The complex pull-apart structures associated with large releasing bends on the major strike-slip fault in East Indonesia revealed by the 2018 Palu supershear-earthquake ruptures (Mw 7.5) in central Sulawesi.

Abstract:

The 2018 Mw 7.5 Palu supershear earthquake in central Sulawesi generated tsunami and triggered massive liquefaction in Palu-bay region, Central Sulawesi, Indonesia, resulting in about 2100 fatalities and economic losses of USD 980 million. The earthquake was produced by the well-known Palukoro fault on the west-end of a major strike-slip fault system in East Indonesia that starts from Papua (Yapen and Sorong Faults), through North Molucca region (Sorong-Sula Fault) and continues to central Sulawesi becomes the Matano and Palukoro Faults. Comprehensive post-disaster surveys have been conducted, including field survey of surface ruptures, LiDAR and multibeam-bathymetry mapping, and seismic-reflection survey. These data are integrated to accurately map the main and secondary fault ruptures. The NNW-SSE main ruptures occurred along 177-km long multi-fault segments, bypassing two large releasing bends. It shows predominantly left-lateral strike slip up to 6-m offset, accompanied by 5-10% dip slip in average. The 7-km wide releasing bend on land is causally linked with the pull-apart Palu basin where the major city in the region lies. Along the main fault in the western margin, the extensional tectonic field is associated with secondary normal faults forming negative flower structures. In the eastern margin, the regional subsidence is accommodated by numerous normal faults trending N-S and NW-SE mapped by their linear fault scarps on Quaternary sediments and associated landscapes. The N-S trending normal faults are also prominent along the neck of the island. Some of them are clearly ruptured during the 2018 earthquake. The cross-section across the Palu alluvial basin shows an asymmetric graben of a large negative flower structures. As part of the mega shear, the normal faults in eastern margin can be ruptured accompanying the major earthquake event on the main Palukoro fault producing smaller earthquakes as the aftershocks, but they may also rupture independently in between the major event on the Palukoro Fault.