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

Abstract

The September 2018, M w 7.5 Sulawesi earthquake occurring on the Palu-Koro strike-slip fault system was followed by an unexpected localized tsunami. We show that direct earthquakeinduced 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.

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

“…This is likely to be the cause of the tsunami. The physics‐based modeling also reveals earthquake displacements are the probable primary source of the tsunami in this earthquake (Ulrich et al, ).…”

Submarine landslide source models consistent with multiple tsunami records of the 2018 Palu tsunami, Sulawesi, Indonesia

“…To our knowledge, only Ulrich et al (2019) compared tsunami simulations with the many tsunami run-up heights obtained by field surveys over the entire bay. They reported that the earthquake was the primary source of the tsunami based on a comparison of tsunami modeling results with the Pantoloan tide gauge record and field survey heights.…”

“…Various tsunami sources have been modeled based on seismic data [Heidarzadeh et al (2019), using data from USGS (2018), Ulrich et al (2019)], Sentinel-2 satellite optical data (Jamelot et al 2019), Sentinel-1 satellite SAR images (Gusman et al 2019), and hypothetical submarine landslides (Pakoksung et al 2019). Although those studies compared calculated tsunami waveforms with the Pantoloan tide gauge record and tsunami heights determined by field surveys, a detailed comparison with tsunami heights over the entire bay has not been performed except for Ulrich et al (2019), and little quantitative consideration has been given to the video footage. Therefore, a source model that can capture the entire observed tsunami data needs to be built.…”

Modeling the Sources of the 2018 Palu, Indonesia, Tsunami Using Videos From Social Media

“…More complex coseismic deformation models have been developed in an effort to reproduce the specific details of the tsunami (e.g., Ulrich et al, ; Yolsal‐Çevikbilen & Taymaz, ) and to combine landslide and coseismic sources (e.g., Gusman et al, ). However, the tide gauge at Pantoloan port and the post‐tsunami surveys measuring the runup heights (e.g., Fritz et al, ; Omira et al, ) which are used to characterize the tsunami do not uniquely constrain the source.…”