TSUNAMI NUMERICAL SIMULATION AROUNDSULU SEA AND CELEBES SEA

Mazni AZISMEE10520

Supervisor:

Bunichiro SHIBAZAKI
Yushiro FUJII

ABSTRACT
We conducted tsunami numerical simulations around the Sulu Sea and the Celebes Sea using amodeling code called TUNAMI-N2. We investigated six scenarios for faults along the Negros Trench(2 segments), Sulu Trench (2 segments) and the Cotabato Trench (2 segments). We compared
simulated results in numerical models based on a uniform grid in Cartesian coordinate system between
using two bathymetry data, namely 1 arc-minute bathymetry data and 30 arc-second bathymetry datafrom GEBCO. The simulated results show that the maximum tsunami heights for simulations using 30
arc-second bathymetry data are mostly larger than those for simulations using 1 arc-minute bathymetrydata. There is a threat of tsunami to the eastern coast of Sabah from an earthquake event of Mw 8.3
along the Sulu Trench with the maximum tsunami height of more than 1 m. Besides, Sabah may
experience much smaller waves, of about less than 0.5 m for the tsunami source from the Negros
Trench and the Cotabato Trench. This is because of two factors such as attenuation at wide shallow
continental shelf and existence of small islands. The biggest maximum tsunami height at thePhilippines coast is more than 4 m at Bayawan in case of the tsunami source from the Negros Trench.
The tsunami arrives earlier for tsunami simulations using 30 arc-second bathymetry data than thoseusing 1 arc-minute bathymetry data for all tide gauge stations except Lahad Datu, Tambisan, Bataraza
and Bayawan. Detailed and accurate bathymetry data at shallower regions and near the coast areimportant to get reliable and much better results for tsunami computations.

Keywords: Tsunami, Maximum tsunami heights, Tsunami arrival times, Sulu Sea, Celebes Sea.

1. INTRODUCTION
Tsunami is one of the most devastating natural coastal disasters. Malaysian waters are comparatively
less exposed to the threat of tsunami but there are many factors to allow the tsunami effects which may
occur depending on the magnitude of the earthquakes, features of the seabed bathymetry, tectonics
boundary between the ocean and the land, and many others. The east coast of Sabah region tends to beaffected by tsunami hazards from earthquakes along the Negros Trench, Sulu Trench and the Cotabato
Trench. There are historical earthquakes in these subduction zones that caused tsunami(http://www.ngdc.noaa.gov). The purpose of this study is to conduct simulations of the trans-oceanic
propagation of tsunami in the Sulu Sea and Celebes Sea, and to investigate the maximum tsunami
heights and tsunami arrival times at eastern coast of Sabah and several areas at the Philippines coast.
This study also is aiming at investigation of the effect of maximum tsunami heights and tsunami
arrival times due to different bathymetry data and different resolution of computation.


Malaysian Meteorological Department (MMD).
International Institute of Seismology and Earthquake Engineering/Building Research Institute, Tsukuba, Japan.



12. DATA AND METHODOLOGY
2.1 Bathymetry data for tsunami simulations
TUNAMI-N2 (Tohoku University’s Numerical Analysis Model for Investigation of Near-fieldtsunamis, No.2) codes are used in this study. TUNAMI-N2 codes were originally developed in the
Disaster Control Research Center (DCRC), Tohoku University, Japan. The model has been
implemented widely for tsunami simulation and run-up for the Tsunami Inundation Modeling
Exchange (TIME) project (IUGG/IOC, 1997). The computation area is bounded by 2oN to 14oN in
latitude and 115oE to 127oE in longitude (Figure 1). We used 1 arc-minute bathymetry data and 30
arc-second bathymetry data from General
Bathymetry Chart of the Ocean (GEBCO) which
can be downloaded from thewebsite(https://www.bodc.ac.uk/data/online_delivery/gebco/). For each bathymetry data we elaborate two
tsunami simulations, firstly with resolution of 1
arc-minute and secondly with resolution of 30
arc-second. The temporal grid sizes (dt) were set to
3.0 sec and 2.0 sec for resolution of 1 arc-minuteand of 30 arc-second respectively which satisfied
the CFL conditions to stabilize the numericalcomputation. The number of grid point is 720 x 720
for resolution of 1 arc-minute and 1440 x 1440 for
resolution of 30 arc-second. The total duration ofcomputation time is 6 hours. Table 1 shows asummary of the data used for simulations.
Tsunami waveforms were calculated at 16 output
points, of which 5 existing tide gauge stations are Figure 1. Computational area in numerical model.
located around the coastal area of Sabah and 11 are Locations of each segment represented byassumed tide gauges along the eastern coast of rectangular blocks. The green and red triangles
represent the location of existing tide gauges andSabah and the Philippines coast (Figure 1).
assumed tide gauges, respectively.

