EDMOND, Okla. (AP) — Utility regulators and geologists said Thursday they suspect a series of earthquakes in the Oklahoma City metro area, home to about a third of Oklahoma's population, were caused by nearby disposal of wastewater from oil and gas production.
The quakes started Tuesday and continued into Thursday, with at least seven of magnitude 3.0 or stronger being recorded in an area about 4 miles (6 kilometers) northeast of Edmond — about 15 miles (25 kilometers) northeast of Oklahoma City. The largest, a magnitude 4.2 temblor, hit Wednesday night.
No injuries or significant damage have been reported, although two Edmond Electric substations were knocked offline Wednesday, leaving about 1,900 customers without power for about an hour.
The Oklahoma Corporation Commission, which regulates utilities, said Thursday that the most likely cause is the injection of wastewater from oil and natural gas production into disposal wells in the area known as the Arbuckle formation, although there is a fault line in the area.
"We are looking at this as an Arbuckle well issue, even though there are no Arbuckle disposal wells right there on that fault," commission spokesman Matt Skinner said.
Jacob Walter, the Oklahoma state seismologist, noted that there are injection wells within 6 miles (10 kilometers) of where the quakes struck.
"It may be a case that this fault has been activated by past injections," Walter said. "Even if you stopped wastewater injection in Oklahoma tomorrow, we would still see seismicity for a number of years ... earthquakes beget other earthquakes."
Heather Warlick, an Edmond resident, said she could see her ceiling fan shaking when the Wednesday night temblor hit.
"I'm from San Diego and I'm kind of used to earthquakes," said Warlick, who has lived in Oklahoma for several years. "But when you get them here, and it's strong like that, it's different."
The power outage in Edmond was due to vibrations causing a safety switch to shut down electric circuits to two substations, said Edmond Electric spokesman Casey Moore, and is the third time in the past 18 months an outage has occurred because of an earthquake.
One substation is less than ½ mile from where the quake struck and the other less than two miles away, Moore said.
Scientists have linked a dramatic increase in quakes in Oklahoma, Kansas and Texas in recent years to the underground disposal of wastewater from oil-and-gas production.
Oklahoma experienced a few dozen earthquakes of a magnitude of 3.0 or great in 2012, and got more than 900 in 2015. After state regulators ordered oil and natural gas producers to close some injection wells or reduce the volume of the fluids they inject, the number dropped to closer to 600 in 2016.
Many of the quakes occurred in north-central and northwestern Oklahoma, with the state's largest-ever recorded earthquake in September when a 5.8-magnitude temblor hit Pawnee, about 90 miles (145 kilometers) northeast of Oklahoma City. That quake damaged buildings across the community of about 2,200 residents. The sandstone facade of some buildings fell, several others were cracked and one man suffered a minor head injury when part of a fireplace fell on him.
Walter said that earthquake occurred along a different fault from the one on which the latest quakes struck.
The tremors are expected to continue across the state, he said.
"The seismic hazard remains high in Oklahoma, despite in the first half of 2017 the seismic activity has gone down," Walter said.