Suraj Bajgain, M.S. 2011 [expand title=”Details”]
Thesis: Gravity and Magnetic Modeling of the Basement Beneath the Alabama Gulf Coastal Plain
Abstract
The southeastern United States has experienced two complete successions of Wilson cycles: (1) the assembly and break up of Rodinia and the opening of the Iapetus ocean; and (2) the closing of Iapetus ocean, the assembly of supercontinent Pangaea and its subsequent break up, and the opening of modern Atlantic Ocean. Evidence of these supercontinent cycles are recorded in the rocks of Alabama and adjacent areas, but in the southern portion of the state, these rocks are covered by as much as 7 km of Coastal Plain sediments. The Grenville-aged basement rocks beneath the Plateaus, the Valley and Ridge, and most of the Piedmont provinces have Laurentian origin, but the Uchee terrane in the Southern Piedmont province to the south is interpreted to have exotic Peri-Gondwanan origin. Also to the south, rocks of the Suwannee terrane are thought to have a Gondwanan origin based on faunal assemblages found in well cores. Triassic to Upper Jurassic sedimentary rocks of South Georgia basin onlap the northern limit of Gondwanan-affiliated Suwannee terrane rocks and obscure the suture between Laurentia and Gondwana. In this study, I use airborne gridded gravity and magnetic data to develop crustal models along three transects that cross major tectonic structures, geophysical anomalies, and the ancient North American (Laurentian) margin. Models derived from gravity and magnetic data are constrained by well-log information, geologic mapping, and previous geophysical studies in Alabama and nearby areas. Results show that a pronounced east-west trending gravity low observed in southern Alabama can be interpreted as the suture between relict Gondwanan crust and Peri-Gondwanan/Laurentian crust. The denser crystalline rocks of the Piedmont and Valley and Ridge provinces correspond to minor gravity highs. Based on its distinctive gravity and magnetic properties, the Wiggins terrane in southwestern Alabama is interpreted as a unique tectonic terrane. The eastern boundary of Wiggins terrane with Gondwanan crust is delineated by another prominent gravity low. Based on its magnetic expression, Laurentian crust is thought to continue beneath the Coastal Plain sediments until it is truncated by the tectonic suture with Gondwanan-affiliated crust. This truncation is marked by the Brunswick magnetic anomaly (BMA). Sharp magnetic gradients and long-wavelength gravity gradients along faults such as the Towaliga fault, Alexander City fault, and Bartletts Ferry fault suggest these structures are major, crust-penetrating features. Gravity and magnetic modeling reveal the thickening of crust from south to north, and a change in crustal thickness near the suture zone. Results suggest that the crust beneath the Wiggins Arch, in western to southwestern Alabama, is similar to that of Mississippi Gulf coast and most closely resembles a transform margin. Unlike the Texas Gulf coast, however, there is no evidence of a volcanic rifted margin in Alabama. The gravity and magnetic data are consistent with the presence of Mesozoic rift basins.[/expand]
Deblina Bose, M. S. 2010 [expand title=”Details”]
Thesis: Limitations of the Microtremor Method: A Case Study in the Los Angeles Basin, California.
