A moderate 5.6-magnitude quake struck at a shallow depth under the sea near Vanuatu in the western Pacific on Friday, the US Geological Survey said.
No tsunami warning was immediately issued and there were no reports of damage.
The USGS said the quake struck at 3:28am (1629 GMT Thursday) with an epicentre 97 kilometres (60 miles) from Isangel, on the island of Tanna, at a depth of just 14 kilometres.
Vanuatu, which lies between Fiji and Australia and north of New Zealand, is in the "Pacific Ring of Fire" known for its seismic and volcanic activity caused by friction between moving plates in the Earth's crust.
Since the start of July, at least four earthquakes measuring 6.0 or above have hit the nation of 83 islands, while in May a 7.2 shaker prompted a brief tsunami alert.