Study of tsunami source as preparation for tsunami modeling in Sulawesi

The Sulawesi region is prone to earthquakes due to the existence of a Subduction Zone which can generate a tsunami so that tsunami modeling is important as a tsunami mitigation effort in the future. The purpose of this research is to study the maximum earthquake potential in the Sulawesi Subduction Zone and to model the tsunami source with a multi-deformation scheme. The research method is a literature study using the 2017 National Earthquake Center book, tsunami modeling using an earthquake scenario with a magnitude of 8.5 Mw, a depth 20 Km, fault dimension modeling using global mapper software based on the Scaling law, fault dimensions with a length of the fault area and a width of the fault area, the earthquake parameters in the study consist of magnitude, epicentre depth, dip, slip, fault area length, the width of the fracture area and the dislocations. Tsunami source modeling uses the multi-fault. The results of this study are the maximum potential for an earthquake of around 8.5 Mw, the shape of the earthquake deformation and parameters as well as the tsunami source model that describes the sea level conditions experiencing an increase of 5,7 meters and for the decrease in sea level reaching -5.9 meters.


Introduction
The Sulawesi Island is a convergence point for three tectonic plates: the Eurasian Plate, the Philippine Sea Plate, and the Indo-Australian Plate.Due to these geological conditions, Sulawesi is prone to earthquakes.Earthquakes in the Sulawesi region originate from various sources such as the North Sulawesi Subduction Zone, the double collision of the Molucca Sea, the Philippine Sea Plate Subduction, and several active faults in the northern part of Sulawesi's mainland [1].The island of Sulawesi, with its high population density in the Eastern Indonesia region, is a primary focus of study.The literacy level regarding disasters significantly influences the positive impact of decisions made by residents living in tsunami-prone areas.
Seismic activity in North Sulawesi has been recorded twice with relatively significant magnitudes.The first earthquake occurred on January 1, 1996, with a magnitude of 7.9 Mw in Toli-Toli, triggering a tsunami.The second one occurred in Gorontalo on November 16, 2008, with a magnitude of 7.3 Mw, which also had the potential to cause a tsunami and resulted in casualties and material losses [2], [3].Additionally, the earthquake that struck Palu City on September 28, 2018, with a magnitude of 7.5 Mw, had the potential to generate a tsunami and caused building damage in the city [4].The convergence of the Eurasian Plate and the Pacific Plate in the Sulawesi Sea forms a subduction zone known as the North Sulawesi Megathrust, with a Geodetic displacement rate of 42-50 mm/year [5].The Indonesian Meteorology, Climatology, and Geophysics Agency (BMKG) states that significant Megathrust Based on the Segmentation Map and Maximum Magnitude of Subduction Zones in Indonesia [6], it is indicated that the North Sulawesi Megathrust can potentially cause a large-scale earthquake with a magnitude of 8.5.With such magnitude, it is highly likely to trigger a tsunami in Sulawesi.The potential for earthquakes and resulting tsunamis around the subduction zone makes the study of tsunami sources important for disaster mitigation and preparation for tsunami source modeling.Therefore, disaster education aims to disseminate knowledge among individuals and groups to help them take actions to reduce their vulnerability to disasters [7].Factors related to the geographical conditions of an area can influence the level of tsunami disaster risk.These aspects are considered in the tsunami modeling process, including land use and the morphology of the region [8].
In the effort to mitigate tsunami disasters, several pieces of information related to the research area are necessary.Mitigation is one of the efforts to minimize disaster risks, either through physical development or increasing awareness of disaster threats [9].The tendency of children to explore and learn in diverse ways [10] serves as a key factor in delivering effective education and training on disaster mitigation measures.In the research conducted by [11], from a disaster perspective, a significant factor in residents' decisions about staying in disaster-prone areas is the level of disaster literacy.Tsunami disaster literacy can include basic knowledge about the disaster itself, its impacts, and evacuation procedures.This study aims to explore the potential for maximum earthquakes in the Sulawesi Subduction Zone and model tsunami sources using a multi-deformation scheme.

Research Methods
The researches was conducted on Sulawesi Island which is directly bordered by the Flores Sea to the north; Sulawesi Sea to the south; Banda Sea to the east; Makassar Strait to the west.Geographically, Sulawesi Island is located between 5.36°N-7.48°Sand 117.02°BT-125.74°BT with an altitude between 200-3000 meters above sea level.The research location can be seen in figure 2.

