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

Abstract

The Mw 7.5 earthquake that struck Central Sulawesi, Indonesia, on September 28, 2018, was rapidly followed by coastal landslides and destructive tsunami waves within Palu Bay. Here, we present new tsunami modeling that supports a dual source mechanism from the supershear strike-slip earthquake and coastal landslides. Up until now the tsunami mechanism: earthquake, coastal landslides, or a combination of both, has remained controversial, because published research has been inconclusive; with some studies explaining most observations from the earthquake and others the landslides. Major challenges are the numerous different earthquake source models used in tsunami modeling, and that landslide mechanisms have been hypothetical. Here, we simulate tsunami generation using three published earthquake models, alone and in combination with seven coastal landslides identified in earlier work and confirmed by field and bathymetric evidence which, from video evidence, produced significant waves. To generate and propagate the tsunamis, we use a combination of two wave models, the 3D non-hydrostatic model NHWAVE and the 2D Boussinesq model FUNWAVE-TVD. Both models are nonlinear and address the physics of wave frequency dispersion critical in modeling tsunamis from landslides, which here, in NHWAVE are modeled as granular material. Our combined, earthquake and coastal landslide, simulations recreate all observed tsunami runups, except those in the southeast of Palu Bay where they were most elevated (10.5 m), as well as observations made in video recordings and at the Pantoloan Port tide gauge located within Palu Bay. With regard to the timing of tsunami impact on the coast, results from the dual landslide/earthquake sources, particularly those using the supershear earthquake models are in good agreement with reconstructed time series at most locations. Our new work shows that an additional tsunami mechanism is also necessary to explain the elevated tsunami observations in the southeast of Palu Bay. Using partial information from bathymetric surveys in this area we show that an additional, submarine landslide here, when simulated with the other coastal slides, and the supershear earthquake mechanism better explains the observations. This supports the need for future marine geology work in this area.

Submarine Landslide Source Modeling using the 3D Slope Stability Analysis Method for the 2018 Palu-Sulawesi Tsunami

“…Moreover, this study found 3 submarine mass failures with a total size of ~0.05 km 3 in the mentioned area. By followingNakata et al (2020),Schambach et al (2021) modeled submarine mass failure with a size of ~0.026 km 3 in a similar area and found good agreement with the nearby observed runups (Fig.6, yellow ellipse). According to these resulting landslides, this study strongly emphasizes the potential submarine landslides in Lower Palu Bay.…”

“…Various modeling efforts have been carried out to 60 simulate tsunami propagation using the COMCOT model (Heidarzadeh et al 2019;Carvajal et al, 2019;Gusman et al 2019;Liu et al, 2020;Sepúlveda et al, 2020). In more recent works, several studies have been performed to simulate landslideinduced tsunamis using other uncoupled methods, in which the landslide mass and tsunami propagation are separately calculated (Nakata et al, 2020, Paris et al, 2020Ulrich et al 2019;Schambach et al, 2021). Some previous works have successfully acquired agreeable results by comparing the surveyed runup heights to the inundation 65 depths, the recorded water levels at the Pantoloan tide gauge, or video-inferred waveforms.…”

“…Although these calculated submarine landslides still cannot fully explain some of the observed tsunami data, they emphasize the possible submarine landslide locations in southern Palu Bay 25 that should be studied and surveyed in the future. Schambach et al, 2021). Moreover, combining seismic fault slip with observed landslide sources can also explain the…”

Tsunami coastal hazard along the US East Coast from coseismic sources in the Açores convergence zone and the Caribbean arc areas

“…Indian Ocean [30] and for the 2011 Tohoku coseismic tsunamis [31], and for the 2018 Palu tsunami [47].…”

“…Tsunami simulations are performed for the 18 selected coseismic sources using the fully nonlinear and dispersive Boussinesq model FUNWAVE-TVD [61,50,31], by one-way coupling in a series of nested spherical or Cartesian coordinate grids. FUNWAVE has been validated for a collection of tsunami benchmarks [53,29], and used in the simulation of many historical [60,22,30,51,24,31,52,21,26,46,47] and hypothetical [23,1,25,54,49,27,45] tsunami case studies. As we only aim at assessing the global coastal tsunami hazard caused by the selected sources along the USEC, we only use two levels of nested grids (Fig.…”