Synopses of Master Papers

Bulletin of IISEE, 47, 127-132, 2013

TSUNAMI INUNDATION MODELING ALONG THE
EAST COAST OF SABAH, MALAYSIA FOR
POTENTIAL EARTHQUAKES IN SULU SEA
Nur Intan Irzwanee Nurashid ∗MEE11623

Supervisor:

Bunichiro SHIBAZAKI ∗∗Yushiro FUJII ∗∗Hideaki YANAGISAWA ∗∗∗ABSTRACT
A tsunami numerical inundation modeling in the east coast of Sabah was developed by consideringlarge earthquakes along the Sulu Trench as the tsunami source. The simulation was conducted using
modified TUNAMI-N2 (Tohoku University’s Numerical Analysis Model for Investigation of Near
Field Tsunami, No. 2) with a nested grid system in spherical coordinate system. Nonlinear shallow
water theory with bottom friction with four different spatial grid sizes was used in the computation.
Three scenarios of different magnitudes, M w 8.2, M w 8.5 and M w 8.8 were assumed. The results
showed that in Tambisan Island the first tsunami wave arrived 22.5 min after the earthquake withmaximum tsunami height of 8.6 m and tsunami run-up of 10.1 m in Scenario 3 (M w 8.8). In Sandakan,
the first tsunami arrived 93 min after the earthquake with maximum tsunami height of 3.8 m andtsunami run-up of 4.0 m in Scenario 3 (M w 8.8). These results show that Tambisan Island andSandakan have a risk of tsunami threat from megathrust events along the Sulu Trench.
Keywords: Tsunami simulation, Inundation, Sulu Trench, Sabah, Sandakan, Tambisan Island.

1. INTRODUCTION
A tsunami is a natural disaster that occurs as a result of the earthquakes in the subduction
zone. Although seismicity is considered low in Malaysia and Malaysian oceans are also
considered less vulnerable to tsunami; nevertheless it can still occur depending on several factors suchas the magnitude of earthquakes, features of the seabed bathymetry, tectonics settings, and manyothers. The east coast of Sabah may be affected by large earthquakes along the Sulu Trench, which iscapable of producing tsunamis. There were historical tsunamis recorded along this subduction zone,
and at least two tsunamigenic earthquakes occurred near the Sulu Trench region. The east part ofSabah is sparsely populated, but its population is predominant in the coastal area. There are water
villages along the coast which increase the vulnerabilities when tsunami occurs in the Sulu Sea.
Mazni (2011) has demonstrated that the largest tsunamis at eastern coast of Sabah can becaused by large earthquakes along the Sulu Trench. Hence, this study is to develop an inundation
model from the trans-oceanic tsunami in the Sulu Sea using a nested grid system in the sphericalcoordinate system. This is required in order to assess the potential of tsunami impact from any future
Sulu Trench earthquakes. This study also is aiming to investigate the maximum tsunami heights andtsunami arrival times at the eastern coast of Sabah.

∗

Malaysian Meteorological Department (MMD).
International Institute of Seismology and Earthquake Engineering/Building Research Institute, Tsukuba, Japan.
∗∗∗
Department of Regional Management, Faculty of Liberal Arts, Tohoku-gakuin University.
∗∗

2. DATA
2.1. Tsunami Simulations, Bathymetry and Topography Data

Tsunami simulation is performed using simulation code which is developed by Yanagisawa(2012) based on the TUNAMI-N2 in nested grid system. Nonlinear shallow water theory withbottom friction in the form of Manning’s formula is applied in the computation. Bathymetry
and topography data for the first to third region are taken from the General Bathymetric Chart of the
Ocean (GEBCO) (https://www.bodc.ac.uk/data/online_delivery/gebco/) 30 arc-second (resampling to
1 min grid size). Bathymetry and topography for the fourth region are interpolated from the nauticalchart(Ridzuan,
2010)
andShuttleRadar
Topographic
Mission(SRTM)(http://www2.jpl.nasa.gov/srtm/). Table 1 shows the boundary, resolution, location and data source foreach region.
Table 1. Boundary, resolution and data source for each computational region.
Longitude

RegionLatitude

Min

Max

Min

Max

o

o

o

o

10

1’

2”

20”

0.6667”

1

115

123

2

2

117o30’119o30’

5o

6o30’

a

o

118 45’o

119 20’

o

5 15’

o

5 45’

b

117o55’118o20’

5o40’

6o05’

a

119o03’

119o11’

5o25’

5o30’

b

118o03’118o09’

5o51’

5o55’

c

o

o

o

o

3

4

Grid Size
SpatialTemporal

118 02’119 08’5 47’

6.6667”0.2222”2.2222”0.0740741”

Data Source
Bathymetry

Topography

GEBCO 30”

GEBCO 30”

NauticalChart

SRTM 3”

5 51’

