ENID, Okla. — In northwest Oklahoma, some scientists are digging for seismic answers as fervently as oil companies have drilled for black gold.

Between 1978 and 1998, the Sooner State averaged fewer than two earthquakes a year, according to U.S. Geological Survey records. But since 2009, the number of Oklahoma quakes has been on the rise, with more than 600 magnitude 3.0 or greater earthquakes recorded in Oklahoma so far this year.

As scientists study the increasing number of earthquakes, oil companies provide proprietary data on faults gathered through exploration, according to U.S. Geological Survey geophysicist George Choy.

In turn, that data is being used by Oklahoma Corporation Commission to regulate the oil and gas industry. Those same industries are accused by critics of causing the increase in Oklahoma quakes through hydraulic fracturing and wastewater injection.

Now a new study indicates that most of the larger earthquakes in Oklahoma over the past century may have been caused by this industrial activity.

Hydraulic fracturing, commonly called fracking, is the process of injecting pressurized fluid deep into the earth around a natural gas or oil well. The fluid, usually a mix of water, sand and chemical additives, fractures the rock, making the gas and oil flow more easily. Wastewater from this process is then injected back into the earth for disposal.

That wastewater injection was pinpointed as a cause for the increased seismicity. Scientists now know there is a link between injection wells and faults but have yet to get to the bottom of why some injection wells are linked to earthquakes and others are not. 

The new study by the U.S. Geological Survey will be published in December’s Bulletin of the Seismological Society of America. It suggests the uptick in earthquakes since 2009 in the central and eastern U.S. is primarily caused by human activity — namely, the injection of wastewater in deep disposal wells.

What is certain is that the ground is shaking in the state of Oklahoma far more than it used to.

But who’s at fault?

“We’ve put ourselves on record saying that there was relationship to the injection of wastewater in deep wells and into the basement, and that’s about as far as we can go right now, because we haven’t been able to really localize that and say, ‘It’s these faults, it’s these faults,’” Oklahoma Geological Survey Director Jeremy Boak said.

According to U.S. Geological Survey records, Oklahoma’s earthquake numbers began to climb significantly in 2009 and the state surpassed a previous 2014 record of earthquakes larger than 3.0 magnitude in August this year.

“I think the growth of the rise of earthquakes over the past few years has been something close to exponential, that is, it increases by a certain percentage every month or so,” Boak said. “But, actually, what it has tended to do is to go in fits and spurts, little bursts, and then slows down and then accelerates.”

Measuring the quakes isn’t an exact science.

As of Aug. 31, the Oklahoma Geological Survey recorded more than 600 magnitude 3.0 or greater earthquakes; OGS has the 2014 earthquake record at 585 magnitude 3.0 or greater quakes.

In contrast, the USGS showed 568 significant earthquakes, which still was below last year’s record. (As of Sept. 10, USGS was reporting 598 quakes — above last year’s total.

“We’ve had more 3.0-plus earthquakes this year than we had in all of last year,” OSG Director Jeremy Boak said. “There was some imprecision in some of the things. I think if they happen close enough together, sometimes it’s hard to be sure.”

Out of all the states in the nation, Alaska is the only state surpassing Oklahoma when it comes to earthquakes, USGS records indicate.

While California, a state commonly associated with large quakes, has recorded 90 magnitude 3.0 or greater earthquakes this year.

Choy noted a sizable body of research correlates the onset of massive injection well activity with the onset of earthquakes.

“There are also a number of papers describing the feasible mechanisms by which injected fluids can cause earthquakes. These mechanisms involve changes in stress caused by injected fluids on pre-existing but previously inactive faults,” Choy said.

Oklahoma Corporation Commission spokesman Matt Skinner said there are about 4,500 disposal wells in Oklahoma, but only 3,200 are operating at any given time.

Bill Ellsworth, a geologist with the U.S. Geological Survey, said an oil drilling operation always produces a mixture of oil and water.

“Any time you’re getting oil, you don’t get pure oil,” he said. “Even in conventional oil fields, you might be five barrels of water and one barrel of oil. This isn’t new.”

The spike in produced water levels started about 10 years ago, Ellsworth said.

“A number of very clever oil men figured out there was oil to be had in formations, rocks, that had never been considered economical to obtain before,” he said. “The oil is distributed through rock with lots of water. With horizontal drilling and big pumps, it’s possible to suck everything out of these formations. You get lots of water and less oil, so you have a huge wastewater issue.”

With more wastewater, companies needed to figure out where to put it; they couldn’t re-inject it into the same formation, a former practice that would help push oil to the surface. And it’s difficult and expensive to treat wastewater to a reusable state, so it has to go somewhere, so the least expensive option is to inject it back into the ground.

Wastewater injection wells are drilled deeper than the rock formations, which hold the oil, gas and wastewater, and groundwater aquifers, down to rock formations that allow the injected wastewater to distribute without the risk of contaminating groundwater or farmland.

As earthquakes increase, the state’s stance on those linking injection wells and seismicity shifted with science.

In Oklahoma, the most popular injection formation is the Arbuckle, the deepest rock formation above the crystalline basement, Ellsworth said.

About 950 injection wells go into the Arbuckle formation, Skinner said.

The high rates of injection have added water pressure in the Arbuckle and crystalline basement, which affects the pressure that holds fault lines together, Ellsworth said.

The issue, Boak said, is to find the medium between injecting enough wastewater to dispose of it in an economical and timely manner, but not enough to cause an earthquake.

“It’s like a slowly leaking balloon,” he said. “Don’t exceed the volume that it can leak.”

Miller and Summars write for the Enid, Oklahoma News and Eagle.