Liquefied gravity flow-induced tsunami: first evidence and comparison from the 2018 Indonesia Sulawesi earthquake and tsunami disasters
On 28 September 2018, a strong earthquake with a moment magnitude of 7.5 occurred on the island of Sulawesi, Indonesia. This earthquake caused extensive liquefaction and liquefaction-induced flow slides inland. Despite a strike-slip fault, which typically displaces land horizontally, being unlikely to produce significant tsunamis, the earthquake in fact caused devastating tsunamis. Our field investigations showed that there was an occurrence of extensive liquefaction in coastal areas. Significant coastal liquefaction can result in a gravity flow of liquefied soil mass that can cause a tsunami. A comparison with a past disaster of the strike-slip fault Haiti earthquake tsunami indicated that essentially the same occurred at the Palu coast of Central Sulawesi. Namely, liquefaction-induced total collapse of coastal land caused liquefied sediment flows, resulting in a tsunami. An important difference between this time and Haiti was that such total collapses and flows of coastal land due to liquefaction occurred at several (at least nine) places, resulting in multiple tsunamis. Analysis of the tidal data implied that less than 20% of the tsunami height was related to tectonic processes, and the majority was caused by the coastal and submarine landslides as characterized by liquefied gravity flows.
KeywordsCoastal and submarine landslide Liquefaction Gravity flow Tsunami Sulawesi earthquake
Coastal inundation surveys showed that the tsunami inundation height was 3–4 m on average around the Palu bay and, was as high as 6.8 m in localized areas above the mean sea level and 6.2 m above the astronomical tide level when the earthquake occurred (the splash was excluded). These inundation heights are consistent with the prior observations made by the International Research Institute of Disaster Science (IRIDeS) at Tohoku University (2018). The average inundation distance was around 200 m, which is very short. This implies that the spatial extent of the tsunami source was much smaller than the scale of tectonic deformation of the Mw 7.5 earthquake.
Below, we present and discuss the liquefaction and liquefaction-induced phenomena that occurred along the Palu coast of Central Sulawesi, Indonesia.
Features of liquefied gravity flows
A gravity flow of liquefied sediment is a phenomenon triggered by a significant coastal liquefaction, which is then followed by the collapse of the liquefied soil under gravity. It is categorized as a coastal/submarine landslide that transforms itself into a high-density gravity flow and subsequently flows out over a long distance, leading to re-deposition. The concurrent processes and the dynamics of liquefied gravity flow that may have a significant impact on tsunami generation are governed by two-phase physics (Sassa and Sekiguchi 2010, 2012).
Liquefied gravity flow-induced tsunami: comparison with the past disaster and analysis
Our field investigation revealed new evidence of extensive liquefaction in coastal areas, not only inland but also along the coast.
Significant coastal liquefaction resulted in liquefied sediment flows, causing tsunami: liquefied gravity flow-induced tsunami.
Integrated use of multiple field evidence, state-of-the-art knowledge of liquefied gravity flows, and comparison with a past disaster involving a strike-slip fault earthquake and tsunami, together with analysis of tidal data, showed that the liquefied gravity flows induced multiple tsunamis, and hence, most of the earthquake and tsunami disasters in the 2018 Sulawesi earthquake stemmed from liquefaction.
Considering liquefaction risks and taking appropriate measures are thus crucial for mitigating the impacts of future earthquakes and tsunamis.
The field investigation was conducted by the authors as members of the mission team for damage and needs assessment of the Japan International Cooperation Agency (JICA) in cooperation with the Ministry of Land, Infrastructure, Transport and Tourism (MLIT), Japan. Both authors would like to thank JICA and MLIT for their assistance and cooperation during the field work.
This work was supported in part by JSPS KAKENHI Grant Numbers JP15H02265 and JP15H04052.
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