Houston Built on Shaky Ground

After finding more than 300 surface faults in Harris County, the University of Houston geologists say the region's builders and city planners would do well to pay attention to the new information they have mapped.

"These shifting fault lines originated millions of years ago during the formation of the Gulf of Mexico," said Shuhab Khan, assistant professor of geology at the university.

"While they are not the kinds that wreak havoc in earthquake-prone California and now the Midwest, they can move up to one inch a year, causing serious damage over the course of several years to buildings and streets that straddle a fault line," he said.

Structures on the subsiding side of the fault line could be more susceptible to flooding due to the lower elevation over time, Khan warned.

Using radar-like laser technology - called lidar for light detection and ranging - Khan and geology PhD student Richard Engelkemeir found that the Houston area is riddled with hundreds of faults. Cracks in pavement and structures are already showing in many of these locations.

The scientists started by looking at data compiled during a 2001 study funded by the Federal Emergency Management Administration, FEMA, and the Harris County Flood Control District.

In 2001, Tropical Storm Allison dumped nearly 40 inches of rain on the Houston area over five days, causing nearly two dozen deaths and billions of dollars in property damage.

To update floodplain maps, FEMA and the flood district employed lidar technology to survey the county.

Using lidar technology, laser beams were directed from an aircraft toward the ground. The time between the laser beam pulse and the return reflection from any point on the ground was used to determine the distance between the instrument and that point.

Buildings and vegetation were then removed from the model to produce a map that shows even the most subtle differences in surface elevation.

When Khan and Engelkemeir refined the grids to identify the more than 300 faults, they found that many were associated with the salt domes in the southeast part of the county.

Other faults were found in the northwest part of the county near highways Texas 6 and I-10, where the ground is subsiding, or sinking.

During the summer of 2005, Engelkemeir visited about 50 of the faults located with the lidar data, looking for signs of displacement where the land on one side of the fault was rising higher than the other.

At many of the faults, he saw cracks in street pavements, and learned that neighborhood residents had foundation problems. At one home there was about three feet of displacement between the garage and the house.

At another site, a building had been so damaged by ground shifts that it was condemned.

Geologists are still studying what causes fault movements and the resulting subsidence in the region. Engelkemeir said some scientists believe land-use practices such as groundwater and petroleum withdrawal are responsible for the faults.

"By knowing the location of surface faults, builders and government planners will be able to avoid those areas or accommodate potential ground shifts in their construction plans," Khan said. "And we must still keep in mind that while lidar has allowed us to identify previously unmapped faults, there still might be faults in the region that have yet to be located."

To view a Houston-area map showing active surface faults, click here.

Copyright Environment News Service (ENS) 2008. All rights reserved.