Coastal landslides in Palu Bay during 2018 Sulawesi earthquake and tsunami
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P. L.-F. Liu I P. Higuera I S. Husrin I G. S. Prasetya I J. Prihantono I H. Diastomo I D. G. Pryambodo I H. Susmoro
Coastal landslides in Palu Bay during 2018 Sulawesi earthquake and tsunami
Abstract In this paper, we investigate the sources for Palu Bay tsunamis, which occurred in September 2018 after a Mw 7.5 earthquake struck on a strike-slip fault. Previous land-based field studies have identified numerous coastal landslides as possible sources for tsunami generation. Here, we present new post-earthquake bathymetry survey data in the bay. We compare the new very detailed nearshore bathymetry with the pre-earthquake data so as to quantify the magnitudes and configurations of coastal landslides and the characteristics of landslide-generated waves. We perform numerical simulations of tsunami propagation, isolating the coastal landslides as the only sources. The results show that, although the landslide-generated waves characteristically have shorter periods than the observed tsunami waves, the combination of the waves is able to produce long waves comparable with those observed, due to the ringing effects of the trapped waves inside the bay. Therefore, we conclude that landslide-generated waves played a significant role in the tsunamis in Palu Bay. Nevertheless, it is likely that the tsunamis were caused by a combination of tectonic and landslide sources. Keywords Palu earthquake . Palu tsunami . Coastal landslides . Bathymetry . Survey Introduction On September 28, 2018 (10:02:45 UTC), a magnitude Mw 7.5 earthquake struck Sulawesi Island, Indonesia. The earthquake’s hypocenter was located in Central Sulawesi (Lat. 0.256° S, Lon. 119.846° E) at 20 km depth. Palu City is 72.2 km south of the hypocenter (USGS 2018). The ruptured fault is a part of the Palu-Koro Fault System, a major active strike-slip fault zone. After the main shock, tsunamis were observed inside the Palu Bay. In Fig. 1, a bathymetry map of Palu Bay, including the known fault lines (from Fig. 4 in Valkaniotis et al. 2018), is presented. On this map, the locations are identified by the nearest villages, and the coastal landslides detected are depicted as red crosses, with the most significant ones labeled by letters A–N. Numerous videos taken at Palu City (e.g., SM Table 3-A) show three leading tsunami waves with a period of 2.5 min attacking the city’s waterfront. These waves broke far from the coast because of shoaling and reached the coast as undular bores (see SM Table 3-A video). Additional videos show that several local waves were generated from discrete locations on both sides of the bay (Omira et al. 2019; Carvajal et al. 2019). Tsunamis were also captured by the tide gauge inside Pantoloan Port, which is located 20 km north of Palu City on the east coast of Palu Bay. The tidal gauge is located inside the harbor basin, which is protected by a slotted seawall so that the water inside is freely connected to the bay. Although the sensor records free surface elevation data every second, data is output every 30 s as the averaged value over that
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