Most unusual. They swarm all over the place, and no fault rupture! With an M7 creating a rupture disk of only 30 km diameter, it is possible to not reach the surface. An M8 at 300 km must have a tabular rupture surface, and will reach the surface.
If the rupture did reach the surface, it could have done so underwater. But since there were no seismometers on the island, we don't know exactly how it ruptured, so my guess is that it started at 15 km depth, and the rupture disk stayed totally underground.
Once again it show that an M7 is too small to make any difference in creating a stable stress shadow, and that an M8 approaches.