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Series of Minor Tremors in Heart of Barnett Shale Intrigues Scientists

Portable seismic stations are being set up in two North Texas counties to determine whether natural gas drilling may be causing some minor tremors in the heart of the Barnett Shale natural gas field.

A team from Southern Methodist University (SMU) plans to deploy 10 portable seismic stations in areas of Tarrant and Johnson counties, where a series of minor earthquakes recently have occurred. Larry Standlee, a geologist who teaches at the University of Texas at Arlington's Department of Earth and Environmental Sciences, also plans to conduct computer modeling with recent seismic data.

"Something is going on," Standlee said. "Exactly what, I'm not sure."

Up to four of the seismographs will be set up in Cleburne, TX, at the request of the mayor and city council, which called for a study of the tremors.

Cleburne experienced two earthquakes last Tuesday night, which brings to five the number of tremors that have shaken the town in less than a week. All of the tremors were mild; none registered above a 2.8 on the Richter scale. However, the tremors may be the first ever recorded in Cleburne, which is about 50 miles southwest of Dallas.

Cleburne, in Johnson County, is near the heart of the Barnett Shale play, and since 2001 more than 200 gas wells have been drilled within the city limits. Johnson County has more than 1,000 gas wells.

According to officials, an estimated 18 minor quakes have struck the North Texas region in the past seven months -- and the region is not historically been known as earthquake-prone. Several of the tremors occurred near Halloween, TX, which is south of the Dallas/Fort Worth International Airport along the border of Dallas and Tarrant counties.

After three quakes shook Tarrant County on May 16, SMU officials decided to conduct a study, said Brian Stump, who teaches earth sciences at the university.

The U.S. Geological Survey tracks earthquakes, but its monitoring equipment isn't close enough to North Texas to provide precise depth measurements, according to area officials. More regional data could help determine if there is a link between the gas drilling and the tremors.

"In this part of the world, any time you have a small earthquake swarm, you usually dismiss it as an act of nature," Standlee told the Dallas Morning News. "But whenever you see three earthquake swarms in a fairly small region over a period of less than a year, and you couple that with enhanced drilling in the Barnett Shale and changes in drilling techniques, and you put all these things together, it makes you curious. It's something that ought to be looked into."

However, there's no cause for the public to be overly concerned, Standlee said. Tremors with a magnitude of 3.0 or less occur at least a million times a year, he said.

"These are small events, very small ground motions," Standlee told the News.

Gas drilling itself doesn't cause earthquakes, according to Cliff Frohlich, a senior research scientists with the Institute of Geophysics at the University of Texas at Austin. Frohlich cowrote and published Texas Earthquakes in 2003.

"However, earthquakes in some parts of Texas may be induced by the pumping of fluids at oil and gas fields or by the injection of fluids to dispose of chemical wastes," the book noted. "Moreover, while there are tens of thousands of oil and gas wells in the state of Texas, in only a few fields does evidence indicate that oil and gas pumping induces earthquakes."

A report by the Governor's Division of Emergency Management's Department of Public Safety said oil and gas drilling had been known to cause earthquakes in some parts of Texas. The report was issued in 1998 and revised in 2001 -- as the Barnett Shale was emerging as a huge gas play.

"In northeastern Texas the greatest hazard is from very large earthquakes (magnitude 7 or above), which might occur outside of Texas, particularly in Oklahoma or Missouri-Tennessee," the report noted. "In south-central Texas the hazard is generally low, but residents should be aware that small earthquakes can occur there, including some which are triggered by oil or gas production."

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