Abstract
Microtremor data from 16 broadband seismic stations across the Los Angeles basin are used to explore limitations of Nakamura’s (1989) method for estimating resonant frequencies, amplification, and ground vulnerability to determine the sources of peaks in horizontal-to-vertical spectra ratios (HVSR) of microtremor data. Spectral peaks observed were analyzed in the context of stratigraphic boundaries inferred from well logs, geologic cross-sections and velocity-depth profiles generated from 3D California velocity model. The temporal stability of observed spectra from each station was tested by comparing time segments from different years. Spectral peaks and amplification values from HVSRs were compared with those from the 2008 Diamond Bar earthquake. The study suggests that microtremor data might be useful for determining resonant periods in sedimentary basins where strata are flat lying; however, a more thorough understanding of the method’s limitations is necessary before it can be used to map amplification factors and ground vulnerability.[/expand]
Gregory Dyer, M.S. 2010 [expand title=”Details”]
Abstract
A new 3D time-dependent pore-pressure diffusion model PFLOW is used to investigate the response of pore fluids to the crustal deformation generated by strong earthquakes in heterogeneous geologic media. Using a carefully calibrated finite fault-rupture model (Ma et al., 2005), the coseismic pore pressure changes and diffusion induced by volumetric strain were calculated for the 1999 Chi-Chi earthquake (Mw = 7.6) in Taiwan. The Chi-Chi earthquake provides a unique opportunity to investigate the spatial and time dependent poroelastic response of near-field rocks and sediments because extensive field data of water level changes and crustal deformation are well documented and readily available. The integrated model provides a means to explore the various mechanisms responsible for hydrologic anomalies observed in Taiwan’s western foothills and the Choshui River alluvial plain. Of special interest is identifying which of the observed hydrologic changes can be explained by a coseismic strain hypothesis and whether the pore-pressure diffusion model can account for observed recovery (dissipation) rates of seismically induced water-level changes in the alluvial fan. Coupled strain-pore pressure modeling results show a strong correlation between areas of coseismic dilatational strain and water-level decline in regions where consolidated rocks are present in the foothills. However, in the Choshui River alluvial fan, water-level rises are observed in regions of dilatational strain, suggesting that other mechanisms, such as seismic shaking, compaction, or faulting-enhanced gravity flow may be responsible for hydrologic changes. Assuming pre-seismic hydraulic conductivity values, our modeling results also show that water-level recovery rates cannot be explained by simple diffusion processes, suggesting that seismic loading may have caused significant re-arrangement and compaction of sediments in the alluvial plain.[/expand]
Kelli Hardesty, M.S. 2008 [expand title=”Details”]
Thesis: Using Microtremors to Assess Site Characteristics in the New Madrid Seismic Zone
Abstract
Results are presented from a study in the New Madrid seismic zone (NMSZ) utilizing the microtremor method. The study analyzes and interprets horizontal to vertical (H/V) spectral ratios to determine resonant periods, wave amplification factors, and liquefaction vulnerability at sites across the Mississippi embayment. Data were collected from areas that experienced earthquake-induced soil liquefaction due to strong ground motion during historic and prehistoric earthquake sequences. Results from 15 sites show resonant fundamental periods of 0.5 s to 4.5 s for embayment thicknesses of ~ 100 m to 900 m and average shear-wave velocity of 800 m/s. These fundamental periods are associated with a strong impedance contrast between embayment sediments and underlying basement rock. The basin configuration and a strong impedance contrast between sediments and basement rocks could contribute significantly to wave amplification at these resonant periods. Other spectral peaks are correlated with major stratigraphic boundaries within basin sediments. Results also indicate higher amplification factors and liquefaction vulnerability at sites located in meandering stream deposits near the basin axis, where embayment thickness is greatest.[/expand]
Sharon Browning, M.S. 2003 [expand title=”Details”]
Thesis: Paleoseismic studies in the New Madrid seismic zone, central United States [/expand]
April Barnes, M.S. 2000 [expand title=”Details”]
Thesis: An interdisciplinary study of earthquake-induced liquefaction features in the New Madrid Seismic Zone, central United States [/expand]
Jonathan Collier, M.S. 1998 [expand title=”Details”]
Thesis: Geophysical investigations of liquefaction features in the New Madrid seismic zone : northeastern Arkansas and southeastern Missouri [/expand]
Todd Grant, M. S. 1998 [expand title=”Details”]
Thesis: Detection of contamination at a closed sanitary landfill using geophysical and hydrogeochemical methods [/expand]
Jochen Flöber(Floesser), M.S. 1996 [expand title=”Details”]
Thesis: Ground water geochemistry of four Upper Cretaceous and Lower Tertiary aquifers in the Gulf Coastal Plain of Southeastern Alabama [/expand]