NUMERICAL SIMULATION AS GUIDANCE IN
MAKING TSUNAMI HAZARD MAP
FOR LABUAN ISLAND
MOHD RIDZUAN bin Adam ∗MEE09199

Supervisor:

Fumihiko IMAMURA ∗∗ABSTRACT
At the northeast end of the South China Sea, tsunamis may be generated by the earthquakes along theeastward dipping subduction zone known as the Manila Trench. Tsunamis generated by major
earthquakes in this zone are expected to affect Malaysian coastal areas especially in Sabah and Labuan
Island. In this study, a numerical simulation of tsunami in Sabah and Labuan Island coast is conductedusing a modeling code called TUNAMI-N2 (Tohoku University’s Numerical Analysis Model for
Investigation of Near-field tsunami, No.2) to calculate the tsunami waveform at the output points. Twotypes of numerical simulations are performed in this study: (1) a numerical model based on uniform
grid systems in Cartesian coordinate with 1 arc-minute and 30 arc-seconds bathymetry data and (2) anumerical model based on the nested grid system in Cartesian coordinate using non-linear theory withfour different spatial grid sizes. Three scenarios of fault rupture are considered in the Manila Trench
for the earthquakes with magnitudes of Mw 8.0, 8.1 and 8.5. Results of the both methods are comparedand it is found that the tsunami amplitude is larger than those with use of smaller grid size ofbathymetry data. However, finer bathymetry and topography data are required for better and moreaccurate modeling. A computational instability problem is introduced and discussed in this paper inorder to obtain a better solution and stable condition in the numerical simulation. Finally, guidance formaking a tsunami hazard map in Labuan Island is discussed.
Keywords: Tsunami simulation, Manila Trench, Sabah and Labuan Island, Tsunami Hazard Map.

1. INTRODUCTION
In view of the tectonic setting, likelihood of tsunamigenic earthquakes occurrence in the South China
Sea which is close to the coastal areas of west Sabah is quite limited. As such, there is no or little
threat of local tsunami to these coastal areas. At the northeast end of the South China Sea, however,tsunami may be generated by the earthquakes along the eastward dipping subduction zone marked bythe Manila Trench. This is a zone with a potential to generate large earthquakes in the South China Sea
and further will generate tsunami which affects coastal area in Sabah and Labuan Island. The purposeof this study is to conduct the trans-oceanic propagation of tsunami expected from the Manila Trench,and then investigate the tsunami height and arrival time based on MMD tidal gauges located in Sabahand Labuan Island area. Next, the tsunami hazard assessment in the Labuan Island will be discussed
for guidance in making tsunami hazard map for the Labuan Island region.

∗Malaysian Meteorological Department (MMD).
Professor, Disaster Control Research Center (DCRC), Tohoku University, Japan.

∗∗12. DATA AND METHOD OF COMPUTATION
2.1. Tsunami Simulations, Bathymetry and Topography Data
We used Tohoku University’s Numerical Analysis Model for Investigation of Near-field tsunamis(TUNAMI-N2) codes developed by Disaster Control Research Center (DCRC), Tohoku University ofJapan (Imamura et al., 2006; Koshimura, 2010) for modeling propagation and inundation of tsunami incoastal area of Sabah and Labuan Island. The tsunami height and arrival time are compared between 3scenario earthquakes which will be described in
the next section; using uniform grid of 1
arc-minute bathymetry data, uniform grid of 30
arc-seconds bathymetry data, and nested grid data
which consists of 4 regions as shown in Figure 1.
We used bathymetry data from General
Bathymetric Chart of the Ocean (GEBCO)(https://www.bodc.ac.uk/data/online_delivery/gebco/). We also used nautical chart for the simulation
using the nested grid. For the calculation ofinundation area for simulation using the nestedgrid, we obtained data from Shuttle RadarTopography
Mission(SRTM)(http://www2.jpl.nasa.gov/srtm/) for Region 3 andtopography data from Advanced Spaceborne
Thermal Emission and Reflection Radiometer(ASTER)(http://asterweb.jpl.nasa.gov/)
forRegion 4. The detailed computation regions andthe data used for the simulations are shown in
Figure 1. Computation area using nested grids.
Table 1.
In this study, theTable 1. Computation regions and data used for simulation
temporal grid sizes (∆t) were set
using nested grids.
to 2.0 s for uniform gridRegion
Region 1Region 2Region 3
Region 4
computations and 0.3 s for nested
5.15°5.235°5.240°grid computation, which satisfy
Latitude5°-20°N
6.65°N
5.735°N
5.400°N
the CFL condition. Total duration
115.05°115.100°115.146°of the calculation is 480 minLongitude
115°-121°E
117.05°E
115.850°E
115.290°E
which has time steps of 14,400
Bathymetry
GEBCO 30 GEBCO 30
NauticalNauticalfor computational time in uniform
data
arc-sec
arc-sec
chart
chart
grid simulations and 96,000 forTopography
GEBCO 30 GEBCO 30
SRTM
ASTER
the nested grid simulation.
data
arc-sec
arc-sec
Tsunamiwaveforms
wereResolution
27 arc-sec
9 arc-sec
3 arc-sec
1 arc-sec
calculated at 18 tide gauge
Grid
800 x 1999
800 x 600
900 x 600
520 x 577
stations along Sabah and Labuan
dimension
Island coast as target points toobtain tsunami heights and arrival times.
2.2. Scenario Earthquakes in the Manila Trench
In this study, we investigated earthquake mechanisms such as seismic faults along the Manila Trench.
One earthquake scenario is proposed by Zaty (2007) in which the magnitude is Mw 8.0. Top depth of
the fault is 25km. On the other hand, the subduction zone of the Manila Trench might reach the west
coast of Mindoro Island. Generally, it is known that the tsunami energy propagates perpendicular tothe fault strike. The trench axis in this area existed from northwest to southeast. Therefore, the2direction perpendicular to the fault strike is expected to become the southwestern direction. The Sabahand Labuan Island coast are located to this orientation. Consequently, the tsunami created by theearthquake along the west coast of the Mindoro Island might affect the Sabah and Labuan Island coast.
Regarding this seismic fault of the Mindoro Island, we assumed the length of the fault to fit the trenchaxis. Due to this, we assume two more earthquakes scenarios (Mw 8.1 and 8.5), in which the fault area
consists of two fault segments. The fault parameters for these scenarios are listed in Table 2.
Single fault was applied in Scenario 1, while multi faults were applied in Scenario 2 andScenario 3. For the worst scenario in this study (Mw 8.5), we compare results obtained from theuniform grid simulation with the ones obtained from the nested grid simulation. In every scenario, assimulation Cases a, b and c we used 1 arc-min of bathymetry data, 30 arc-seconds of bathymetry dataand nested grid, respectively. The fault parameters are estimated using equations for dip-slip faults in
subduction regions proposed by Papazachos et al. (2004). The dip angle of the fault plane is assumed
equal to 30° proposed by Bautista et al. (2001).
Table 2. Faults parameters for each scenario earthquake.
No.

