Abstract
The Long Point Fault is one of the most active urban faults in Houston, Texas, which belong to a complex system of normal growth faults along the Texas Gulf Coast. To assess the activity of the Long Point Fault, a GPS array with 12 permanent stations was installed along the two sides of the 16-km-long fault scarp in 2013. GPS datasets were processed with the Precise Point Positioning (PPP) and Double-Difference (DD) methods. The daily PPP solutions with respect to the International Global Navigation Satellite System (GNSS) Reference Frame 2014 (IGS14) were converted to the Stable Houston Reference Frame (Houston16). The six-year continuous GPS observations indicate that the Long Point Fault is currently inactive, with the rates of down-dip-slip and along-strike-slip being below 1 mm/year. The Long Point Fault area is experiencing moderate subsidence varying from 5 to 11 mm/year and a coherent horizontal movement towards the northwest at a rate of approximately 2 to 4 mm/year. The horizontal movement is induced by the subsidence bowl that has been developing since the 1980s in the Jersey Village area. Current surficial damages in the Long Point Fault area are more likely caused by ongoing uneven subsidence and its induced horizontal strains, as well as the significant seasonal ground deformation, rather than deep-seated or tectonic-controlled fault movements. The results from this study suggest a cause-and-effect relationship between groundwater withdrawals and local faulting, which is pertinent to plans for future urban development, use of groundwater resources, and minimization of urban geological hazards.
Highlights
The Houston metropolitan area, more broadly the Gulf Coast region, has numerous gravitationally induced “down-to-the-coast” faults that represent slow sliding of the land mass towards the Gulf of Mexico
The scatter of the DD solutions is smaller than the scatter of the Precise Point Positioning (PPP) solutions at all stations, which verified that the DD method achieved a higher accuracy than the PPP method for the case study
According to our previous study, the subsidence will continue until the Evangeline groundwater level approaches the regional preconsolidation head, which is approximately 40 meters below the ground surface in the Houston area [45]
Summary
The Houston metropolitan area, more broadly the Gulf Coast region, has numerous gravitationally induced “down-to-the-coast” faults that represent slow sliding of the land mass towards the Gulf of Mexico. A stable local scale reference frame is needed to precisely trace the movements of permanent GPS stations over time and space [36,37,38]. Houston was realized by over 7 years of continuous observations from 15 permanent GPS stations outside the Houston metropolitan area Those reference stations were not affected by subsidence and faulting problems. The scatter of the DD solutions is smaller than the scatter of the PPP solutions at all stations, which verified that the DD method achieved a higher accuracy (repeatability) than the PPP method for the case study For this reason, the DD method is employed to delineate the relative displacements within the Long Point Fault GPS Array. The detailed process for calculating 24-hour average displacements using the DD method is addressed in Wang [22]
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