Las Vegas Valley is a fault-bounded basin containing hundreds of metres of Tertiary and Quaternary sediments derived from lacustrine, paludal, and

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1 Land Subsidence (Proceedings of the Fourth International Symposium on Land Subsidence, May 1991). IAHS Publ. no. 200, Elevation Changes Associated with Subsidence in Las Vegas Valley, Nevada JOHN W. BELL Nevada Bureau of Mines and Geology, University of Nevada, Reno, NV 89557, USA ABSTRACT Subsidence in Las Vegas Valley, Nevada, is characterized by a primary regional subsidence bowl punctuated by several localized bowls. Previous studies indicate that from 1963 to 1980 the valley-wide bowl had subsided more than 49 cm with the secondary bowls subsiding as much as 79 cm. Vertical-control data obtained from releveling benchmarks in indicate that the same patterns and trends of movement have continued to occur since Nearly constant rates of subsidence on several benchmarks outside of major pumping areas suggest that subsidence does not simply reflect the location of major pumping or the location of major water-level decline. Releveling of nine short vertical-control lines from 1978 to 1989 indicates that preexisting geologic faults are the sites of preferred differential subsidence, and that these faults should be regarded as zones of high subsidence risk. INTRODUCTION Location Land subsidence in the Las Vegas area, Nevada, is primarily related to groundwater withdrawal. Although the rate of subsidence has remained relatively constant for the last decade, recent rapid urban development has highlighted the longer term effects, in particular, the appearance of fissures and an increase in structural damage. The metropolitan Las Vegas area is located in southern Nevada, within a 1295 km 2 (500 mi 2 ) alluvial valley which receives between 12 to 20 cm (5 to 8 in) of average annual precipitation. Nevada is the fastest growing state in the USA and Las Vegas is the most rapidly growing city in Nevada. The present permanent population (1990 statistics) of the Las Vegas metropolitan area is about 800,000. Hydrogeologic setting Las Vegas Valley is a fault-bounded basin containing hundreds of metres of Tertiary and Quaternary sediments derived from lacustrine, paludal, and 473

2 John W. Bell 474 alluvial deposition. These sediments consist predominantly of coarse-grained alluvial fan deposits around the periphery of the valley and predominantly of fine-grained (silt and clay) sediments in the central portion of the valley. Areas presently exhibiting significant amounts of subsidence are underlain by compressible deposits having fine-grained contents ranging from 25% to more than 75% by volume (Plume, 1984). Poorly permeable clay and calcium carbonate (caliche) beds occur throughout the alluvial sequence. A series of linear and curvilinear north- to northeast-trending tectonic Quaternary faults cut the valley floor creating a succession of prominent scarps as much as 50 m (160 ft) high, with displacements all down to the east. Nearly all of the groundwater supply in Las Vegas Valley comes from a zone of confined and semi-confined principal aquifers at depths of m (Maxey & Jameson, 1948; Harrill, 1976). Recharge to the principal aquifers is primarily through an artesian flow system. Between 31 and 43 hm 3 /year (25,000 and 35,000 acre-feet/year) of water are estimated to enter the Las Vegas Valley hydrologie system in the recharge area (Maxey & Jameson, 1948). Water has been withdrawn from the valley-fill reservoir through artesian discharge and pumped wells since In 1946, annual groundwater withdrawals began to exceed annual recharge (Maxey & Jameson, 1948). In 1955, withdrawals and discharge from the principal aquifers were estimated to be about 49.3 hm 3 /year (40,000 acre-feet/year) (Malmberg, 1965); by 1968 this had increased to an all-time maximum of hm 3 /year (88,000 acrefeet/year) (Harrill, 1976). Since 1968, annual groundwater withdrawals have gradually been reduced (Katzer, 1977), and since 1980, withdrawals have remained at about 82.7 hm 3 /year (67,000 acre-feet/year) (T. Katzer, 1990, personal communication). Increased demand in the last decade has been satisfied by the importation of Colorado River water. PREVIOUS SUBSIDENCE STUDIES Using vertical control data from the 1935 National Geodetic Survey (NGS) first-order network through Las Vegas Valley, Maxey & Jameson (1948) first noted as much as 7.5 cm (3 in) of movement in the central portion of the valley. By 1963, a distinct subsidence bowl with a maximum depth of more than 1 m (3.3 ft) had affected an area of 518 km 2 (200 mi 2 ) (Mindling, 1971). Releveling of the first-order NGS network in 1980 showed that the affected area had enlarged to more than about 1030 km 2 (400 mi 2 ), and that three distinct secondary subsidence bowls had developed, superimposed on the larger valley-wide bowl (Bell, 1981). From 1963 to 1980, the valley-wide bowl subsided more than 49 cm (1.6 ft) with the secondary bowls subsiding as much as 79 cm (2.6 ft). SUBSIDENCE DATA- POST-1980 Elevation data acquired since the last tabulation in 1980 (Bell, 1981) consist of two sets: a) vertical-control data obtained from releveling of selected NGS

