Submarine landslide-induced tsunami generation in Makassar strait, Indonesia using Non-hydrostatic wave model

Tsunami modelling of potential landslide-induced tsunami in Makassar Strait is carried out to quantify possible damage to the nearby cities. The numerical model is used to represent the wave generation by using NHWAVE model. The simulations consist of a series of scenarios based on distinct size of the landslide volume. Four landslides with volume 5, 8, 70, and 200 km3 are used as tsunami sources in the initiation stage. The sources are evenly distributed in the strait addressing different landslide location. Maximum wave heights of 1.5 m are found in the area between Palu and Bangkir from case 1 and around Talok from case 2 simulations. The empirical run-up calculation of 7.5 m is estimated at the land for the presented wave height. The value significantly elevates the case 3 and 4 proportional to the volume values. The waves impact more than half of coastline with maximum value found in the Sulawesi side. Interestingly, wide and narrow shelf next to Kalimantan Island plays an important role in reducing the tsunami hazard level.


Introduction
A tsunami is a major threat to Indonesia archipelago due to its location surrounded by the active subduction zones.The tectonic stresses that are released from the subduction of Indo-Australia plate beneath the Sunda shelf are potentially generated tsunami [1].For instance, one of the most devastating catastrophic occurred in December 2004, which triggered by Mw = 9.3 Sumatra-Andaman earthquake.This tsunami generates 30 m run-up height in Aceh, Indonesia [2] and approximately 20 m in Thailand [3].
Despite the majority of large tsunamis are produced by underwater earthquakes [4], the latest tsunami event in Indonesia revealed the importance of studying other tsunami sources (such as subaerial, submarine landslide and earthquake-induced landslide).For instance, the flank collapse of Anak Krakatoa on 22 December 2018 generated tsunami and affected certain areas around Java and southern Sumatra, killed 437 people [5].Field survey reported that this tsunami produced wave run-up more than 85 m at the northern coast of Rakata and 83 m at the southern coast of Sertung [6].Rakata and Sertung 1250 (2023) 012020 IOP Publishing doi:10.1088/1755-1315/1250/1/012020 2 islands located less than 5 km from Anak Krakatoa and facing directly its Southwest flank.A numerical study to investigate the effect of landslide volume in Anak Krakatoa tsunami has been done by [7].By using an assumption that a rigid body with a volume of 0.15 km3 sliding into water as a granular flow under gravity forces, destructive wave of 1.7 m height is observed in the Tanjung Lesung (approximately 50 km south east of Anak Krakatoa) after 28 minutes of propagation.This phenomenon is described as the most damaging volcanically-resulted tsunami since the eruption of Krakatoa in 1883 [8].It's worth to note that the Anak Krakatoa tsunami shows that the tsunami induced by a subaerial or a submarine landslide can generate destructive tsunami waves [9,10].
Generation mechanism of tsunami induced by a landslide is more diverse compared with the earthquake-tsunami, in which composed of the impulsive waves due to subaerial landslides falling into the water with high velocities [11,12,13] to huge submerged landslide [14].Furthermore, a combination of earthquake triggers subaerial or submarine landslide-induced tsunami has elevated its threat to the coastal area.Particularly, submarine landslides can be triggered by a moderate magnitude of earthquake which take place in the continental slope [15].The example of such mechanism occurs on 28 September 2018 tsunami in Palu, Sulawesi.An earthquake of 7.5 Mw struck at the land of Sulawesi followed by a destructive and deadly tsunami producing measured run-up of 9.1 m at Benteng village.Ten large coastal sectors have reportedly collapsed into the sea after the earthquake.The measured tsunami data collected around Palu Bay clearly show the contribution of secondary non-seismic local sources generating tsunami [16].Similar finding has been reported in another research [17].They concluded that among the arrived tsunami waves there are two initial waveforms most likely generated by a submarine landslide with approximately volume of 0.02-0.07km 3 at the Southwest part of Palu [18].As a result of 3.2 million m 3 of materials lost from the seabed causing a maximum decrease in the seabed elevation of 40 m.These results were supported by [19] which highlighted that the generation of Palu tsunami was a combination of sea floor changes as a result of earthquake followed by landslides.More importantly, tsunami heights from a combination of small to large submarine landslides along the Palu shores could reach up to 7.0 m [20].
