Scientists are assessing whether injecting wastewater from fracking can cause earthquakes, and if it can, whether the quakes are severe enough to cause surface damage.

Art McGarr, a geophysicist with U.S. Geological Survey, is one of the nation’s lead experts investigating the cause and effect between disposal of the wastewater and earthquakes.

“The first thing to be clear about is that we’re almost certain that fracking itself does not lead to earthquakes,” McGarr said. “What we’re looking into is the deep injection disposal of the wastewater.”

Deep injection disposal is the fourth part of the process in horizontal drilling in the shale.

First the well is drilled, and then highly pressurized fluid is injected to break up the rock. This increases the rate at which natural gas can be produced from the well.

To avoid contaminating nearby groundwater, the wastewater is sometimes injected back, deep into the well.

It is this step, scientists believe, that can trigger earthquakes.

“We believe that the New Year’s Eve 4.0 [temblor] in Ohio is the result of this process, and other smaller scale earthquakes over the years,” McGarr said. “But to date we can’t connect the process to any surface damage.”

McGarr also said that the Richter scale is a limited predictor of damage. It would take less of a jolt in the eastern United States than the west to cause damage, for instance.

“A tremor travels much further in the east than the west,” McGarr said. “And so potentially causes more damage. The 5.8 [temblor] in August with the epicenter in Virginia travelled all the way up the east coast.”

McGarr added that the USGS is certain the Virginia earthquake was not caused by drilling or disposal.

Pennsylvania has six underground injection control wells. Kevin Sunday, a spokesman for the Department of Environmental Protection, said the disposal and pressure rates are set by EPA, which, under the federal Safe Drinking Water Act, have primary responsibility to regulate the wells.

“No seismic activity has been observed in Pennsylvania as the result of deep underground injection control wells or as a result of natural gas extraction,” Sunday wrote in an e-mail. “Many operators in the state are recycling more and more of the flowback fluid and produced water — some at 100 percent.”

­— J.L.K.