Subsidence of the US Gulf Coast and crustal loading, geophysics, and geodesy
|Location||International Geological Congress,oslo 2008|
|Author||Dokka, Roy۱; Ivins, Erik۲; Blom, Ronald۲|
|Holding Date||07 October 2008|
Hurricanes Katrina and Rita focused attention on the vulnerability of the U.S. Gulf Coast. Several processes have been documented to contribute to this vulnerability, including wetland loss due to lack of present day sediment flux, land subsidence due to sediment compaction and oxidation, fluid withdrawal, salt evacuation, tectonics, and also crustal loading. Relative contributions of each, and their net subsidence effects, are poorly constrained and controversial. Meanwhile, it is urgent to provide policy makers and the civil engineering community with accurate estimates of subsidence rates to protect lives and infrastructure.
One of the least studied subsidence driving phenomena is the effect of crustal loading due to Mississippi River sediments deposited during deglaciation, and also the geologically recent ~130 m rise in sea level. Crustal response to these loads continues today. We model subsidence rates expected from sediment and sea-level rise loads using methods developed for, and validated by, research on post-glacial rebound. We use geodetic data from recently validated traditional surveying and continuously operating GPS stations to constrain the geophysical modeling. We consider time-varying Pleistocene and Holocene sediment loading of the Gulf of Mexico which forces the solid earth into isostatic disequilibrium. Viscoelastic and gravitationally reasonable values are here considered with a detailed loading history to model and predict subsidence due to ocean and sediment loading. We do not consider other factors in this analysis. Holocene depositions in the delta and shore-zone of the Mississippi River, and upper mantle viscosities, are key ingredients for modeling a load response that broadly corresponds to geodetically controlled vertical motions. Upper mantle viscosities inferred from recent studies of Laurentide glacial isostatic adjustment are consistent with the viscoelastic ’memory’ required for this unique sensitivity to Holocene and post-glacial transition era sediment loading. We can vary crustal properties and compare and contrast model predictions with present day geodetic observations to refine the geophysical modeling.
Our model-predicted, and geodetically-observed, vertical subsidence rates vary between 2 - 8 mm per year over areas of 30,000 to 750 square kilometers, respectively. This viscoelatistic flexure is the background crustal deformation field, upon which larger amplitude, but smaller spatial scale, subsidence occurs due to the other factors mentioned. These results are integrated into GIS data bases for the region from which one can assess impact of predicted subsidence. Additionally, current sea-level rise rate must also be considered by policy makers and the civil engineering community.