The 2018 Palu Tsunami: Coeval Landslide and Coseismic Sources

Abstract

On 28 September 2018, Indonesia was struck by an MW 7.5 strike-slip earthquake. An unexpected tsunami followed, inundating nearby coastlines leading to extensive damage. Given the traditionally non-tsunamigenic mechanism, it is important to ascertain if the source of the tsunami is indeed from coseismic deformation, or something else, such as shaking induced landsliding. Here we determine the leading cause of the tsunami is a complex combination of both. We constrain the coseismic slip from the earthquake using static offsets from geodetic observations and validate the resultant “coseismic-only” tsunami to observations from tide gauge and survey data. This model alone, although fitting some localized run-up measurements, overall fails to reproduce both the timing and scale of the tsunami. We also model coastal collapses identified through rapidly acquired satellite imagery and video footage as well as explore the possibility of submarine landsliding using tsunami raytracing. The tsunami model results from the landslide sources, in conjunction with the coseismic-generated tsunami, show a greatly improved fit to both tide gauge and field survey data. Our results highlight a case of a damaging tsunami the source of which is a complex mix of coseismic deformation and landsliding. Tsunamis of this nature are difficult to provide warning for and are underrepresented in regional tsunami hazard analysis.

Anatomy of strike-slip fault tsunami genesis

“…Bao et al (13) and Socquet et al (14) were among the first to recognize the supershear nature of this earthquake (13,14). Several authors (13,(15)(16)(17) have postulated that submarine landslides, triggered by the earthquake's strong ground motion, were the primary source for the devastating tsunami. Ulrich et al (18) argued that the earthquake displacements were critical to the tsunami generation.…”

Field Insights and Analysis of the 2018 Mw 7.5 Palu, Indonesia Earthquake, Tsunami and Landslides

“…The Palu earthquake unexpectedly generated multiple tsunamis (Williamson et al, 2020) that caused severe damage and, in some cases, complete coastal inundation to low-lying settlements along Palu Bay (Fig. 7A and B).…”

“…The question considered was whether the Palu tsunami was triggered primarily by coseismic deformation beneath Palu Bay or shaking-induced landslides. Several studies (Sepulveda et al, 2020;Williamson et al, 2020;Liu et al, 2020;Carjaval et al, 2019;Gusman et al, 2019;Ulrich et al, 2019) have suggested the answer may be a combination of the two processes.…”

“…The tsunami source models for the Palu event rely predominantly on three key variables: (1) the tide gauge records (2) field survey data of runup heights; and (3) an accurate rupture model of the mainshock. Many models have demonstrated success in fitting one or more of these key observations (Carvajal et al, 2019;Heidarzadeh et al, 2019;Gusman et al, 2019;Takagi et al, 2019;Ulrich et al, 2019;Williamson et al, 2020).…”

“…If fault displacement were the main source, a significant component of horizontal motion of steep topography or dip slip motion within the fault structure hidden beneath the bay would be required to produce the vertical seafloor displacement necessary to generate the tsunami. However, analysis of the Pantoloan tide gauge record used in various tsunami models as well as integrated field surveys proves that the fault displacement alone was insufficient to generate a large tsunami (Sassa and Takagawa, 2018;Gusman et al, 2019;Takagi et al, 2019;Heidarzadeh et al, 2019;Carvajal et al, 2019;Omira et al, 2019;Sepulveda et al, 2020;Williamson et al, 2020).…”