8 October 2018
The Palu landslides – the worst landslide disaster in five years?
It is becoming increasingly clear that the lass of life from the three Palu landslides, triggered by the earthquake in Sulawesi, is very high. In particular, the two landslides that struck main parts of the town, at Balaroa and Petobo, are thought to have exacted a terrible toll. A spokesman from Indonesia’s disaster agency, Sutopo Purwo Nugroho, was quoted thus on Sunday:-
“Based on reports from the (village) heads of Balaroa and Petobo, there are about 5,000 people who have not been found. Nevertheless, officials there are still trying to confirm this and are gathering data. It is not easy to obtain the exact number of those trapped by landslides, or liquefaction, or mud.”
Given the scale of the Palu landslides, and the very high number of houses involved, this level of loss is entirely possible. It is clear from the videos that the flow was highly mobile, which will have made escape from the crush of houses and debris extremely challenging, especially in a dense network of streets. The mobility of the debris will have meant that buildings will have filled with mud, even where there were voids that might have protected a person. It is therefore unsurprising that no survivors have now been recovered in a week. This AFP video shows the magnitude of the damage:-
The search for the missing will continue until Thursday; after that it is likely that the landslide sites will need to be classified as mass graves.
There is considerable confusion in the reporting as to what actually happened at Balaroa and Petobo. This is Petobo, as seen from the air:-
Whilst this is Balaroa:-
Various newspaper articles have described these events as being liquefaction, even nonsense such as:
“Petobo, a cluster of villages in Palu, was virtually wiped out by the powerful quake and wall of water that devastated Palu. Much of it was sucked whole into the ground as the vibrations from the quake turned soil to quicksand.”
In both cases these were landslides, of that there is no doubt. The mechanism of failure might have been liquefaction, but this is just a hypothesis. The houses were not sucked into the ground, they were buried and crushed by a flow of debris coming from upslope, albeit down a very low gradient slope. Whilst this might seem academic, it is important in the context of understanding future hazards.
To understand why these slopes flowed under the effects of earthquake shaking will require detailed investigations, in due course, using field data collection, lab testing and modelling. But seismically induced flowslides are not a new phenomenon by any means, and in previous events they have caused huge numbers of fatalities.