OKLAHOMA CITY — Mary Catherine Sexton has been rattled enough.
This fall her neighborhood in the northeastern part of this city has been shaken by dozens of minor earthquakes.
“We would just have little trembles all the time,” she said.
Even before a magnitude 4.5 quake Saturday knocked objects off her walls and a stone from above her neighbor’s bay window, Sexton was on edge.
“People are fed up with the earthquakes,” she said. “Our kids are scared. We’re scared.”
Oklahoma has never been known as earthquake country, with a yearly average of about 50 tremors, almost all of them minor. But in the past three years, the state has had thousands of quakes. This year has been the most active, with more than 2,600 so far, including 87 last week.
While most have been too slight to be felt, some, like the quake Saturday and a smaller one in November that cracked a bathroom wall in Sexton’s house, have been sensed over a wide area and caused damage. In 2011, a magnitude 5.6 quake — the biggest ever recorded in the state — injured two people and severely damaged more than a dozen homes, some beyond repair.
State officials say they are concerned, and residents accustomed to tornadoes and hail are now talking about buying earthquake insurance.
“I’m scared there’s going to be a bigger one,” Sexton said.
Just as unsettling in a state where more than 340,000 jobs are tied to the oil and gas industry is what scientists say may be causing many of the quakes: the widespread industry practice of disposing of billions of gallons of wastewater that is produced along with oil and gas, by injecting it under pressure into wells that reach permeable rock formations.
“Disposal wells pose the biggest risk,” said Dr. Austin Holland, the seismologist with the Oklahoma Geological Survey, who is studying the various clusters of quakes around the state.
Oklahoma has more than 4,000 disposal wells for waste from tens of thousands of oil and gas wells.
“Could we be looking at some cumulative tipping point? Yes, that’s absolutely possible,” Holland said. But there could be other explanations for the increase in earthquakes, he added.
Scientists have known for years that injection wells and other human activities can induce earthquakes by changing pressures underground. That can have the effect of “unclamping” old stressed faults so the rocks can slip past each other and cause the ground to shake.
The weight of water behind a new dam in China, for example, is thought to have induced a 2008 quake in Sichuan province that killed 80,000 people. In Australia, a 1989 quake that killed 13 people was attributed in part to the opposite effect — the removal of millions of tons of coal during more than two centuries of mining.
In other places, including California and Switzerland, enhanced geothermal projects, in which water is pumped into hot rocks deep underground to produce energy, have caused quakes.
In Texas, some earthquakes have been connected to the industry practice of “water flooding,” increasing the yield of older oil wells by pumping water into nearby wells to force the oil out, said Dr. Cliff Frohlich, a University of Texas scientist. In other cases, Frohlich said, just the extraction of oil and gas from a long-producing field has been seen to induce quakes.
The practice of hydraulic fracturing, or fracking — injecting liquid at high pressures into shale rock — causes very small tremors as the rocks break, releasing trapped oil or gas. The technique has also been linked to a few minor earthquakes — in Oklahoma about a year ago, and in England and British Columbia. Yet unlike the continuing clusters of quakes elsewhere, the fracking-related earthquakes occurred only over short time periods, scientists say.
Of greater potential concern, scientists say, is wastewater disposal — from fracked or more conventional wells. Disposal wells linked to quakes have been shut down in a few states, including Arkansas and Ohio.
Along with oil and gas, water comes out of wells, often in enormous amounts, and must be disposed of continuously. Because transporting water, usually by truck, is costly, disposal wells are commonly located near producing wells.
The oil and gas industry points out that many of Oklahoma’s disposal wells are in areas with no earthquake activity, and that the practice of injecting wastewater has been going on for years.
“We’ve been doing this for a long time and it hasn’t been an issue before,” said Chad Warmington, president of the Oklahoma Oil and Gas Association.