The Pacific Tsunami Warning Center says there is no threat of a widespread destructive tsunami, but local tsunamis may be generated from the quake.
The US Geological Service says the tremor struck 77 kilometres north-west of the capital, Port Villa, on Thursday afternoon
The epicentre of the quake was fairly shallow, 26 kilometres deep.
Tonga police tell their boss to quit
Tonga's cabinet is considering a peition from almost half the police force calling for the police commissioner to step down.
Media there says up to 200 police have signed the peition calling for Chris Kelley, a New Zealander, to resign.
They are dissatisfied with recent reforms aimed at increasing accountability and transparency.
Commissioner Kelley says he is not going to quit.
"That issue is not being contemplated," he said.
Police Minister Viliami Latu says it is too early to say if he will consider removing the commissioner.
"The commissioner is doing a good job for Tongan police."
A Cabinet sub-committee is considering the petition and is expected to make a recommendation next week.