The Athens Geodynamic Institute said the quake had a preliminary magnitude of 6.1 and occurred at 9:10 a.m. (0710 GMT) off Greece's western mainland, some 300 kilometers (186 miles) west of Athens. The U.S. Geological Survey put the preliminary magnitude at 6.5. Different agencies often have varying preliminary magnitudes in the hours and sometimes days after a quake.
The temblor was felt across western Greece, with people on Lefkada and nearby Ionian Sea island of Kefalonia rushing out into the streets. The fire brigade said it was sending crews to Lefkada to assess the situation.
State-run television said there were reports of damage to buildings on Lefkada and a possible report of a fatality, but authorities could not confirm the reports until their crews arrived on the scene.
Earthquakes are common in Greece, which is one of the world's most seismically active areas, though serious injuries and deaths are rare. More than 100 people died in a severe quake near Athens in 1999.
The Ionian is particularly seismically active, and new buildings on the area's islands are constructed to strict anti-seismic standards. Kefalonia was struck by a series of strong earthquakes, two of them with magnitudes of around 6, in January 2014, causing damage and minor injuries, but no fatalities.
Those temblors awakened memories of catastrophic 1953 quakes that flattened nearly all the islands' structures, killing hundreds of people.
Related story
Death toll reaches 27 in quake that hit Turkey, Greek island
