IDENTIFICATION OF TSUNAMI DEPOSIT CHARACTERISTICS AND RECONSTRUCTION TSUNAMI RUN UP AND VELOCITY THE 2018 PALU TSUNAMI IN THE COASTAL LOLI PESUA, DONGGALA REGENCY, CENTRAL SULAWESI

• Main Author: wahyuni, Rizni
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/64479
• Institution: Institut Teknologi Bandung

Based on BMKG data, an earthquake occurred on the 28th of September 2018 at 18.02 WITA caused by the Palu-Koro Fault with 7.6 RS (Generated a tsunami in Palu- Donggala). The cause of the tsunami in Palu was submarine landslides and coastal collapse resulting from a great earthquake proved by 10 coastal collapses along Palu Bay. Field surveys of the tsunami had been conducted, but there is no further analysis to determine the characteristics of deposits. The purpose of the research was to determine the characteristics of the tsunami deposits, the characteristics of foraminifera, and the reconstruction of tsunami run-up and velocity. The research location is on the west coast of Palu Bay, in Loli Pesua Village, Banawa District, Donggala. A total of 22 samples taken at 5 spots sampling were taken every 10-15 m on a 75 m transect away from the shoreline. The analysis used is megascopic, grain size, geochemistry (X-Ray fluorescence and loss on ignition), and foraminifera assemblages. This research also reconstructs the tsunami run-up and velocity using Srisutam’s equation with the result of grain size analysis. The characteristics of the Palu tsunami deposit on 28th of September 2018 for the Loli Pesua area shows contact with pre-tsunami, dominated medium sand and coarse sand, dark-brownish, poor sorted. At the top of the tsunami deposit has fragments gravel-sized stones, laterally varying thickness of sediment 1–5.2 cm, presence of coral fragments and shells, relatively unimodal in mode 0.037phi - 5.764phi, coarse sand - coarse silt and poorly sorted - very poorly sorted. The tsunami deposit also has an abundance of elements Ca and Sr (18108.4 - 120808.1 ppm and 98.16 - 1456.17 ppm), organic material content 4.72% - 21.47% , and carbonate material content 5.83% - 55.83%. The results of vertical analysis of tsunami deposits concluded that there were 1-2 patterns of values in mean grain size, the number of foraminifera and the elemental Ca content which indicated the change was formed from 2 waves. The deposition of gravel fragments at the top of the tsunami deposit indicates the second wave is bigger so it carries bigger material. The third wave carries some of the sediment, leaving thin sediment and gravel. Foraminifera is formed by a transitional - middle neritic environment. Species of foraminifera found are similar to foraminifera species in coastal deposits. Foraminifera size test and abundant broken foraminifera test are used to support tsunami deposit characteristics. Broken foraminifera and many shell fragments are at the bottom of the tsunami deposit which is directly adjacent to the pre-tsunami deposit. The characteristics of the Palu landslide tsunami deposit in 2018 are based on a comparison of the results of previous research studies (in Lesehan and Kayumalue) showed that the tsunami deposit has a thin thickness, the curve mode is unimodal relatively, the foraminifera is formed by a transitional - middle neritic. The values of Ca, Sr, and the content of carbonate material in deposit tsunami has decreased vertically. In general, the characteristics of tsunami deposits due to landslides have a thin thickness and have relatively smaller values of Ca and Sr elements compared to tsunami deposits caused by earthquakes. The results of the calculation of tsunami mean run-up velocity is 1.703 m/s, run-up hight is 6.67m, and the inundation distance is 167 m. There is a significant difference between the results of the inundation calculation and field measurements. The results of this calculation that Srisutam’s equation uses a simple assumption which is the mean grain size of the tsunami deposit is fining landward and the tsunami deposit is no re-erosion of sediment by backwash so that the calculation results are not accurate.