A worker checks water levels at a hydraulic fracturing operation. Companies typically need several million gallons of water to frack a single well. (BRENNAN LINSLEY/ASSOCIATED PRESS)

The rate of earthquakes in Oklahoma has dropped dramatically since late May, when the state limited injections of wastewater from fracking into underground wells, an Associated Press statistical analysis shows. And a new scientific study says the state is on its way back to calmer times that prevailed before a huge jump in man-made quakes.

Over the past couple of years, scientists have linked a dramatic increase in earthquakes in Oklahoma, Kansas and Texas to the practice of injecting wastewater from hydraulic fracturing, or fracking, back underground after drilling for oil and gas.

For quake-prone parts of Oklahoma, the state ordered what is essentially a 40 percent reduction in injection of the saltwater that scientists generally blame for the massive increase in earthquakes.

This year, before the new rules went into effect on May 28, Oklahoma averaged 2.3 quakes a day. Since then, the average dropped to 1.3 per day, based on AP’s analysis of U.S. Geological Survey data of earthquakes of with a magnitude of 3.0 or more. But some of those fewer post-regulatory quakes have been large and damaging.

Some big ones still

“Definitely the rate of quakes have gone down,” said USGS geophysicist Robert Williams. “At the same time, we had more magnitude 5s this year than ever before historically in Oklahoma. It’s good news on one hand. It’s heading in the right direction, but troubling to see these large damaging quakes.”

Higher volumes of injected wastewater are connected to more quakes, with the fluids adding more pressure to tiny faults. After Kansas regulated wastewater volume in March 2015, a January AP analysis showed, that state felt fewer quakes, while less-regulated Oklahoma got more. In response, Oklahoma announced new regulations.

Last month, Oklahoma had an average of less than a quake per day, though a 5.0-magnitude quake shook the vulnerable Cushing area, where massive oil reserves are stored.

Williams said it is important to put even the one-a-day rate in context. Before 2009, Oklahoma averaged one 3.0-magnitude earthquake a year; now it is good news that the rate is down to one a day, he said. In 2015, Oklahoma averaged 2.3 quakes with a magnitude of 3.0 or larger quakes a day. In 2014, it was 1.6 per day.

A study published Wednesday in the journal Science Advances by another Stanford seismologist, Mark Zoback, used intricate computer simulations to project that the rate of earthquakes in Oklahoma will continue to go down and eventually return to near levels seen before 2009.

Oklahoma has had three quakes 5.0 magnitude 5.0 or greater since 2014, and two of them occurred after the May regulations took effect.Pent-up pressure may be partly to blame, Zoback said. And Williams noted that some injecting continues.

Mainly, though, “we don’t understand the plumbing down there,” Williams said.

— Associated Press