A strong undersea quake measuring 5.4 on the Richter scale occurred shortly before 9 a.m. near Pylos, in southern Greece.
There were no immediate reports of injuries or serious damage.
According to initial data, the quake occurred 34 kilometers southwest of Pylos at a depth of 10 kilometers.
The Euro-mediterranean Seismic Center estimated the magnitude of the quake at 5.8 Richter.
The head of Greece's Earthquake Planning and Protection Organization, Efthymios Lekkas, told Kathimerini that it was likley the main quake with aftershocks not expected to be stronger.
The quake was not unusual for the region which has high seismic activity, the head of research at the Athens Geodynamic Institute, Gerasimos Papadopoulos, told the Athens-Macedonia news agency.