Subsidence of South Central North America Posters
Presiding: R K Dokka, Louisiana Spatial Reference Center and Center for GeoInformatics; G Sella, Northwestern University
G43A-01 1330h
Geological implications of recently derived vertical velocities of benchmarks of the south-central United States of America
It has been long-recognized that the south-central United States of America bordering the Gulf of Mexico (GOM) is actively subsiding, resulting in a slow, yet unrelenting inundation of the coast from south Texas to southwestern Alabama. Today's motions are but the latest chapter in the subsidence history of the GOM, a region that has accommodated the deposition of over 20 km of deltaic and continental margin sediments since mid Mesozoic time. Understanding the recent history of displacements and the processes responsible for subsidence are especially critical for near-term planning for coastal protection and restoration activities. Documentation of the true magnitude and geography of vertical motions of the surface through time has been hampered because previous measurement schemes did not employ reference datums of sufficient spatial and temporal precision. This situation has been somewhat improved recently through the recent analysis of National Geodetic Survey (NGS) 1st order leveling data from >2710 benchmarks in the region by Shinkle and Dokka (NOAA Technical Report 50 [2004]). That paper used original observations (not adjusted) and computed displacements and velocities related to NAVD88 for benchmarks visited during various leveling surveys from 1920 through 1995. Several important characteristics were observed and are summarized below. First, the data show that subsidence is not limited to areas of recent sediment accumulation such as the wetland areas of the modern delta (MRD) of the Mississippi River or its upstream alluvial valley (MAV), as supposed by most current syntheses. The entire coastal zone, as well as inland areas several hundred km from the shore, has subsided over the period of measurement. Regionally, vertical velocities range from less than -52 mm/yr in Louisiana to over +15 mm/yr in peripheral areas of eastern Mississippi-Alabama. The mean rate is ~-11 mm/yr in most coastal parishes of Louisiana. In the Mississippi River deltaic plain, subsidence was 2-3 times higher than estimates based on long-term geologic measurements. The data also indicate that adjacent alluvial ridges where the population is concentrated have been similarly affected. In the Chenier plain of southwest Louisiana, a region previously thought to be subsiding at slowly, rates of sinking are similar to those of the deltaic plain. Second, spatial patterns suggest that motions at most locations may have both long (10-100 km) and short (<5 km) wavelength components. Gross aspects of some long wavelength motions can be explained by flexure produced by late Quaternary sediment loads such as the MRD and the MAV. Short wavelength spikes in motions correlate well with areas of fluid withdrawal, faults, and salt structures. Third, motions at many benchmarks have not been linear through time. For example, subsidence in ~10-30 km wide zones surrounding some active normal faults of south Louisiana declined as faulting has slowed (and vice versa). Subsidence in these areas reached a peak in 1970 and declined thereafter. Some local changes also correlate with changes in human-related activities (e.g., reduced groundwater pumping and slower subsidence in the Lake Charles area beginning in the late 1980s).
