OKLAHOMA, USA (AP) — Oklahoma oil-and-gas regulators yesterday issued their most far-reaching directive yet in response to a surge in earthquakes when they asked the operators of nearly 250 injection wells to reduce the amount of wastewater they inject underground by 40 per cent.

The Oklahoma Corporation Commission wants operators, over the next two months, to reduce injections by more than 500,000 barrels of wastewater daily in an area that covers more than 5,200 square miles of north-west Oklahoma.

The commission’s plan has been in the works since late October and was not influenced by a 5.1-magnitude quake that hit the area Saturday, said commission spokesman Matt Skinner. People reported feeling Saturday’s quake — the third-strongest in state history — in as many as 13 other states, including in Georgia, 900 miles away.

“Obviously the events of the weekend are clear (and) underscore the need to put a plan like this in place,” Skinner said.

As the corporation commission was preparing to announce its move, the Sierra Club filed a lawsuit asking that three major Oklahoma energy producers reduce wastewater volume.

The number of earthquakes with a magnitude 3.0 or greater has risen in Oklahoma from a few dozen in 2012 to more than 900 last year. Recent peer-reviewed studies suggest injecting high volumes of wastewater could aggravate natural faults. In Oklahoma’s six most earthquake-prone counties, the volume of wastewater disposal increased more than threefold from 2012 to 2014.

Most operators comply with commission directives, though one — SandRidge Energy Inc — initially refused to comply before reaching an agreement with the agency last month. Oklahoma House Speaker Jeff Hickman, whose home is 20 miles from the epicentre of Saturday’s quake, is pushing a Bill to make clear the corporation commission has the power to order wells to shut down or reduce volume.

“We will remove any doubt at all that the commission has complete authority in these emergency situations, without so much as a notice or hearing, to take whatever action they believe is necessary in these emergency situations,” said Hickman, a Fairview Republican whose Bill is set for review in a House committee today.

Alarm has swelled among Fairview residents after Saturday’s earthquake, said George Eischen, 51, who works at the city’s Chevrolet dealership. He was delivering a pickup when the truck started “rocking back and forth like a ship”, he said. The quake rattled doors in the dealership.

Eischen said Fairview residents shrugged off a previous 4.8-magnitude quake recorded January 6 a half-mile from the city, but they aren’t shrugging off this one.

“Everybody kind of just laughed and said, ‘Ah, it was just a fluke’,” Eischen said. “And now, months later, we’re sitting here with a 5.1, and now everybody’s getting concerned.”

In its lawsuit, the Sierra Club wants to see immediate, substantial reductions to three Oklahoma energy companies’ wastewater-injection levels. The lawsuit claims wastewater disposal from hydraulic fracturing operations at Chesapeake Operating, Devon Energy Production Co and New Dominion is contributing to the increased number of earthquakes.

Devon spokesman, John Porretto, said it would be inappropriate to discuss the litigation. The other companies didn’t immediately reply yesterday to requests for comment.

Chesapeake operates wells affected by the new corporation commission directive, but Devon and New Dominion do not.