Insights on the source of the 28 September 2018 Sulawesi tsunami, Indonesia based on spectral analyses and numerical simulations

Abstract

Copyright © The Author(s) 2018. The 28 September 2018 Sulawesi tsunami has been a puzzle because extreme deadly tsunami waves were generated following an Mw 7.5 strike-slip earthquake, while such earthquakes are not usually considered to produce large tsunamis. Here, we obtained, processed and analyzed two sea level records of the tsunami in the near-field (Pantoloan located inside the Palu Bay) and far-field (Mamuju located outside the Palu Bay) and conducted numerical simulations to shed light on the tsunami source. The two tide gauges recorded maximum tsunami trough-to-crest heights of 380 and 24 cm, respectively, with respective dominating wave periods of 3.6−4.4 and 10 min, and respective high-energy wave duration of 5.5 and >14 h. The two observed waveforms were significantly different with wave amplitude and period ratios of ~16 and ~3, respectively. We infer tsunamigenic source dimensions of 3.4–4.1 km and 32.5 km, for inside and outside of the Palu Bay, respectively. Our numerical simulations fairly well reproduced both tsunami observations in Pantoloan and Mamuju; except for the arrival time in Mamuju. However, it was incapable of reproducing the maximum reported coastal amplitudes of 6–11 m. It is possible that these two sources are different parts of the same tectonic source. A bay oscillation mode of ~85 min was revealed for the Palu Bay through numerical modeling. Actual sea surface disturbances and landslide-generated waves were captured by two video recordings from inside the Palu Bay shortly after the earthquake. It is possible that a large submarine landslide contributed to and intensified the Sulawesi tsunami. We identify the southern part of the Palu Bay, around the latitude of -0.82oS, as the most likely location of a potential landslide based on our backward tsunami ray tracing analysis. However, marine geological data from the Palu Bay are required to confirm such hypothesis.