The Athens Geodynamic Institute said the quake struck this morning, 89 kilometres south of the country's second-largest city of Thessaloniki, and 271 kilometres north of Athens.
Greece is in a highly seismically active part of the world, but the vast majority of earthquakes do not cause severe damage or casualties.
In January and February, two quakes with magnitudes of about 6 struck the western Greek island of Kefalonia, damaging buildings and leaving many homeless for weeks, but causing no serious injuries.