Two things can now be said with certainty about the Youngstown Earthquake near an EPA Class II deep well injection site.
First, the earthquake and seismic activity was caused by the injection of liquids--in this case drilling wastewater--deep underground for the purpose of disposing them.
Second, hydraulic fracturing to break open shale rock to release gas had nothing to do with the earthquakes.
In this sense "fracking" did not cause the earthquake. But disposal of drilling wastewater at the Youngstown deep well injection site did, though the earthquake would like have happened, even if the material being disposed at this site came from a non-gas industry source.
EPA has permitted 144,000 class II deep well injection sites around the country, where 2 billion gallons per day of liquid are disposed. The sites have been operating for many decades. This extensive experience shows both that the overwhelming majority of these sites operate without triggering seismic activity and that the injection underground of liquids can cause seismic activity in rare cases.
Ohio state government has reacted to the Youngstown earthquake by closing one deep well injection site and promulgating new regulations, governing the siting and operation of future sites. The goal is to reduce the risk of seismic activity further by more stringent siting and operation criteria.
To date, Ohio has appropriately tightened deep well regulations to protect public safety. In Pennsylvania, less than 10 deep well injection sites operate, out of the national total of 144,000, and the EPA has full responsibility for their operation.