COLUMBUS, Ohio — Just as the momentum of fracking was gaining strength in Ohio, a new study released last month suggests the practice is to blame for hundreds of small earthquakes in eastern Ohio last year – months before the state first linked seismic activity to the much-debated oil and gas extraction technique.

“The industry is looking at the issue nationally, but we want more information on what caused the Ohio earthquakes,” said Shawn Bennett, a spokesman for the Ohio Oil & Gas Assoc. “Human-caused earthquakes are extremely rare, whereas 1.3 million tremors of similar magnitude happen naturally around the globe every year.”

The report on these earthquakes, “Characterization of an Earthquake Sequence Triggered by Hydraulic Fracturing in Harrison County, Ohio,” appeared in the November/December issue of the journal Seismological Research Letters.

The report identified nearly 400 tremors on a previously unmapped fault in Harrison County between Oct. 1-Dec. 13, 2013.

The research was conducted by seismologists Paul Friberg and Ilya Dicker of Instrumental Software Technologies in New York and Glenda Besana-Ostman of the Geophysics and Seismotectonics Group in the Bureau of Reclamation, U.S. Department of Interior, in Denver. It itemized 10 quakes of magnitudes of 1.7-2.2 on the Richter magnitude scale.

Under Ohio’s new drilling permit rules those measures are intense enough to have temporarily halted activity, even though most in the industry say these quakes are “minor.”

The seismologists revealed a previously undiscovered fault line approximately two miles below three horizontal gas wells near the town of Uhrichsville. Friberg said they detected 190 tiny earthquakes below one of those wells during a 39-hour period starting just after that well was fractured; however, none of the quakes were reported as felt by people.

“Hydraulic fracturing has the potential to trigger earthquakes, and in this case small ones that couldn’t be felt,” Friberg said, adding, “however, the earthquakes were three orders of magnitude larger than normally expected.”

The technique of fracking works, essentially, by “fracturing” underground shale rock. Companies drill a well underground, then inject a high-pressure mixture of water, sand and chemicals into that well to crack the rock and let gas flow out more easily.

According to Friberg, the process of cracking the rocks causes “micro-earthquakes,” the magnitudes of which actually come up as negative numbers on the Richter scale – usually in the range of -3 to -1.

Ohio Department of Natural Resources spokeswoman Bethany McCorkle said the state has installed seismic monitoring equipment throughout eastern Ohio over the past year and is keeping close watch for earthquakes strong enough to be felt.

Rowena Lohman, an assistant professor of geophysics at Cornell University who was not involved in the study, said some faults cannot be discovered until underground activity is attempted. She said the latest findings can be used to try to prevent worse quakes.

“We’ve known for a really long time, going back to the (19)70s, that when you do any subsurface manipulation you cause small earthquakes,” she said. “The big question is, are we doing something now that increases the probability that it will induce larger quakes?”

Ohio determined a probable link between fracking and five small tremors in eastern Ohio in April, in the first such tie announced in the northeastern United States.

Since those quakes, the state government started requiring oil and gas companies to install earthquake monitors before drilling within three miles of a known fault line, or in any area that has ever experienced an earthquake greater than a 2.0 magnitude. Under the set guidelines, detection of a quake of 1.0 or more will force the company involved to halt its operations. At that time regulators will investigate whether drilling was the cause for the tremors.

Blame wastewater injection?

Some studies on artificial earthquakes don’t blame fracking itself. Instead, they credit the process of wastewater injection – that is, taking the leftover water used to fracture the well and disposing of it by injecting it back underground.

Scientists increasingly believe the underground fluid migrates along dormant fault lines, reactivating them and causing earthquakes. These scientists say larger quakes triggered by wastewater injection from fracking could become the norm as more water is stored underground. “I think ultimately, as fluids propagate and cover a larger space, the likelihood that it could find a larger fault and generate larger seismic events goes up,” Western University earth sciences professor Gail Atkinson said at a meeting of the Seismological Society of America in May.

The research linking hydraulic fracturing and the fracking wastewater disposal process to earthquakes is still preliminary, but evidence seems to be getting stronger. Seismologists say more research needs to be done on the subject, particularly because it’s so difficult to tell which seismic events are natural and which are caused by human activity.

Meanwhile, U.S. Geological Survey scientists in September found evidence “directly linking” the uptick in Colorado and New Mexico earthquakes since 2001 to wastewater injection. A group of more than 50 earthquakes in 2009 and 2010, in Cleburn, Texas, was also linked also to the injection of fracking wastewater into the ground. Before 2008, the area had never recorded an earthquake.