Land subsidence in Las Vegas Valley is primarily related to groundwater withdrawal. As early as the mid-1940's, it was observed that groundwater pumping in the valley was exceeding the estimated annual recharge to the system. Between 25,000 and 35,000 acre-feet per year of water is believed to enter the hydrologic system of the valley, but annual withdrawals have consistently exceeded this annual recharge by a factor of approximately two since accurate tabulations were begun in 1955. Groundwater pumping reached a peak of about 88,000 acre-feet per year in 1968, and it has gradually been reduced since then through the construction of the Southern Nevada Water Project, which imports Lake Mead water into the valley. Since 1980, groundwater withdrawals have remained at about an average of 68,000 acre-feet per year.
Much of Las Vegas Valley is underlain by more than 1,000 feet of interbedded fine- and coarse-grained sediments ranging in age from Miocene to Quaternary. The Tertiary Muddy Creek Formation and the overlying Plio-Pleistocene basin fill comprise the valley-fill groundwater reservoir. Most wells draw upon aquifers within the upper 1,000 feet of these sediments. Groundwater moving laterally into the basin is confined or partially confined beneath fine-grained, low-permeability sediments, resulting in artesian flow conditions.
The result of continued overdrafting of the groundwater system has been the decline of water levels and a reduction of the artesian pressure throughout the valley. A reduction of the artesian pressure is accompanied by an increase in the vertical loads, or effective stresses, imposed upon the sediments by the weight of the overlying deposits. As the effective stresses increase in the basin fill, the sediments are compacted and the ground surface subsides.
Subsidence in the valley has been geodetically monitored since 1935. On the basis of this original leveling network, it was found that by 1963 the center of the basin (near downtown Las Vegas) had subsided as much as 3.4 feet. From 1963 to 1980, the downtown area continued to sink as much as 2.0 feet, and other areas near the Strip and the North Las Vegas Airport subsided as much as 2.4 feet and 2.6 feet, respectively, during this I 7-year period (see figure). On the basis of the last detailed elevation survey of the basin in 1980, as much as 5.0 feet of subsidence has occurred in the center of the valley since 1935. Since 1980, subsidence has continued at a more or less constant rate, but the actual amounts are not presently known.
The surface effects of continuing land subsidence are displayed in several characteristic ways. Broad downwarping of the valley floor is occurring; that is, much of the valley is sinking at a relatively uniform rate so that most structures are not adversely affected. Locally, however, the subsidence is focussed on preexisting geologic faults, which serve as points of weakness for ground movement. In 1978, the Nevada Bureau of Mines and Geology, in cooperation with the Nevada Department of Transportation, established nine geodetic control lines across selected faults in areas of high subsidence to determine the amount of movement occurring on the faults. A ten-year (1978-1987) record of annual releveling of these lines indicates that several faults are moving as much as 2 inches per year. In the North Las Vegas area, one fault shows a vertical displacement of 1.3 feet for the period 1978 to 1985 (when the line was destroyed).
In the absence of leveling data, subsidence-induced movement on the faults can be detected by the presence of ground fissures, or cracks, on or in close proximity to the faults. Fissures originate as relatively small (less than 1/2 inch wide) linear cracks as the sediments are pulled apart through subsidence. Once formed, however, they are enlarged through erosion. In many areas of the valley, eroded fissures are as much as 6 feet wide and are frequently characterized by subsurface tunnels or pipes as much as 3 feet in diameter. Total fissure depths are unknown, but may be on the order of tens of feet.
Structural damage or distress has typically been located in areas on or close to the geologic faults in the valley, with most damage having been reported since the mid-1960's in the central and northern parts of the valley. Since these faults continue to show evidence of differential movement, they should be regarded as zones of high risk for construction as long as groundwater withdrawal remains at the present level.
In 1981, the Nevada Bureau of Mines and Geology published a compilation of land subsidence data for Las Vegas Valley (NBMG Bulletin 95); it is now in the process of updating this database. This update will include a new geodetic survey of the valley, the addition of new vertical control lines across more faults, the mapping of new fissures, and a tabulation of new areas of structural damage.
---John W. Bell, Engineering Geologist