Forensic Evaluation of Long-Distance Flow in Gently Sloped Ground During the 2018 Sulawesi Earthquake, Indonesia

The Central Sulawesi Province of Sulawesi Island of Indonesia was hit by a powerful earthquake of moment magnitude (Mw) 7.5 on September 28, 2018. The earthquake was triggered by the left lateral Palu–Koro fault at a shallow hypocentral depth of 20 km. The event resulted in major geotechnical failures and structural damages in Palu city and Sigi Regency, causing thousands of deaths and injuries to more. Areas such as Balaroa, Petobo, Jono Oge and Sibalaya suffered enormous damage due to long-distance flow-slides and mud flows. It was the first of its kind of a large-scale flow-slide event triggered by an earthquake, surprisingly on a very gently sloped ground, displacing ground to hundreds of meters. The objective of this paper is to provide a brief insight on the outcomes of the field investigations launched by teams of researchers from Japan, immediately after the earthquake to delineate the key factors responsible for triggering such intensive flow-slides. Findings from the field investigations performed by means of unmanned aerial vehicle (UAV), in situ testing using portable dynamic cone penetration test (PDCPT) and trench survey, are described here. In addition, subsequent data interpretation and some probable mechanism of the flow-slides are discussed.