A magnitude 2.4 earthquake shook Youngstown early Saturday morning, but this one was a little different than the rest.
The earthquake -- the ninth this year with an epicenter in Mahoning County -- struck at about 1:24 a.m.
The shake, like eight of the nine before it, occurred in close proximity to a brine-injection well on Ohio Works Drive near Girard.
A brine-injection well is one method of disposal for fracking fluids. Fracking is a process in which water, chemicals and sand are blasted into rocks thousands of feet below the ground to unlock natural gas and oil.
But unlike the previous earthquakes, the depth of Saturday's shake was shallower than the previous eight.
The preliminary determination on the 2.4 earthquake on Christmas Eve morning was that it happened 4 kilometers below the ground. All other shakes registered depths of about 5 kilometers.
A state geologist told The Vindicator that multiple seismographs are needed to pinpoint exact depths.
That prompted the state to entrust the Lamont- Doherty Earth Observatory at Columbia University to set up four seismographs in the Valley.
The approximate depth of the latest quake is about 4,000 feet below the bottom of the D&L Energy Inc. brine-injection well. The earthquake depth, however, occurred in the deep-rock formation known as the Precambrian, a nearly impermeable formation.
About 300 feet of the D&L injection well reaches the depth of the Precambrian, according to the Ohio Department of Natural Resources.
ODNR will mandate D&L to plug the bottom 250 feet of that well to "alleviate any perceived [earthquake] accusations."
That plugging hasn't yet taken place; D&L will plug it once another well, in Coitsville, is fully operational.
The state has contended there is no definitive correlation between this particular injection well and earthquakes.
Brine-injection wells have been known to cause earthquakes, including at Colorado's Rangely Oil Fields in the 1960s.