Magnitude
Mw

Length(km)Width(km)Strike(°)

Dip(°)

Rake(°)

Slip(m)Top
Depth(km)Latitude(°)

Longitude(°)

1

8.0
North8.1
SouthNorth
8.5
South

162.2
162.2
96
305.5
96

70.8
70.8
41.9
101.2
41.9

00315
0315

30
30
30
30
30

9090
90
90
90

2.19
2.19
1.29
4.57
1.29

25
25
20
25
20

13.8
13.8
13.2
13.8
13.2

119.5
119.5
120.2
119.5
120.2

2
3

3. RESULTS AND DISCUSSION
3.1. Tsunami Propagation toward Labuan Island
Here we show the snapshots of tsunami propagation only for Cases 1a, 2b and 3c in Figure 2. For
Scenario 1, in shallow coastal area especially near Mindoro, tsunami wave propagation moves wider
and crosses faster to Sulu Sea. The first negative wave reaches Manila Bay while positive wave
reaches Mindoro coast facing the tsunami source. The positive wave reaches Sabah coast at 2 hours
and 40 minutes after the earthquake and Labuan Island coast 3 hours after the earthquake.
For the magnitude of Mw 8.1 in Scenario 2, the tsunami wave traveling to southwestern
direction is a bit higher. The coastal area will be badly hit by the tsunami in the Philippines becoming
more extensive especially at the coast facing the tsunami source. The positive wave reaches Sabah
coast at 2 hours and 40 minutes after the earthquake and Labuan Island coast almost at 3 hours after
the earthquake. The small difference in magnitude of the earthquake and tsunami propagation direction
is not much influential to the tsunami arrival time in Sabah and Labuan Island because much of thetsunami energy have been dissipated by the time while they approach to this area.
In the worst scenario (magnitude Mw 8.5), the tsunami propagation is compared between
simulations using bathymetry of 1 arc-minute (Case 3a), 30 arc-seconds (Case 3b) and nested grid
simulation (Case 3c). Most Philippine coast area facing the earthquake will be hit badly by the tsunami.
Subsequently, the tsunamis will move towards Sabah coast and reach Kudat at after 2 hours and 38
minutes. Because of the shallow coastal area in Labuan Island, the tsunami wave arrive Labuan Island
coast at about 3 hours after the earthquake.

33.2. Maximum Tsunami Heightsand Travel Times

Figure 2. Snapshots of tsunami propagation at 0 (initialcondition), 30, 60, 120 and 180 min for Case 1a (top), Case2b (middle) and Case 3c (bottom). Tsunami height scale iscrimped to 0.2 m.

