Abstract
Tsunami waves severely damaged the densely populated coast of Palu City immediately after the 2018 Mw 7.5 Sulawesi earthquake. Among the several tsunami waves that arrived to the city, the two initial waveforms were most likely generated by a landslide at the south-western shore of Palu Bay, about 5 km away from one of the city’s shopping malls. The authors accurately identified the arrival time and direction of the waves by comparing multiple videos taken by a pilot from the cockpit of a plane and local people who witnessed waves approaching the coast. Although the authors’ bathymetric survey only covered a limited area of 0.78 km2, it was found that about 3.2 million m3 of mass disappeared from it, causing a maximum decrease in the seabed elevation of 40 m. A landslide scarp up to 5 m height was also investigated in the southwestern shore of the bay, which seems to be relatively minor compared to the submarine mass failure. Visible clue for liquefaction was not observed at this particular site. A simplified numerical model suggests that the landslide tsunami propagated as an edge wave and split into two separate waves due to the presence of an underwater shallow area just north of Palu City. Both waves arrived to the coast of this city within several minutes: one from North-West and the other from the North. Three major waves were witnessed by residents, who felt horizontal and vertical ground movements and heard the sound of an explosion just after the earthquake. Wave splash exceeded the height of trees on the beach. Given the results, the authors conclude that any modern early warning system is unlikely to work well against such short-warning time tsunamis, and thus, it is necessary for disaster risk managers to consider a way to help people quickly become aware of the potential disaster and evacuate.
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Acknowledgements
The authors would like to thank the citizens in Palu who kindly took part in the interviews after such a traumatic disaster. Special thanks should also be given to Mr. Icoze Ricochet, Mr. Hendri Dwi Selamet, and Ms. Sri Wachyuni for allowing the use of their invaluable videos, which enabled the precise identification of landslide locations and tsunami travel time. Tomoya Shibayama, Koichiro Ohira, Tomoyuki Takabatake, Takahito Mikami, Yuta Nishida, Harman Ajiwibowo, and Hendra Achiari also contributed useful discussion to this paper.
Funding
This research was funded through the grants from Tokyo Institute of Technology (Japan Society for the Promotion of Science, 16KK0121) and CONICYT (Chile) (FONDAP 15110017). A part of the present work was performed as a part of activities of the Research Institute of Sustainable Future Society, Waseda Research Institute for Science and Engineering, Waseda University.
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Takagi, H., Pratama, M.B., Kurobe, S. et al. Analysis of generation and arrival time of landslide tsunami to Palu City due to the 2018 Sulawesi earthquake. Landslides 16, 983–991 (2019). https://doi.org/10.1007/s10346-019-01166-y
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DOI: https://doi.org/10.1007/s10346-019-01166-y