The seismic epicenter of the quake -- which had a magnitude of 5.0 on the Richter scale -- was 33 miles north of Gatineau, Quebec, Canada, the US Geological Survey said on its website.

Vermont Yankee continued to operate at 100% power and no damage was detected in a plant walk down, Entergy Nuclear said.

An unusual event is the lowest of four NRC emergency classifications.

NRC spokesman Neil Sheehan said in an e-mail June 23 that "plant conditions are stable." Two NRC resident inspectors and their branch chief are onsite and will walk around the plant to perform "independent verification," Sheehan said.

A spokesman for Bruce Power, which operates nuclear power plants in Ontario, said the earthquake had no effect on the company's plants.

--Steven Dolley, steven_dolley@platts.com