The U.S. Geological Survey says five minor earthquakes originated in an area just west of New Castle in a 22-hour period on Monday, all small tremors between magnitude 1.7 and 1.9, which is below what humans can feel.
The timing and proximity of the Lawrence County earthquakes — originally the geological survey had identified just two — suggest a link to a nearby natural gas fracking operation, but seismologists were being cautious Thursday, saying it is too early to tell definitively if fracking triggered the quakes.
The Pennsylvania Department of Environmental Protection and the Department of Conservation and Natural Resources are investigating whether the quakes are associated with hydraulic fracturing, or fracking, the natural gas extraction technique that was ongoing at a Hilcorp Energy Co. well pad about a mile from the closest earthquake.
Hilcorp shut down its operations at the site at noon on Monday.
A DEP spokeswoman said Wednesday that the wells that were being fracked have horizontal wellbores headed northwest from the pad, which is the general direction of the closest earthquakes.
The wells are targeting the Utica Shale and Point Pleasant formation, gas-rich layers that can lie a mile deeper than the better known Marcellus Shale, according to DEP records.
The depth, time and location of the fracking operations suggest a link with the nearby seismic events, but far more evidence would need to be analyzed to establish, or rule out, a clear connection, researchers said Thursday.
“I think anyone looking at the situation would say there are earthquakes very close to where the well is at the time the well is being stimulated with hydraulic fracturing. That’s suggestive that there is a link,” said Michael Brudzinski, a geology professor at Miami University in Ohio. “But I don’t think any of us are ready to say anything more conclusive than that.”
Andrew Nyblade, a geosciences professor at Penn State University, said monitors in the state’s newly expanded seismic network picked up the events on Monday.
The network, which will increase the number of seismic monitoring stations in Pennsylvania to 42, is funded by DCNR and DEP and operated by Penn State, which will provide open data on a website starting this summer.
“Certainly, the locations and the timing of the sequence is consistent with the drilling activity on the well pad,” Mr. Nyblade said.
Northwestern and southeastern Pennsylvania are comparatively more seismically active than other regions of the state, but Mr. Nyblade said, “I think it is fair to say that this is probably not related to natural events.”
Fracking has rarely been implicated in triggering recorded or felt earthquakes, which have been induced far more commonly in some regions by oil and gas wastewater disposed of in deep wells.
But researchers have linked fracking to earthquakes in England, Canada, Oklahoma and, notably, several sites in eastern Ohio not far from the recent Pennsylvania quakes.
Earthquakes with a magnitude up to 3.0 in Mahoning County, Ohio, were linked to fracking at a Utica Shale well operated by Hilcorp in 2014. That earthquake series was about four miles west of this week’s quakes in Lawrence County.
Researchers also have identified earthquakes as large as magnitude 2.8, as well as swarms of tiny seismic disturbances, associated with fracking in the Utica Shale and Point Pleasant formation in Harrison County, Ohio, since 2013.
Mr. Brudzinski, who has investigated both of those cases, said there is some indication that certain areas might be more susceptible to triggered seismicity from some operations, but that predicting earthquakes in areas with unknown faults is very difficult.
“I think that these operators are trying hard to identify where these faults might be to try and avoid them, and it’s just a very tough thing to do,” he said.
Mr. Brudzinski said there is encouraging evidence that Hilcorp was proactive about assessing the risks of induced earthquakes in Lawrence County and responded quickly when it detected them, which is what he recommends when operators ask for his advice.
The company was apparently operating its own seismic network in the area, he said.
A Hilcorp spokesman did not respond to a request for comment.
In the Ohio cases, Mr. Brudzinski and his colleagues suspect that fracking in the Utica Shale and Point Pleasant layers somehow triggered movement on a fault about half a mile deeper in the geological foundation known as the basement.
Researchers will have to do considerably more work to determine the depth of the quakes in Lawrence County, which will tell them more about their relationship, if any, to the fracking operations.
“Determining the depths of these earthquakes when they’re quite shallow like this is not trivial,” he said.
Laura Legere: firstname.lastname@example.org.
First Published April 29, 2016 12:19 AM