The number of “felt” manmade earthquakes induced by hydraulic fracturing (fracking) in oil and natural gas operations is negligible, the chair of a major report on seismicity activity told a Senate panel Tuesday. However the disposal of the water associated with fracking of shale gas wells is linked to increased quake activity, an official with the U.S. Geological Survey (USGS) said.

Of the 35,000 horizontally drilled and fracked unconventional wells in the United States, only one case of “felt” seismicity is “suspected, but not confirmed,” said Murray Hitzman, professor of economic geology at the Colorado School of Mines. He testified at a hearing of the Senate Energy and Natural Resources Committee about a report he chaired by the National Research Council, which was released Friday. The report, which examined the impact of different energy technologies on earthquake activity, was requested by Committee Chairman Jeff Bingaman (D-NM).

Outside of the United States only one case of “felt” earthquake activity has been attributed to fracking operations, Hitzman noted. “The very low number of ‘felt’ events relative to the large number of hydraulically fractured wells for shale gas is likely due to the short duration of injection of fluids and the limited fluids used,” he said.

Tens of thousands of wells to dispose of wastewater from fracking operations currently are active in the United States. “Although only a few induced seismic events have been linked to these disposal wells, the occurrence of these events has generated considerable public concern,” Hitzman said. “The majority of hazardous and nonhazardous wastewater disposal wells do not pose a hazard for induced seismicity. However the long-term effects of any significant increase in the number of wastewater disposal wells in a particular area on induced seismicity are unknown.”

Responding to a question from Bingaman, Hitzman said he believed that other energy technologies — such as carbon capture and storage — were a bigger threat to causing manmade earthquakes than fracking.

Of the number of earthquakes reported in the central and eastern United States since 2011, only one — a quake that occurred last August in central Virginia — was “unequivocally a natural Teutonic earthquake,” said William Leith, senior science adviser for Earthquake & Geologic Hazards at the USGS. “In all the other cases, there arises the possibility that the earthquakes were induced by wastewater disposal,” he noted.

“The disposal of fluids by deep injection is occurring more frequently in recent years. The occurrence of induced seismicity associated with fluid disposal for natural gas production in particular has increased significantly since the expanded use of hydraulic fracturing. Although there appears to be very little hazard associated with hydraulic fracturing itself, the disposal of the waters that are produced with the gas does appear to be linked to increased earthquake activity,” Leith said.

These manmade earthquakes, and the issue of whether they could lead to much bigger earthquakes in the future, are being collaboratively examined by the USGS, Department of Energy and the Environmental Protection Agency as part of President Obama’s interagency working group on fracking.

Mark Zoback, geophysics professor at Stanford University, believes the quake events can be properly managed. “I think that it is clear that with proper planning, monitoring and response, the occurrence of small to moderate earthquakes associated with waste injection can be reduced and the risks associated with these events effectively managed.”

He recommended that several steps be taken to avoid induced seismicity, which include avoiding injecting fluids into faults, selecting formations that minimize pore pressure changes, installing seismic monitoring arrays when there is a potential for triggered seismicity and establishing in advance protocols to define how operations would be modified in the event of seismicity.