Abstract

The coastal region of the northwestern Gulf of Mexico remains tectonically active. In the Houston area alone there are over 300 active surface faults. Figure 1 shows a map of the Houston area which includes major faults and salt domes. The faults in northwest Houston are regional faults while the faults in southeast Houston are associated with salt domes (Verbeek and Clanton, 1978). The faults cause damage to man-made structures such as roads, pipelines and buildings. We used LIDAR DEM (Light Detection And Ranging Digital Elevation Model) images from the 2002 Tropical Storm Allison Recovery Project (TSARP) to examine known faults and to search for others that may have been overlooked in previous studies. Our work focused on the faults in northwest Houston.

Figure 2 is a LIDAR DEM for a portion of northwest Houston. In this location the elevation difference on the Longpoint Fault is distinct. We used hill-shading as the primary visualization method for locating the faults. Figure 3 illustrates the elevation aspects of the interpretation process. The scarp is evident in the photo. Figure 4 illustrates the value of using the hill-shaded DEMs for interpretation. The fault is clear in Figure 4A, while not detectable in the aerial photo in Figure 4B. Later we examined the faults in the field (e.g., Fig. 3C). A field visit helps explain the kink of the fault in Figure 5A. The east-west portion of the fault runs along the property line indicating the lot to the south was excavated to a constant elevation. Figure 5B is a field photo of the fault ramp at this location. At some locations fault deformation and associated damage were evident, while in other locations field expression of the fault was subtle and the presence of a fault was difficult to confirm. In some areas we used refined grids, using both raw data and supplied DEM data, to define known faults better and to identify previously unknown faults. Figure 6A shows a refined DEM grid which was hill-shaded and interpreted. The fault splits into several branches in this area. Figure 6B shows two of the fault ramps.

Study of these faults may also help in understanding the underlying tectonics. Figure 7 shows our model for the relationship between faults and salt (based upon Jackson et al., 2003). The regional faults are tied to the withdrawal of salt. Further work is needed to confirm this model.

Proper documentation of active surface faults is important so that developers can avoid building in the zone of disturbed ground along them. In some cases developers and builders have taken steps to avoid construction on fault traces, often by leaving the land as an open greenbelt or as a storm water detention pond. In other cases structures have been built unknowingly within fault zones.