Geodetic Observations of the 2018 Mw 7.5 Sulawesi Earthquake and Its Implications for the Kinematics of the Palu Fault

The ascending and descending interferometric synthetic aperture radar data are used to investigate the fault rupture and slip model of the 2018 Mw 7.5 Sulawesi, Indonesia, earthquake. The best fitting slip model indicates that this earthquake ruptured not only a segment extending the Palu fault to the north but also a northwestern segment offshore. The slip on the onshore fault is predominant left‐lateral strike slip. The slip on the offshore fault is dominated by normal faulting with a maximum slip of ~6.3 m. The newly discovered offshore normal faulting is likely to be the cause of the tsunami after the shock. Combined with previous geomorphic, tectonic, geodetic, and modeling studies, we suggest that the kinematics of the Palu fault maintained the same style of faulting from north to south, which resulted from an oblique extension occurred on an east dipping fault at depths. The deformation pattern of NW Sulawesi is dominated by this slip mechanism. Plain Language Summary On 28 September 2018, a large earthquake (Mw 7.5) struck the Palu city, Sulawesi Island, eastern Indonesia, where three tectonic plates, the Indo‐Australian, Philippine Sea and Sunda Plates, interact. Using a joint analysis of ALOS‐2 and Landsat 8 satellite images, we found coseismic slip partitioning during this earthquake. We propose a conceptual structural model to interpret the slip mechanism, where oblique extension occurred on an east dipping fault at depth and the slip is partitioned into left‐lateral strike‐slip motion at the ground surface and normal faulting at the seabed surface. A similar small‐scale geological system is observed in the Palu basin along the same fault system, which provides good evidence for this mechanism. Our study has important implications for understanding the geohazards and deformation mechanism of faults in eastern Indonesia. Key Points This earthquake ruptured one onshore segment with predominant strike‐slip motion and an offshore fault with a significant normal slip Offshore normal faulting is responsible for the tsunami after the earthquake This event revealed the kinematics of the Palu fault, which can be used to interpret the geomorphological expression in the Palu basin