GPS Observations of Subsidence in South Louisiana over Seasonal and Sub-seasonal Timescales Associated with Hydrologic Loading

Abstract

Louisiana’s primary environmental concern is arguably coastal wetland loss due to high subsidence rates, previously estimated to range from 1-14 mm/yr across much of the coast. Spatially and temporally varying strong short-term and seasonal components of deformation associated with changes in hydrologic loading throughout the year can give information about the elastic properties of the shallow crust, having implications for better understanding long-term subsidence trends across the region. Therefore, we quantify and characterize vertical deformation that occurs on annual to sub-annual timescales across southeast Louisiana. Vertical deformation is evaluated using publicly available daily GPS positions from 2016–2020 at 16 stations across southeast coastal Louisiana. Calculated subsidence rates from 2016-2020 range from -8.1-2.6 ± 0.2 mm/yr, with rate increases towards the coast, and seasonal amplitudes of approximately 20 ± 1.5 mm each year, where the calculated amplitudes are fairly stable over time and across the region. Hydrologic load is inferred from publicly available information including daily CRMS marsh water level observations, USGS Mississippi River stream gauge height, and NOAA ocean water level from tide gauges. Continental water storage and ocean loading is considered using monthly mascons from GRACE and GRACE-FO. Correlation coefficients demonstrate that some GPS stations are correlated to tide or stream gauge data, but are unable to distinguish or quantify contributions from multiple loading sources. To account for this, we perform reconstruction independent component analysis (rICA) on the GPS data across southeast Louisiana, which seeks to identify and measure statistically independent contributions to the network-wide signals. Results indicate that rICA is able to detect the independent contributions from hydrologic loads from both the main Mississippi River channel and the Gulf of Mexico, with correlation coefficients of 0.43 and 0.59 respectively. Overall results indicate that coastal Louisiana does not act uniformly as multiple hydrologic loads and near-surface processes from distinct basins influence the complicated vertical motion observed across the region. These observations will provide the basis for future modeling of elastic crustal loading response across the region, and will aid researchers in extending our understanding of how short-term seasonal signals affect subsidence rates in coastal Louisiana.