MONITORING SUBSIDENCE IN FORT BEND COUNTY, TEXAS USING GPS

The Houston metropolitan area is the sixth largest in America, with a population of nearly 6 million people, according to the 2010 U.S. census. Fort Bend county is the southwest margin of this area, which has been experiencing rapid development since the 70’s. A shift in land-use from agricultural purposes, such as rice paddies and cattle fields, to subdivisions and commercial buildings has lead to an increased dependence on groundwater.

Subsidence of the land surface in the greater Houston metropolitan area has mainly been attributed to aquifer compaction related to groundwater withdrawal. As pore pressure decreases, unconsolidated sediments will compact. This compaction is irreversible in the silt and clay rich lenses, but rebound can occur in the sandy units. Depressurizing has also been suggested as a mechanism for reactivation of growth faults in the area. Problems are mainly concentrated around the urban center where demand for water is high and the risk of damage due to flooding and active faulting is greater. The Texas legislature created the Fort Bend Subsidence District (FBSD) in 1989 to regulate groundwater withdrawal and mitigate related flooding.

New geodetic data from the FBSDs GPS network offer insight on the spatial patterns and magnitude of subsidence through 2012. Specifically, GPS data shows that rates decrease away from the metropolitan areas, while areal extent has increased with time. There has been a steady rate of compaction at about 2 cm yr^-1 near Sugarland since 1994, and 0.2 cm yr^-1 near Needville. This data will be used to examine the relationship between the activity and processes of growth faults, the role of salt domes, and the possible role of aquifer depletion and petroleum extraction in sediment compaction and the reactivation of listric faults in the area.