© Robert Nickelsberg/Getty ImagesFive small earthquakes this month have become the talk of Cleburne, Texas, where a natural gas boom has brought drilling closer to residential areas
This small city at the epicenter of the region's natural-gas boom has been shaken by another arrival from underground: earthquakes.
Five small temblors this month have some people pointing the finger at technology that drilling companies use to reach deep into the earth to shatter rock and release new stores of natural gas -- the same technology that has made many of the locals rich.
Thousands of wells have been drilled in the past five years. Now, a wave of small earthquakes is leading some residents in the north Texas town to link the two developments and some seismic experts to wonder about the cause.
The industry says there isn't any evidence linking the quakes to gas production. Even geophysicists, who take the residents' concerns seriously and are deploying seismic sensors in the area, say they can't prove a connection between the drilling and the quakes.
On Halloween, eight temblors struck the Dallas-Fort Worth area. Another cluster of quakes hit the area in mid-May. In all, more earthquakes have been detected in the area since October than in the previous 30 years combined, according to the U.S. Geological Survey, although survey geologist Russ Wheeler said there could have been small quakes that weren't detected. The USGS monitors seismic activity with sensors across the country, but the }devices are most heavily concentrated in quake-prone areas.
© U.S. Geological SurveyThe U.S. Geological Survey has detected more earthquakes in the Dallas Fort Worth area since 2008 than in prior decades.
"We're not used to worrying about this kind of thing," said Cleburne resident Ben Oefinger, a retired school principal who felt the first quake June 2 while lying on his couch following an afternoon of yard work. "Nothing ever happens in Cleburne. That's why people live here."
Prior to that week, Cleburne, about 30 miles south of Fort Worth with a population of about 30,000, hadn't registered a single earthquake in its 142-year history.
After a fourth and fifth quake struck within 90 minutes Tuesday evening, registering 2.4 and 2.1, the city council held an emergency session and voted to a hire a geophysicist to investigate. City Manager Chester Nolan ordered his staff to start drawing up emergency plans in case a more serious quake strikes.
"After the fourth one, there wasn't any doubt we needed to do something," Mr. Nolan said.
Earthquake chatter has dominated conversation in recent days at the Chaf In, a local coffee shop where some patrons joked that the earthquakes are God's retribution for the town's recent reversal of its 106-year-old ban on liquor sales.
The drilling boom has minted dozens of new millionaires in the area. Cleburne has received more than $25 million for allowing drilling on municipal land. The money has helped pay for a new golf course and a revamped civic center.
"They've been such a boon to the community," Mr. Nolan said.
More than 200 wells have been drilled within Cleburne's 30-square-mile border in recent years, and hundreds more have been drilled in surrounding Johnson County. Virtually all the wells have undergone hydraulic fracturing -- or "fracking," for short -- that injects them with millions of gallons of a high-pressure water mix to crack open gas-bearing rock. Wells are generally drilled to depths of about 5,000-7,500 feet.
"If it's not that, it just seems like the biggest coincidence in history," Patty Hughes said over lunch with friends at the Chaf In.
Oil and gas production has been suspected of causing earthquakes in the past, including in Texas, particularly when it involves injecting fluids into the ground.
"As a scientist, I can't prove that they were related to the gas production ... but I think most reasonable people would conclude they were related," said Cliff Frohlich, a University of Texas geophysicist and the co-author of a 2003 book on Texas earthquakes. In his book, Mr. Frohlich concluded that 22 of the 130 Texas earthquakes he studied were "probably" caused by oil and gas production or other human activity.
Others, such as Brian Stump, a geophysicist at Southern Methodist University in Dallas, said he isn't ready to jump to that conclusion. "I don't think we have enough information," he said.
Jeff Eshelman, a spokesman for the Independent Petroleum Association of America, said he hasn't seen "any overwhelming, scientific evidence other than anecdotes to suggest a connection between energy exploration and major seismic events."
Texas isn't regarded as a high-risk earthquake area. The state's largest quake struck the town of Valentine in 1931, destroying a schoolhouse and rotating tombstones at the local cemetery. The 5.8 magnitude quake ranked as "severe" but not "violent," in the U.S.G.S.'s intensity rankings.
Still, the sudden spate of activity in Texas has made scientists take notice. Seismic sensors deployed in the area in November by Prof. Stump and his colleagues picked up several dozen low-level quakes within a few weeks. Now, they are deploying 10 more sensors to monitor the area the rest of the year.
The quake concerns come at a sensitive time for the industry, which is battling proposed legislation in Congress that would more heavily regulate hydraulic fracturing. Some environmental groups are concerned that the fluids used to fracture wells -- mostly water, but also chemicals such as hydrochloric acid and guar gum -- could contaminate drinking water. The industry says the process is safe.
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