3 475 Elevation changes associated with subsidence in Nevada benchmarks by the Clark County Surveyors Office in , and b) vertical-control data derived from a 12-year leveling program by the Nevada Department of Transportation designed to monitor localized differential movement across geologic faults. Clark County data During , the Clark County Surveyors Office releveled most of the existing city, county, and NGS benchmarks in Las Vegas Valley in order to update a major portion of the valley-wide vertical-control network. Leveling procedures were comparable to second-order, class II accuracy standards. Since the last NGS survey in 1980, many of the NGS benchmarks have been destroyed by urbanization. A total of 28 NGS benchmarks are used here to determine the elevation change throughout Las Vegas Valley for the period (Fig. 1). Many of these points were releveled by the NGS in 1963 but not in 1980; too few 1980 points remain to allow a comparison. The Clark County survey assumed that benchmark W51 was fixed and determined all other elevations relative to this point. Since benchmark W51 is situated within alluvial fill and therefore susceptible to subsidence, all Clark County elevations were normalized to benchmark A109 located in bedrock about 10 km (6 mi) FIG. 1 Location of NGS benchmarks FIG. 2 Contour map showing subused in study. sidence for the period , with Quaternary fault base.

4 John W. Bell 476 southeast of benchmark W51. The elevation adjustment applied to all Clark County points based on this assumed fixed datum is about 2.1 cm (0.07 ft). Subsidence in Las Vegas Valley for the period is shown in Figure 2. Contouring is based on proportional interpolation, and since the data set consists of only 28 points, the subsidence contours are only approximately located. In particular, the centers of the secondary subsidence bowls are contoured on the basis of maximum measured change on a benchmark within the existing network. Because of the relatively sparse coverage, other areas of large elevation change may be present but undetected. The outer 15 cm (0.5 ft) contour is relatively well defined in the eastern, western, and southern parts of the valley. The location of the 1986 zero-change contour is not precisely known, but it is outward at least as far as in 1980, which in general was near the perimeter of the valley fill. In order to analyze long-term subsidence rates and trends on individual benchmarks, the 1986 data were compared to a combined set of elevation data derived from several different surveys. The second-order county data were combined with first-order data from the 1935, 1963, and 1980 NGS surveys, and with second-order data from a survey by the Nevada Department of Transportation in Five benchmarks (K169, P169, Z368, N366, and H369) are shown here (Figs. 3 & 4) to illustrate the trend of subsidence with time. Benchmark K169 has the most complete vertical-control record dating back to the original 1935 survey. Benchmark Z368 shows the greatest amount of vertical movement for the period. YEAR VEAR FIG. 3 Subsidence trend for FIG. 4 Subsidence trend for benchmarks K169 and P169 for benchmarks Z368, N366 and H369 the period for the period Nevada Department of Transportation Level Lines Much of the subsidence in Las Vegas Valley is preferentially focussed on the numerous Quaternary faults which cut the alluvial sediments (Fig. 2) and serve as preexisting planes of weakness. In , a series of short (1.5-4 km [ mi]) vertical-control lines were established by the Nevada Department of

