New research shows the role of earthquake motions in causing devastating 2018 tsunami in Palu, Sulawesi, Indonesia

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An international team of geologists, geophysicists, and mathematicians presented in a newly published study how coupled computer models can exactly display the conditions that resulted in the 2018 Palu earthquake and tsunami – one of the deadliest natural disasters in the world in 2018. 

The tsunami stirred near an active plate boundary where earthquakes typically occur. Large-scale tsunamis are commonly triggered by vertical earthquake motions, but this particular tsunami in Indonesia was caused by a horizontal ground offset. The team was briefly astonished as to how the waters were stimulated into a tsunami. To them, it could be landslides, faults, or both.

Satellite data of the surface rupture indicated that it was straight faults but these do not cover areas offshore. This lead to the next hypothesis– the shape of the faults beneath Palu bay. The researchers argued whether it is significant for stimulating the tsunami since the earthquake moved rapidly. Furthermore, they also theorized if the rupture speed triggered the tsunami to be more powerful.

The scientists used a supercomputer operated by the Leibniz Supercomputing Centre, a member of the Gauss Centre for Supercomputing. It was shown that the movement of the ocean floor beneath Palu bay was prompted by the earthquake and could have stimulated the tsunami. This information also meant that landslides data are not needed to identify the main features of the tsunami.

Using a combined earthquake-tsunami on LRZ computing resources, LMU researchers were able to uncover the cause of the 2018 Palu Bay earthquake's devastation. Video credit: LMU.

In the model, the slip is lateral and downward along the fault, causing a vertical seafloor change of 0.8 meters to 2.8 meters (31 inches to 110 inches) with an average of 1.5 meters (59 inches). The tilted fault geometry and combination of lateral and extensional strains that occurred on the region are critical to triggering this tsunami source.

As part of the conclusion, the researchers used a detailed earthquake-tsunami model based on earthquake physics that is different from the usual data-driven ones. It incorporates models of the complex physical processes happening at the fault and off the fault.

This output enables the scientists to come up with a realistic model that works with regional tectonics and earthquake physics. Prior to this, the team also evaluated the scenario against various available sets of information. The modeled tsunami wave amplitudes corresponded the available measurements and the modeled inundation elevation.

This output provided a quick physics-based evaluation of the interactions between earthquake and tsunami especially on this complex order of events.

Visualization of the modeled coupled earthquake and tsunami across Palu Bay, from Ulrich et al., 2019: Left: Seismic waves being generated while the earthquake propagates southward in a 'superfast' manner. Warm colors denote higher movements across the geological faults and higher ground shaking (snapshot after 15 seconds of earthquake simulation time). Palu Bay generates a 'surprise' tsunami (snapshot of the water waves, 20s of the time of the tsunami). Image credit: LMU.

Thomas Ulrich, PhD student at Ludwig Maximilian University of Munich and the lead author of the study stated, "finding that earthquake displacements probably played a critical role generating the Palu tsunami is as surprising as the very fast movements during the earthquake itself,"

He added, "We hope that our study will launch a much closer look on the tectonic settings and earthquake physics potentially favoring localized tsunamis in similar fault systems worldwide."

Reference:

"Coupled, Physics-Based Modeling Reveals Earthquake Displacements are Critical to the 2018Palu, Sulawesi Tsunami" – T. Ulrich et al. – Pure and Applied Geophysics – https://doi.org/10.1007/s00024-019-02290-5 – OPEN ACCESS

Abstract

The September 2018, Mw7.5 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 to6 m left-lateral slip and up to 2 m normal slip on the straight fault segment dipping 65East 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 gaugeand inundation that reproduces observations from field surveys. We conclude that a source related to earthquake displacements is probable and that landslide may not have been the primary source of the tsunami. These results have important implications for forsubmarine strike-slip fault systems worldwide. Physics-based modeling offers rapid response specifically in tectonic settings that are currently underrepresented in operational tsunami hazard assessment.

Featured image credit: LMU

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