Trigger of the NTT Megaquake: What Is the Flores Back-Arc Thrust?

A powerful tectonic earthquake measuring magnitude 7.7 struck the Flores region of East Nusa Tenggara (NTT) on Saturday morning (15/8/2026) at 04:58 WIB.

BMKG initially issued a tsunami early warning before officially ending it at 07:30 WIB.

BMKG analysis shows that the earthquake was a shallow quake caused by activity on the Flores Back-Arc Thrust, with a thrust fault movement mechanism.

“Taking into account the epicentre location and hypocentre depth, the earthquake that occurred is a shallow earthquake due to Flores Back-Arc Thrust activity with a thrust fault movement mechanism,” BMKG stated in an official release.

So, what is the Flores Back-Arc Thrust?

To the north of Flores lies the Flores Back-Arc Thrust structure. This structure is highly active and can produce shallow earthquakes of large magnitude.

This is one of the reasons why the Flores region and surrounding islands repeatedly experience strong earthquakes.

The Flores Back-Arc Thrust (FBT) is an active fault stretching from north of Bali, through Nusa Tenggara, to the waters of Wetar. This fault was formed by the compression of tectonic forces from the subduction of the Indo-Australian Plate beneath the Eurasian Plate.

The combination of a very shallow hypocentre depth and thrust faulting mechanism on the seabed is the main trigger of destructive vertical deformation of the Earth’s crust as well as the generation of tsunami waves.

Daryono, a member of the Indonesian Association of Disaster Experts (IABI), explained that the Flores Thrust Fault has high vulnerability due to the accumulation of strain that has been locked for years. The release of M 7.7 energy confirms the estimated maximum magnitude potential in the NTT back-arc segment.

The Flores Thrust Fault has repeatedly triggered major earthquakes and deadly tsunamis in the Bali to Nusa Tenggara region.

According to him, to date no technology is capable of predicting the exact timing of an earthquake. However, the threat from the Flores Thrust Fault north of Bali to Nusa Tenggara is permanent.

“Seeing the large number of damaged houses, there needs to be a real and serious effort to realise earthquake-resistant and safe buildings. If not, every time a strong earthquake occurs, the public will continue to become victims,” Daryono said in his writing.