Nat Hazards DOI 10.1007/s11069-016-2661-x ORIGINAL PAPER
Tsunami simulation due to seaquake at Manila Trench and Sulu Trench N. H. Mardi1 • M. A. Malek1 • M. S. Liew2
Received: 14 July 2015 / Accepted: 2 November 2016 Ó Springer Science+Business Media Dordrecht 2016
Abstract Seaquake is a phenomenon where there are water disturbance at the sea, caused by earthquake or submarine eruption. The scope of this study focuses on tsunami simulation due to Manila Trench and Sulu Trench seaquake which is prone to harm Malaysia offshore areas. Manila Trench is a highly potential earthquake source that can generate tsunami in South China Sea. Meanwhile, Sulu Trench could be a threat to east of Sabah offshore areas. In this study, TUNA-M2 model was utilized to perform tsunami simulation at South China Sea and Sulu Sea. TUNA-M2 model applied Okada source model to create tsunami generation due to earthquake. It utilized linear shallow water equation during tsunami propagation with its radiant boundary condition. Five simulations performed at each study region. Forecast points at South China Sea areas were divided into three separate locations which are at the Peninsular Malaysia, west of Sabah and Sarawak offshore areas. Forecast points at Sulu Sea were focused at the east of Sabah offshore areas. This paper will present the simulation results of tsunami wave height and arrival time at various forecast points. The findings of this study show that the range of tsunami wave height at Sulu Sea is higher than that of South China Sea. The tsunami arrival time at Sulu Sea is less than South China Sea. It can be concluded that Sulu Sea poses worse tsunami threat than South China Sea to the Malaysian offshore areas. Keywords Tsunami TUNA-M2 Malaysia Manila Trench Sulu Trench Seaquake
& N. H. Mardi
[email protected] M. A. Malek
[email protected] M. S. Liew
[email protected] 1
Civil Engineering Department, College of Engineering, Universiti Tenaga Nasional, Kajang, Selangor, Malaysia
2
Faculty of Geoscience and Petroleum Engineering, Universiti Teknologi Petronas, Tronoh, Perak, Malaysia
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1 Introduction Tsunami is described as a series of ocean waves formed when there is sudden disruption at the sea floor. Water will be vertically shifted, which will then create an abrupt vertical displacement of huge volume of seawater. Malaysia is a country located in South-east Asia where it is situated on the Eurasian Plate where seismic activities occur daily (Mardi et al. 2015). These seismic activities occur due to the instability of adjacent plates. The earthquake and volcanic activities are concentrated near to this boundary. This phenomenon could lead to the occurrence of tsunami events. Figure 1 shows the seismic hazard and relative plate motion at Philippine Sea plate by Smoczyk et al. (2013). The figure indicates that currently, the west part of Philippine Sea Plate is subducting beneath the Eurasian Plate. Manila Trench was located at the subduction zone. The relative motion of Philippines plate is *800 mm/year relative to Eurasian plate. This relative motion could increase; hence, it could cause earthquake and tsunami to occur in the near future. The Philippine Institute of Volcanology and Seismology has performed studies on tsunami prone areas in the Philippines as shown in Fig. 2. There are several trenches located around the Philippines namely the Manila Trench, Negros Trench, Sulu Trench, Cotabato Trench, Philippine Trench and East Luzon Trough. This study focuses on tsunami simulation from Manila Trench and Sulu Trench which could be a harm to South China Sea and Sulu Sea, respectively. Cotabato Trench and Negros Trench were not included in
Fig. 1 Seismic hazard and relative plate motion at Philippine Sea plate (Smoczyk et al. 2013)
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Fig. 2 Tsunami prone areas in the Philippines (PHIVOLCS 2008)
this study because these trenches are not harmful to Malaysian offshore areas. Cotabato Trench generate tsunami hazard within the Celebes Sea. Meanwhile, tsunami wave generated by Negros Trench will affect the Palawan Island.
