Numerical Analysis of Overpressure Development in the New Madrid Seismic Zone
Numerical Analysis of Overpressure Development in the New Madrid Seismic Zone
Credit:
Lorraine W. Wolf, Ming-Kuo Lee, Sharon Browning, Martitia P. Tuttle/IRIS Consortium
Description
Calculated groundwater flow and overpressure distribution in the Mississippi Embayment in response to regional topography without (a) and with (b) a vertical fault zone. Arrows show predicted flow directions. Colors map abnormal pore pressures from underpressure in the recharge area to overpressure in the discharge area. Gray color maps a permeable fault zone and Upper Cretaceous aquifer. The permeability of non-faulted portion of confining units was held constant as 10-5 darcy for both simulations. Vertical fault in (b) (permeability = 1 darcy) extends from Cambrian strata through the Porter’s Creek Clay near well 3. The predicted magnitude of overpressure (< 4 atm) in fault model agrees better with field data of artesian well heads, suggesting that confining units may be breached by faults. Major confining units (Porter’s Creek Clay and Clairborne group) inhibit upward groundwater flow, allowing overpressures to develop in underlying Cretaceous sands and Cambrian-Silurian carbonate rocks.
We use mathematical and numerical modeling techniques to evaluate the mechanism of overpressure development in the Mississippi Embayment and discuss potential implications for seismic hazards in the New Madrid seismic zone (NMSZ). The mathematical model explores how the magnitude of excess pore pressure in the basin’s discharge area may be explained by the geometry and hydraulic conductivity of basin strata. Our modeling results demonstrate how excess pore pressures of up to 4 atm could be sustained in a wide discharge area of the NMSZ by regional gravity flow. The predicted magnitude of excess pressure is generally consistent with observed elevation heads (10-30 m) of artesian wells that penetrate the Upper Cretaceous and Paleozoic aquifers in the basin. The modeling results demonstrate that overpressures developed at depth in the Mississippi Embayment could be communicated to shallower layers if basin-wide confining units are breached by faults. The model shows that the greatest overpressures develop in Early Paleozoic carbonate rocks and overlying Upper Cretaceous sand aquifers that are capped by regionally extensive confining units (e.g., Porter’s Creek Clay). Other researchers note geophysical anomalies, such as low P-wave velocities and low resistivity, in the Paleozoic strata and speculate that the anomalies may reflect elevated pore pressures at shallow depths (< 5 km). The study emphasizes the importance of permeability distributions in overpressure development and suggests that the conditions favoring elevated pore pressures in areas of the NMSZ may significantly influence the hydrologic response of the basin during large earthquake events and possibly reduce the stress required for deformation.
Photographer / Contributor: Lorraine W. Wolf, Ming-Kuo Lee • Auburn University; Sharon Browning • University of Memphis; Martitia P. Tuttle • M.P. Tuttle and Associates
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