The shallower an earthquake is, the more destructive its effects on the surface tend to be.
Strong aftershocks were felt in the same area, including one with a magnitude of 6.1.
Video taken at the port in Maumere showed large chunks of concrete breaking off the terminal building onto passengers waiting to board an inter-island ferry. Some were thrown into the sea when the gangway to a ship collapsed.
Emanuel Melkiades Laka Lena, governor of East Nusa Tenggara province, told a news conference that they were killed in their sleep by collapsing rubble.
A stream of people could be seen moving inland on motorbikes after authorities issued a tsunami warning. BNPB said 2,000 people had evacuated or relocated on their own.
But it will take time to assess the full extent of the destruction in the region, which is remote and mountainous.
BNPB advised people to remain calm and stay away from coastal areas.
“Residents should also avoid entering buildings, especially those that have already sustained cracks or structural damage,” BNPB said.
“A rapid assessment is currently underway to determine the number of casualties and the extent of the impact. We will provide that information after the data has been verified.”