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Manila Trench is identified as the most hazardous tsunami source in South China Sea (Liu et al. 2009). Manila Trench has also been classified as the highest earthquake tsunami source at the USGS tsunami workshop 2006. Meanwhile, Sulu Trench is located at the southern part of Philippines near Mindanao Island. Based on past tsunami events, at least two tsunamigenic earthquake occurrences have been recorded near the Sulu Trench Region (Nurashid et al. 2013). As a result, Sulu Trench has been categorized as one of the trenches in the Philippines that could be a threat to Sabah waters. The aim of this study is to determine tsunami wave height and arrival time. Results generated from TUNA-M2 simulation at Manila Trench and Sulu Trench will be compared. This study is significant to Malaysian offshore areas, since there are various industrial activities at the offshore areas of Sabah such as oil and gas industries located at the west of Sabah offshore areas. Fisheries and tourism industries are the most popular activities at the east of Sabah offshore areas (Malek et al. 2013). Therefore, the findings from this study can be used by the communities located at the offshore areas of Sabah for their tsunami preparedness plan. There are three stages in tsunami process namely tsunami generation, tsunami propagation and tsunami run-up and inundation (Mardi et al. 2015). This study is limited to tsunami generation and propagation because all forecast points are located at the offshore areas of Malaysia. The tsunami simulation was performed using TUNA-M2 tsunami numerical simulation model. There are five different earthquake moment magnitude applied during tsunami simulation that varies from 7.0 to 9.0 moment magnitude with increment of 0.5. The reason is to create different scenario of tsunami event. The tsunami wave heights are expected to increase as the earthquake moment magnitude increases.
2 Literature review South China Sea is a marginal sea that is part of the Pacific Ocean. The countries surrounding the South China Sea include Malaysia, Singapore, Thailand, Vietnam, China and the Philippines. Recently, the possibility of tsunami hazard occurrences at South China Sea has become an important global issue. Many research studies have been carried out to explore this phenomenon. At the USGS tsunami source workshop, Kirby et al. (2006) identify the three identified subduction zones that could harm the areas within South China Sea. Manila Trench has been found to give the most significant impact on areas in the South China Sea for tsunami hazard. Since then many researchers have been studying the possibilities of tsunami occurrences at the South China Sea. Presently, there are various studies conducted pertaining to tsunami at South China Sea. Each study has contributed to new finding. For example, Megawati et al. (2009) and Huang et al. (2009) perform tsunami research from Manila Trench towards Singapore water. Huang et al. (2009) has identified that based on the worst case scenario there could be earthquake of magnitude 9.0 generate from Manila Trench. In the other hand, Megawati et al. (2009) has found that GPS geodesy measurement indicates the convergence rate of Manila Trench is about 8 cm/year. Ha et al. (2009) has identified a wave with leading crest generate from Manila Trench would move through South China Sea towards China, Vietnam, Brunei, Malaysia and Singapore. Meanwhile, the trough head wave would propagate to eastern side of Philippines and Taiwan. Wu and Huang (2009) performed modelling tsunami hazard to Taiwan regions. Manila Trench is located about 100 km from Taiwan. From the study, it is found that tsunami
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hazard from Manila Trench are dangerous to the south-west and north-east coast of Taiwan. Further, Liu et al. (2009) performed a study to investigate the potential of tsunami hazards at the South China Sea. It aimed to establish a procedure for tsunami warning system for South China Sea. The warning system is capable to issue early warning information such as tsunami wave height and arrival time at surrounding countries of the Manila Trench earthquake generated tsunami. Ruangrassamee and Saelem (2009) had performed study on return period of earthquake event at each moment magnitude in the Philippines. The data used to construct the return period are collected from Advanced National Seismic System (ANSS) from 1963 to 2006. The results show earthquake return period of each moment magnitude. The probability of earthquake to occur at moment magnitude of 7.0 is at every 6 years. The highest earthquake at moment magnitude of 9.0 could happen at the probability of 667 years. The results of tsunami wave may varies depending on the moment magnitude of earthquake occurred. Zaharah Saleh (2012) has found that high magnitude could lead higher tsunami wave height. Teh and Koh (2010) has identified that Manila Trench that could become a serious risk to Malaysia, especially for Sabah and Sarawak. Tsunami simulation at South China Sea was performed using TUNA model. The result indicates that the Island