A Three-Dimensional Modeling Approach of the Role of Faulting in Land Surface Deformation: The Case of the Eglington Fault in Las Vegas Valley

Overpumping in Las Vegas Valley has caused an increase in effective stress and the accumulation of strain in critical zones. As a consequence of this stress-strain configuration, surface deformation expressed in the form of earth fissures and land subsidence has been occurring most commonly in zones adjacent to quaternary faults. The Eglington Fault, located in the north-west portion of Las Vegas Valley, has had a profound effect on surface deformation patterns by restraining the migration of land subsidence and creating complex hydromechanical failure mechanisms. Fissures typically result from horizontal displacements that occur in zones where extensional stress derived from groundwater flow exceeds the tensile strength of the near-surface sediments. A series of hypothetical numerical models using the finite-element code ABAQUS, and based on the observed conditions of the Eglington Fault zone, were developed to gain a better understanding of the mechanisms responsible for the observed land subsidence and fissuring patterns. Model simulations were developed using three time steps to reproduce the (1) long-term period of natural recharge and discharge, (2) heavy pumping and (3) artificial recharge that reflect the conditions of Las Vegas Valley over the past 95 years. The simulated hydrostratigraphy consists of three aquifers, two aquitards and a relatively dry vadoze zone. In addition, a normal fault zone that reflects the Eglington Fault was included in the models. The fault zone was simulated using four different widths and two different types of materials with contrasting hydromechanical properties. Numerical results suggest that a 100 m wide fault zone composed of sand-like material represents: (1) conditions most similar to those observed in Eglington Fault zone and (2) the most favorable conditions for the development of fissures forming on the surface adjacent to the fault zone.