Coastal Subsidence Due to Faults: Insights from Elevation Profiles of Vehicular Bridges, Southeastern Louisiana, U.S.A.

Abstract

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 6 0.06 mm/y to 3.2 6 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.

Does Load‐Induced Shallow Subsidence Inhibit Delta Growth?

“…Similarly, the subsidence rate of the Larose transect located seaward of the Lake Hatch Fault (−1.31 ± 0.41 mm/yr) is lower than that of the Raceland transect located landward of this fault (−1.71 ± 0.49 mm/yr). Despite uncertainty in the exact surface expression of faults in our study area, the lack of a seaward increase of subsidence suggests that faulting is producing at most only limited, local effects in the Mississippi Delta (e.g., Hopkins et al, 2021) and is not driving broad-scale subsidence in this part of the delta, consistent with the slow long-term rates of fault slip inferred by Frederick et al (2019).…”

Variations in Subsidence Patterns in the Gulf of Mexico Passive Margin From Airborne‐LiDAR Data and Time Series InSAR: Baton Rouge Case Study

“…The faults cut compacted Pleistocene strata reducing the number of variables contributing to ground subsidence. The study of Hopkins et al (2021) indicates that fault slip rates are within the range of detection of both LiDAR and InSAR.…”

“…Fault motion and subsidence near the Golden Meadow fault zone (GMFZ) have been related to hydrocarbon extraction (e.g., Chan & Zoback, 2007;Morton et al, 2002). There is visual evidence of building and road displacements along the BRF and other coastal faults with measured rate estimates of ∼3 mm/y (Hopkins et al, 2021;McCulloh, 2001). The BRF and DSF reactivated during the Pleistocene due to depositional loading (McCulloh & Heinrich, 2013).…”

“…The Baton Rouge region east of the Mississippi River in southern Louisiana is crossed by several E-W striking growth faults that have topographic relief of ∼5 m, with Quaternary slip rates of <1 mm/y (Shen et al, 2017) (Figures 1-3). Hopkins et al (2021) measured deformation of man-made structures of known age in the Baton Rouge-Lake Pontchartrain region to estimate fault slip rates of ∼3 mm/y. These rates are 10 times faster than time-averaged rates from Shen et al (2017) (Figure 1).…”

Novel Integration of Geodetic and Geologic Methods for High‐Resolution Monitoring of Subsidence in the Mississippi Delta

“…Faulting is another plausible driver of deep subsidence. For example, recent fault activity 50–80 km north of the superstation has been observed (Hopkins et al., 2022). However, studies of fault slip rates in coastal Louisiana over geologic time scales generally show low rates, typically <0.1 mm/yr (e.g., Frederick et al., 2019; Shen et al., 2017).…”