of Hainan, Hong Kong and the coast of Vietnam are directly exposed to the risk of tsunami from the Manila Trench (Teh and Koh 2011). It is found that the risk occurring at the coast of Sabah can be mitigated by the presence of Palawan Islands. The tsunami wave may be deflected by the existing Palawan Islands instead of heading directly towards Sabah. Mardi et al. (2015) perform tsunami forecasting at the offshore areas. The study areas are focus on the South China Sea offshore areas where the source of tsunami is located at Manila Trench. TUNA-M2 model was utilized in the study. The tsunami simulation varies according to three different moment magnitude only which are 7.0, 7.5 and 8.0. The result show the range of wave height at Malay basin is 0.002–0.122 m. Sabah basin record the range of wave height is 0.004–0.168 m. The range of wave height at Sarawak basin is 0.004–0.230 m. From this study, it is found that Sabah basin has higher risk of tsunami wave follow by Sarawak and Malay basin. Sulu Trench is located at Sulu Sea. Its specific position is at the southern part of the Philippines near Mindanao Island. Cruz Salcedo (2011) have identified the set of earthquake parameters for events that could lead to the occurrences of large tsunami. This study showed six zones of tsunami sources causing tsunami in the Philippines where Sulu Trench is one of it. There are only few studies on tsunami occurrences performed specifically at Sulu Sea such as (Pedersen et al. 2010) and (Azis 2012). Pedersen et al. (2010) conducted a study on tsunami modelling and risk map for the east coast of Sabah. The risk map can be used by planners and authorities to be considered in the local planning. The risk map was developed from vulnerability and hazard mapping factors. The main concern of this study was the centre population within the coastline areas of more than 1000 km. It was found that the population is largely located at the east coast of Sabah where there were mostly fishing villages also known as the ‘Water Village’. This study has identified that the east coast of Sabah is a potentially high risk area. This is due to its location being within the active tectonic zone at Sulu Sea and Celebes Sea. Another study on tsunami numerical simulations at Sulu Sea and Celebes Sea was conducted by Azis (2012). Several trenches have been identified to be potential sources of earthquake tsunami which are Negros Trench, Sulu Trench and Cotabato Trench. Tsunami threats from these trenches could affect the east coast of Sabah. Azis (2012) concluded that among the three trenches that could generate tsunami, Sulu Trench is considered as the
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trench that has the highest impact as compared to others. The study also suggested that communities located at the east coast of Sabah need to be prepared for potential upcoming disasters. Based on reviews of tsunami simulation at the Manila Trench source generation, it can be concluded that this area is currently experiencing rapid growth. Many researches have performed various research works at countries surrounding the region of South China Sea. These studies strongly suggest that there is potential for the Manila Trench to generate tsunami events. Malaysia is one of the countries that could be affected. Results of this study are therefore pertinent, since there is no specific study carried out thus far that looks into the depth of Malaysia offshore areas and impact of tsunami to oil and gas industries located within South China Sea. Since very few studies on tsunami occurrences at Sulu Sea have been carried out, the studies reviewed in this section are considered as pertinent to shed light on potential of tsunami occurrences at the Sulu Trench. Based on the literature review, it shows that Sulu Trench has the potential to generate tsunami wave with due to its historical characteristics. Bautista et al. (2012) has found that there is quake of 7.5 moment magnitude within the Sulu Sea area occurred on 21 September 1897. The quake has led to tsunami event, and it has been reported by a few eyewitnesses. The tsunami has record maximum tsunami wave height of 6 m at the Zamboanga City. Meanwhile, the east of Sabah offshore area has record 2 m tsunami wave height. The location of east of Sabah which facing towards the Sulu Sea cause this area exposed to a high risk of tsunami threat from the Sulu Sea if there is tsunami event occurred from Sulu Trench.
3 Study region There are two tsunami case studies identified: (1) tsunami simulation at South China Sea and (2) tsunami simulation at Sulu Sea. Tsunami generated from Manila Trench will propagate towards the Peninsular Malaysia (PM), west of Sabah (WS) and Sarawak offshore areas (SK). Meanwhile, tsunami generated from Sulu Trench will propagate towards east of Sabah offshore areas (ES). There are twelve forecast points determined at South China and four forecast points located at Sulu Sea as shown in Fig. 3 and Table 1. The forecast points were determine based on the location of oil and gas offshore platforms and also based on the location of village water communities along the east coast of Sabah.