G43A-02 1330h
Determination of Recent Fault Activity in Southwestern Louisiana from Geodetic Leveling and LIDAR Data
An analysis of geodetic leveling data and LIDAR digital elevation models in southwestern Louisiana has led to the identification of several surface scarps in the area that exhibit measurable differential movement over the past half-century. Computation of relative benchmark velocities has shown that some of these faults had slip rates as much as 6 mm/yr during the 1960's and 70's. However, leveling data obtained as recently as January 2005 has revealed that these faults are currently moving at much slower rates. This study identifies the presence of several previously mapped fault-line scarps in addition to some unidentified surface features that exhibit similar characteristics. High-resolution mapping with LIDAR data has provided evidence of numerous linear topographic features that clearly offset and truncate alluvial ridges and terraces in the study area. Also, field observations have led to the discovery of significant damage to built structures due to differential movement across these scarps. These lines of evidence, combined with the leveling data, suggest that these faults are currently active and have moved episodically over the past fifty years on time scales of months to years. Previously mapped faults in this region of the state have been known to exhibit episodic motion since the late Tertiary and the driving force behind this movement is thought to be the enormous load of sediments along the Gulf Coast. This mechanism for fault movement acts on much longer time-scales, however, and an alternative source for the shorter wavelength fault activity is proposed. A possible cause for the accelerated slip rates that were calculated during the 60's and 70's could be the large amounts of fluid withdrawal that occurred in the southwestern part of the state during that time. From 1965 to 1980, groundwater withdrawals for the study area were the highest recorded in the state and petroleum production also peaked during that period. A reduction in pore pressure due to fluid withdrawal results in an increase in the effective stress in the subsurface and compaction of the underlying sediments. Differential compaction of sediments adjacent to faults accelerates fault activity and this motion is detected by the leveling data. Similar faults in the nearby Houston area also exhibit this spatial and temporal relationship of increased slip rates coinciding with increased subsurface fluid withdrawal. This suggests that in addition to the natural forces driving fault motion, anthropogenic activities may have also affected faulting in this area of Louisiana.
G43A-03 INVITED 1330h
Present-day deformation along the Northern Gulf of Mexico as observed by continuous and episodic GPS observation
Characterizing the northern Gulf Coast as a passive plate margin based on the lack of seismicity, low relief, and geological history does not reflect all the processes that are at work. Additional processes include: gravitational loading and consequent flexure from the Mississippi deltas (and earlier rivers), compaction of thick Quaternary sediments, growth normal faulting, salt migration, and local fluid withdrawal. The net effect of these processes results in significant horizontal and vertical deformation. This deformation contributes to coastal land loss that reaches maximum rates in Louisiana of 35-40 square kilometers a year. To ascertain present day rates of horizontal and vertical motion along the northern Gulf of Mexico we have analyzed GPS data from both our own GULFNET network consisting of 18 continuously operating stations (CGPS) and 25 episodic stations that have been monitored every two-three years. In addition we have also included over 20 NGS CGPS sites. Results from the continuous GPS sites provide limits on southward directed horizontal displacement with respect to the stable interior of North America, with maximum rates of 5 mm/yr. Subsidence estimates from CGPS generally are up to 15mm/yr
G43A-04 1330h
Observation of Subsidence in New Orleans Using Permanent Scatterers
In Louisiana, areas along the coast are sinking as much as one inch a year, and the state loses approximately 35 square miles per year due to ground subsidence and coastal erosion. Subsidence in the region has leads to loss of coastal wetlands and increased vulnerability to flooding and coastal storm inundation. Some of the technology for monitoring statewide elevation change is already in place. The Louisiana Spatial Reference Center (LSRC) is capable of measuring land movement as small as a few millimeters per year using Global Positioning System satellites. However, the lack of precise and high resolution quantitative spatial data on subsidence has limited development of a satisfactory theory to explain all aspects of modern surface motions. In this study, we apply the method of permanent scatterer SAR interferometry (PSInSAR) to detect and quantitatively measure land subsidence in New Orleans. PSInSAR is a fully operational tool for millimeter-scale accuracy ground deformation mapping on a dense grid of persistent targets. High accuracy and spatially dense subsidence data lead to enhanced understanding via mechanical modeling. Our results will be tested against GPS and another InSAR result that may be derived from RADARSAT-1.
