By Karl Henkel
The depth of the city’s latest earthquake was the shallowest to date, according to data from the U.S. Geological Survey.
A magnitude 2.4 earthquake — the ninth since March with an epicenter in Mahoning County — shook Youngstown at 1:24 a.m. Saturday.
But unlike the previous eight earthquakes, the depth, or hypocenter, of Saturday’s shake was more shallow.
The preliminary determination from USGS was that the depth was 4 kilometers or 2.4 miles below the ground. All of the other shakes registered depths of about 5 kilometers or 3.1 miles.
Later on Saturday, the USGS revised the depth to 4.9 kilometers, but maintained a margin of error of 3.1 kilometers.
USGS collected data from 31 stations as of Saturday afternoon.
The quake, like all eight before it, occurred near a brine-injection well on Ohio Works Drive near Girard and was felt by many area residents.
Scott Williams of Youngstown was one of them.
“I was laying in bed just about to fall asleep when the bed started shaking and the items on my shelves started rattling,” Williams said.
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.
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 7,000 feet below the bottom of the D&L Energy Inc. brine-injection well.
About 300 feet of the D&L injection well reaches the depth of the Precambrian, a nearly impermeable formation, 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 taken place yet; D&L will plug it once another well, in Coitsville, is fully operational.
The well site remained open Saturday.
The state has contended there is no definitive correlation between this injection well and earthquakes.
Brine-injection wells have been known to cause earthquakes, including at Colorado’s Rangely Oil Fields in the 1960s.