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SURFACE OF THE EARTH: NORTH AMERICA 2006 IRIS 5-YEAR PROPOSAL Investigating Crust and Mantle Structure with the Florida-to-Edmonton Broadband Array Karen Fischer, Stephane Rondenay, Ellen Syracuse, Christine McCarthy, Catherine A. Rychert, Lindsey Doermann, Mariela Salas, Margaret Welsh • Brown University Michael Wysession, Ghassan Aleqabi, Patrick Shore, Jesse Fisher Lawrence, Brian Shiro, Moira Pyle, Garrett Euler, Tara Mayeau • Washington University During the Florida to Edmonton Broadband Seismometer Experiment (FLED), 28 broadband IRIS/PASSCAL STS-2 seismometers were deployed (May, 2001 to October, 2002) from central Florida to Alberta, Canada (Figure 1). Together with adjacent permanent stations from the IRIS/GSN, USNSN, CNSN, and New Madrid Seismic Network, the roughly linear array crossed diverse tectonic provinces and features, including the rifted continental margin, the Appalachian orogen, the Proterozoic and Archaean provinces of the continental interior, the Mid-Continent Rift and the Williston Basin. Data quality was in general very good.One project involved constraining crustal structure beneath the array using Ps phases scattered from the Moho and crustal reverberations (Figure 2). Significant features include zones of thickened crust beneath the southern Appalachian mountains, the late-Proterozoic Mid-Continent Rift in Iowa, and the Williston Basin. Beneath FLED in the southern Appalachians, the ratio of surface topography to excess crustal thickness matches the value found in the northern Appalachians beneath the Missouri-to-Massachusetts Broadband Seismometer Array (MOMA). However, both Appalachian ratios are small when compared to topography/crustal root ratios in young orogens. These results are consistent with global trends in orogenic crustal thickness, mountain topography and gravity anomalies that suggest a systematic decrease with age in the buoyancy of crustal roots relative to the mantle. Such an increase in crustal root density can be explained by gradual metamorphic reactions over hundreds of millions of years. Figure 1. The station locations of the Florida-to-Edmonton (FLED) broadband seismic array. Other projects completed or underway involve examining the shear-wave splitting in SKS and related phases and their constraints on mantle anisotropy, transition zone discontinuity topography with scattered wave migration, lateral variations in upper mantle heterogeneity and anisotropy using Rayleigh and Love waves, ultra-low velocity zones at the core-mantle boundary using SPdKS phases, mantle attenuation structure beneath North America, lateral variations in the lowermost mantle using ScS-S waves, mantle discontinuities using SS precursors, and D" thermal boundary layer structure (see later). NSF Grant #: EAR- 9903385, EAR-9903260 Figure 2. Image of the Moho beneath the FLED array from Ps phases (red or positive arrivals). Waveforms at each station were corrected with an inverse free surface transform, simultaneously deconvolved, and migrated to depth in 1D using a uniform velocity model. True crustal thicknesses (not shown) were modeled allowing for lateral variations in crustal velocities. 86
2006 IRIS 5-YEAR PROPOSAL SURFACE OF THE EARTH: NORTH AMERICA Numerical Analysis of Overpressure Development in the New Madrid Seismic Zone Lorraine W. Wolf, Ming-Kuo Lee • Auburn University Sharon Browning • University of Memphis Martitia P. Tuttle • M.P. Tuttle and Associates 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 basinʼs 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., Porterʼs 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. 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. 87
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