G43A-05 1330h
The Role of Hydrocarbon Production on Subsidence and Fault Slip in the Louisiana Coastal Zone
Pore pressure reductions and the associated stress changes due to subsurface hydrocarbon production can lead to reservoir compaction and fault reactivation. These deformations may result in a significant vertical elevation drop on the land surface. When elevation change occurs in a sensitive environment, such as a coastal wetland, the impact of hydrocarbon production cannot be ignored. Using a simple analytical solution and a numerical model, the impact of reservoir compaction and fault slip on surface subsidence is investigated. The Lapeyrouse field in southern Louisiana is chosen as the study site due to its relatively complete pressure data and structural maps. A releveling survey, showing elevation changes for a 30-year period transects the field, is used as a quantitative comparison between the model predictions and observed subsidence. Despite using stress and rock mechanics data from offshore oil fields to compensate for the absence of such data in Lapeyrouse, the degree and extent of subsidence estimated from the analytical method generally agrees with the order of magnitude of elevation drop measured from the releveling survey. Using different constitutive rheological laws to describe the producing sand formation changes the predicted magnitude of subsidence as expected. While the general trend and magnitude of subsidence predicted generally matches the observed there is one notable exception at the northern end of the releveling line adjacent to the Golden Meadow Fault. Numerical models allow for investigation of the impact of compaction-induced slip along the Golden Meadow Fault, located north of the Lapeyrouse field, on surface subsidence. When incorporating the Golden Meadow Fault, the subsidence pattern changes when the fault is allowed to slip freely as a response to deformation associated with reservoir compaction. The magnitude and location of slip along the Golden Meadow Fault are estimated due to reservoir compaction in the Lapeyrouse field. Subsidence predicted through analytical and numerical methods in this study do not take into account contributions of natural subsidence and other human activities, however, the magnitude of induced subsidence and fault slip suggest that hydrocarbon production can have an impact on land subsidence on a local scale. We suggest that production-induced land subsidence is one of the many mechanisms that should not be ignored when evaluating subsidence and land loss in the Louisiana Coastal Zone.
G43A-06 1330h
Estimates of historic, present, and future rates of surface displacement due to hydrodynamic autocompaction of Holocene sediments in the Louisiana delta plain
Subsidence and resulting rapid relative sea level rise and wetland loss in Louisiana have been attributed to natural geologic (isostatic sediment loading, faulting, sediment compaction, dewatering) and human (subsurface fluid withdrawal, salt mining) processes, but we lack quantitative estimates of the relative contributions of each factor, and their role in future subsidence is largely unknown. Such information is critical for ongoing regional-scale wetland restoration plans. We attempt to isolate the component of subsidence attributable to compaction (gravity-driven reduction in porosity and resulting surface displacement) of sediments deposited above the Lower Wisconsin unconformity (<~18 k.y.) in the delta region. One-dimensional, multi-lithology compaction models based on Darcy flow and Terzaghi effective stress principles are solved using a finite difference technique, allowing calculations of cumulative sedimentation and surface displacement, as well as historic, present, and future (in the absence of further loading) rates of vertical surface displacement. Detailed modeling of the USGS/LGS P-1-90 boring (47 m recovered, 13 radiocarbon dates), at the LUMCON facility in Cocodrie, revealed relationships among sediment type, geotechnical parameters, depositional rate, time, and compaction rate. Results indicate that, regardless of variations in depositional history, present rates of surface displacement can be constrained by two end-member stratigraphic models: 100% sand and 100% mud. Shallow (0-200 m) borehole data from the U.S. Army Corps of Engineers have been used to interpolate a regional isopach of Holocene sediments for the Mississippi River alluvial valley and adjacent delta plain and to extend lithologic generalizations (topstratum/substratum) and suitable geotechnical parameters from the USGS boring. Regional-scale analysis allows comprehensive testing of the hypothesis that higher subsidence rates in the alluvial valley can be attributed to compaction of thicker Holocene deposits.
