A new study suggests fracking triggered hundreds of small, unnoticeable earthquakes in eastern Ohio late last year, months before the state first linked seismic activity to the much-debated oil-and-gas extraction technique.

The report, which appears in the November issue of the journal Seismological Research Letters, identified nearly 400 tremors on a previously unmapped fault in Harrison County between Oct. 1 and Dec. 13, 2013.

That included 10 quakes of magnitudes of 1.7 to 2.2. That’s intense enough to have temporarily halted activity under Ohio’s new drilling permit rules had they been in place at the time, but is still considered minor.

The quakes fell along a fault lying directly under three hydraulic fracturing operations and tended to coincide with nearby activity, researchers found. About 190 quakes were detected in a single three-day period last October, beginning within hours of the start of fracking. None of the quakes was reported felt by people.

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

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.

Fracking involves the high-pressure injection of water, sand and chemicals into shale to break up the rock and release trapped oil and gas.

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 ’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?”

Study co-author Paul Friberg, a seismologist at New York-based Instrumental Software Technologies Inc., said that fracturing rock results in micro-earthquakes but that some of the Harrison County tremors were significantly larger than expected.

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 U.S. The Ohio DNR in mid-Aprilenacted stricter regulations for oil and gas drilling near faults or areas of past seismic activity.