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Our understanding of pull-apart basins and their fault systems has been enhanced by analog experiments and simulations. However, there has been no opportunity to compare the faults that constitute pull-apart basins with surface ruptures during earthquakes. In this study, we investigated the effects of a 2018 earthquake (Mw 7.5) on a pull-apart basin in the Palu-Koro fault system, Sulawesi Island, Indonesia, using geomorphic observations in digital elevation models, optical correlation with pre- and post-earthquake satellite images. A comparison of active fault traces determined by geomorphology with the locations of surface ruptures from the 2018 earthquake shows that some of the boundary faults of the basin are inactive and that active faulting has shifted to basin-shortcut faults and relay ramps. We also report evidence of lateral spreading, in which alluvial fan materials moved around the end of the alluvial fan. These phenomena may provide insights for anticipating the location of future surface ruptures in pull-apart basins.

Supplementary Files in Shortcut faults and lateral spreading activated in a pull-apart basin by the 2018 Palu earthquake, central Sulawesi, Indonesia

Earthquakes are considered to be important triggers of deep-seated gravitational slope deformation (DGSD) in seismically active regions, and if that is the case, DGSD features could be candidates for off-fault paleoseismology investigations. On the basis of pit excavations and sediment cores at an off-fault DGSD site and a trench excavation across the active Neodani fault at a nearby site, we examined the records of DGSD and surface-rupturing paleoearthquakes of the Neodani fault. The four most recent DGSD events were dated at after 240 cal BP, 1710–340 cal BP, 4730–3970 cal BP, and 5570–5340 cal BP. The four most recent surface-rupturing earthquakes were dated at 1891 CE (the Nobi earthquake), 2010–1220 cal BP, 7180–2110 cal BP, and before 9540 cal BP. We conclude that the ages of the four DGSD events are consistent with surface-rupturing earthquakes on the adjacent Neodani fault. We infer that static crustal strain from repeated seismogenic faulting plays an important role in the occurrence of DGSD events, at least in the immediate vicinity of active faults, although coseismic severe shaking would have at least some effect on them. Our case study suggests that off-fault DGSDs can be used to reconstruct or refine the paleoseismic history of a nearby active fault. We propose that an ideal DGSD for that purpose would be located in an area of concentrated strain near a termination, bend, or stepover of the target fault.

Additional file 1: Fig. S1. Schematic diagram showing the differences in incidence angles and resulting LOS values between PALSAR (2011 event) and PALSAR-2 (2016 and post-2016 event). Estimated difference in vertical offsets is ~ 2.7%, which is considered negligible. Fig. S2. Geologic map of the study area adapted from Kubo et al. (2007); location in Fig. 1. Also shown are small notches and wind gaps on ridge crests as well as fluvial terraces, identified in aerial photographs and a lidar DEM. Fig. S3. MPI-RRIM maps (locations in Fig. 2a) based on 2-m lidar DEMs. Location of 2016 surface ruptures and minor cracks, FFZs, subtle small notches, and wind gaps on ridge crests are shown. Fig. S4. Photographs of a the Mochiyama trench excavation, b the Koyama Dam FFZ, and c the Wakaguri FFZ, fresh slickenlines in gouge of d the Mochiyama trench and e the Koyama Dam FFZ, f displaced clay fragments within gouge of the Koyama Dam FFZ, and g subtle slickenlines in gouge of the Wakaguri FFZ. Fig. S5. Photomosaics of the a east wall and b west wall of the Mochiyama trench with fiducial grid overlay. Sketch log of this trench is shown in Fig. 7.