G43A-07 1330h
Using Holocene Relative Sea-Level Data for High-Precision Measurement of Differential Crustal Movements in the Mississippi Delta
Due to the immense concern about wetland loss and coastal erosion in southern Louisiana, there is vigorous debate about the driving mechanisms that cause this environmental catastrophe. One frequently invoked component is tectonic subsidence of the Mississippi Delta and its surroundings as a consequence of lithospheric flexure due to ongoing sediment loading by the deltaic depocenter. We have collected relative sea-level data covering the past 8500 years from three study areas in different sections of the Mississippi Delta, to assess whether significant differential crustal movements occur. Our sea-level index points were obtained from basal peat that accumulated during the initial transgression of the pre-existing, consolidated Pleistocene basement, thus ruling out the role of compaction of Holocene strata. The study areas differ in their distance to the present shoreline; in addition, a presumed major growth-fault system may be located between two of them. The rationale of our analysis is that given spatially uniform eustatic and glacio-hydro-isostatic signals, any difference between relative sea-level curves from the three study areas can be attributed to differential tectonic subsidence rates. The extremely favorable conditions for sea-level research on the US Gulf Coast (largely due to the low tidal range) and the long time span of observation allow us to calculate tectonic movements with exceptionally high accuracy and precision. Our results show that differential crustal movements among the three study areas have been on the order of ~0.1 mm/yr, values that are statistically indistinguishable. We compare our new evidence with a recently published compilation of relative sea-level data from the Caribbean, to a large extent based on data from areas that are widely believed to be tectonically very stable (e.g., Florida, Bahamas, Belize). All our sea-level index points nearly coincide with the Caribbean data, showing that considerable parts of the Mississippi Delta may be surprisingly tectonically stable. Thus, we suggest that the rapid rates of coastal-wetland loss are largely due to a combination of compaction of the thick Holocene strata, as well as human action, like the extraction of oil, gas, and groundwater.
G43A-08 1330h
GPS geodetic measurements of subsidence from groundwater drawdown in central Arkansas
The state of Arkansas ranks fifth in the nation in ground water consumption, largely fueled by its robust rice and manufacturing industries. The region centered on Lonoke, Arkansas, Jefferson and Monroe counties in the center of the state has seen dramatic drawdown of its ground-water resources and development of significant cones of depression. Subsidence of 1.5 meters over a few decades was postulated for portions of Arkansas county on the basis of elevation changes of benchmarks. These results were preliminary and measurements were not made with geodetic grade equipment. Validation of these results is the primary goal of our on-going work. We are developing an extensive network of suitable benchmarks throughout central and eastern Arkansas, whose vertical position can be monitored using high-precision Global Positioning System geodesy. Campaign GPS measurements were first obtained in summer 2003. Second epoch observations were acquired throughout 2004. Although errors are high due to the limited temporal dataset, preliminary GPS geodetic results are consistent with subsidence of the ground surface above the cones of depression on the order of tens of millimeters per year.
G43A-09 1330h
Quantifying the Uncertainty in Land Loss Estimates in Coastal Louisiana
For the past twenty-five years the land loss along the Louisiana Coast has been recognized as a growing problem. One of the clearest indicators of this land loss is that in 2000 smooth cord grass (spartina alterniflora) was turning brown well before its normal hibernation period. In 2001 data were collected using low altitude helicopter based transects of the coast, with 8,400 data points being collected. The surveys contained data describing the characteristics of the marsh, including; latitude, longitude, marsh condition, marsh color, percent vegetated, and marsh die-back. The 2001 data were compared with previously collected data from 1997. Over 100,000 acres of marsh were affected by the 2000 browning. Satellite imagery can be used to monitor changes in coastlines, vegetation health, and conversion of land to open water. An unsupervised classification was applied to 1997 Landsat TM imagery from the Louisiana coast. Based on the classification, polygons were delineated surrounding areas of water. Using the Kappa Classification Statistical Analysis extension in ArcView, kappa statistics were calculated to quantify the amount of agreement between the unsupervised classification and field checked data while correcting for agreement due to chance. Numerical results reveal that a straightforward unsupervised classification does a reasonable job of approximating the actual field checked data. Kappa values of 0.57 and higher have been obtained, which is considered fair to good agreement. This agreement adds credibility to imagery based estimates of coastal land loss, which affords the opportunity for significant savings of time, labor, and cost compared to field based monitoring. Refined classifications and use of higher resolution imagery are expected to yield improved costal land loss estimates.