5 477 Elevation changes associated with subsidence in Nevada Transportation (Fig. 5) to verify the hypothesis that the faults may be at the sites of incipient vertical rupture due to large amounts of differential subsidence in the immediate vicinity LAS VEGAS RANGE FIG. 5 Location of level lines established by the Nevada Department of Transportation in across geologic faults. of the faults (Holzer, 1978). Benchmarks were set at m ( ft) intervals across nine faults and monitored at second-order accuracy annually or semiannually through All elevation measurements were based on assumed fixed benchmarks along each line, so that only relative differential changes were determined. All lines show differential movement during the monitoring period with total displacements across the fault zones ranging from a few centimeters to more than 40 cm (1.3 ft) (Varnum, 1987). The results from four of these lines are presented here. Level line 1 extended across a prominent Quaternary fault - the Eglington fault- in the northern part of the valley (Fig. 6). The line was destroyed in Line 2 (Fig. 7) crosses a compound scarp and lies 1 km (0.6 mi) northeast of a subdivision which recently experienced more than $12 million in subsidence-related damage. Lines 3 and 10 (Figs. 8 & 9) cross a large scarp in downtown Las Vegas. DISCUSSION Subsidence in Las Vegas Valley is characterized by two distinct modes: a) regional vertical movement primarily centered in the valley and secondarily centered in three localized areas, and b) differential vertical movement associated with preexisting geologic faults. Regional and localized horizontal strains induced by the vertical subsidence may be present, based on the distribution of earth fissures (D. Helm, 1990, personal communication), but have not been systematically measured. The pattern of valley-wide subsidence for the period follows the same trend delineated for the period (Bell, 1981), with the exception that the localized subsidence bowl in the northwest part of the valley is much more pronounced. This deep secondary bowl was present in 1980 but undetected because the 1980 survey did not relevel benchmarks in this area.

6 John W. Bell 478 The 1986 Clark County survey did include these benchmarks, which are here compared to 1963 NGS and 1972 Nevada Department of Transportation data. Individual benchmark trends indicate that in many cases subsidence has continued at an approximately constant rate at least since Trends of subsidence in each of the localized bowls are reflected by benchmarks K169 and P169 (Fig. 3) and Z368 and N366 (Fig. 4). Benchmark K169 has the most detailed record of any point in the actively subsiding part of the valley, and it indicates that the downtown Las Vegas area has been subsiding uniformly since The average rate of movement on this benchmark has been 3.6 cm/year (1.4 in/year). The rate was substantially lower because significant over drafting of the groundwater system did not commence until the mid-1940's (Maxey & Jameson, 1948). Several benchmarks (P169, N366, & H369) show reduced rates of movement since 1972, probably in response to reduced pumping. Benchmark Z368, which is located in the deep localized subsidence bowl in the northwest part of the valley, shows the most movement (1.5 m [5 ft]) for the period (Fig. 4) with an average subsidence rate of 6.6 cm/year (2.6 in/year). It lies within a few kilometers of two other benchmarks (T365 & TOPOGRAPHY TOPOGRAPHY DISTANCE (1000 FT) DISTANCE (FT, X1000) DISTANCE (1000 FT) DISTANCE (1O00 FT) FIG. 6 Topography and corresponding FIG. 7 Topography and corresponding relative movement for level line 1 relative movement for level line 2 for the period for the period

7 479 Elevation changes associated with subsidence in Nevada Y368) which subsided comparable amounts: 1.2 m (3.9 ft) and 1.4 m (4.7 ft), respectively. The subsidence trend data suggest that the amounts and rates of subsidence may be semi-independent of the location of major pumping and the location of major water-level decline. The relatively constant rate of subsidence on benchmark K169 is significant because of the lack of intense groundwater pumping in this part of the valley (Harrill, 1976; Wood, 1988). The location of the deep subsidence bowl around benchmark Z368 does not clearly coincide with either the location of major pumping or the location of major waterlevel decline. The closest heavily pumped area lies 3-5 km ( mi) to the southwest (Wood, 1988) where it is associated with a zone of large (>55 m [180 ft]) water-level decline. Such observations suggest that the relations between pumping, water-level decline, and long-term subsidence are complex, and may be related to other variables such as thickness and compressibility of sediments, residual compaction, and hydraulic partitioning. DISTANCE (1000 FT) DISTANCE (1O0O FT) DISTANCE (1000 FT) FIG. 8 Topography and corresponding relative movement for level line 3 for the period FIG. 9 Topography and corresponding relative movement for level line 10 for the period Localized differential vertical movements are occurring along the preexisting geologic faults. The direction of movement across the faults is controlled by the location of adjacent subsidence bowls, resulting, in several instances, in the movement being antithetic to the original sense of displacement along the fault. For example, level line 1 shows a large (52 cm [15.6 in]) displacement across

