Schools have been closed and rescue teams have been sent to Santorini, where hundreds of smaller earthquakes are causing concern.
The tremors could be a warning of a larger earthquake on the Greek holiday island, says earthquake expert Gerasimos Papadopoulos.
Over the past three days, over 200 undersea earthquakes have been recorded off Santorini, with several measuring over 4.5. Schools will be closed on Monday on Santorini and the nearby islands of Amorgos, Anafi and Ios.
Rescue teams have been sent to Santorini, and Greek media reports that some residents have chosen to sleep outdoors. Others have left the island by plane or ferry.
The tremors are not believed to be linked to the underwater volcano that surrounds Santorini and some smaller islands. The volcano had a massive eruption around 1620 BC.