Strong Earthquakes Shake NTT to North Sumatra in a Single Day, Here Is BMKG's Explanation

Three strong earthquakes occurred in a single day on 15 August. The Meteorology, Climatology and Geophysics Agency (BMKG) has explained that there is no direct correlation among the three earthquakes.

BMKG Director of Earthquakes and Tsunamis Wijayanto explained that the three strong earthquakes occurred in NTT, Simalungun and Parigi. However, based on his analysis, the three were not directly correlated.

“There is no direct correlation. The three earthquakes, namely the NTT M 7.7 caused by the Flores Back Arc Thrust Fault, the Simalungun M 6.4 caused by the subduction of the Indo-Australian Plate, and the Parigi Moutong M 5.9 caused by intraplate deformation within the Sulawesi Sea Plate, occurred in three different source systems and tectonic settings. Mechanically, they did not trigger one another,” he told reporters on Sunday (16/8/2026).

He also explained that the three earthquakes had different sources and characteristics. The details are as follows:

• NTT M 7.7: A shallow earthquake caused by activity on the Flores Back Arc Thrust Fault. The mechanism was thrust.

• Simalungun M 6.4: An intermediate-depth earthquake caused by the subduction of the Indo-Australian Plate beneath the Eurasian Plate. It did not originate from a land fault, but from the subduction interface.

• Parigi Moutong M 5.9: A shallow-to-intermediate earthquake at 60 km depth caused by intraplate compressional deformation in Tomini Bay with an oblique thrust mechanism.

Furthermore, he revealed that the NTT earthquake had a very large number of aftershocks. Meanwhile, no aftershocks from the Simalungun earthquake had been recorded to date. Likewise, no aftershocks had been recorded from the Parigi Moutong earthquake.

“The difference in the number of aftershocks also reinforces that each earthquake released energy according to the characteristics of its respective fault,” he said.

According to him, the phenomenon of three earthquakes in a single day constitutes seismic clustering. This is because Indonesia indeed has many active zones.

“These closely spaced events represent ‘seismic clustering’. Indonesia has many active zones, so it is natural for several segments to release stress within a short period, yet independently,” he stated.