Earthquakes keep rocking Azle.

On Nov. 26, another earthquake jolted the small town in northwest Tarrant County. And on Nov. 29, yet another one hit nearby. About 20 quakes rattled North Texas in November -- five of them in or near Azle.

(Update: Another quake struck Dec. 3 in Reno, which is near Azle. The quake registered 2.7 on the Richter scale, according to the U.S. Geological Survey. A 3.6 quake struck near Azle Dec. 8. And a 3.7 quake hit near Mineral Wells Dec. 9.)

The minor earthquakes haven't caused significant damage, but residents in Azle are getting nervous and seismologists are trying to get to the bottom of what’s going on.

Some point to natural gas drilling that’s happening in the Barnett Shale, a massive geological formation that covers about 20 North Texas counties. But a geophysicist with the National Earthquake Information Center says more testing is needed to make such a connection.

Azle isn't the kind of place with a Starbucks or a quaint coffee shop. But at the popular gas station, Centerpoint Kwik Stop, the morning coffee crowd could only talk about one thing:

"Bam. It was like something hit the side of my house -- and it wasn't nothing but the earthquake,” Janice Hammond said.

She can still feel the tremors.

“Fifteen days now, we've had that little one every night, it seems like,” she said.

"Real spooky"

Donna Luce is worried.

"I was sitting there watching TV, and my house just shook, and I've never felt that before, ever,” she said. “Now, actually, I'm afraid of sinkholes."

Debbie Raub and her husband, Fred, described last Tuesday's quake with a 3.6 magnitude as “real spooky.”

"Real loud boom, and then it just started shaking the ground under our mobile home,” she said. “Just rocking it. And our dogs, they go totally insane. They know it before it happens."

Each time there's a quake, their two-bedroom trailer has to be re-leveled.

"You just have to get under there, and jack it up in spots, put a level on underneath the trailer, and level it,” Debbie Raub said. “It's just pulling the ground out beneath our stands."

Fred Raub, who was born in Azle, says he doesn't think the quakes are natural. Several residents blame local natural gas drilling -- and the use of disposal wells to store wastewater from the drilling.

"They say it ain't what happens, but till then, we didn't have it,” Fred Raub said. “You gotta drill about every mile. You just start looking around at all the drills they're putting down."



View North Texas Earthquakes in a larger map

This map from NPR's StateImpact Texas shows recent earthquakes (in red) in North Texas. Active disposal wells are in green; inactive wells are in yellow.

Natural gas drilling to blame?

Don Blakeman, a geophysicist with the National Earthquake Information Center, says the quakes could be the result of oil and gas production. But he says more testing is needed to know that for sure.

"Obviously, if it is oil and gas production related, they might continue until that activity stops,” he said. “The other possibility is that it's just a natural swarm, because we do see that sort of thing occurring sometimes in areas where we haven't had quakes before."

A natural swarm is a series of small quakes. And Blakeman says they aren't as dangerous as a quakes with bigger magnitudes – quakes at 5.0 magnitudes can produce structural damage. The strongest North Texas earthquake in November was 3.6. No damage has been reported.

"The good news is based on what typically happens, they'll die down in a while,” Blakeman said. “You can't predict one way or the other with absolute certainty."

But StateImpact Texas reports that Cliff Frohlich, associate director of the Institute for Geophysics at the University of Texas at Austin, has lead research into links between oil and gas drilling activity and manmade earthquakes. His study of earthquakes in the Barnett Shale found that disposal wells were responsible.

(Update: On Tuesday, SMU scholars released research that suggests disposal wells, also called injection wells, may be connected to earlier earthquakes that rattled Cleburne in 2009 and 2010. Injection wells store wastewater from drilling.)

Azle Mayor Alan Brundrett told The Dallas Morning News that if wells from natural gas drilling is causing the earthquakes, "then the disposal wells need to stop."

Officials are conducting daily inspections on water reservoir dams in North Texas for fear that the quakes may have caused cracks or loosened foundations. A dam at Eagle Mountain Lake near Fort Worth is among those being inspected, the Fort Worth Star-Telegram reports. It's also expected that the dam at Possum Kingdom Lake will be inspected after two quakes were registered in that area.

Be prepared

In the meantime, in case the ground starts to shake again, Blakeman advises individuals and families to be prepared if a bigger quake strikes North Texas.

"Find out about the kind of preparations they can do,” he said. “Take things off high shelves, know where to stand if an earthquake occurs, to be safer in their house."

Scientists and other experts are reportedly teaming up with the U.S. Geological Survey to collect more data to learn more about why these quakes keep occurring in North Texas.

Timeline: Across North Texas, many earthquakes in November

Nov. 29 – near Azle, 3.2 magnitude

Nov. 28 – near Jacksboro, 2.8 magnitude

Nov. 28 – near Mineral Wells, 3.6 magnitude

Nov. 26 – near Springtown, 2.8 magnitude

Nov. 26 – near Azle, 3.0 magnitude

Nov. 25 – near Reno, 2.7 magnitude

Nov. 25 – near Azle, 3.3 magnitude

Nov. 23 – near Reno, 2.9 magnitude

Nov. 19 – near Azle, 3.6 magnitude

Nov. 19 – near Azle, 2.8 magnitude

Nov. 19 – near Reno, 2.5 magnitude

Nov. 13 – near Eagle Mountain, 2.6 magnitude

Nov. 11 – near Briar, 2.8 magnitude

Nov. 9 – near Springtown, 3.0 magnitude

Nov. 7 – near Springtown, 2.9 magnitude

Nov. 6 – near Springtown, 2.7 magnitude

Nov. 5 – near Newark, 2.3 magnitude

Nov. 5 – near Reno, 2.6 magnitude

Nov. 1 – near Richland Hills, 2.1 magnitude

Source: U.S. Geological Survey