Oklahomans rang in the New Year with a reminder of a growing consequence of the drilling boom underway beneath the surface of their state.

Less than six hours after the stroke of midnight on Jan. 1, a magnitude 4.2 earthquake struck directly under the city of Edmond, jolting residents from sleep across the state and triggering an hour-long local power outage for nearly a half-million people.

The event came exactly 72 hours after a slightly larger magnitude 4.3 earthquake rocked the same town, representing the latest in hundreds of seismic events felt by Oklahomans since 2009, when oil and gas production increased in the state. State energy regulators and scientists say these quakes are likely caused by drillers’ injection of wastewater deep into wells nearby.

Responding to last week’s two major events, state oil and gas regulators ordered operators of the five injection wells within 10 miles of the earthquake epicenter to scale back their activity. Also, operators farther away from the events were ordered to conduct additional testing of their wells. It is at least the twelfth time in 10 months that the state has restricted drillers because of earthquakes.

“We are working with researchers on the entire area of the state involved in the latest seismic activity to plot out where we should go from here,” Tim Baker, director of the Oklahoma Corporation Commission’s Oil and Gas Division (OGCD), said in a statement.

Pedestal Oil Company Inc., the operator of the only well within 3.5 miles of the two similar events, is being asked to cut the well’s injection in half. The company has already volunteered to suspend all activity at the well for now.

The four other operators of wells within 10 miles of the event epicenters are being asked to slash their activity by 25 percent. One of those companies, Devon Energy Corporation, has agreed to suspend activity at its single impacted well.

The two major morning earthquakes felt in the Oklahoma City suburb of Edmond (pop. nearly 85,000) on Tuesday, Dec. 29, and Friday, Jan. 1, both knocked out power for about an hour and caused only minor property damage such as falling pictures and shelves. This is the first time earthquakes have been blamed for knocking out the power in this region, according to Casey Moore, a spokeswoman for power provider Edmond Electric.

Last week, many smaller earthquakes were also observed in the area. Over the weekend, a few more small quakes were felt in the Oklahoma town of Stillwater (pop. approximately 47,000). The home of Oklahoma State University, it is about 50 miles northeast of Edmond.

“We are looking not only at the Edmond area, but the surrounding area as well, including the new seismic activity that has occurred in the Stillwater area,” said Baker, from the OGCD.

Edmond overlaps the Arbuckle formation, where most of the state’s drillers dispose of the wastewater that comes up after their wells start producing oil and gas. A single well over the course of its lifetime can produce hundreds to thousands gallons of what’s called “produced water”—the waste that is later injected underground for disposal.

In Oklahoma, oil production nearly doubled between 2009—the year when earthquakes started occurring more frequently in the state—and 2014. Natural gas production also increased during this time. Both regulators and scientists agree the increased levels of wastewater injection are likely triggering the earthquakes.

Last March, state regulators stepped up their oversight of the earthquake-wastewater problem in the Arbuckle, stipulating a wave of well testing and later requiring numerous energy companies to reduce the volume of their waste disposal—or to dispose of the waste at a shallower depth. And it’s possible these methods are already starting to work, said Jeremy Boak, director of the Oklahoma Geological Survey. At least 11 similar orders followed. In one case, Sandridge Energy Inc. disregarded a regulator’s request to shut down six disposal wells despite their risk of triggering earthquakes.

Oklahoma experienced around 907 earthquakes at or above magnitude 3.0 in 2015, a more than 50 percent increase since 2014, according to Jeremy Boak, a scientist at the Oklahoma Geological Survey. About 480 such earthquakes were observed in the first half of 2015, and 427 events in the second half, Boak told InsideClimate News.

Boak suspects the “flatline or slight downturn” in events during the second part of the year was due to either decreased production of oil and gas—which in turn led to a decrease in wastewater production—or the effectiveness of operators’ shutting down or reducing their injected wastewater volume. Or it could have been some combination of the two factors, he noted. Boak’s team expects to pinpoint the exact cause of the slight earthquake decline as more energy company wastewater disposal data becomes available.