Distribution and morphology of the surface ruptures of the 2018 Donggala-Palu earthquake, Central Sulawesi, Indonesia

A large Mw 7.5 earthquake occurred at 18.03 on 28 September 2018 in the Donggala-Palu area of Central Sulawesi, Indonesia, which triggered a tsunami, liquefaction, and landslides. A 160-km rupture zone was formed on the surface along the NS to NNW-SSE trending Palu-Koro fault (PKF) system through three regencies: Donggala, Palu, and Sigi. Obvious surface ruptures associated with the earthquake were not observed in the epicentral area. Surface ruptures with a small displacement of both sinistral and westerly downward slip components appear south of the epicentral area in the Donggala area. The event induced widespread subsidence of the Donggala coastal area on the west side of the rupture zone. Passing across the Palu bay, the rupture cuts through the urban area of Palu city and extends across alluvial fans along the western side of the Palu basin showing large displacements with a predominant sinistral slip up to 4.2 m and subordinate easterly downward slip components. Approximately 5 m of anomalously high vertical displacement with 3 m of horizontal displacement occurs in an extensional bend in the southern area of the Palu basin. In the Sigi valley, the rupture zone follows the previously known PKF trace showing predominant sinistral and subordinate easterly downward slip, of which displacement is 0.9 m sinistral and 0.5 m easterly downward at the site 60 km south of Palu city and gradually smaller toward the south. These distributions of rupture zones and displacements suggest that rupture of the 2018 earthquake unilaterally propagated southwards and resulted in the movement of the eastern side of the fault towards the NNW.