Table 1. Summary of data used for simulation
Name of the case

Grid dimension

Case aGEBCO
1 arc-min1 arc-min(1850 m)
720 x 720

Case b
GEBCO
1 arc-min
30 arc-sec(925 m)
1440 x 1440

Case cGEBCO_08
30 arc-sec
1 arc-min(1850 m)
720 x 720

Case dGEBCO_08
30 arc-sec
30 arc-sec(925 m)
1440 x 1440

Temporal Grid Size (dt)

3s

2s

3s

2s

Bathymetry Data
Resolution

2.2 Earthquake sources in the Philippines
The Philippines have experienced many earthquakes including some earthquakes that can cause
tsunamis. For this study we used earthquake source parameters proposed by Salcedo (2010) from the
Negros Trench (NT), Sulu Trench (ST) and Cotabato Trench (CT). Six cases of fault are investigated
and the top depth of the fault is assumed to be 1.0 km for all cases. Table 2 shows a list of theparameters set for each segment in the Negros Trench, Sulu Trench and Cotabato Trench. The
locations of each segment shown in Figure 1.

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Table 2. Fault parameters for each scenario earthquake (Salcedo, 2010)

Number of
Scenario
Scenario 1
Scenario 2
Scenario 3
Scenario 4
Scenario 5
Scenario 6

SourceMagnitude
(Mw)

NT1
NT2
ST1
ST2
CT1
CT2

8.2
8.1
8.0
8.3
7.9
8.1

Location of
the fault
corner
Long
Lat
( o)
(o)
121.5
122.7
121.4
119.6
124.5
123.6

9.0
7.8
7.2
6.2
4.9
5.7

LengthWidth

Strike

Dip

Rake

Slip

(km)

(km)

(deg)

(deg)

(deg)

(m)

206
174
167
230
135
190

81.01
73.66
71.97
84.00
63.84
77.40

20
310
30
45
315
355

32
32
45
45
25
35

100
90
129
90
79
92

2.89
2.37
2.26
3.16
1.77
2.63

3. RESULTS AND DISCUSSION
3.1 Tsunami propagation
We investigate tsunami propagation considering four different cases as shown in Table 1 for 6
scenarios as shown in Table 2. Generally tsunami propagation in cases a and b are slightly different.
Tsunami propagation in cases c and d also have slight differences. We noted that tsunami propagationin cases a and b is significantly different from cases c and d in several scenarios. This is caused by thefact that, the tsunami propagation path depends also on bathymetry data. Tsunami propagation in casesb and d is smoother than in cases a and c, because tsunami simulation in cases b and d used a grid
resolution of 30 arc-second.
3.2 Maximum tsunami heights
3.2.1 Maximum tsunami heights for the tsunami source from the Negros Trench 1
For the tsunami source from the Negros Trench 1, the biggest maximum tsunami heights arecalculated at Bayawan for all cases. This is because Bayawan is located very close to the tsunami
source. At Bayawan, the maximum tsunami height for case 1a is 3.37 m while for case 1b it is 4.32 m.
For cases 1c and 1d the maximum tsunami heights are 3.19 m and 3.77 m respectively at Bayawan.
The maximum tsunami heights at Sabah coast were less than 0.5 m for all tide gauge stations while
they were more than 0.5 m for tide gauges located at the Philippines coast except for Puerto Princesa
and Balabac tide gauges. This is probably because of the characteristics of the sea bottom topography
and the tsunami energy directivity. The maximum tsunami height is larger for simulations using 30
arc-second bathymetry data (cases 1c and 1d) except the tide gauges at Lahad Datu, Tambisan,
Balabac and Bayawan compared with simulation using 1 arc-minute bathymetry data (cases 1a and1b).
3.2.2 Maximum tsunami heights for the tsunami source from the Negros Trench 2
In case of the tsunami source from the Negros Trench 2, maximum tsunami heights more than 0.5 m
were calculated at Bayawan, Iloilo (except for case 2b), Sindangan, Sibuco and Roxas (only for case2d) while others show maximum tsunami heights less than 0.5 m. The biggest maximum tsunami
height is 4.01 m at Bayawan for case 2d. This is because Bayawan is located close to the tsunami
source. The maximum tsunami heights are lower at coastal areas of Sabah even though tsunami energy
directly propagated to this area. This is due to a wide shallow continental shelf and existence of small
islands such as Kagayan de Sulu Island, Nunuyan Island and Berhala Island facing to the Sabah coast.
For this reason the tsunami heights will be reduced before arriving at the Sabah coast.

