Even though some state officials recently denied a link between earthquakes and oil and gas drilling, human-cased tremors have been widespread in Texas since 1925, according to a new study that looked at historical records and current seismic data.
The report, published in Seismological Research Letters, found that the earthquakes have been caused by different kinds of petroleum and gas production methods, according to Cliff Frohlich, the study's lead author and senior research scientist and associate director at the Institute for Geophysics at the University of Texas at Austin.
In recent years, Texas has seen a big jump in the frequency of earthquakes tied to a boost in oil and gas drilling.
"Prior to 2008, Texas averaged two magnitude 3 earthquakes for last 25 years," Frohlich said. "Since 2008, it's been 12 a year. Almost certainly the change is due to manmade earthquakes."
The rise in earthquakes have occurred within a mile or so of petroleum production wastewater disposal wells where large amounts of drilling wastewater is injected back into the ground.
Some of these more recent earthquakes include the Dallas-Fort Worth International Airport sequence between 2008 and 2013; the May 2012 Timpson earthquake; and the earthquake sequence near Azle that began in 2013.
Some of the quakes have caused minor damage, but so far no injuries. Texas is not a naturally occuring seismic area, and most cities and towns are not prepared for the effects of earthquakes.
Frohlich's study looked at the history of the oil industry. In the 1920s and 1930s, oilmen would drill hundreds of wells, suck oil out of the ground as fast as they could, and there would be noisy slumps that shook the earth as the oil was sucked out.
When those fields were mostly depleted, in the 1940s through the 1970s, petroleum operations started tried to drive out water by flooding underground deposits with large amounts of water contributed to seismic activity, said Frohlich.
In the past decade, enhanced oil and gas recovery methods have produced briny wastewater that is disposed by injection back into the ground through special wells, triggering nearby earthquakes. Most earthquakes linked to this type of wastewater disposal in Texas are smaller (less than magnitude 3) than those in Oklahoma, the study concludes.
Still, in November 2015, the Texas Railroad Commission, which overseas the oil industry in Texas, said it found no conclusive evidence that the recent spate of several dozen quakes were the result of oil drilling operations, despite a peer-reviewed study from geologists at Southern Methodist University showing industry activity was "most likely" the cause of the temblors.
Lawmakers in Oklahoma and Kansas have recently begun restricting oil and gas drilling in certain areas, and as a result, the number of small earthquakes has dropped, according to Justin Rubinstein, research geologist at the U.S. Geological Survey in Menlo Park, Calif.
Texas has not curtailed the oil and gas business, but the state Legislature approved money to purchase additional seismic sensors to help researchers collect more data.
Rubinstein says scientists are still trying to understand why some regions are getting hit with lots of these human-induced quakes -- places like Oklahoma, Kansas, Texas and Ohio. While other areas, such as North Dakota, West Virginia and Pennsylvania, have not.
"It's an important question we are trying to understand," Rubinstein said. Maybe there aren't faults that are big enough. There are a lot of variables we are trying to consider. Understanding in these areas that don't have earthquakes is a lot more challenging."
Given that oil prices have fallen along with drilling activity in the past year, Rubinstein is curious about the a real-world experiment underway throughout the Oil Patch states.
"We've obviously seen a big downturn in oil and gas," he said. "When the price of oil goes back up, how long is it going to take to reactivate these (underground) structures? It gives us an experiment to see how long it takes for the geological system to return to its natural state."