(AP) — Geologists in Ohio have for the first time linked earthquakes in a geologic formation deep under the Appalachians to hydraulic fracturing, leading the state to issue new permit conditions Friday in certain areas that are among the nation's strictest.

A state investigation of five small tremors last month in the Youngstown area, in the Appalachian foothills, found the injection of sand and water that accompanies hydraulic fracturing, or fracking, in the Utica Shale may have increased pressure on a small, unknown fault, said State Oil & Gas Chief Rick Simmers. He called the link "probable."

While earlier studies had linked earthquakes in the same region to deep-injection wells used for disposal of fracking wastewater, this marks the first time tremors in the region have been tied directly to fracking, Simmers said. The five seismic events in March couldn't be easily felt by people.