4 Methodology In this study, TUNA-M2 model is chosen to perform the tsunami simulation at Manila Trench and Sulu Trench. This model is able to simulate tsunami generation and tsunami wave propagation at offshore areas due to earthquake. TUNA-M2 is chosen to perform the tsunami simulation due to the speed of its computational period (Mardi et al. 2015). This model has been used to simulate tsunami events at the Indian Ocean (Cham et al. 2006), the Straits of Malacca, the Andaman Sea (Koh et al. 2009) and the South China Sea (Teh and Koh 2010, 2011). It is pertinent to highlight that TUNA has also been validated with other tsunami numerical models. Koh et al. (2009) had conducted studies on tsunami simulation at Andaman Sea based on 2004 case study where the first tsunami wave hits the northern part of Malaysia. The
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Fig. 3 Location of forecast points at South China Sea and Sulu Sea Table 1 Location of forecast points at South China Sea and Sulu Sea Study case South China Sea
Sulu Sea
Location
Forecast points
Peninsular Malaysia offshore areas
PM1, PM2, PM3, PM4, PM5 and PM6
West of Sabah offshore areas
WS1, WS2 and WS3
Sarawak offshore areas
SK1, SK2 and SK3
East of Sabah offshore areas
ES1, ES2, ES3 and ES4
simulation from TUNA model was compared with COMCOT model based on their computational domain using initial Gaussian hump. Next, the wave height result from both model was compared at three different locations. The simulation results from TUNA model show satisfactory performances compared to others model such as COMCOT (Cornell Multi-grid Coupled Tsunami Model) and on-site survey data for validation purposes (Koh et al. 2009). Tsunami generation TUNA-M2 model works on the basis of Okada source model in order to create initial displacement of tsunami wave. Okada model was developed in 1985 by Yoshimitsu Okada. This model is used as a function to calculate analytical solution for surface deformation due to shear and tensile faults in an elastic half-space. This model is
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widely used to simulate ground deformation produced by tectonic faults (earthquakes). There are two source model discussed in Okada (1985) which are point source and finite rectangular source. Several values of fault parameters are required in Okada source model in order to simulate tsunami source generation due to earthquake events. TUNA-M2 model utilizes shallow water equation (SWE) during tsunami wave propagation. Based on Intergovernmental Oceanography Commission (UNESCO 1997), a linear shallow water equation wave is valid since tsunami wave height is smaller in the deep ocean with large wavelength. Therefore, TUNA-M2 model is capable in simulating 2D tsunami generation and wave propagation. Equations (1), (2) and (3) are hydrodynamic equations where the conservation of mass and momentum is at average depth since tsunami propagation occurs in the deep ocean (Koh et al. 2009). og oM oN þ þ ¼0 ot ox oy ffi oM o M 2 o MN og gn2 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi M M2 þ N 2 ¼ 0 þ þ gD þ þ ot ox D oy D ox g7=3 ffi oN o MN o N2 og gn2 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi þ N M2 þ N 2 ¼ 0 þ þ gD þ ot ox D oy D ox g7=3
ð1Þ
ð2Þ
ð3Þ
In Eqs. (1), (2) and (3), the values of M and N are the discharge flux in x and y directions related to u and v velocities as shown in Eqs. (4) and (5), g is gravitational acceleration, h is sea depth, g is water elevation associate with tsunami and D is total water depth. D value can be expressed as (h ? g). M ¼ uðh þ gÞ ¼ uD
Fig. 4 Computational points for staggered scheme (Teh and Koh 2010)
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ð4Þ
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N ¼ vðh þ gÞ ¼ vD
ð5Þ
The shallow water equation was solved using explicit finite difference method with staggered grids. Figure 4 shows the computational points for staggered scheme where g, u and v are located. The partial derivatives were replaced by finite difference as shown in Eq. 6 where i and j denote as index at x and y direction, respectively, and k denote as time index. Staggered scheme involves half-integer spatial steps (i ? 0.5, j) (i, j ? 0.5) and half-integer time steps (k ? 0.5) (Teh et al. 2009). i Dt h i Dt h kþ0:5 kþ1 kþ0:5 kþ0:5 kþ0:5 Miþ0:5;j Mi0:5;j Ni;jþ0:5 ¼ gki;j Ni;j0:5 gi;j Dx Dy i Dt h k giþ1;j gki;j Dx h i ¼ hiþ0:5;j þ 0:5 gkiþ1;j þ gki;j
kþ0:5 k0:5 ¼ Miþ0:5;j gDkiþ0:5;j Miþ0:5;j
Dkiþ0:5;j
i Dt h k gi;jþ1 gki;j Dy h i ¼ hi;jþ0:5 þ 0:5 gki;jþ1 þ gki;j
kþ0:5 k0:5 ¼ Mi;jþ0:5 gDki;jþ0:5 Ni;jþ0:5
Dki;jþ0:5
ð6Þ
The time step Dt was restricted by the Courant criterion as shown in Eq. 7 (Cham et al. 2006). This process is required in order to ensure that the numerical scheme is stable. The stability condition is when the time step Dt must equal to be or smaller than the time required for the disturbance to travel the spatial grid size Dx (Kenji 2007). Dx Dt p 2gh
ð7Þ
Figure 5 shows the numerical approximations using staggered scheme. TUNA-M2 model had implemented radiant boundary condition at the study domain. This is to allow
Fig. 5 Numerical approximations using staggered scheme (Teh and Koh 2010)
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wave disturbance to pass through the open boundary without reflection (Cham et al. 2006). The wave energy can pass through the boundary and travel away from the system to avoid wave reflection or otherwise it would induce disturbance inside the computation domain (Koh et al. 2005).
