The quake, which had a depth of 47 kilometres (29 miles), struck at 0508 GMT with an epicentre in the sea 48 kilometres southwest of Arvi, on the south coast of the island, the Observatory said.
No damage was reported but Greek firefighters were dispatched to the scene as a precautionary measure.
Some 10 hours later, a 5.3 magnitude aftershock struck from the same epicentre with a depth of 59 kilometres.
Greece is located on a number of fault lines, and is sporadically hit by earthquakes.
In October 2020, a 7.0 magnitude quake hit in the Aegean Sea between the Greek island of Samos and the western Turkish city of Izmir.
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