In the debate over fracking of oil and gas wells, opponents often cite the risk that the process can set off nearby earthquakes. But scientists say that in the United States, fracking-induced earthquakes are not common.

In Canada, however, a spate of earthquakes in Alberta within the last five years has been attributed to fracking, or hydraulic fracturing, in which water, chemicals and sand are injected at high pressure into a well drilled in a shale formation to break up the rock and release oil and gas.

Now, scientists at the University of Calgary who studied those earthquakes, near Fox Creek in the central part of the province, say the quakes were induced in two ways: by increases in pressure as the fracking occurred, and, for a time after the process was completed, by pressure changes brought on by the lingering presence of fracking fluid.

“The key message is that the primary cause of injection-induced seismicity in Western Canada is different from the central United States,” said David W. Eaton, a professor of geophysics at the University of Calgary and co-author of a paper in the journal Science describing the research. The findings could help regulators take steps to avoid such induced earthquakes, he said.