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Subsidence

 

Subsidence is a lowering of part of the Earth's crust including the surface. This can result from
a number of causes including tectonic downwarping or faulting, settling of fill, the collapse of a
cave or mine workings, and fluid withdrawal from an underground reservoir.

Tectonic processes are natural and tend to be very slow and generally unnoticed, except during
the formation of a fault scarp during an earthquake, on a human time scale. However, the other
causes of subsidence are another matter.

The settling of landfill can be a major problem. When an area is filled, especially for construction
purposes, the material should be properly compacted. Otherwise, any structures built on the fill
will sag and risk structural failure as the fill material naturally compacts. Fill such as that covering
an old dump or naturally unconsolidated soils in swampy or boggy areas where rotting vegetation
in the soil naturally compacts should also be viewed with caution. Few geologic maps show areas
of fill, and NBMG does not have a database of such areas, artificial or natural, for the state. If
you have concerns about fill or the use of fill, you should check with the local county or city
engineer or hire a consulting geologist or engineering firm to do a study.

Sagging or collapse of the surface due to the collapse of an underground cavity can be another
major problem. Parts of eastern Nevada are commonly underlain by carbonate formations and locally underground caverns form such as Lehman Cave in White Pine County. When underground caverns collapse, the surface expression is a sinkhole. Few geologic maps show sinkholes, though carbonate is always mapped when present. Also, NBMG does not have a database of caves or sinkholes for the state. If one has concerns about sinkholes in a particular area, then they should hire a consulting geologist or engineering firm to do a study. The USGS does have a number of general and site specific publications concerning caves and sinkholes, though few are Nevada specific.

In areas where underground mining was conducted, the collapse of underground workings can be a problem. In the 1990s, in Virginia City on the Comstock, fill over the 3000-foot-deep Obistan shaft collapsed one night. The Obistan shaft is near the high school. Also in Virginia City, part of the middle school parking lot collapsed into an old mine shaft. NBMG does not have a comprehensive database of all mines and underground workings in the state, and miners especially in the early days were never required to turn over detailed plans of their operations. However, NBMG does have a number of surface and underground mine maps in the Mining District Files, and NBMG Open-File Report 96-4, Nevada Abandoned Mined Database Compilation Project, consists of a database of all mining symbols plotted from the nearly 2,000 topographic maps that cover the state of Nevada. Many mines are also described in various NBMG bulletins and reports, and all NBMG publications are available from the NBMG Publication Sales Office. Numerous mining related publications, especially including those of the USGS and the now closed U.S. Bureau of Mines discuss various mining operations in Nevada, and these can be found through library searches. Also, the Nevada Historical Society, Special Collections in the Knowledge Center on the University of Nevada, Reno campus, and the Nevada State Archives in Carson City have some mining related materials. If you have concerns about collapsed mine workings or the potential of such in a particular area, you should hire a consulting geologist or engineering firm to do a study.

Subsidence due to underground fluid withdrawal can be another problem. The main area of the state suffering from this is Las Vegas Valley. Las Vegas (Spanish for "the marshes") naturally contained areas of a high water table and artesian springs, and was a stopping off point on the Old Spanish Trail. After an aborted effort by the Latter Day Saints to settle the area in the 1850s, ranches were reestablished by the late 19th century. Las Vegas was founded in 1905 as a railroad town and has since grown into a gambling mecca of almost a million people and continues to grow explosively. Las Vegas Valley receives less than 8 inches of precipitation annually, and despite receiving a share of the water from Lake Mead, gets most of its water from wells. The large removal of groundwater from the generally unconsolidated alluvial sediments underlying Las Vegas has resulted in surface subsidence of locally as much as 6 feet since the 1930s. This has also resulted in local fissuring of the ground.

NBMG has studied the problem and produced a number of reports dealing with it such as Bulletin 95, Subsidence in Las Vegas Valley, and Open-File Report 93-4, Subsidence in Las Vegas Valley, 1980-1991. These publications are available from the NBMG Publication Sales Office. Our website also contains a couple of dozen reports on water, subsidence, and fissuring in the Las Vegas Valley. Also, the USGS has a number of publications dealing with water, subsidence, and fissuring in the Las Vegas Valley and elsewhere. If you have concerns about subsidence from underground fluid withdrawal whether in Las Vegas Valley or not, you should also check with the local county or city engineer or hire a consulting geologist or engineering firm to do a study.

Research on land subsidence in Las Vegas Valley provides valuable information about the rates of subsidence resulting from groundwater withdrawals and the development of fissures that can cause considerable damage to buildings. NBMG researchers, in collaboration with other experts, are using the most current technologies to attack this problem—geodetic measurements using the global positioning system (GPS) and interferometry using synthetic aperture radar (InSAR), a remote-sensing technique. NBMG is also evaluating concerns regarding subsidence and fissures in other desert valleys, where groundwater is being pumped to supply the needs of expanding populations or for mines.