5 Fault plane parameters The fault plane parameter is important component in order to create the tsunami generation. Tables 2 and 3 exhibit the hypothetical fault plane along Manila Trench and Sulu Trench. The fault plane along Manila Trench has been divided into six segments with total of 990 km length, and Sulu Trench has been divided into two segments with total length of 397 km length. For the purposed of this study, the longitude, latitude, length of segment, strike angle and dip angle was obtain from Liu et al. (2009) and Cruz Salcedo (2011) study. The rake angle was set at 90° in order to create the worst case scenario (Wu 2012). The parameters from Tables 2 and 3 will next determine the width of fault parameter using empirical equations. Equation (8) and (9) were obtained from study performed by Papazachos et al. (2004). Empirical equations developed by Papazachos et al. (2004) was applied in this study since it is one of the most recent studies performed and applied worldwide. It considered three mechanisms that include subduction zones and applicable for events ranging from 6.7 to 9.2 magnitude. log L ¼ 0:55M 2:19
ð8Þ
log w ¼ 0:31M 0:63
ð9Þ
where L length of fault plane, w width of fault plane, M moment magnitude. Based on the calculations, it is found that the width of each segment at Manila Trench is ranges between 53.00 and 88.00 km. The average of the calculated width is chosen as one value for all fault widths. In this study, 71.0 km is used as the width of fault plane at Manila Trench. Meanwhile, the width of each segment at Sulu Trench is ranges between 86.46 and 71.81 km. The average calculated width is 79.00 km used as the width of fault plane at Sulu Trench. Tables 4 and 5 exhibit the slip amount of fault parameter along Manila Trench and Sulu Trench. Usually, the actual value of slip motion, length and width of fault plane is measured after the earthquake event. Since this is forecast study, the value of slip amount was
Table 2 Hypothetical fault planes along Manila Trench (Liu et al. 2009) Segment
Longitude (°)
Latitude (°)
Length (km)
M1
120.5
20.2
160
M2
119.8
18.7
180
M3
119.3
17.0
240
M4
119.2
15.1
170
3
20
90
M5
119.6
13.7
140
320
22
90
M6
120.5
12.9
100
293
26
90
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Strike (°)
Dip (°)
Rake (°)
10
10
90
35
20
90
359
28
90
Nat Hazards Table 3 Hypothetical fault planes along Sulu Trench (Cruz Salcedo 2011) Segment
Longitude (°)
Latitude (°)
Length (km)
Strike (°)
Dip (°)
Rake (°)
S1
120.63
6.58
230
45
45
90
S2
121.72
7.95
167
30
45
90
Table 4 Slip amount of fault parameter along Manila Trench
Table 5 Slip amount of fault parameter along Sulu Trench
Segment
Slip (m) Mw: 7.0
Mw: 7.5
Mw: 8.0
Mw: 8.5
Mw: 9.0
MT1
0.104
0.585
3.292
18.514
104.112
MT2
0.093
0.520
2.926
16.457
92.544
MT3
0.069
0.390
2.195
12.343
69.408
MT4
0.098
0.551
3.099
17.425
97.988
MT5
0.119
0.669
3.763
21.159
118.985
MT6
0.167
0.937
5.268
29.622
166.579
Segment
Slip (m) Mw: 7.0
Mw: 7.5
Mw: 8.0
Mw: 8.5
Mw: 9.0
ST1
0.065
0.366
2.058
11.575
65.091
ST2
0.090
0.504
2.835
15.942
89.647
calculated using empirical equation where the rupture area is assumed as rectangular. Equation (10) and (11) were used to calculate the slip amount. Mo ¼ lDLW
ð10Þ
2 Mw ¼ log10 Mo 10:7 3
ð11Þ
where l rigidity of earth mantle (3.0 9 1010 N/m), D amount of slip motion, L length of fault plane, W width of fault plane. In this study, the slip amounts were determined according to five different moment magnitudes selected (Mw: 7.0, 7.5, 8.0, 8.5 and 9.0). The increment of moment magnitude represents the different amount of energy released at the source. Using Hanks and Kanamori (1979) relation as shown in Eq. (11), the value of seismic moment could be determined. Next, Eq. (10) was applied to obtain the value of slip amount. Seismic moment (Mo) of the earthquake is equal to the rigidity of the earth multiplied by the average amount of slip on the fault and the size of the area that slipped (William 1989). In this study, a grid dimension of 1851 9 1851 was used at Manila Trench case study. It contains a study domain within a rectangle bounded by 100°E–125°E in longitude and 0°N–25°N in latitude. The grid size chosen is 1500 m. Bathymetry data used in the simulation are obtained from ETOPO1 using 1 arc-min. Meanwhile, the grid dimension used for Sulu Trench study case is 1444 9 1111. The study domain is within a rectangle
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bounded by 115°E–125°E in longitude and 0°N–25°N in latitude. The grid size chosen is 1000 m. Bathymetry data used in the simulation are the same as in Manila Trench case study which is ETOPO1 at 1 arc-min.
