Seismological and Geological Review of the 2018 Palu Earthquake and Tsunami Mechanism

Seismological and Geological Review of the 2018 Palu Earthquake and Tsunami Mechanism

On September 28 2018, after a magnitude 7.5 earthquake, an unexpected tsunami hit the coast of Palu Bay, Sulawesi, Indonesia, causing many casualties. The detected tsunami waves reached 8-10 meters in several locations (Muhari, 2018). Earthquake modeling based on teleseismic and geodetic data estimates the tsunami height to be no more than 5 m. Existing tsunami models cannot explain tsunami heights higher than 5 meters obtained from surveys nor tsunami wave heights reconstructed from video footage.
However, the most remarkable feature of the tsunami that devastated Palu was its extremely brief and instantaneous arrival. Typically, the time lag between an earthquake and its associated tsunami ranges from tens of minutes to several hours (Cienfuegos et al., 2018). The tsunami that occurred in Palu approached the coast in less than 2 minutes. Even in some videos it can be seen that the tsunami came in less than 100 seconds.
The 2018 Palu tsunami wave propagation modeling conducted by Liu et al., (2020) using COMCOT (Cornell Multigrid Coupled Tsunami Model) simulation
Carvajal, et al., (2019) found that the Palu tsunami that devastated the coastal area of Palu city had an unusually short wave period, which cannot be explained by earthquake faults alone. It is suspected that there was another mechanism in the occurrence of the 2018 Palu tsunami. The post-tsunami survey showed changes in the coastline, and this combined with video footage concluded that there had been an underwater landslide that caused a large tsunami in Palu Bay.

The epicenter of the earthquake was located in North Donggala Regency, Central Sulawesi, Indonesia. The quake had a magnitude (Mw) of 7.5 and broke the segment of the Palu-Koro shear fault that connects Sulawesi and connects with Palungasa in the north. The fault that occurs is approximately 200 km along the Palu-Koro shear fault.

Tectonic settings and the hypothesis of earthquake rupture in a horizontal fault earthquake in Palu. (Socquet et al., 2019). The black dotted line indicates the surface break or surface fracture proposed by Socquet et al. (2019); the red star indicates the center and mechanism of the earthquake focus (Carvajal, 2019).

Tectonic settings and the hypothesis of earthquake rupture in a horizontal fault earthquake in Palu. (Socquet et al., 2019). The black dotted line indicates the surface break or surface fracture proposed by Socquet et al. (2019); the red star indicates the center and mechanism of the earthquake focus (Carvajal, 2019).

Located 50 km southeast of the epicenter, the resulting tsunami had a flow depth of 8 m and a maximum height of 10 m. Since the earthquake was a horizontal fault, there is uncertainty about the creation of the resulting tsunami mechanism (Muhari et al., 2018). A total of 14 landslides have been detected on the main beach which extends above sea level along Palu Bay. Most of the landslides are located near river mouths, where sediments have been deposited for a long time. Other minor landslides occur away from river deltas, especially on the west coast, where land reclamation is carried out (Liu et al., 2020).
 

Tsunami observation and modeling results. images from Carvajal, etc., (2019) data from Socquet, etc., (2019)

Tsunami observation and modeling results. images from Carvajal, etc., (2019) data from Socquet, etc., (2019)