Due to its strongly varying initiation of motion and complex interplay between different triggers mechanism, the quantification of the tsunami genesis from the submarine landslides are highly important.Using bathymetric data collected from MERAMEX (2004) and SINDBAD (2006) surveys, six submarine landslides in the eastern part of Sunda margin which span from central Java to Sumba Island have been identified [21].The volume ranges from 1 km 3 in the Java basin to 20 km 3 in the Sumba trench.A series of simulations by using the aforementioned landslides unveiled run-up height of 7 m, 6 m, 3 m, and less than 2 m at Sumba, Lombok, Bali, and Java, respectively.
While major effort has been invested to study [22] and developing a tsunami early warning system, mitigation plans, and significant amount of works in tsunami modelling and risk assessments in the western part of Indonesia, less attention has been given to the eastern part of Indonesia.Numerical simulation of potential tsunami shows that the maximum tsunami height is more than 10 m locally and exceed 20 m next to the source [23].This condition strongly indicates that the eastern part has similar order of tsunami threat compared with the western side of Indonesia.It should be noted that, more active regions strongly describe that the tsunami hazard and risk in the eastern side are alarmingly high.
Among the eastern area, the highest frequency of tsunami events for Indonesian archipelago takes place in Makassar Strait.Collaborative research has evaluated slope stability in the Makassar Strait as a result of a huge amount of sediment being transported from the Mahakam Delta [24].The strait, in which is being known as the pathway for transferring water mass from Pacific to Indian Ocean can trigger slope failure due to massive sedimentation or erosion.Approximately 100-600 km 3 of alluvial sediment were observed on the west side of the Makassar Strait.This is a tectonically active region and a place where the sediment deposited.Previous tsunami study has revealed the necessity to account the secondary generation mechanism, submarine landslide [25].In his research, submarine landslides are believed has strong contribution to add more energy of tsunami.Another work shows similar agreement indicated the importance role of landslide-induced tsunami in the Makassar Strait [26,27].
The Makassar Strait separates western and eastern Indonesian archipelago.This area is classified as a marginal basin occupying the continental slope and rise regions within Sulawesi and Kalimantan Islands [25].It's located at the intersection of four major plates, such as the Indo-Australian Plate in the southern part, Eurasian Plate lied in the west, Pacific Plate in the east, and Philippine Sea Plate in the north-west area.Developing a complex system of subduction, back-arc-thrusting, extension and major transform zones [24].The seabed features of Makassar Strait distinctly describe two steep gradient zones as the southern and northern boundaries representing the Pastenoster and Palu-Koro transform fault.Historical data describe that seismic activities are mostly dominated by shallow depth earthquakes as a result of back-arc-opening zone in the strait and north-south Palu-Koro fault movements [1].As expected, a previous study unveils lower magnitudes (5.5-6.6) of tsunamigenic earthquakes at the Pasternoster than at the other faults (6.3-7.7)[25].He concluded that both empirical and numerical calculations of initial tsunami wave coming from Pasternoster sources are too small for destructing the coastline.Hence, a secondary mechanism such as submarine landslide is important to consider.In fact, Due to its narrow width, it is possible that far-field earthquake is the triggering mechanism for the landslide inside the strait.Furthermore, the research domain is in Makassar strait with the observation point around Kalimantan and Sulawesi (Fig 1).

Fig 1. Study location of tsunami modelling in Makassar strait.
A high rate of sediment transport results in under-consolidated accumulation of the Mahakam Delta is considered as the prominent contributor to the slope instability.It is fine grained sediment composed of high organic contents and gas charge, which significantly recede the sediment shear strength produced instability slope condition.Furthermore, interpretation of seismic, bathymetry and gravity core offshore the Mahakam Delta indicates high susceptible landslide.Following the aforementioned findings, the primary focus on this research would be on tsunami generation which account for submarine landslide.To the best authors knowledge, this study is the first landslide-induced tsunami simulation in Makassar strait.1250 (2023) 012020 IOP Publishing doi:10.1088/1755-1315/1250/1/0120204 2. Ease of Use