6 Results of tsunami wave height and arrival time 6.1 South China Sea For this study, the result of tsunami wave height and arrival time is taken when the first wave hits each of forecast point. Table 6 shows the tsunami wave arrival time at each forecast point. Based on these results, it can be summarized that the tsunami wave height from Manila Trench will first move through the west of Sabah offshore areas followed by Sarawak offshore areas and lastly towards Peninsular Malaysia offshore areas. These results indicated that the tsunami wave will arrive at west of Sabah offshore areas after 2.03 h (forecast point WS2). It also indicated that the first tsunami wave will reach Sarawak offshore areas after 2.92 h (forecast point SK1) and reach Peninsular Malaysia offshore areas after 7.07 h (forecast point PM1). Figure 6 shows the time sequence of tsunami wave propagation at South China Sea. It exhibits the tsunami wave at its generation and the movement of tsunami wave propagation. The result of tsunami wave at Peninsular Malaysia, west of Sabah and Sarawak offshore areas was shown in Tables 7, 8, 9. The results of tsunami wave height at Peninsular Malaysia, west of Sabah and Sarawak offshore areas show similar pattern where tsunami wave height increases as the moment magnitude increase. The results indicate that tsunami wave height at Mw: 9.0 are higher compared to tsunami wave height at Mw: 7.0. For example, at offshore areas of Peninsular Malaysia record the wave height between 0.002 and 0.004 m during Mw: 7.0. As the moment magnitude increases to Mw: 9.0, the tsunami wave height increases to 1.925–3.872 m. The explanation on these increments is that large
Table 6 Result for tsunami arrival time at South China Sea Location
Forecast point
Peninsular Malaysia offshore areas
PM1
West of Sabah offshore areas
Sarawak offshore areas
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Arrival time (h) 7.07
PM2
9.04
PM3
10.61
PM4
8.17
PM5
8.10
PM6
9.22
WS1
2.34
WS2
2.03
WS3
2.55
SK1
2.92
SK2
5.09
SK3
2.66
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Fig. 6 Time sequence of tsunami wave propagation at South China Sea Table 7 Results of tsunami wave height at Peninsular Malaysia offshore areas
Moment magnitude
Wave height (m) PM1
PM2
PM3
PM4
PM5
PM6
Mw: 7.0
0.003
0.002
0.002
0.002
0.004
0.002
Mw: 7.5
0.016
0.013
0.011
0.011
0.022
0.011
Mw: 8.0
0.090
0.072
0.062
0.062
0.122
0.061
Mw: 8.5
0.508
0.403
0.350
0.348
0.689
0.342
Mw: 9.0
2.855
2.264
1.970
1.956
3.872
1.925
value of moment magnitude will produce high value of energy release during the earthquake event. The large energy released may lead to high tsunami wave.
6.2 Sulu Sea Table 10 shows the tsunami wave arrival time at Sulu Sea. Based on results obtained from this study, it can be summarized that the tsunami wave from Sulu Trench travels at higher speed since the area is smaller than South China Sea. The simulation reveals that it takes
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Nat Hazards Table 8 Results of tsunami wave height at west of Sabah offshore areas
Table 9 Results of tsunami wave height at Sarawak offshore areas
Table 10 Tsunami arrival time at Sulu Sea
Moment magnitude
Wave height (m) WS1
WS2
WS3
Mw: 7.0
0.005
0.003
0.003
Mw: 7.5
0.030
0.016
0.015
Mw: 8.0
0.168
0.092
0.084
Mw: 8.5
0.943
0.519
0.473
Mw: 9.0
5.302
2.919
2.659
Moment magnitude
Wave height (m) SK1
SK2
SK3
Mw: 7.0
0.005
0.007
0.004
Mw: 7.5
0.028
0.041
0.020
Mw: 8.0
0.157
0.230
0.112
Mw: 8.5
0.880
1.291
0.629
Mw: 9.0
4.951
7.261
3.538
Location East of Sabah offshore areas
Forecast point
Time of arrival (h)
ES1
1.33
ES2
1.11
ES3
0.55
ES4
0.35
only 0.35 h for a tsunami wave to reach forecast point ES4. It also indicates that after approximately 2 h, all offshore areas at the east of Sabah will experience the tsunami wave. Figure 7 shows the time sequence of tsunami wave propagation at the Sulu Sea. Table 11 exhibits the results of tsunami wave height at east of Sabah offshore areas. The simulation results of tsunami wave height at east of Sabah offshore areas show similar pattern to the results at Peninsular Malaysia, the west of Sabah and Sarawak offshore areas. The results of tsunami wave height at east of Sabah offshore areas also show tsunami wave height at Mw: 9.0 is higher compared to tsunami wave height at Mw: 7.0. The results show that at Mw: 7.0 the tsunami wave height is between 0.007 and 0.013 m. As the moment magnitude increases to Mw: 9.0, the tsunami wave height has increased to 6.713–12.858 m. Again, this is due to energy released where large energy released may lead to high tsunami wave height.