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3.2.3 Maximum tsunami heights for the tsunami source from the Sulu Trench 1
For the tsunami source from the Sulu Trench 1, the biggest maximum tsunami heights were calculated
for all cases at Sibuco. The maximum tsunami heights are 2.33 m for case 3c and 2.35 m for case 3d.
The maximum tsunami height at Sibuco varies very much between two bathymetry data. In this case,when the bathymetry data was details or precise, the obtained maximum tsunami height was bigger.
The difference in tsunami height is about two times. At Labuan tide gauge station, no computed
tsunami heights were available. This is because of the location of Labuan at which much of thetsunami energy has been dissipated by the time tsunami waves approach to this area. Thus, we canconclude that Labuan is not an affected area if an earthquake generates a tsunami from the Sulu
Trench 1.
3.2.4 Maximum tsunami heights for the tsunami source from the Sulu Trench 2
Figure 2 shows maximum tsunami heights for scenario 4 at each tide gauge station. It is observed thatthe maximum tsunami heights computed through simulations using 30 arc-second bathymetry data islarger than the one using 1 arc-minute bathymetry data except for stations of Lahad Datu, Tambisan,
Balabac and Sindangan.
The biggest maximum tsunami height is calculated at Narra (about 2.4 m for case d).
Other than that, at Roxas, Bataraza, Puerta Princesa and Sibuco were calculated the biggest maximum
tsunami heights with the heights of more than 1.0 m in cases c and d. This is mainly because thesestations are located directly along the path of the tsunami directivity. In view of the maximum tsunami
heights at the Sabah coast, they were about 1.3 m at Tambisan in case a, and 0.73 m at Sandakan incase c while less than 0.5 m maximum tsunami heights calculated at other tide gauge stations.
Sandakan may experience much smaller waves, of about 0.3-0.73 m even though it is located close to
the source. This is due to a wide shallow continental shelf in Sabah coast and small islands such as
Nunuyan Island and Berhala Island facing to the Sandakan.

Tsunami Heights (m)

2.5
2.0
1.5

case 4acase 4b1.0case 4c

0.5case 4d

0.0

Tide Gauge Stations

Figure 2. Maximum tsunami heights at each tide gauge for the tsunami source from the Sulu Trench 2.
3.2.5 Maximum tsunami heights for the tsunami source from the Cotabato Trench 1
The computed tsunami heights are very small at all tide gauge stations with less than 0.11 m for thetsunami source from the Cotabato Trench 1. This is because of small islands along stations of Sibuco
to Tambisan as well as the tsunami energy directivity towards Sulawesi. The biggest maximum
tsunami height calculated through this scenario is about 0.103 m (case c) at Lahad Datu.

43.2.6 Maximum tsunami heights for the tsunami source from the Cotabato Trench 2
For the tsunami source from the Cotabato Trench 2, the biggest maximum tsunami height is about 0.2
m (case 6d) at Lahad Datu. The maximum tsunami heights computed at Tawau and Tambisan areabout 0.13 m (case 6a) and 0.17m (case 6c) respectively. We can conclude that tsunami generated
from the Cotabato Trench 2 will not affect the eastern coast of Sabah area.