Figure 2. Research locations on Sulawesi Island
The approach in this research involves using secondary data, particularly literature studies about the maximum potential in the subduction zone from the National Earthquake Center's book in 2017.Additionally, fault dimensions are modeled based on the Scaling Law, which is applied in Global Mapper to compare earthquake-triggering parameters used for estimating fault geometry dimensions [12].This study an earthquake strength of magnitude 8.5 due to presence of an active segment spanning 300 km [13] and includes nine earthquake-triggering parameters: magnitude, epicenter, depth, strike, dip, slip, L (length of fault area), W (width of fault area), and dislocation.Furthermore, there is also SRTM DEM data used to determine the coarse resolution of the sea surface and bathymetric data depicting the shape of the seafloor, explaining the presence of water covering the surface down to 0 meters.These data can be obtained from the Geospatial Information Agency and accessed through the website https://tanahair.indonesia.go.id [14], [15].Other parameters are based on the Slab2 USGS, which involves models for subduction zones that experience sparse earthquakes but possess a high level of danger.This aims to facilitate the development of methodologies related to subduction zone geometry [16].These parameter values are processed to create the initial tsunami source of the earthquake using a multi-deformation scheme as previously explained.The earthquake parameter values from this research are presented in Table 1 and the visualization map of the Location, Size and Shape of the 8.5 Mw Scenario Fault is shown in figure 3.This subduction zone experiences continuous activity, where each tectonic plate keeps moving, resulting in the potential for deformation on the seafloor [17].This study utilizes a potential magnitude of 8.5, which could generate a tsunami from the North Sulawesi Megathrust [18].The fault dimensions are divided into 5 segments, and the detailed fault directions for each segment can be seen in Figure 3.The construction of the tsunami source is conducted by identifying earthquake parameters due to vertical deformation on the seafloor [19].According to the Scaling Law by [20] the fault dimensions for an 8.5 Mw earthquake have a length (L) of 57.68 km and a width (W) of 108.4 km.Because of the significant initial momentum, tsunami waves experience varying heights, allowing them to propagate at specific 5 points on the Earth's surface and damage various human-made structures in the path of tsunami wave propagation [21].

Tsunami Source Model
Tsunami source planning in this context refers to the initial point of an earthquake undergoing changes in the seafloor depth.The source area is located along the subduction zone where the oceanic plate pushes down the continental plate, resulting in an upward movement manifested as a fault in the simulation of that area.The source file is used to correct spatial data, particularly in terms of bathymetry, as it is assumed that there are vertical changes in bathymetric data due to earthquake or fault activities [22].The simulation of earthquake scenarios that trigger tsunamis is carried out around Sulawesi Island.The goal is to provide an overview of the tsunami generation process that could impact Sulawesi Island, especially in Toli-Toli Regency and its surroundings, as it is the area closest to the tsunami point with an earthquake magnitude reaching 8.5 on the scale.The process of creating a tsunami source model (source) or commonly referred to as the initial model or initial condition [23], and the hypothetical tsunami model with multi-fault or multi-scenario [24], is focused on the potential maximum magnitude that could occur in the Sulawesi subduction zone.The results of the source with an earthquake strength of 8.5 depict the sea surface conditions with a rise of up to 5.7 meters and a decrease of -5.9 meters.The visualization of the dimensions and profile of the source can be seen in Figure 4.

Tsunami Modeling Preparation
Based on the tsunami source modeling mentioned by Hasan et al. (2015) [25], tsunami wave modeling can be divide into three stages, namely: 1. Modeling the earthquake source: Initial simulation of the tsunami induced by the tectonic plate movement of the Earth.2. Modeling tsunami wave propagation: The propagation of tsunami waves from the earthquake's epicenter to coastal areas.3. Modeling tsunami inundation: Simulation of the spreading of tsunami waves from the shore to the inland areas.The first stage, which is modeling the underwater earthquake situation, is formed using geophysical information and geological data, including data about potential faults in the subduction zone area.In this case, several faults with a history of producing tsunamis will be identified.Consequently, there be several earthquake sources with the potential for generating tsunamis.The next stage is the dispersion in the tsunami simulation, which describes the spread of the tsunami from the initial sea surface conditions after the earthquake to its propagation in all directions.This process is illustrated through the relationship between tsunami wave height and time.The propagation phase depends on how much the seafloor uplifts during the earthquake.As the tsunami approaches the coastline or land, especially in shallow waters, it functions to slow down or reduce the speed of the bottom part of the wave while the upper part of the wave remains fast.As a result, the sea wave height becomes higher, and the wave length becomes shorter.

Conclusions
Sulawesi Island was declared as a potential area for future earthquakes and tsunamis because the subduction in the Sulawesi submarine zone caused an earthquake with a magnitude of 8.5, resulting in a potential tsunami disaster.The results of the tsunami source model stated that Toli-Toli district, Central Sulawesi experienced a decrease in sea level reaching -5.9 meters and an increase reaching 5.7 meters.Based on these results, it can be estimated that the potential changes in sea level that can trigger tsunami waves with a height of more than 5 meters at the coastline.Based on these findings, mitigation efforts can be used as information to anticipate tsunami disasters, in addition to coordinating tsunami disaster mitigation efforts through mapping, monitoring, disseminating early warning information and conducting rehabilitation, reconstruction and spatial planning of coastal areas.

Figure 3 .
Figure 3. Visualization of Location, Size and Shape of 8.5 Mw Scenario Faults

Figure 4 .
Figure 4. Visualization of Dimensions and Profile of 8.5 Mw Scenario Source

Table 1 .
Seafloor Earthquake Parameters on Each Fault Sumber: Compiled by Researcher based on Blasé et al Scaling-Law (2010); BMKG Earthquake History; Study Earthquake Center (2017)