Article updated at 4:33 p.m. EDT.
More than half the earthquakes in Texas in the past 40 years were triggered by oil and gas production, according to a new study documenting quakes caused by drilling in the state back to 1925.
"There's a long history of this if you go back and look," said lead author Cliff Frohlich, senior research scientist at the University of Texas' Institute for Geophysics. "When oil practices changed, the human-induced quakes changed."
The study is being published online today in the journal Seismological Research Letters.
The review looked at 162 earthquakes since 1975 of magnitude 3 or greater, which is the size at which most can be felt. Using a five-pronged test, the scientists classified 26 percent as "almost certainly" man-made and 33 percent as "probably" triggered by drilling activity. Another 28 percent were deemed "possibly" human-induced. Thirteen percent, or 21 quakes, were found to be natural, or "tectonic."
The test looked at the proximity of the quakes and drilling activities in time and space, how close the quakes were to the surface and how close the drilling operations were to faults. On top of that, they looked at whether other scientists had linked the quake to drilling.
Frohlich and his co-authors from Southern Methodist University attributed none of the quakes to the practice of hydraulic fracturing, in which water, sand and chemicals are blasted underground to free up oil and gas. Most of the recent quakes, such as a swarm near Azle in 2013 and one at the Dallas-Fort Worth airport in 2008, were attributed to deep injection of wastewater from drilling.
The researchers classified a more recent series of quakes in nearby Irving as "probably" man-made (EnergyWire, Jan. 13, 2015).
While the authors acknowledge that their approach is subjective, they said strictly objective criteria wouldn't work when going back to decades long before there was precise recording of seismic activity.
But the study documents several other ways that activities linked to production have been linked to what's called "induced seismicity" over the decades. There's water flooding, in which water is injected underground to enhance oil production. Use of carbon dioxide for the same purpose has been linked to other quakes. In the Eagle Ford Shale region, a magnitude-4.8 event in 2011 has been linked to rapid extraction of oil and gas.
And in 1925, Frohlich's study says, earthquakes near what's now called Baytown in the Houston area were linked to rapid extraction of oil and gas that caused the land to sink underwater.
The assertion that drilling had caused the subsidence and quakes was made by the producing company, Exxon forerunner Humble Oil and Refining Co. The subsidence submerged much of the land, and the state of Texas sought rights to the oil because underwater oil is publicly owned. Humble won in court by arguing that the subsidence was not natural, but an act of man.
"In 1925, it was in Humble Oil's interest to argue that they were causing earthquakes," Frohlich said.
Don't mind the shaking
It's notable, he said, that many of the links between drilling and quakes were made not by industry critics, or even by disinterested academics, but by people associated with the oil industry.
But he also noted that many Texans didn't mind the shaking. The quakes were often in rural areas populated by people making money in the oil business. That changed, he notes, when the rumbling hit urban and suburban areas.
"Throughout most of Texas history, it wasn't considered a major problem," he said. "When you have earthquakes in the Permian Basin, it's a lot different than Dallas-Fort Worth."
Man-made earthquakes has been one of the oddest side effects of the drilling boom that has swept across parts of the country in the past decade. Quakes have been linked to drilling activity -- usually wastewater disposal -- in Arkansas, Colorado, Ohio and West Virginia. But no place has been affected as much as Oklahoma, where earthquakes have gone from one or two a year before 2009 to more than 900 last year.
Texas, however, has perhaps the most chronicled history of quakes linked to oil and gas, thanks in large part to Frohlich. He's been studying the phenomenon of man-made quakes in Texas for years -- as the paper notes, "long before this was fashionable in Texas."
He said he and his colleagues decided to do an in-depth review of Texas' history of man-made earthquakes for two reasons -- continuing denial by some public officials and looming changes in earthquake research.
The study notes that, despite extensive documentation of links between drilling and quakes, the seismologist at the state agency in charge of oil and gas last year dismissed the concept. David Craig Pearson, staff seismologist with the Texas Railroad Commission, told The Dallas Morning News in June 2015 that there was "no substantial proof" that recent quakes were man-made.
Texas Railroad Commission spokeswoman Gaye McElwain responded that the agency has implemented "some of the most stringent rules in the nation on seismicity" (EnergyWire, May 11, 2015). Since the rules were adopted, she said, 10 of the 51 disposal well applications have been returned or withdrawn. Another 22 have had special conditions put on them.
"The commission will continue to use objective, credible scientific study as the basis for our regulatory and rulemaking functions," McElwain said. "However, this new study acknowledges the basis for its conclusions are purely subjective in nature and, in fact, admits its categorization of seismic events to be arbitrary."
The agency ruled last fall that an SMU study linking two disposal wells to the quakes near Azle didn't provide enough proof for it to take action against the well operators (EnergyWire, Nov. 4, 2015). The SMU paper was authored by many of Frohlich's co-authors on the paper published today.
Frohlich acknowledges that some parts of Texas state government are taking earthquakes seriously. The state has put $4.5 million toward developing a seismic network and a system to oversee it. That means that studying seismic activity in Texas is about to change.
"Before 2016 will be one era. After 2016 will be another era," Frohlich said. "I thought it was a good time to do a review."