The same fault that produced the 4 magnitude earthquake in May 2015 in Johnson County — the strongest ever recorded in North Texas — could create an even larger one in the future, a recent study has found.
Heather DeShon, a seismologist at Southern Methodist University, led the study that focuses on the quake that struck near the town of Venus. The quake in the Bend Arch-Fort Worth Basin was triggered by the underground disposal of wastewater from oil and gas operations, the report concluded.
And it wasn't the first earthquake on that fault, which is a weakness in the earth's crust. Earthquakes have been happening in the area since 2008, DeShon said. Her team has also studied the slew of earthquakes in Azle, Reno and Irving.
She says there are many similarities among the earthquakes that happen in the Fort Worth Basin. For one: They all occur in granitic basement rocks.
“If you went down to Austin and you were climbing around the Hill Country and you went to Llano and saw those big, pink Texas rocks, those are what we’re studying,” she said. “Except down in the Fort Worth Basin, those are buried under a large amount of old sediments.”
Those sediments produce the oil and gas that we use for energy purposes in the area, DeShon said.
On the connection between fracking and earthquakes:
Starting in 2004, we went through the "shale gas revolution" in the U.S. Shale is a type of rock that has a very tight space where oil and gas is trapped. It’s really hard to get out. Horizontal drilling in the process of hydrofracking created space in those rocks for the oil and gas to come out. One of the ways you hold the cracks open is to put water and sand down into the rocks and then pull that back up. The wastewater that comes out is the water that was put into the rocks during the hydrofracking process. That water needs to be disposed of safely. It’s full of chemicals.
The oil and gas industry gathers the water from multiple production wells. You might have hundreds of production wells and then one wastewater disposal well, where that water’s placed back very deep into the earth. The Ellenburger Limestone Unit, which you can see in the Austin area, is actually deep below the Dallas-Fort Worth area, and that’s the unit — this really thick limestone unit — where the wastewater is pumped into.
On continued skepticism:
Man-made earthquakes in Texas have been documented since the 1920s. I don’t think there’s any argument over whether or not humans, and particularly the oil and gas industry, can cause earthquakes. The U.S. government ran experiments in the 1960s that showed that wastewater injection in particular could clearly induce earthquakes on pre-existing faults. Where some of the disagreement comes in is where you start arguing about one specific well causing one specific set of earthquakes.
This interview has been edited for clarity and length.