8 John W. Bell 480 the Eglington fault scarp for the period (Fig. 6), with the northwest end of the line moving down relative to the southeast end. The sharpest relative elevation deflection coincides with the location of both the topographic fault scarp and the location of numerous fissures lying along the toe of the scarp. Similar, but less striking, antithetic movements are also seen along faults on lines 2, 3, and 10 (Figs. 7, 8, and 9). The displacement along level line 1, although coinciding in part with the deep localized subsidence bowl, indicates that the area to the northwest of the fault trace is subsiding more rapidly than the area to the southeast. Since benchmark Z368 is situated on the southeast (downthrown) side of the fault, deeper portions of the subsidence bowl may exist to the northwest but be undetected because of sparse benchmark density. Line 2 is situated between the subsidence bowl surrounding benchmark Z368 and the bowl surrounding benchmark K169 in downtown Las Vegas. The elevation change profile along this line is an irregular pattern of individual benchmark displacements across a compound set of fault scarps, probably reflecting an influence from both subsidence bowls. The largest displacement between adjacent benchmarks across any fault occurs on this line, where more than 15 cm (6 in) of differential movement has occurred between two points for the period. Lines 3 and 10 both cross a large (45 m [150 ft]) fault scarp that cuts through downtown Las Vegas. The patterns and magnitude of the elevation change for both of these lines are similar; displacement is down to the west, antithetic to the original geologic offset, with about 30 cm (1 ft) of cumulative movement measured on line 3 for the period , and about 24 cm (0.8 ft) of cumulative movement measured on line 10 for the period Differential offset on these lines is distributed across zones about 1.6 km (1 mi) in width, and the largest differential offset between adjacent benchmarks is about 4.5 cm (1.8 in) for the monitoring period. Although differential elevation changes related to subsidence are observed across several principal faults in the valley, the patterns of displacement are not suggestive of imminent surface faulting. The elevation changes are, as noted, either antithetic to the dip of the existing fault plane, or are distributed across relatively broad zones. The location of deep secondary subsidence bowls is the primary factor influencing the sense and style of renewed movement on the faults. The fault traces, however, are clearly preferred sites of enhanced movement (and fissuring) in comparison to the regional subsidence and thus are regarded as zones of high subsidence risk. REFERENCES Bell, J.W. (1981) Subsidence in Las Vegas Valley. Nevada Bureau of Mines and Geology Bulletin 95. Harrill, J.R. (1976) Pumping and ground-water storage depletion in Las Vegas, Nevada, Nevada Department of Conservation and Natural Resources. Water Resources Bulletin 44.

9 481 Elevation changes associated with subsidence in Nevada Holzer, T.L. (1978) Documentation of potential for surface-faulting related to ground-water development in Las Vegas Valley, Nevada. U.S. Geological Survey Open-file Report Katzer, T. (1977) Water-level changes associated with ground-water development in Las Vegas Valley, Nevada, March 1976 to March Nevada Department of Conservation and Natural Resources. Water Resources Information Series Report 27. Malmberg. G.T. (1965) Available water supply of the Las Vegas ground-water basin, Nevada. U.S. Geological Survey Water-Supply Paper Maxey, G.B. & 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. Mindling, A.L. (1971) A summary of data relating to land subsidence in Las Vegas Valley. University of Nevada Desert Research Institute Publication. Plume, R.W. (1984) Ground-water conditions in Las Vegas Valley, Clark County, Nevada: Part I. Hydrogeologic framework. U.S. Geological Survey Open-file report Varnum, N.C. (1987) Results of leveling across fault scarps in the Las Vegas Valley, Nevada, April April Nevada Bureau of Mines and Geology Open-File Report Wood, D.B. (1988) Water-level changes associated with ground-water development in Las Vegas Valley, Nevada, U.S. Geological Survey Water-Resources Information Report 31.

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