Geologist Reza Syahputra, a geologist from the Faculty of Mathematics and Natural Sciences, University of Indonesia (UI), stated that there were two possible causes for the 7.4 magnitude earthquake that triggered the tsunami in Palu, Central Sulawesi, on Friday (September 28th).

He previously explained that the earthquake in Palu was caused by faults in the earth’s plates moving and pushing against each other. There is a major fault, the Palu-Koro fault, which cuts through the “neck” of Central Sulawesi.

However, Reza explained that a number of the earthquakes did not originate from the main fault, but rather from smaller faults around the Palu-Koro fault.

“The epicenter of the earthquake was not exactly on the Palu-Koro fault, but in the surrounding area, there was shifting or movement in the area of smaller faults than the Palu-Koro fault, which likely caused the earthquake,” Reza explained.

Regarding the tsunami, which reached a height of up to 1.5 meters, that struck Palu and Donggala, Reza explained that there were two possible causes.

First, it was caused by liquefaction, or landslides of sand or soil on the seabed of Palu Koro Bay that had not been properly compacted. This was triggered by the movement of faults around Palu Koro. This, Reza continued, is suspected to be the main factor in the tsunami.

“This earthquake did not directly trigger a tsunami. However, the strong earthquake vibrations caused by the shifting of these faults are suspected to have been a factor in the tsunami,” he said.

The second possibility is purely due to faults, where the faults triggered the movement or uplift of faults elsewhere. However, Reza continued, the fault pattern in Central Sulawesi is not actually a fault pattern that can cause tsunamis.

Typically, tsunamis are caused by thrust or reverse fault movements. This means vertical rock movement. This vertical movement causes seawater to be shaken and generates waves that reach land.

The earthquake that occurred in Central Sulawesi, however, was caused by strike-slip fault movement, which is horizontal rock movement

However, Reza continued, research over the past few years has revealed that an event has caused the land in Central Sulawesi to rise several centimeters per year, and the rate of land uplift is greater than in other areas.

So, when a fault shifts in one area, it triggers uplift on top of the fault in another area.

“It’s possible that the fault, which initially moved only horizontally, also had an element of vertical movement,” he added.

The Palu earthquake was actually centered on land, but the formation of Palu Bay amplified the tsunami energy, making it even more powerful.