The 2018 Mw7.5 Palu ‘supershear’ earthquake ruptures geological fault's multi-segment separated by large bends: Results from integrating field measurements, LiDAR, swath bathymetry, and seismic-reflection data

Abstract

Summary On 28 September 2018, 18:02:44 local time, the Magnitude 7.5 earthquake accompanied by a tsunami and massive liquefaction devastated Palu region in Central Sulawesi, Indonesia. Comprehensive post-disaster surveys have been conducted, including field survey of surface ruptures, LiDAR, multibeam-bathymetry mapping, and seismic-reflection survey. We used these data to map fault ruptures and measure offsets accurately. In contrast to previous remote-sensing studies, suggesting that the earthquake broke an immature, hidden-unknown fault inland, our research shows that it occurred on the mappable, mature geological fault line offshore. The quake ruptured 177-km long multi-fault segments, bypassing two large releasing bends (first offshore and second inland). The rupture onset occurred at a large fault discontinuity underwater in a transition zone from regional extensional to compressional tectonic regimes. Then it propagated southward along the ∼110-km submarine fault line before reaching the west side of Palu City. Hence, its long submarine ruptures might trigger massive underwater landslides and significantly contribute to tsunami generation in Palu Bay. The rupture continued inland for another 67 km, showing predominantly left-lateral strike-slip up to 6-m, accompanied by a 5–10% dip-slip on average. The 7km sizeable releasing bend results in a pull-apart Palu basin. Numerous normal faults occur along the eastern margin. They cut the Quaternary sediments, and some of them ruptured during the 2018 event. Our fault-rupture map on mature straight geological fault lines allows the possible occurrence of early and persistent ‘supershear’, but significant asperities and barriers on segment boundaries may prohibit it.

New High-Resolution Modeling of the 2018 Palu Tsunami, Based on Supershear Earthquake Mechanisms and Mapped Coastal Landslides, Supports a Dual Source

“…However, as mentioned above, the initial interpretations of the high-resolution deeper water bathymetric data of Frederik et al (2019), and the separate high resolution bathymetric survey by Liu et al (2020) did not show any evidence of a clear rupture trace in the bay. In their recent interpretation of this data, Natawidjaja et al (2020), in contrast, suggest several meters of vertical fault movement. Notwithstanding these new interpretations and, as noted by Liu et al (2020), since the Palu-Koro fault was not previously mapped underwater, the existing earthquake models have adopted various assumptions regarding where the fault trace is located.…”

“…Without a comparison of pre-and post-earthquake leveling data, it is not clear which, if any, of the earthquake models is most appropriate. The recent work by Natawidjaja et al (2020), published too late to include for consideration here, reinterpreted Frederik et al (2019)'s multibeam bathymetry and identified the major, meandering, submarine channel in the center of Palu Bay, as the seabed expression of the 2018 movement of the strike-slip fault. Based on this study, the fault could be considered to be more effective in tsunami generation than previously proposed.…”

“…They estimated slide parameters based on differences between their new and the Badan Informasi Geospasial's (BIG; Geospatial Information Agency, Indonesia) pre-earthquake bathymetric contours. In a study published after our work was completed, Natawidjaja et al (2020) reanalyzed the published bathymetries (Frederik et al, 2019;Liu et al, 2020) and interpreted the margins of the deep central channel in Palu Bay to be faults that were activated during the 2018 earthquake. They proposed several meters of vertical displacement for these, although this movement is within the m resolution of the data.…”

Shortcut Faults and Lateral Spreading Activated in a Pull-Apart Basin by the 2018 Palu Earthquake, Central Sulawesi, Indonesia

“…The earthquake was triggered by the rupture of more than 200 km of the Palu-Koro fault extending from the Tanimbaya Peninsula in the north to near Labua in the south [34][35][36][37][38][39][40]. Post-earthquake field surveys mapped a surface rupture 70 km long in the Palu basin [30][31][32][33], and surface ruptures on the seafloor of Palu Bay were documented after the earthquake [33]. The earthquake caused left-lateral displacements as great as about 6 m and vertical displacements of about 5 m at the surface [30][31][32][33].…”

“…In Sulawesi Island, Indonesia, a pull-apart basin is present near the city of Palu along the Palu-Koro fault (Figure 2), which ruptured during the Mw 7.5 earthquake on 28 September 2018 ('the 2018 earthquake' hereafter), causing extensive damage mainly in the Palu basin, the southward structural continuation of Palu Bay, due to tsunami waves and liquefaction landslides [27][28][29]. The earthquake produced distinct surface ruptures in the Palu basin [30][31][32][33], and many studies have used satellite and seismic data to estimate fault models and rupture propagation [34][35][36][37][38][39][40][41][42]. However, few studies have adequately compared the distribution of active faults, defined on the basis of geomorphic features, with the location of 2018 surface ruptures.…”