, M.D., 1992, Subsidence-induced fissuring along preexisting faults in Las Vegas Valley, Nevada: Proceedings, Association of Engineering Geologists, 35th Annual Meeting, Los Angeles, p. 66-75.
Bell, J.W., and Ramelli, A.R., 1991, in Bell, J.W., and Price, J.G., eds., Subsidence in Las Vegas Valley, 1990-1991, final project report: unpublished Nevada Bureau of Mines and Geology Report, p. E1-E31.
Longwell, C.R., 1960, Interpretation of the leveling data, in Smith, W.O., Vetter, C.P., Cummings, G.B., and others, Comprehensive survey of sedimentation in Lake Mead, 1948-49: U.S. Geological Survey Professional Paper 295, p. 33-38.
Maxey, G.B., and Jameson, C.H., 1948, Geology and water resources of the Las Vegas, Pahrump, and Indian Springs Valley, Clark and Nye Counties, Nevada: Nevada Department of Conservation and Natural Resources, Water Resources Bulletin 5, 128 p.
Mifflin, M.D., Adenle, O.A., and Johnson, R.J., 1991, Earth fissures in Las Vegas Valley, 1990-91 inventory, in Bell, J.W., and Price, J.G., eds., Subsidence in Las Vegas Valley, 1990-91, final project report: unpublished Nevada Bureau of Mines and Geology Report, p. C1-C31.
Mindling, A.L., 1971, A summary of data relating to land subsidence in Las Vegas Valley: University of Nevada Desert Research Institute Publication, 55 p.
Patt, R.O., and Maxey, G.B., 1978, Mapping of earth fissures in Las Vegas Valley, Nevada: University of Nevada Desert Research Institute Project Report 51, 19 p.
Poland, J.F., and Davis, G.H., 1969, Land subsidence due to withdrawal of fluids, in Varnes, D.J., Reviews in engineering geology, v. II: Boulder Colorado, Geological Society of America, p. 187-268.