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-centralbasin migration of faulting activity from basin-bounding fault to intra-basin fault.

Shortcut Faults and Lateral Spreading Activated in a Pull-Apart Basin by the 2018 Palu Earthquake, Central Sulawesi, Indonesia

“…Comparisons with the surface ruptures mapped by field surveys [30][31][32][33] may also shed light on the development of the Palu basin in the Palu-Koro fault system.…”

“…To the south, the faults bend to southeast and display a stairstep series of east-facing scarps near the sampling point of Bellier et al [53]. Also, Patria and Putra [31] pointed out that several pressure ridges in the interior of the basin from Palu Bay to Loc. 15 (Figure 5c), but we could not determine whether they are pressure ridges with certainty, except for the clear one in Loc.…”

“…The earthquake was triggered by the rupture of more than 200 km of the Palu-Koro fault extending from the Tanimbaya Peninsula in the north to near Labua in the south [34][35][36][37][38][39][40]. Post-earthquake field surveys mapped a surface rupture 70 km long in the Palu basin [30][31][32][33], and surface ruptures on the seafloor of Palu Bay were documented after the earthquake [33]. The earthquake caused left-lateral displacements as great as about 6 m and vertical displacements of about 5 m at the surface [30][31][32][33].…”

“…Post-earthquake field surveys mapped a surface rupture 70 km long in the Palu basin [30][31][32][33], and surface ruptures on the seafloor of Palu Bay were documented after the earthquake [33]. The earthquake caused left-lateral displacements as great as about 6 m and vertical displacements of about 5 m at the surface [30][31][32][33]. Liquefactioninduced landslides within the Palu basin also caused significant damage [27][28][29].…”

“…In Sulawesi Island, Indonesia, a pull-apart basin is present near the city of Palu along the Palu-Koro fault (Figure 2), which ruptured during the Mw 7.5 earthquake on 28 September 2018 ('the 2018 earthquake' hereafter), causing extensive damage mainly in the Palu basin, the southward structural continuation of Palu Bay, due to tsunami waves and liquefaction landslides [27][28][29]. The earthquake produced distinct surface ruptures in the Palu basin [30][31][32][33], and many studies have used satellite and seismic data to estimate fault models and rupture propagation [34][35][36][37][38][39][40][41][42]. However, few studies have adequately compared the distribution of active faults, defined on the basis of geomorphic features, with the location of 2018 surface ruptures.…”