AZLE, Texas — For 48 days, the ground shook like clockwork.
When the first tremor hit the evening of Nov. 5, 65-year-old Jerry Parker thought it was someone banging on the door. No one in Azle could remember ever feeling earthquakes. But by Dec. 23, when a 3.3-magnitude tremor shook the Christmas lights dotting the surrounding countryside, the count was up to 30.
The mysterious series of small earthquakes in the Barnett Shale, one of the largest natural gas fields in the country, has mirrored events in other drilling areas including Arkansas and Oklahoma. As scientists look for answers, the farming community around Azle has quickly found itself at the center of a national debate over the dangers of disposing of drilling wastewater thousands of feet underground.
The Texas Railroad Commission, which oversees the state's oil and gas industries, is in discussions with geologists at the University of Texas to study the phenomena. And last week the commission announced it was hiring its own in-house seismologist after a meeting in Azle drew hundreds of residents to the high school auditorium.
There have been no earthquakes reported by the U.S. Geological Survey since Dec. 23. But Julie Hutcheson, a local hairdresser whose home lies close to the epicenter of a number of the recent earthquakes, said concern persisted about whether they might return, and stronger than before.
"Everybody's talking about it," she said. "I had a guy from Devon Energy in the other day. And he was saying how it was nothing to do with the gas industry, and they only had two disposal wells in the county. And then there was another customer fighting with him, saying she'd looked it up and there were 5,000. I was like, 'children.' "
For decades, scientists have connected earthquake activity to disposal wells used by drilling companies to store the large volumes of saltwater and drilling fluids that come out of the ground along with oil and natural gas. The wells can reach more than 10,000 feet underground, crossing through rock and sometimes active fault lines.
In 1968, the U.S. Army Corps of Engineers triggered a series of earthquakes by pumping wastewater into a disposal well outside Denver. But there are tens of thousands of injection wells around the country and only a handful of definitive findings. And the exact dangers of injection wells remain a subject of debate, said Steve Horton, a seismologist at the University of Memphis Center for Earthquake Research and Information.
Horton studied a series of earthquakes close to gas-drilling operations in central Arkansas that began in 2009. He found many of the tremors stemmed from three injection wells drilled into a previously unknown fault line. But others were the result of natural fault activity, he concluded.
"In my paper there was very strong evidence, but that's not always the case," Horton said. "It's not entirely clear to me what's happening in Oklahoma and Texas."
One of the questions in Texas is why now. Drilling activity in the Barnett Shale peaked in 2008 and is at its lowest level in a decade, according to the railroad commission.
Similar questions persist in Oklahoma, which has seen earthquake activity increase one hundredfold over the last three years.
In an interview this week, Scott Tinker, director of the Bureau of Economic Geology at the University of Texas, who is expected to head the state study, said it was still too early to say whether the earthquakes were the result of abundant waste-disposal wells in the area.
"There's lots of potential things that could be causing that, and anything I say would be speculation. We need to study it," he said.
In 2009, Oklahoma City-based Chesapeake Energy shut down two injection wells near Dallas/Fort Worth International Airport and Cleburne after scientists connected them to a series of small quakes in North Texas.
Other states, including Arkansas and Ohio, have pursued broader regulation, even placing a moratorium on injection wells across a large swath of Arkansas' Fayetteville Shale.
That has huge financial implications for drilling companies, which are then forced to truck out the waste fluid and find another area for disposal, said Ed Ireland, executive director of the industry-funded Barnett Shale Energy Education Council.
"There are 50,000 injection wells in Texas. And only a handful have had any seismic activity near them. To make a broad swath area where there can't be injection wells, I think would be a mistake," he said. "It's well known there are other events that have induced seismicity, including the construction of large dams and other activities."
State and local politicians have been careful not to cast blame too quickly.
That has aggravated some residents. At last week's public meeting, an aide for state Rep. Phil King was jeered after reading a statement urging patience.
But among many in town, the earthquakes have come to represent another odd quirk of country life. A recent edition of the weekly Azle News featured a masthead of off-kilter letters as if trembling in an earthquake. And at a propane dealer downtown Wednesday, customers bantered and laughed about the town's new reputation as a seismic hot spot.
"My first thought was, 'incoming,'" Parker quipped.
Alan Lobaugh, minister at Azle Christian Church, said that while the tremors might make for interesting gossip around town, they had done little to raise anxiety. The highest recorded earthquake had a magnitude of 3.7.
"A lot of these people are farmers and ranchers. It's just one more thing to put up with," he said.