Numerical Analysis of Gravity Flow and Overpressure Development: Implications for Liquefaction Susceptibility in the New Madrid Seismic Zone

We used mathematical modeling techniques to evaluate relations involving topographic-driven flow, overpressure development, and susceptibility to seismically-induced liquefaction in the Mississippi Embayment. The mathematical model assumes that the magnitude of excess pore pressure in the discharge area is determined by a basin's geometry and the distribution of hydraulic conductivity. The numerical simulations show the development of both shallow and deep groundwater flow (< 2 km) that discharges toward the Mississippi River near the New Madrid seismic zone (NMSZ). The modeling results indicate that excess pore pressures up to 7 atmospheric pressures could be sustained in a wide discharge area of the NMSZ by gravity flow. The predicted magnitude of excess pressures is generally consistent with the observed elevation heads (10-20 m) of artesian wells that penetrate the Upper Cretaceous aquifer in the study area. Numerical simulations examine the degree of overpressure downgradient in the discharge area. The high pore fluid pressure reduces effective stress, or the stress borne by the aquifer skeleton through grain to grain contacts. A drop in effective stress and a change in shear strength in matrix can influence strong ground motion in areas with water-saturated sediments. The prevailing overpressure conditions in the shallow NMSZ may result in a degree of ground shaking that is different than that expected under hydrostatic conditions. Reconstruction of the hydrology and overpressures of the deep seismic zone is difficult because critical hydrologic data of crustal rocks and faults are not available. Further theoretical modeling is needed to assess the overpressure conditions resulting from large-scale compressive stress in the crust of the NMSZ. We argue that ambient pore pressure distribution may be an important, but often ignored, component of earthquake and liquefaction studies in the NMSZ and elsewhere.