Sulawesi Island is located at the triple junction between the converging Australian, Sunda, and Philippine plates. The magnitude (Mw) 7.5 Palu earthquake occurred on 28 September 2018 on Sulawesi Island and caused serious casualties. The causative fault of the Palu earthquake was the left-lateral, strike-slip Palu-Koro fault, which has a rapid slip rate. We experienced this earthquake in Palu City and conducted field investigations on coseismic surface ruptures 1 d after the earthquake. Field surveys revealed that the coseismic surface ruptures were characterized by left-lateral offset, en echelon tensional cracks, mole tracks within a narrow zone, and large areas of sand liquefaction that increased the damage and losses. We measured the coseismic displacements along surface ruptures and observed a maximum coseismic offset of ∼6.2 m. The rupture traces in the north Palu Basin near Palu City mark the previously unmapped Palu-Koro fault. Based on the field investigations, we determined the exact location of the Palu-Koro fault within the Palu Basin and found that the Palu-Koro fault zone can be divided into three branches: F1, F2, and F3, forming a typical flower structure.

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