Triggering mechanism of long-distance flow-type landslides caused by 2018 Sulawesi Earthquake, Indonesia

On 28 September 2018, the Mw 7.5 Sulawesi earthquake occurred in Indonesia, triggering long-distance flow-type landslides on very gentle slopes in and around Palu City. In order to investigate the triggering mechanism of these landslides, this study firstly compiled the field investigations that have been conducted since immediately after the earthquake and a soil profile of the landslide areas. Secondly, a groundwater flow analysis was carried out for the landslide in Petobo on the basis of the estimated soil cross-section. The results showed that, prior to the 2018 earthquake, there was confined groundwater with a water pressure of 40–60 kPa above what would be expected from the hydrostatic conditions in the flow zone of the landslide area. Finally, a simplified liquefaction analysis was performed using the groundwater pressure obtained by the groundwater flow analysis. The results indicated that, although the flow zone in the landslide area consisted of subsoils, including a relatively dense silty sand layer, it is likely that the significant liquefaction that occurred during the 2018 earthquake was due to the presence of confined groundwater. Furthermore, the authors concluded that the liquefaction probably contributed to the prolonged upward flow of large quantities of confined groundwater from aquifers located more than 20 m in depth below the ground surface, which resulted in a long-distance flow-type landslide by muddying the surface layer. It was also shown that the presence of an irrigation channel just above the landslide areas on the eastern side of Palu Valley had little effect on the sequence of landslide mechanism.