Last Christmas Eve, residents of Youngstown, Ohio, were shaken by a rare earthquake, followed by another a week later on New Year's Eve.
On Friday, the state's Department of Natural Resources confirmed what many Ohio residents suspected: The earthquakes could be traced to re-injection wells used by oil companies to discard wastewater from shale gas wells, primarily from neighbouring Pennsylvania.
Ohio has now imposed tough new regulations on the re-injection operations, which are a key method for companies involved in shale gas and tight oil development to dispose of contaminated wastewater.
The tremors sparked political aftershocks across the state, which oil companies are targeting as a promising source of shale gas and oil. Opponents seized on the seismic shocks to illustrate the environmental risks that come with development, while the state legislature held hearings on the seismic activity.
It was not the first time that re-injection wells have been blamed for local earthquakes. Occurrences have been reported in Texas, Arkansas, the United Kingdom and northeastern British Columbia, though none were strong enough to cause serious damage or injury.
Many in the energy industry argue that small-scale earthquakes happen frequently, and are strong enough only to be noticed by sensitive seismic monitoring equipment. The rattling, proponents argued, began long before they arrived.
Stanford University geologist Mark Zoback said the risks of seismic activity can be managed by not drilling injection wells in fault zones, monitoring seismic activity, and managing the flow of injections to avoid pressure buildup.
"It should be pointed out that no injection-triggered earthquake has ever caused serious injury or significant damage," Mr. Zoback told an industry conference in Houston this week. "But nonetheless, when people are feeling the earth shake they have every right to be concerned."
Geologist John Clague, from Simon Fraser University, said several minor tremors in northeastern B.C. have been caused by the re-injection of oil industry wastewater, notably around Encana Corp.'s Horn River operations.
Mr. Clague said more work needs to be done to understand the risk, but he's concerned that the federal government – which has the country's main seismology capability – is absent from the field.
"I wouldn't totally dismiss it; just because past earthquakes have all been small doesn't mean you couldn't get a larger one," he said.