A&M, USGS experts explain reasoning behind recent South Texas earthquakes

“Industry activities can sometimes cause earthquakes and this is definitely related to the local geology information and also related to the stress caused by the industrial activity,” said Xiaowei Chen, an A&M professor who studies earthquakes.

Since Feb. 8, 12 earthquakes with a magnitude of 3.0 or higher have occurred within a 10-mile area near Falls City, according to Justin Rubinstein, a research geophysicist for the U.S. Geological Survey. This sequence of seismic activity was highlighted by a magnitude 4.7 earthquake on Feb. 16. A magnitude 4.4 earthquake occurred near the 4.7 quake just 12 minutes prior. Rubinstein did say it’s too early to know the exact cause of these earthquakes, but noted others in the area have been induced by petroleum production.

“Data on these techniques is often delayed by months, so we won’t be able to make a determination until this data is available,” Rubinstein said in an email response. “One interesting thing to note is that the [magnitude] 4.7 earthquake is within 1 mile of a M4.0 that occurred in 2018. That M4.0 is the largest known frac-induced earthquake to have occurred in the United States.”

There are several fault zones, including the Balcones and Luling, that stretch through South Texas. Falls City is also in the middle of the Eagle Ford Shale, which is an active region for oil and gas production. What’s happening in Falls City is likely the reactivation of an existing fault, according to Nicholas Perez, an associate professor in A&M’s Department of Geology & Geophysics whose research focuses on tectonics and mountain building.

“Because of where it’s located, which is far from an active plate tectonics boundary, you wouldn’t expect all that many natural earthquakes there, but it is right in the middle of a known petroleum resource [area] where there’s definitely wastewater injection happening. It seems very likely that event is related to wastewater injection,” Perez said.

Perez said Texas and Oklahoma have seen an increase in seismicity over the past two decades that’s been linked to the injection of fluids into the subsurface as part of the petroleum process. There is some debate over whether these earthquakes are attributed to hydraulic fracturing or wastewater injection.

Hydraulic fracturing, commonly known as fracking, is the process of injecting fluids such as water, sand and/or chemicals into the subsurface to break up bedrock and free up oil or gas reserves. The fracking field was pioneered by the late George P. Mitchell, A&M Class of 1940. The USGS website says reports of hydraulic fracturing causing noticeable earthquakes are extremely rare and notes wastewater injection is more likely to induce earthquakes.

Wastewater disposal well injections are done after oil or gas wells are used and produce mineralized water that is pumped back into the ground to safely and efficiently dispose of the fluid, according to the Texas Railroad Commission. Perez said the hydraulic fracturing that likely causes earthquakes is distinct from fracking and more related to wastewater disposal well injections. He noted this process works like how an air hockey table operates.

“If you have the little puck sitting on the table and you don’t turn it on, well the puck is just sitting there and if you try to push it, there’s forces resisting it,” Perez said. “It’s the mass of puck and the friction on the surface. If you turn on the air, the air is pushing up against the bottom of the puck so it’s kind of counteracting the mass of the puck so it’s changing the forces, it’s changing the state of stress in that area, so that makes the puck slide more easily along the surface.

“It’s the same analogy: if it’s air for the hockey table or if it’s a liquid in the subsurface, and that liquid basically counteracts some of the forces of the rock above it and that reduces the amount of force needed to make that fault slip.”

Falls City is about 180 miles southwest of the Bryan-College Station area. Although seismic activity has increased there, Chen said she doesn’t believe similar occurrences will happen in the Brazos Valley.

“As far as I know, there doesn’t seem to be any similar industry activities in this area and this area has been historically quiet in terms of seismic activity,” Chen said. “So, I don’t think this will happen around this area any time soon.”