Coupled, Physics-based Modeling Reveals Earthquake Displacements are Critical to the 2018 Palu, Sulawesi Tsunami

Abstract

<p>The September 2018, Mw 7.5 Sulawesi earthquake occurring on the Palu-Koro strike-slip fault system was followed by an unexpected localized tsunami. We show that direct earthquake-induced uplift and subsidence could have sourced the observed tsunami within Palu Bay. To this end, we use a physics-based, coupled earthquake-tsunami modeling framework tightly constrained by observations. The model combines rupture dynamics, seismic wave propagation, tsunami propagation and inundation. The earthquake scenario, featuring sustained supershear rupture propagation, matches key observed earthquake characteristics, including the moment magnitude, rupture duration, fault plane solution, teleseismic waveforms and inferred horizontal ground displacements. The remote stress regime reflecting regional transtension applied in the model produces a combination of up to 6 m left-lateral slip and up to 2 m normal slip on the straight fault segment dipping 65° East beneath Palu Bay. The time-dependent, 3D seafloor displacements are translated into bathymetry perturbations with a mean vertical offset of 1.5~m across the submarine fault segment. This sources a tsunami with wave amplitudes and periods that match those measured at the Pantoloan wave gauge and inundation that reproduces observations from field surveys. We conclude that a source related to earthquake displacements is probable and that landsliding may not have been the primary source of the tsunami. These results have important implications for submarine strike-slip fault systems worldwide. Physics-based modeling offers rapid response specifically in tectonic settings that are currently underrepresented in operational tsunami hazard assessment.</p>

Stress partition on the Palu-Koro fault controlling the 2018 Mw 7.5 Palu earthquake and its seismic hazards

“…This value was subsequently modified to assess its effect on the calculated ΔCFS. Previous studies reported the 2018 Palu Mw 7.5 earthquake fault parameters using seismic wave, GPS, and INSAR data inversions (Bao et al, 2019;Fang et al, 2019;Socquet et al, 2019;Song et al, 2019;Ulrich et al, 2019;Wang et al, 2019aWang et al, , 2019bCevikbilen et al, 2019;USGS). The employed fault-plane segments, fault geometries, and slip distributions differed due to inversion differences in the modelling techniques and data sets.…”

“…Previous fault slip model reported that dip angle of the Palu-Koro fault varied from 67° to 85° with distance (Bao et al, 2019;Fang et al, 2019;Socquet et al, 2019;Song et al, 2019;Ulrich et al, 2019;Wang et al, 2019;Cevikbilen et al, 2019;USGS). Based on these observations, the present study calculated the Palu-Koro fault ΔCFS using the dip angle 85° in another case.…”

“…Previous studies have revealed the fault rupture process of the 2018 Palu Mw 7.5 earthquake using seismic wave, GPS, and InSAR data inversions (Bao et al, 2019;Fang et al, 2019;Socquet et al, 2019;Song et al, 2019;Ulrich et al, 2019;Wang et al, 2019aWang et al, , 2019bCevikbilen et al, 2019;USGS). The present study used the fault slip model developed by Song (2019) as it exhibited a geometrical simplicity and comparable slip distribution as compared to the other models (Bao et al, 2019;Socquet et al, 2019;Song et al, 2019;Wang et al, 2019).…”

“…Following the first year of the shock, many researchers explored the earthquake rupture process through geodetic and seismic data inversion. The earthquake rupture is highly suspected to present predominantly unilateral southward propagation (Bao et al, 2019;Fang et al, 2019;Socquet et al, 2019;Song et al, 2019;Ulrich et al, 2019;Wang et al, 2019;Cevikbilen et al, 2019;USGS). The along strike rupture on the Palu-Koro fault propagated southward for about 130 km to 140 km.…”

“…The Palu-Koro fault extends around 460 km N-S from the west of Minahassa trench to the north of Matano fault. The 2018 Palu earthquake only ruptured the central part (about 160 km) of the Palu-Koro fault (Bao et al, 2019;Fang et al, 2019;Socquet et al, 2019;Song et al, 2019;Ulrich et al, 2019;Wang et al, 2019;Cevikbilen et al, 2019;USGS). Two seismic gaps were left to the north offshore and the south on shore of the Palu-Koro fault with the length of about 180 km and 120 km, respectively (Figure 1).…”