Tsunami Simulation
The characteristics of tsunami generated by submarine landslide are mainly determined by the volume, initial acceleration, maximum velocity, and possible retrogressive behaviour of the landslide [28].Tsunami generation in this research is simulated using a three-dimensional shock capturing Non-Hydrostatic Wave (NHWAVE) [29].The model domain covers most part of the Makassar Strait (see Fig 2) composed of uniform rectangular grids.Eights layers are imposed in the vertical direction.The horizontal grid resolution is 100 m yielding in total 277 grids in x-direction and 295 grids in y-direction.Four tsunami sources were distributed in the Strait with the total volume 4, 8, 70, and 200 km 3 following geographic, oceanographic, and seismic study [24].These sources are modelled as a rigid body falls into the water and it excludes the deformation effects.The simulation time for each source is 30 minutes.Tsunami characteristic time (tc) is hopefully achieved in NHWAVE.Besides that, the driving forces such as surface elevation, velocity component in x-direction, and velocity component in y-direction that obtained from NHWAVE solutions after tc is fulfilled can be used in FUNWAVE-TVD model to simulate the tsunami propagation.Bathymetry data with 6" resolution used inside the domain was obtained from National Hydrographic Chart (BATNAS).In addition, detailed information about the landslides can be seen at Table 1.Bathymetry and topography data were obtained from The Geospatial Information Agency of Indonesia (BIG).Landslide locations were defined based on geographic, oceanographic, and seismic study [24] (grey diamonds).Earthquake data were retrieved from Global CMT https://www.globalcmt.org/(redcircles).Figure was made by using generic mapping tools (GMT) version 6.1

Case 1: Submarine Landslide at the Palu-Koro Fault
The first tsunami scenario is caused by a hypothetical slope failure at the Palu-Koro fault.The slope failure occurred in water depths of 2000 m (Fig. 2) with plane underwater slope of 15 o (θ = 15 o ).The characteristics of submarine landslide modelled by NHWAVE illustrate an initial acceleration of 0.08 m/s 2 (ao), a terminal velocity of 25 m/s (vt), and a characteristic time (tc) of 312.5 s.The characteristic time (tc) which defined as tc = vt/ao plays an important role to specify the moment after the landslide occurred.It marked the time to introduce the initial condition from tsunami generation model to the tsunami propagation model (NHWAVE to FUNWAVE).More importantly, it is a time when the buildup of tsunami wave finish and transformed from potential to kinetic energy [21].
The first reporting positive wave was at Bangkir synthetic wave gauge with amplitude of 0.7 m, in which followed by -0.8 m wave trough.The Fig 3b1 clearly describes that subsequent wave arrived periodically at 10-12 minutes interval with significantly lower amplitudes.After approximately 120 minutes since it's generation, the tsunami waves were observed at Palu synthetic gauge.Wave amplitude of 0.15 m and 10-12 minutes wave period were recorded in this site (Fig 3b2).The tsunami waves required more time to reach the Kalimantan Island compared to the Sulawesi Island.The first-arrived tsunami wave at the Talok synthetic wave gauge was the minimum surface elevation of -0.2 m followed by the maximum free surface elevation of 0.15 m as shown in Fig 3b4.
The maximum surface elevation plot (see Fig 3a ) shows the maximum value of surface elevation in each grid during whole simulation time.For the sake of clarity, the maximum surface elevations less than 0.3 m were removed from the plot.The values were in the range of 0.8 to about more than 5 m in the simulation domain.The maximum value of 1.8 m was observed in the area between Bangkir and Palu with the tsunami time arrival less than 30 minutes.In the southern part of Palu, the maximum surface elevations of 0.3 -0.8 m were recorded with the arrival time approximately 50 minutes.Analysis from the maximum surface elevation plot explains that there is no coastal region in the Kalimantan Island observed surface elevation more than 0.3 m (see Fig 3b4).It is worth to note that, the plot not only unveils the wave coming from direct tsunami but also the tsunami reflections, diffractions and refractions.

Fig 3.
Maximum wave amplitudes of case 1 (left) and recorded surface elevation at synthetic observation points (right).Plots were made by using matplotlib python library.

Case 2: Submarine Landslide at the western part of the Makassar strait
The source of the tsunami in this scenario is a slope failure at the West side of North Makassar Strait.This event took place in water depths of 1500 m as a result of massive depression near the shelf edge involved 8 km 3  Interestingly, the maximum values are in the range of 0.6-0.8m, in which two times higher than other coastline areas (less than 0.3 m).We believe that bathymetry condition around Palu Bay was the key point in this condition.The average value of the tsunami time arrival in this region is 90 minutes.

