Geologists at Columbia University say the number of earthquakes in the middle of the United States is up 1,000 percent in the last 30 years. Although North Texas is still trembling from recent quakes, ours are by no means the first... or worst.
"We don't get earthquakes here," Sandy Murrell told News 8 last month. It's a common refrain echoing around Irving these days.
They didn't get them in Denver, either. But in 1967 — when the Rocky Mountain Arsenal injected millions of gallons of hazardous waste 3,600 feet underground — quakes rumbled the mile-high city.
Three magnitude 5 tremors damaged highways and homes.
Scientists in Japan took note of what happened in Denver, because that island nation is always in peril from deadly quakes. In 1970 a young seismologist named Masakazu Ohtake drilled an experimental well 120 miles northwest of Tokyo to see what would happen. Fifty quakes resulted.
"We did understand that water injection does cause earthquakes," Ohtake told News 8.
Ohtake, it turns out, studied at the University of Texas, where Dr. Cliff Frolich is now the resident expert on Texas quakes.
"In the last five or six years, there's been a lot of cases where there have been earthquakes within a couple of miles of injection wells," Frolich told News 8. "And there's quite credible evidence that the injection is related to that."
An injection waste disposal well is a product of oil and gas drilling. When modern wells are drilled, they bore into an energy deposit horizontally. They are fractured — "fracked" — to free the oil and gas.
Millions of gallons of water, sand and chemicals are used in the process.That water — now turned into salt water — has to be removed for the energy to be harvested. And all that water has to be disposed of.
Injection waste disposal wells are drilled for that purpose. There are dozens of salt water disposal wells in North Texas where millions of gallons of salt water waste is pumped into underground formations.
Texas Railroad Commission permits show that it's common for a million gallons of water to be injected at disposal well on any given day.
News 8 created an interactive map that shows where quakes have hit North Texas and where injection wells are.
INTERACTIVE MAP: North Texas earthquakes and injection sites
"Some disposal wells are two guys, a truck, and a well." Cliff Frolich told News 8.
Is it possible to drill a disposal well without knowing what's down there and what might cause an earthquake?
"That's true," Frolich said. "It's true that you can have a disposal well and not have studied that."
Problems might arise when liquid is pumped into a crack below the earth where two formations come together, which geologists call a fault. Liquid — salt water — might lubricate the fault and cause it to slip, resulting in an earthquake.
Do scientists know where all the faults in an area are? "No," Dr. Frolich said. "Definitely not."
Beginning in January 2008, 13 gas wells were drilled on the property of Dallas/Fort Worth International Airport. To handle the waste salt water, two injection disposal wells were drilled. At the south end of the airport, a disposal well began pumping an average of 400,000 gallons of salt a day in September.
Over the next seven months, 10 earthquakes — the strongest of which registered a magnitude of 3.3 — rumbled through North Texas.
"That's one of the stronger cases where injection probably triggered earthquakes," Dr. Frolich told News 8. Frolich and a UT colleague, along with Chris Hayward and Brian Stump of SMU prepared a detailed study of the quakes.
"There are disposal wells within about a kilometer of where these earthquakes were, and they were injecting at about 4 kilometers (about 2.4 miles) depth. And injection started about a month before the earthquakes started."
The DFW Airport disposal well scientists linked to the quakes is now shut down. The other airport well is still in operation.
There are 7,500 injection disposal wells in Texas, according to the Texas Railroad Commission. Until recently, there have been very few earthquakes, so — statistically — the evidence of wells creating quakes is small.
And the cause of the recent tremors in Irving remains to be answered.