Development of the Palu–Koro Fault in NW Palu Valley, Indonesia

Abstract The 220-km-long Palu–Koro Fault, Central Sulawesi, is a major fault with prominent expression in Eastern Indonesia. Many studies about the Palu–Koro Fault have shown its capability of generating large earthquakes, but how the Palu–Koro Fault has evolved remains enigmatic. This study is to investigate the geomorphology of NW Palu Valley based on DEMNAS (Digital Elevation Model of Indonesia) and field observations to understand the development of the Palu–Koro Fault. The study area comprises a high mountain in the west and a valley in the east. There are two major normal faults and a strike–slip fault observed in NW Palu Valley. The western normal fault is a basin-bounding fault, which marks the topographic break between mountain and valley. To the east, another normal fault is observed cutting the old alluvial fans and expressed by planar fault scarps. The strike–slip fault is observed within the basin and crosses the distal part alluvial fans. It is expressed by intra-basin ridges in places which are slightly uplifted from the adjacent surface. The surface rupture of the 2018 Mw 7.5 Palu earthquake in NW Palu Valley also shows left-lateral movement up to 4 m. We consider that the development of the Palu–Koro Fault in NW Palu Valley is characterized by toward-central-basin migration of faulting activity from basin-bounding fault to intra-basin fault.