7 Discussion 7.1 Range of tsunami wave height at South China Sea In this study, there are three study locations identified at South China Sea. The locations are at the Peninsular Malaysia, west of Sabah and Sarawak offshore areas. Based on the
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Fig. 7 Time sequence of tsunami wave propagation at Sulu Sea Table 11 Results of wave height at east coast of Sabah
Moment magnitude
Wave height (m) ES1
ES2
ES3
ES4
Mw: 7.0
0.013
0.009
0.010
0.007
Mw: 7.5
0.072
0.051
0.059
0.038
Mw: 8.0
0.407
0.287
0.332
0.212
Mw: 8.5
2.287
1.611
1.870
1.194
Mw: 9.0
12.858
9.061
10.513
6.713
tsunami wave height results at Tables 7, 8, 9, the range of tsunami wave height and arrival time were identified at each forecast point. The minimum and maximum value of tsunami wave height at each forecast point was simplified in terms of range. Table 12 shows the
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range of tsunami height at South China Sea. Sarawak offshore areas have the highest range of tsunami wave height. This is followed by the west of Sabah and Peninsular Malaysia offshore areas. Table 13 shows the range of water depth at offshore areas of Peninsular Malaysia, Sabah and Sarawak. Data on water depth were collected using SEAFINE (SEAMOS-South Fine Grid Hindcast). Based on the water depth, it shows that Sarawak has crucial water depth which affects the results of tsunami wave. The water depth at forecast points in Sarawak has vast differences that could lead to high tsunami wave height. Comparison performed between the west of Sabah and Peninsular Malaysia offshore areas reveals that the range of tsunami wave height at the west of Sabah offshore areas is higher than tsunami wave height at Peninsular Malaysia offshore areas. Based on Table 13, it shows that water depth at the offshore areas of Sabah is shallow compared to the offshore areas of Peninsular Malaysia. Another additional factor is that the offshore areas of Sabah are located near to tsunami source generation which is Manila Trench. Therefore, these areas will experience large impact of tsunami wave compared to the offshore areas Peninsular Malaysia. Based on the findings of this study, the overall results of tsunami wave height at South China Sea suggested that the Sarawak offshore areas are the most hazardous area as it is exposed to the highest range of tsunami wave height and velocity. This is followed by the west of Sabah and Peninsular Malaysia offshore areas. It is also found that out of three areas in South China Sea, the west of Sabah offshore areas is classified as the most risk offshore areas since the tsunami wave from the Manila Trench could reach the offshore areas of Sabah after 2 h. The west of Sabah offshore areas is nearer to the location of tsunami sources generation. Therefore, this area will feel the tsunami impact first compare to Sarawak and Peninsular Malaysia offshore areas.
7.2 Range of tsunami wave height at Sulu Sea The east of Sabah offshore areas is the concern area at Sulu Sea. Based on the tsunami wave height result at east coast of Sabah, the ranges of tsunami wave height were identified
Table 12 Range of tsunami wave height at South China Sea Location Peninsular Malaysia offshore areas
West of Sabah offshore areas
Sarawak offshore areas
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Forecast point
Range of tsunami wave height (m)
PM1
0.003–2.855
PM2
0.002–2.264
PM3
0.002–1.970
PM4
0.002–1.956
PM5
0.004–3.872
PM6
0.002–1.925
WS1
0.005–5.302
WS2
0.003–2.919
WS3
0.003–2.659
SK1
0.005–4.951
SK2
0.007–7.261
SK3
0.004–3.538
Nat Hazards Table 13 Range of water depth at South China Sea
Location
Range of water depth (m)
Peninsular Malaysia offshore areas
51.00–70.00
West of Sabah offshore areas
14.00–37.50
Sarawak offshore areas
30.00–120.00
at each forecast point as shown Table 14. Again, the minimum and maximum value of tsunami wave height at each forecast point was simplified in terms of range. Comparing the results at Tables 12, 13, 14 it shows that the range of tsunami wave height at Sulu Sea is higher than South China Sea. This is because Sulu Sea has larger impact due to the fact that the Sulu Trench is located nearer to east of Sabah. Another reason is Sulu Sea has smaller areas compared to South China Sea; therefore, tsunami wave at Sulu Sea moves faster than tsunami wave at South China Sea. The simulation shows that it takes only 2 h for tsunami wave to cover the offshore areas at the east of Sabah. The first tsunami wave will hit forecast point ES4, 0.35 h later. Compared to South China Sea, it takes 11 h for the tsunami wave to cover the offshore areas at South China Sea. The first tsunami wave will hit forecast point WS2, 2.03 h later.