Case 3: Submarine Landslide next to Mahakam Delta
The scenario is a failure triggered by massive sedimentation in Mahakam Delta.The total volume of the landslide is 70 km 3 following previous study of [24].Similar landslide parameters with the previous cases are applied.
The first-observed tsunami wave was at Bontang station with -2.0 m wave trough 50 minutes after the landslide, followed by 3.0 m high wave approximately 10 minutes later as shown in Fig 5b3 .The amplitude of this wave is much higher than the previous two cases.Then, a 1.

A subsection Case 4: Worst case scenario of submarine landslide at Mahakam Strait
In this scenario, a total of 200 km3 sediments is used as the driving force for tsunami generation.The number of sediment volume is derived from comprehensive study by considering sediment deposit coming from Mahakam Delta [24].As a result, the landslide is categorised as the failure due to the massive sedimentation and moved as a translational slide.This slump occurred at 1500 m water depths, with 0.25 m/s 2 initial acceleration, and 67.5 m/s terminal velocity.The value of terminal velocity is equal with the Storega landslide simulation and it is assumed as the possible value of maximum velocity.

Discussion
An initial standpoint of submarine landslide as secondary tsunamigenic mechanism in Makassar Strait has been previously reported [25].In their report, seismic profile data of the Southern Makassar Strait described the condition of an older trench filled with slumping materials.Unfortunately, there is no further study to unravel the tsunamigenic sources in the Strait until recently geological study of Makassar strait is published [24].By using comprehensive data comprised of bathymetric, oceanographic, and seismic, their research concluded that significant amount of sediments has been deposited along the shelf next to the Mahakam Delta.The analysis of tsunami hazard due to the landslide failure in this study is a scenario-based tsunami hazard assessment (STBHA) since there is no primary data of landslide in this region.Such procedure has shown satisfying result in some instances, if the imperfection and uncertainty in the assumed scenario is properly considered.The assumptions in this research are there is only single landslide for each scenario (i.e.there is no subsequent slump during simulation) and only accounts for four landslides.Another critical point is that no direct run-up calculation originating from the model because of poor resolution of the grid.Horizontal grid size at least 50 m is required to accurately resolved computation of tsunami inundation and run-up [30].Hence, the run-up calculation in this study used empirical formula [31], in which significantly influenced by slope condition at nearshore and wave height at continental basin.
In order to understand the threat from tsunami to the surrounding coastline, it is important to provide a tsunami wave height threshold.Experienced from previous tsunamis 2018 in Palu and Krakatoa, extreme damage to houses and vehicles may occur due to strong water mass flux and currents in the nearshore.The minimum tsunami wave amplitude or run-up at 1.5-2.0m has ability to caused severe damage [32].Another study chose tsunami run-up of 1.5 m for potential coastal damage and 3 m for having significant consequence to the coastal [33].
The simulations carried out in this study indicate potential severe damage to the local coastal area in Makassar strait.Impacts are varied depend on the generating mechanism, landslide characteristics and regional bathymetry.Simulations with landslide volume less than 10 km3 (case 1 and case 2) reveal few sites with wave more than 1.5 m high.These areas are the Southern side of Bangkir (case 1) and the bottom part of Kalimantan Peninsula (case 2).By assuming the slope in that region is 10 o and 5 m of water depth (d), the calculated run-up from [31]'s formula is 7.5 m which is believed has enough energy to produce severe damage at the land.Notice that, these are resulted from mid-size landslide and even less than 1/10 volume of the worst scenario.The third and worst-case simulations will significantly elevate the hazard of tsunami to the both islands.
The landslide tsunami presents more challenges compare with the tsunamis generated by earthquake.For instance, the subduction faults are generally well known and has been studied by broad communities.By comparison, landslides generated tsunami are nature local features and very difficult to be uncovered except with collective research.Since Palu tsunami 2018, the government has deployed many sensors around the archipelago, but still there is no particular technologies to deal with the landslide-induced tsunami.As a result, a number landslide-tsunami simulations are required to provide more understanding and alert to the society.

