Dramatic surface crack on the Palu-Koro fault marked by the lateral shift of embankment line

Central Sulawesi Reconnaissance Survey: Geological Impacts

The four meter subsidence on the Palu-Koro Fault
The four meter subsidence on the Palu-Koro Fault

Two months following the earthquake, tsunami, liquefaction, and landslide in Central Sulawesi in September 2018, a collaborative team from TDMRC and EEFIT UK conducted a field investigation on the affected sites. The aim of the mission was to map the Palu-Koro fault, the source of the earthquake, and to identify the main cause of the liquefaction and landslide phenomena in Balaroa, Petobo, and Jono Oge. The result of this investigation will provide critical information for the future development planning of the region.

The Fault Trace Investigation

The team successfully located the trace of the Palu-Koro fault. It spanned 150 kilometers long from the earthquake epicenter to the sourthern part of Palu. The team also found normal fault trace that became an extensional bend between the Palu and Saluki fault segments. A four-meter-deep subsidence was also found along the extensional bend.

Liquefaction and Landslide

The two phenomena occurred in Petobo, Balaroa, Jono Oge, and Sibalaya. Geologically, the area of Balaroa lies in an alluvium and colluvium fan zone. The alluvium sediment consists of gravels, sand, and silt. Underneath the alluvium layer is colluvium sediment formed by ancient landslides. It consists of boulders, cobbles, pebbles, and granite chunks. Since Balaroa is very close to the Palu-Koro fault, the earthquake vibration caused the ground to lose its cohesion, resulting in landslides. Balaroa is so close to the Palu-Koro fault that the landslide toe lay just on the fault line.

Similarly, the area of Petobo, Jono Oge, and Sibalaya lie upon broadly gradated, gravelly sand deposit, besides clay layers. On the higher part of Petobo, Jono Oge, and Sibalaya, there were irrigation lines for paddy fields. During the earthquake, the liquefaction spread the ground laterally and cracked the fully filled irrigation structures. The water then leaked and flowed entirely into the ground. The great infuse of water and the clay layer ten meters under became the main cause of earth flow in the area.