Project researchers: Rina Schumer and Scott McCoy
The relationship between poor water pressure and aquifer matrix stress in balancing the force imposed by an overburden is a fundamental concept in hydrogeology because it explains why excessive groundwater withdrawal has led to surface deformation and subsidence. In the Las Vegas Valley, groundwater withdrawal has led to cracks near preexisting faults and ground subsidence of more than six feet. Although local subsidence issues resulting from pumping are well-known, recently it has been determined that changes in surface mass (glaciers, surface water bodies) and groundwater withdrawal can affect regional fault motion and seismicity (González et al., 2012; Hampel and Hetzel, 2006). This can been explained by the unloading of mass from the crust affecting the vertical stress state of the lithosphere as induced flexure and rebound alters the horizontal stress (Hetzel and Hampel, 2005).
In 2012, the Nevada state engineer approved the withdrawal of 61,127 acre-feet of groundwater from Spring Valley, 5,235 acre-feet from Cave Valley, 11,584 acre-feet from Dry Lake Valley, and 6,042 acre feet from Delamar Valley in the counties of Lincoln and White Pine for transfer to the Las Vegas water system. The pumping will occur in the valleys between eastern Nevada mountain ranges that are bounded on at least one side by a listric normal fault that has been active with large earthquakes during the last 1.6 million years (Price, 2004). Therefore, it is important to evaluate if withdrawals of this magnitude will drive crustal unloading stresses to induce seismicity or motion in normal regional faults.
Although the effects of altering eastern Nevada surface hydrology and ecology have been a focus of environmental impact statements, the possibility of affecting fault slip rates and inducing seismicity has not. The technology to evaluate this possibility has been developed over the past decade and will provide a cutting-edge research project for a graduate student who will be trained with cross-disciplinary expertise in structural geology, rheology, and hydrogeology. The graduate student will use numerical simulations to address how much withdrawal is sufficient to cause changes in regional lithospheric stress fields that can lead to fault motion. The graduate student will be co-advised by Rina Schumer and Scott McCoy, who is a new faculty member (earth surface processes) in Geological Sciences and Engineering at University of Nevada, Reno. Dr. McCoy’s participation in the Graduate Program of Hydrologic Sciences will broaden the program’s expertise in hydrogeomorphology. A finite element model of the crust and lithospheric mantle will be used to evaluate the evolution of the regional stress field that is subject to a reduction of groundwater mass near the surface. Strain and slip rates under various mass reduction scenarios will also be evaluated. The critical withdrawal volumes will be compared with proposed extraction plans for eastern Nevada and an evaluation of the long-term mass reduction that pumping capture zones will create. The software package ABAQUS (Hibbitt et al., 2002) has been used in the past for similar applications and will likely be used for this project.
The objectives of this project are:
- To determine it the magnitude of groundwater withdrawal and the reduction of the hydraulic head within the large capture zones in eastern Nevada basins is sufficient to modify the crustal stress field and affect basin faults.
- To determine the time scale of structural response to a reduction in water storage.