Tsunami Height (meter)

3.2.7 Comparison of maximum tsunami heights for all scenarios of case d
Figure 3 shows the maximum tsunami heights at each tide gauge station for case d with different
scenarios (different sources of tsunami). As a result, the biggest maximum tsunami height was
obtained at Bayawan station from scenario 2 with the tsunami source from Negros Trench 2. The
Sabah coast may experience tsunami heights less than 0.5 m for all sources except for a tsunami
source from the Sulu Trench 2 by which Sandakan and Tambisan may experience tsunami heights
more than 0.5 m. It is observed that tsunami generated at the Cotabato Trench 1 (case 5d) andCotabato Trench 2 (case 6d) would generate lower tsunami heights at each tide gauge station.
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0

case 1dcase 4dcase 2dcase 5dcase 3dcase 6d

Tide Gauge Stations

Figure 3. Maximum tsunami heights at each tide gauge station with different scenarios.
3.3 Tsunami arrival times
The tsunami arrival times were obtained at 16 output points along the Sabah coast and several areas at
the Philippines coast. In this paper, the tsunami arrival time is defined as the beginning time of the firstwave computed at tide gauge station. From this study, we find that tsunami arrived earlier for thetsunami simulations using 30 arc-second bathymetry data (cases c and d) than for those using 1
arc-minute bathymetry data (cases a and b) for all tide gauge stations except Lahad Datu, Tambisan,
Bataraza and Bayawan. Tsunami arrival times did not change much in cases simulation with the grid
resolution of 1 arc-minute and the resolution of 30 arc-second for the same bathymetry data.
Figure 4 shows the tsunami arrival times at each tide gauge station for case d withdifferent scenarios (different sources of tsunami). As a result, tsunami will arrive at Iloilo, Sindangan
and Sibuco just after the earthquake occurred at the Negros Trench 1, Negros Trench 2, and the Sulu
Trench 1 respectively. At the Sabah coast, tsunami arrived earlier at Tambisan (less than 25 min) for
the tsunami source from the Sulu Trench 2.

5Arrival Time (minutes)

360

NT1
ST1

300
240

NT2
ST2

180
120
60
0

Tide Gauge Stations

Figure 4. Tsunami arrival times at each tide gauge station with different scenarios.

4. CONCLUSIONSThe simulated results show that the biggest maximum tsunami height at Sabah coast is 1.271 m in case
4a (the tsunami source from Sulu Trench 2) at Tambisan while the biggest maximum tsunami height at
the Philippines coast is 4.322 m in case 1b (the tsunami source from Negros Trench 1) at Bayawan.
Sabah may experience much smaller waves, of about less than 0.5 m for all case scenarios (except
Sandakan and Tambisan in scenario 4). This is because of two factors such as attenuation at wide
shallow continental shelf and existence of small islands. The maximum tsunami heights for
simulations using 30 arc-second bathymetry data (GEBCO_08) are mostly larger than those forsimulations using 1 arc-minute bathymetry data (GEBCO).
The earliest tsunami arrival time is expected to be in less than 25 min at Tambisan in
scenario 4 (the tsunami source from Sulu Trench 2). At the Philippines coast, tsunami will arrive justafter the earthquake occurs in scenarios 1, 2 and 3 at Iloilo, Sindangan and Sibuco respectively. The
tsunami arrives earlier for the tsunami simulations using 30 arc-second bathymetry data (case c andcase d) than for those using 1 arc-minute bathymetry data (case a and case b) for all tide gauge stations
except Lahad Datu, Tambisan, Bataraza and Bayawan. Tsunami arrival times do not change much insimulations with the grid resolution of 1 arc-minute and the resolution of 30 arc-second for the same
bathymetry data.
It is finally concluded that, there is a threat of tsunami to the eastern coast of Sabah from
Sulu Trench 2. However, detailed and accurate bathymetry data at shallower regions and near the coast
are important to get more a reliable and much better result for tsunami computation. Hence, theresidents living the vulnerable areas should be well prepared for the disaster.
AKNOWLEDGEMENT
We used TUNAMI-N2 code (Imamura et al., 2006) for numerical simulation in this study. We thankto Ms. Salcedo and Dr. T. Hara for gives permission used earthquake source parameters data to
conduct tsunami simulation.
REFERENCES
Imamura, F., et al., 2006, Tsunami Modeling Manual, DCRC, Tohoku University, Japan, 1-72.
IUGG/IOC, 1997, IOC Manuals and Guides No. 35. UNESCO.
Salcedo J. C., 2010, Individual studies by participant at IISEE, Japan, 1-38.

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