As shown in Figure 3, themaximum
tsunamiheightcalculated by using bathymetry
data of 30 arc-seconds is always
higher than the one obtained byusing bathymetry data of 1
arc-minute. From all scenarios,
tsunami heights simulated at tidegauges facing to the South China
Sea are slightly higher than otherareas. In the Sabah coast, thehighest tsunami obtained is 0.47 mat TG03 which is located near
Kota Belud. The second highest
tsunami obtained is 0.36 m at
TG02 which is located near Kota
Kinabalu. In Labuan Island coast,
the highest tsunami was calculated
at TG12 with 0.45 m which islocated near Kerupang Village atthe east part of Labuan Island.
As shown in Figure 4,
the first positive wave of tsunami
could reach the coastal area inabout 2 hours and 38 minutes. The
earlier tsunami arrives at thenorthern part of Sabah, the further
it will propagate to the west part.
In each scenario, the tsunami will
arrive at each target point slightly
earlier with the use of smaller gridsize in the simulations.

Figure 3. Maximum tsunami heights at each tide gauge station for all cases.

4Figure 4. Tsunami positive wave travel times estimated at each tide gauge station for all cases.
3.3. Computation Problems of Instability and Results in Labuan Island
Figure 5 shows some instability for Case
3c of simulation due to joining of two
different spatial grids between Region 3and Region 4 in the nested gridcomputation. For this result, we neglectobservation of tide gauges TG07, TG08,TG09, TG16, TG17 and TG18 for theCase 3c which are affected by thiscomputation instability, which could notcause low accuracy for the tsunami
Figure 5. Snapshots with instability using the nested grid ofevaluation in Labuan Island. Although weRegion 4.
have the instability problem at theboundary, the result for the analysis
should be reasonable. Therefore, basedon the numerical simulation, we could
obtain the result that the highest tsunami
calculated in Labuan Island is 0.45 m at
TG12 for Case 3b using bathymetry of 30
arc-seconds in uniform grid. So, there isno observation of inundated area inLabuan Island area obtained from thesimulation. We used a transform software
which is developed by Fortner Software
to investigate boundary condition
between coastal and land area of Region
4 in the nested grid as shown in Figure 6.
In the raster data for Region 4 which
covers the area of Labuan Island, theminimum elevation step for theFigure 6. Analysis of boundary condition between land
topography data form ASTER is 1 m. For
and sea in the numerical computation.
that reason, there will be no inundated
area shown in the simulation. In order to obtain more consistent and better results for the tsunami
computation, the use of detailed and accurate bathymetry as well as topography data should be
essential with a small grid size, and is much more effective in the numerical computation.

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3.4. Tsunami Hazard Map
From the simulation, we can obtain parameter such as tsunami travelling time, maximum andminimum water level at each output point and tsunami velocity which is important to estimate thedamage that may occur as a result of the tsunami. Lifeline facilities and residents living near thecoastal zone should be well prepared for the disaster. Hence, the tsunami hazard map containing theinformation of the evacuation area, the route for evacuation and the information about the tsunami
should be prepared and disseminated to the residents especially in the vulnerable areas.

4. CONCLUSIONS
We found that the fault ruptures along the shore of Philippines will generate a strong directivity oftsunami energy to propagate toward countries around the South China Sea and a very small effect tothe coastline of Sabah and Labuan Island because most of their energy would have been dissipated by
the time while they approach the west coast of Sabah. It is noticed that the tsunamis arrived earlier ineach scenario with use of smaller grid size in the simulations. As the results, the tsunami heights ofabout 0.47 m are calculated at the west part of Sabah and 0.45 m are calculated at the eastern part inLabuan Island facing the South China Sea.

5. RECOMMENDATION
The most important aspect to be tackled in the future is to improve the computed results for tsunami
height and travel time in the model by using more detailed and accurate bathymetry as well as
topography data which is essential for tsunami numerical computation. A more efficient technique andmodification to the program may be necessary to reduce computational instability in the simulation. Inaddition, few other scenarios should be studied to determine the effect of changes in top depth of faults
to estimate tsunami risk in the Sabah and Labuan Island coast.

AKNOWLEDGEMENT
I would like to express my sincere gratitude to Prof. Shunichi Koshimura, my advisor Dr. YushiroFujii and tsunami course leader Dr. Bun’ichiro Shibazaki for their continuous support, valuablesuggestion and guidance during my study. I would like to extend my gratitude for all DCRC laboratory
members especially to the PhD students for their support and valuable comments during my study inTohoku University.

REFERENCES
Bautista, B. C., Baulista, M. L. P., Oike, K., Wu, F. T. and Punongbayan, R. S., 2001, Tectonophysics,
279 -310.
Imamura, F., Yalciner, A.C., Ozyurt, G., 2006, DCRC, Tohoku University, Japan.
Koshimura, T., 2010, IISEE Lecture Note 2009-2010, 1-20.
Papazachos B. C., Scordilis, E. M., Panagiotopoulos, D. G., and Karakaisis G. F., 2004, Bull. Geo. Soc.
Greece, 36, 1482-1489.
Zaty, A. M., 2007, GRIPS, Tokyo, Japan, 1-45.

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