I agree with Jane that it is more like a swarm than a quake and aftershocks. The M6 today was followed after 20 mins by a M6 in Fiji area. As to whether the Rat Island quakes are volcanic or directly subduction or something else, it would be interesting to look at focal mechanisms, hypocenters (epicicenters with depths, if the depth are not just assigned). I don't know the area, and am not going to look into this in detail. Maybe you can do a search on these quakes and someone would already have done the work. Don used to be good at finding this sort of info but he does not seem to be posting...
Chris