Indonesia quake of 7.7 magnitude triggers tsunami warning, kills 20. Why is this one different?

At least 20 people have died after a magnitude 7.7 earthquake struck off northern Flores Island of Indonesia on Saturday (Aug 15). While Indonesia is known for frequent earthquakes, this one is different in where and how the rupture took place. The quake occurred on the unusual Flores back-arc thrust, a fault capable of producing very large earthquakes close to populated parts of the island. The shallow rupture produced severe shaking and landslides, while a tsunami warning was triggered but later lifted after only minor waves were recorded.

What happened in the north Flores earthquake

The earthquake struck at about 4:58 a.m. local time. The epicentre was at around 30 km northeast of Mbay in Nagekeo, according to Indonesia’s BMKG. It was 15 km deep.

At least 20 people were reported dead in Maumere, with six injured and two still trapped under rubble.

Authorities warned that the toll could rise as rescuers reach areas closer to the epicentre.

About 2,000 people were evacuated in Nagekeo.

Buildings, homes and government facilities were damaged, while landslides blocked roads and disrupted communications and power.

The unusual faultline

The Flores Thrust is a back-arc thrust. This is a fault system that accommodates compression behind Indonesia’s volcanic arc. It is driven by the broader convergence between the Australian and Sunda tectonic plates, but its movement is in the opposite direction to the main subduction geometry.

That makes it particularly important for Flores: the fault runs close to the island and can generate powerful earthquakes directly beneath or near coastal communities.

BMKG said the fault has a maximum estimated earthquake potential of about magnitude 7.8–7.9, making Saturday’s 7.7 event close to that upper range.

Why the shaking was so damaging: The fault has a dangerous history

The combination of a large magnitude, shallow depth and proximity to land meant intense shaking reached populated areas quickly.

Landslides became a major secondary hazard, cutting access to Nagekeo and other areas. Strong shaking was reported across East and West Nusa Tenggara and parts of South Sulawesi.

The 1992 Flores earthquake, of around 7.8 magnitude, struck the same system and generated a devastating tsunami that killed more than 2,000 people.

Tsunami risk — and why this time was different

BMKG initially issued a tsunami warning covering parts of East Nusa Tenggara, West Nusa Tenggara and South Sulawesi. Waves below one metre were subsequently recorded, including a maximum of 0.94 metre at Maurole, and the warning was lifted about three hours later.

That limited tsunami does not make the earthquake less significant. The crucial difference from 1992 was the secondary hazard profile: this time, the greatest immediate threat came from violent ground shaking, collapsing structures and landslides rather than a catastrophic tsunami.