2018 Sulawesi tsunami explained by sub marine landslide

Published: 12 December 2018

In the early morning of September 28 2018, a strike-slip earthquake occurred near Donggala (Sulawesi, Indonesia) which caused for the collapse of buildings as well as a phenomenal liquefaction event in Palu City. At Pantoloan harbour the tsunami amplitude reached 4 meters, which could not have been foreseen. Deltares experts made a new computer simulation, presented yesterday by coastal expert Ap van Dongeren at the AGU Fall Meeting in Washington.

Ap van Dongeren explaining the 2018 Sulawasi tsunami at the AGU Fall Meeting in Washington.

Tsunami land slide model

The reported tsunami heights cannot be explained by conventional earthquake slip-displacement mechanism. Initial computer simulation made by experts from the Dutch research institute Deltares, the peak tsunami wave caused by the earthquake reached only ~0.4 m at Palu and ~0.3 meter at Donggala. Videos taken from the airplane that has just departed Palu city just after the event showed a number of coastal landslides. However, these appear to be relatively small slides incapable of generating the observed tsunami heights and run-up. Deltares experts explain that one or more submarine landslides with larger dimensions must have occurred in the area.

Multiple source locations

Deltares experts Deepak Vatvani, Maarten van Ormondt and Ap van Dongeren localized a number of possible large landslide sources using visual inspection of sonar charts of the bottom sea obtained from internet (www.navionics.com). These sources are input into a Delft3D hydrodynamic computer model as instantaneous bed displacements, without considering the landslide kinematics.  By combining these sources with the initial earthquake source large tsunamis are generated in the Bay of Palu, which match the observed flooding pattern and flood heights in Palu.

Deepak Vatvani (tsunami expert Deltares): “Up till now, most tsunami warning systems only account for earthquake mechanisms. This event in Sulawesi shows that tsunami caused by landslides need to be included as well”.

Next steps: mapping underwater topography

In order to improve the computer models and forecasting in the future, there is a need for accurate mapping underwater topography and a better understanding of submarine landslide mechanism and also how to incorporate these effects in tsunami warnings.