April 26 (UPI) -- Scientists have uncovered more evidence linking fracking to earthquakes, this time in the central and eastern United States.
At the 2019 Seismological Society of America meeting, held this week in Seattle, researchers detailed the connection between hydraulic fracturing wells and some 600 earthquakes in Ohio, Pennsylvania, West Virginia, Oklahoma and Texas.
Previous studies have detailed the link between oil and gas operations and earthquakes in Oklahoma, Canada and China. But these studies have mostly focused on the connection between wastewater disposal and seismic activity.
Hydraulic fracturing involves the injection of high-pressure fluids deep into rock, most commonly shale, in order to trigger the release of oil and gas trapped between the rock layers. Often, wastewater left over from the activity is treated and then injected back into the ground. The layers from which oil and gas are extracted are wetter in places like Oklahoma, resulting in larger quantities of wastewater and a greater need for disposal efforts.
In states like Ohio, Pennsylvania and West Virginia, hydraulic fracturing wells are more common than wastewater injections. Wells in Appalachia have become increasingly common over the last decade.
"The wells are more widely spaced when they're active, and there isn't as much wastewater disposal going on," Michael Brudzinski of Miami University in Ohio said in a news release. "So you can see a bit more specifically and directly when wastewater disposal is generating seismicity and when hydraulic fracturing is generating seismicity in the Appalachian Basin."
Brudzinski and his colleagues used a method called multi-station template matching to scan seismic records for the signature of small, shallow quakes. They compared the timing of the seismic signatures with the schedule of regional fracking well operations.
"The [fracking] seismic signature when you look at it in a sort of timeline shows these bursts of seismicity, hundreds or sometimes thousands of events over a couple of days or weeks, and then it's quiet again. You don't tend to see that pattern with wastewater disposal," Brudzinski said.
The research team is currently studying the relationship between different fracking variables -- the type of solution injected into rocks, the depth of the targeted rock layers -- and the risk of an earthquake.
"The one that has stuck out to us the most is that the depth of the well is more tied to likelihood of seismicity than we expected," Brudzinski said.