ABSTRACT

Hopkins, M.; Lopez, J., and Songy, A. . Coastal subsidence due to faults: Insights from elevation profiles of vehicular bridges, southeastern Louisiana, U.S.A.

Normal faults are common geological structures in the Mississippi River Delta within the underlying thick Cenozoic sedimentary section. Some faults have been identified as active, have deformed artificial features in the past century, and are concurrent with Louisiana's massive loss of coastal wetlands. Subsidence due to vertical motion, or slip, on faults, however, is poorly understood spatially and over decadal to centennial timescales. Therefore, how faults contribute to subsidence and subsequent land loss over these timescales must be considered for coastal wetland restoration efforts. Here, the elevation profiles of seven vehicular bridges that cross five known faults were analyzed to investigate the spatial and temporal behavior of the faults. Bridge elevation profiles are an effective method to analyze these faults over the timescale of interest for several reasons: (1) the bridges are of known age (10–89 y), (2) they were initially constructed horizontally, and (3) they are constructed on a foundation anchored below compacting Holocene sediments. Faults were projected to the surface using subsurface data such as seismic reflection data and well logs, and abrupt vertical offsets in the elevation of the bridges near a known fault are the result of slip. Four of the five faults examined show an offset in the bridges that cross them, and the offsets are largest at the traces. Offset rates, which are interpreted to be fault-slip rates, were calculated utilizing bridge age and the total offset amounts at the traces. Slip rates range from 0.6 ± 0.06 mm/y to 3.2 ± 0.2 mm/y. Restoration project viability may face significant risks from fault-related subsidence. The biggest threat appears to be limited spatially to within hundreds of meters of the faults, however, that risk cannot be totally precluded at greater distances from the faults.

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