Abstract
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.
Highlights
The Sulawesi Island of Indonesia was the site of a large earthquake (Mw 7.5) on 28 September 2018 which was followed by a deadly tsunami
It is possible that these two sources are different parts of the same tectonic source because the isolated segment of the tectonic source located inside the Palu Bay may act independently towards tsunami generation within the bay
This may indicate that the timing of the Mamuju tide gauge station is not correct or possibly a different local source was responsible for the early tsunami in Mamuju
Summary
The Sulawesi Island of Indonesia was the site of a large earthquake (Mw 7.5) on 28 September 2018 which was followed by a deadly tsunami. Based on media reports and results of preliminary field surveys (Figure 2), the Sulawesi tsunami reached a maximum height of approximately 6-11 m along the coast of Palu within the Palu Bay (Fig. 1) (Muhari et al 2018). As of 10 October 2018, the reported number of deaths has been 1763, making the Sulawesi event as the deadliest event in Indonesia since the December 2004 Sumatra-Andaman earthquake and tsunami. Indonesia has experienced several deadly tsunamis since the December 2004 tragedy ([ 130,000 deaths; Synolakis and Kong, 2006) including the tsunamis of October 2010 Mentawai (408 deaths) (Satake et al 2013), the 2006 West Java (668 deaths) (Fujii and Satake, 2006), and the March 2005 Nias-Simeulu ([ 915 deaths, mostly from earthquake) (Borrero et al 2011)
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