Late 20th and early 21st century deep-seated vertical motions of New Orleans and implications for hydrologic and tectonic influences on causationDokka, Roy K.1; Dixon, Timothy H.2; Ivins, Erik R. and Blom, Ronald G.3 Land- and space-based geodetic data and water level measurements in the New Orleans-Lake Pontchartrain area were analyzed to infer vertical motions and to test models of causation. Measurements from water gauges attached to bridge foundations, InSAR targets on homes resting on piles, and benchmarks affixed to deep rods that avoid the effects of shallow processes such as compaction and consolidation of Holocene sediments yield consistent results after gauge adjustments and datum reconciliation. Results show that the entire sampling area subsided during the late 20th century, with the maxima of deep subsidence occurring in the urbanized and industrialized sections of eastern New Orleans. Subsidence due to processes originating below the Holocene section locally exceeded 0.9 m between 1955 and 1995. Relative sea-level rise during the same time interval was not uniform, varying between ~0.25 and ~1.2 m. Subsidence decreased away from urbanized areas and north of the belt of active basin margin normal faults; this decrease in subsidence continued to the north and east along the Mississippi coast. These independent, yet consistent measures of vertical motions provide insights into the complexity and causes of modern landscape change in the region. First, the magnitude and horizontal extent of deep subsidence are much greater than can be explained by models that consider modern subsidence to be mainly due to compaction and consolidation of Holocene sediments. Previous models are extrapolations of middle and late Holocene sea-level measurements that lack sufficient horizontal and vertical sampling, spatial and temporal precision, and accuracy to capture the variability late 20th century vertical motions. Thus, processes operating beyond the footprint of the Mississippi River delta (MRD) such as lithospheric loading and relatively short-lived drivers of subsidence such as groundwater withdrawal and active faulting are missed. Second, deep subsidence occurring east and north of the basin margin faults can be explained by regional tectonic loading of the lithosphere by the modern Mississippi River delta and local groundwater withdrawal. Third, sharp, local changes in subsidence coincide with strands of the basin margin normal fault system. Displacements are consistent with modern activity and show motion behavior consistent with ancient growth faults. Fourth, deep subsidence of the region to the south, including New Orleans, can be explained by a combination of groundwater withdrawal from shallow upper Pleistocene aquifers, the aforementioned lithospheric loading, and non-groundwater-related faulting. Near Michoud, substantial local subsidence also originates at depths >1.8 km below producing aquifers, suggesting that active regional faulting occurs within the MRD. Subsidence due to groundwater extraction from aquifers ~160 to 200 m deep dominated the urbanized areas from ~1960 to the early 1990s and is likely responsible for lowering flood protection structures and bridges in the area by as much as ~0.8 m. The slowing of subsidence from the early 1990s to the present in all areas, except Michoud, was coincident with the stabilization of groundwater level in a regional reference well. This suggests that lithospheric loading may be the dominant current process causing deep subsidence. |