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CLEBURNE — After the fourth earthquake in a week rocked Johnson County on Tuesday afternoon, the City Council hastily called a special meeting to consider hiring a geologist to determine what is causing tectonic activity.
Before the meeting could be called to order at 8 p.m., a fifth tremor was felt.
The council voted 4-0 to hire the specialist.
"Our residents are asking questions," Mayor Ted Reynolds said. "They are asking if there are any connections with the [natural gas] drilling. The prudent thing to do is to come up with any answers that we can as quickly as we can."
The day’s first quake, registering a magnitude of 2.6, struck at 5:10 p.m. while the council was in a workshop. Members immediately gave notice of an emergency meeting at 8 after their regular meeting.
Then, at 6:19 came the day’s second quake. That measured 2.1, according to the U.S. Geological Survey.
They were the fourth and fifth tremors — called microquakes by the survey — in the area in a week. The others were a 2.8 on June 2, a 2.6 on Sunday and a 2.3 on Monday west of Cleburne.
The city has not determined how much a geologist might cost or what options the city has if drilling causes the tremors.
"We have more than 200 gas wells permitted in the city," city spokesman Charles Hodges said.
"We will err on the side of caution. We can withstand charges that we overreacted. What we can’t live with is if something bad were to happen, and we stood by and did nothing."
Quake clusters of this kind are not unprecedented, said Dale Grant, a geophysicist with the Geological Survey.
"You just can’t tell with ol’ Mother Earth," Grant said. "Sometimes these will come one right after another and then stop, and you won’t get another one for years."
Chris Hayward, a geophysics research project director, said it will take several weeks for seismic stations to be placed in Johnson County. Hayward said the depth of the quakes will be a central question.
"If the quakes are happening much deeper than the drilling, then we can probably rule it out," Hayward said. "If the quakes are happening at the same depth or a shallower depth than the drilling, then we can’t rule out drilling as a possible cause."
SMU researchers are still studying quakes measured in the Dallas/Fort Worth Airport area this year and haven’t reached any firm conclusions.
“It’s very hard to say what causes any earthquake,” said Cliff Frohlich, associate director and senior research scientist with the Institute for Geophysics at the University of Texas at Austin. “That said, I’ve spent a fair amount of time looking at these events over the past several months. I think most reasonable people with earthquake training probably would say they’re related to the gas production.”
He said there have been about 100 tiny earthquakes picked up since October 2008.
The largest quakes caused by oil and gas production in North Texas have been in Lamar, Gregg, Denton and Cooke counties, among others. The largest earthquake registered 4.2 and struck in April 1934, according to the report.