Sediment analysis and historical context of the 2018 Palu-Donggala tsunami deposit, Indonesia

On 28th September 2018, a magnitude 7.5 earthquake occurred on the Palu-Koro strike slip fault near Palu, Indonesia. Shortly after, large tsunami waves generated by the earthquake and the submarine landslides it triggered, washed into Palu Bay. Here, we describe sediment characteristics of the tsunami deposits and present wave height models to provide a modern analogue for tsunamis occurring on strike slip faults. We analyzed 51 sediment samples from 21 sampling points on two transects at Palu City and one transect at Pantaloan. At Palu City, the tsunami inundated between 250 and 270 m inland. Sediments were massive, with landward fining from coarse sands (phi 0) to fine sands (phi 3.7) and weak upward fining sequences (by up to 1 phi). At Pantaloan the tsunami inundated up to 275 m inland. Sediments were massive, with landward fining from a few grains of gravel (phi -2) and coarse sands (phi 0.5) to fine sands (phi 2.5) and weak upward fining sequences (by up to 1 phi). Based on the sediment data the TSUFLIND model predicted: at Palu city flow depths of 1 to 8 m, and flow velocities up to 3.9 m/s; and at Pantoloan flow depths of 2 to 8 m, and flow velocities up 3.8 m/s. The boulder transport model was applied to data from heavy concrete blocks (0.8 to 4.9 tons) deposited by the tsunami between 46 and 125 m inland, and predicted flow depths of 0.8 to 4.3 m and flow velocities up to 5.6 m/s. The rapid attenuation inland of the tsunami flow depth is consistent with the event being partially generated by landslides caused by the PaluDonggala earthquake. A study of Dutch colonial archives and historical data reveals at least six tsunami events have occurred in or near Palu Bay since 1920, suggesting a very short return period for such events, and a significant tsunami hazard to the area.