Spatial and Temporal Variability in Vertical Deformation in Willowbend, TX Derived from Long-Term GPS and Extensometer Observations
Burrough, Toby M 1987-
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This study examines the long history of surface deformation observed in the Willowbend neighborhood of Houston, Texas utilizing seven Houston-Galveston Subsidence District GPS stations (2000-2013) and one USGS borehole extensometer (1980-2012). The Willowbend neighborhood is located southwest of downtown Houston, near an area of recent and rapid subsidence (the Jersey Village subsidence bowl). GPS positions are generated in reference to a global reference frame, IGS08, using the GIPSY-OASIS (version 6.2) software package, which employs the Precise Point Positioning with a Single Receiver Phase Ambiguity (PPP-SPRA) method. GPS observations were then transformed from a global reference frame to a local reference frame, the Stable Houston Reference Frame, in order to highlight motion on a local scale. Rapid subsidence events were observed in both the GPS vertical component and extensometer record, in 2005 and 2011. From 2004-2013, GPS and extensometer recorded compaction in the vertical component below one centimeter per year. During 2005 the GPS stations and extensometer recorded vertical compaction between three and four centimeters; for 2011 vertical compaction was up to five centimeters. Both of the rapid vertical deformation events (2005 and 2011) are coincident with droughts that occurred in Texas. Water levels in the area of Willowbend were investigated further. A shallow monitoring well in the vicinity of Willowbend registered a loss in water level for both 2005 and 2011 at a depth of 588-590 meters. Groundwater data were obtained from the USGS Active Groundwater Watch for water wells in the vicinity of Willowbend. Along with observing vertical deformation of the GPS stations, horizontal motion was also analyzed to determine a three dimensional perspective on surface deformation in Willowbend. For 2005 and 2011, no discernible pattern was observed from the GPS data in the horizontal components. An advantage of referencing GPS observations to a local reference frame is the ability to assess whether motion observed is localized versus a regional trend. Current data suggest that surface deformation in the Willowbend area (and to a larger extent, the Houston metropolitan region) is a local, site specific hazard. Varying rates of motion between GPS stations is explained by the complex, heterogeneous nature of the Texas Coastal Plain’s geology. The aquifer system in the Houston area is composed of facies that alternate cyclically from continental sediments to marine sediments. This is important for future regulation to consider when dealing with surface deformation in the Houston area.