8 Perspective This study has successful simulate tsunami wave from Manila Trench and Sulu Trench. The simulation was performed using TUNA-M2 model. There are two outcomes from the simulation which are tsunami wave height and arrival time. It is found that tsunami from Sulu Trench is more hazardous than Manila Trench towards the offshore areas of Sabah. The reason is because tsunami from Sulu Trench produces higher wave height at east of Sabah offshore areas compared to tsunami from Manila Trench heading towards west of Sabah offshore areas. Based on the overall results, it can be concluded that the offshore areas of Sabah has higher risk to be affected by tsunami wave if tsunami event occur. The west of Sabah offshore areas experiences tsunami from Manila Trench. And meanwhile, the east of Sabah offshore areas experiences tsunami from the Sulu Trench. Both places are identified as hazardous areas since they are located near the source generation. But from analysis the Sabah offshore still safe when tsunami occurred at moment magnitude of 7.0–8.5. This is because at moment magnitude 7.0–8.5, the west of Sabah offshore areas will experience less than 1 m wave height and the east of Sabah offshore areas will experience less than 3 m wave height. Nevertheless, if there is tsunami event occurred at 9.0 moment Table 14 Range of tsunami wave height at Sulu Sea Location East of Sabah offshore areas
Forecast point
Range of tsunami wave height (m)
ES1
0.013–12.858
ES2
0.009–9.061
ES3
0.010–10.513
ES4
0.007–6.713
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magnitude, the offshore areas of Sabah is considered dangerous due to tsunami wave height that is higher than 6 m. The analysis of tsunami wave at South China Sea is significant because it has been confirmed through literature review of this study, that there are possibilities for tsunami events to be generated from the Manila Trench. Currently, the Malaysian oil and gas industries are growing actively at South China Sea. However, with current studies on tsunami waves in Malaysia thus far, there has been little focus on the offshore communities. Therefore, it is hoped that this study could provide the Malaysian offshore communities a better view on tsunami events and the risks that they face in the event of tsunami. The results from this study can be used to conduct further research on the impact of tsunami waves to the Malaysian oil and gas offshore platforms. Findings from this study are also significant to Sulu Sea where the east coast of Sabah can be affected by tsunami generated from the Sulu Trench. Currently, the east coast of Sabah offshore areas is popular with fisheries and tourism industries. The impact of tsunami wave on the offshore areas at the east coast of Sabah could affect the growth of these industries. Besides, the east coast of Sabah offshore areas are also known for their water communities. Results from this study can be used to raise awareness among the offshore communities pertaining to tsunami events. Offshore community awareness as well as, awareness among the people in the fisheries and tourism industries at the east coast of Sabah is especially necessary since this study has revealed that huge tsunami impacts can be expected from the Sulu Sea more than the South China Sea. A huge tsunami event can be expected within the Sulu Sea since; the area is much smaller as compared to South China Sea. Further analysis should be conducted at Sabah onshore areas where the safety of onshore communities is the main concern. More studies are recommended to be carried out on tsunami events at Sulu Sea. This is because Sulu Sea is identified as the most critical region that could affect Malaysian offshore areas. In the case of tsunami occurrences in the Sulu Sea, results from this study have proven that it only takes a few minutes for tsunami wave to reach the offshore areas of east Sabah. Acknowledgements The authors wish to express their sincere gratitude to Universiti Teknologi Petronas (UTP), Yayasan Universiti Teknologi Petronas (YUTP) and PETRONAS Carigali Sdn. Bhd on financial assistance to this study. The authors would also like to thank Assoc. Prof. Dr Teh Su Yean and Prof. Koh Hock Lye of Universiti Sains Malaysia for providing a good support on TUNA-M2 model.
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