Conclusions
Tsunami generation simulation is conducted in order to analyse the potential tsunami hazard in Makassar Strait caused by a submarine landslide.The Strait is defined as a highly tectonic active region and its location where a significant amount of sediment being deposited.The sea bed features of this area clearly show two alignments of the continental shelf in the side of Kalimantan and Sulawesi Islands.Based on the aforementioned condition, both deposition or erosion has possibilities to trigger slope failure and due to the narrow width, far-field earthquake can also destabilize the slope.
Different wave heights are produced from the simulation, which depend on the volume size demonstrating various damage levels to the shore.A landslide with volume less than 10 km3 is calculated and produced 0.8 to 1.5 m wave height at 5 m depths.Estimated run-up of 7.5 m is obtained for 1.5 m wave heights.Case 3 and case 4 produced maximum tsunami wave in the range of 1.5-3.0m in the Kalimantan side and 3.0-7.8m in the Sulawesi side.As the result, higher run-up and stronger tsunami waves are predicted from the two worst scenarios.Due to the lack of landslide parameters in Makassar Strait, a scenario-based study is used which underlie on the assumption that only single source of the landslide exists for each scenario.There is no subsequent slope failure in the simulation.The assumption might lead to under-predicted results.Therefore, it is important to conduct a comprehensive survey as a way to provide primary data.

Fig 2 .
Fig 2. Study location comprises of NHWAVE and FUNWAVE model domains in Makassar Strait.Bathymetry and topography data were obtained from The Geospatial Information Agency of Indonesia (BIG).Landslide locations were defined based on geographic, oceanographic, and seismic study[24] (grey diamonds).Earthquake data were retrieved from Global CMT https://www.globalcmt.org/(redcircles).Figure was made by using generic mapping tools (GMT) version 6.1 of sediment materials.The failures of the materials are in parallel direction with the slope indicating a typical slide style.The submarine landslide features are similar with the case 1, except the initial acceleration, ao = 0.25 m/s 2 , produced tc = 100 s.The wave crest and trough initial value after the landslide occurred are 3 and -6 m at the centre mass.The first-recorded tsunami wave at Bontang station was -0.14 m of trough wave about 50 minutes after initiation (see Fig 4b3).These waves were then followed by 0.13 m high wave 10 minutes later.The Palu station observed a 0.4 m high wave 90 minutes after the landslide as shown in Fig 4b2.The maximum elevations plot distinctively shows higher value at the southern part of Palu coastline (Fig 4a).

7 Fig 4 .Fig 5 .
Fig 4. Maximum wave amplitudes of case 2 (left) and recorded surface elevation at synthetic observation points (right).Plots were made by using matplotlib python library The time characteristic produced by this scenario is 270.0 m/s.Amplified surface elevation is observed after more than 1 hour simulation as the wave commenced to reach the Shelf of Samarinda and Southern part of Palu coastline (see Fig 6b4).The wave trough with magnitude less than 0.05 m was observed at Bontang station after 110 minutes which followed by 0.18 m high wave 10 minutes later (Fig 6b3).Higher wave amplitudes were recorded at Samarinda and Palu station after 120 minutes.The first arrived waves at Samarinda station were -0.5 m trough wave followed by 1.5 m high wave 10 minutes later.These waves clearly showed typical of tsunami waveform.Fig 6b2 describes that high wave with 2.0 m amplitude is found at Palu station.The second observed wave at this station is -3.8 m wave about 10 minutes after the first wave.The following wave trains at Palu station are still high, even after the second wave with 1.0 m wave magnitude as indication of amplification process due to wave reflection, undulation, and refraction.The maximum surface elevation showed disparity values in the range of 3.0-9.3m, particularly in the side of Sulawesi Island (southern part of Sulawesi) as clearly depicted in Fig 6a.These sites are directly hit by the waves during simulation.Interestingly, there are no areas with surface elevation maximums exceed 0.3 m in the side of Kalimantan Island despite some waves propagated directly to the shore.Overall, the patterns are similar with the case 3 where the amplification at Sulawesi Island exist.

Fig 6 .
Fig 6.Maximum wave amplitudes of case 4 (left) and recorded surface elevation at synthetic observation points (right).Plots were made by using matplotlib python library

Table 1 .
Landslide characteristics used by NHWAVE model and coordinate of each coordinate.The north direction is pointed by 90 o .