Total computation time is 360 min which has the time steps of 10,800. Tsunami waveform
is calculated at several assumed tide gauge stations along the Tambisan Island and Sandakan coast to
obtain tsunami height and arrival time. For inundation modeling, several assumed coastal points havebeen selected on land in the target areas.
2.2. Scenario Earthquakes in the Sulu Trench
In the present study, multi-segment earthquake from the Sulu Trench are taken as an earthquake
source of which top depth is assumed to be 1.0 km for all scenarios. The parameters used here areproposed by Salcedo (2010) with trench segmentation given by Bautista (2001). Three scenarios areinvestigated in this study. Two more earthquakes of magnitude Mw 8.5 (with a slip of 10 m) and Mw
8.8 (with a slip of 20 m) are assumed as the worst case scenarios. Table 2 describes the parameters foreach scenario.
Table 2. Faults parameters for each scenario earthquake.
Scenario
1
2
3

Source
ST1
ST2
ST1
ST2
ST1
ST2

Mw
8.2
8.5
8.8

Fault Location

LengthWidth

Strike

Dip

Rake

Slip

Long
121.4
119.6
121.4
119.6
121.4
119.6

(km)
167
230
167
230
167
230

(km)
71.97
84.00
71.97
84.00
71.97
84.00

(deg)
30
60
30
60
30
60

(deg)
45
45
45
45
45
45

(deg)
129
90
129
90
129
90

(m)
2.26
3.16
10.0
10.0
20.0
20.0

Lat
7.2
6.2
7.2
6.2
7.2
6.2

3. THEORY AND METHODOLOGY
3.1. Shallow Water Theory
A spherical coordinate shallow water model has been applied to simulate the propagation of tsunami
originated from the Sulu Trench. It can express as follows (Dao and Tkalich, 2007):∂η ∂M ∂(N cos θ ) = 0+

∂t R cos θ  ∂λ ∂θ∂M∂  M 2 
1
∂
1
gD ∂η
MN τ bx+++=0(cos θ)+∂tρ
R cos θ ∂λ R cos θ ∂λ  D  R cos θ ∂θ
D
∂N
1
∂ 
∂ MN τ by1
gD ∂N 2 
+(cos θη ) +
cos θ
+
=0
(
)+
∂t R cos θ ∂θ
R cos θ ∂λ 
D  R cos θ ∂λ D
ρ
+

1

(1)

(2)

(3)

where θ and λ are the latitude and longitude respectively, η is elevation of water surface, D is
still water depth and R is the earth rotation. M and N are the components of discharge along the

τ by
τ bx
and
are the bottom friction terms express asρρτ by gn 2and= 7 N M 2 + N 2 respecti vel y ( Imamur a, 2006).
ρ
D 3

latitude θ and longitude λ .

τ bx gn 2= 7 M M2 + N2
ρD 3

4. RESULTS AND DISCUSSION
4.1. Tsunami Propagation

Figure 1. Snapshot of tsunami propagation at 0 min (initial time),
30 min and 120 min for all scenarios.

Figure 1 shows thesnapshot of tsunami
wave
propagation
generatedfromamulti-segment
fault
along the Sulu Trenchfor each scenario. The
initial waves for eachscenario are shown in
the left panel while theother two panels showthesnapshotsoftsunami propagation at
30 and 120 min after theearthquake. The firstwave
reaching
theSabah coast for all
scenarios is the negativewave.

In Scenario 1 (M w 8.2), the positive wave reaches Sabah in Tambisan Island approximately
35 min after the earthquake. In less than 1 hour after the earthquake, tsunami wave reaches the entire

coast of Tambisan Island in all scenarios. Then tsunami wave continues to propagate in the Sulu Sea
and reaches the water village in the eastern Sandakan (in Region 4b) at approximately 1 hour 38 minafter the earthquake occurred. For the earthquake of magnitude M w 8.5 (Scenario 2) and M w 8.8(Scenario 3), the tsunami wave height is higher compared to Scenario 1 (M w 8.2). The difference inmagnitude of the earthquake in this study does not influence much to the tsunami arrival time in all
scenarios.
4.2. Maximum Tsunami Heights and Travel Times

Tsunami Height (m)Tsunami waveforms and the travel times are obtained from the tsunami simulation. We assumed 19
tide gauges which are located near the coastal area. Figure 2 shows the maximum tsunami height ateach assumed tide gauge station for all scenarios. TBN5 recorded the maximum tsunami height inTambisan Island for all scenarios as the wave concentrates along the coastline which has acharacteristic of shallow circular bay. Tsunami wave height is about double in parallel with theincrease in earthquake magnitude. In Sandakan, SDN1 always recorded the maximum tsunami height
for all scenarios. SDN1 recorded a wave height of 1.0 m, 2.2 m and 3.8 m for Scenario 1, 2 and 3
respectively. The nearest assumed tide gauge to the Sandakan City is SDN5 which has the maximum
tsunami height of 1.6 m in Scenario 3. The coast of Sandakan City has a threat from tsunami wave
height of 1.0 m in Scenario 2. The nearest assumed tide gauges to the Sandakan Port is SDN 6a and it
recorded a tsunami height of 2.1 m in Scenario 3. SDN 7 to SDN 8a represented Sandakan Industrial
Zone which is located along the coast near the Sandakan Port, recorded maximum tsunami height of
1.0 m to 1.2 m in Scenario 3.
10.0

Mw 8.28.0

Mw 8.56.0

Mw 8.8

4.0
2.0
0.0

Tide Gauge Stations

Figure 2. Maximum tsunami heights at each assumed tide gauge for all scenarios.
The earliest positive wave reaches the Sabah coast at 22.5 min (in Scenario 1, M w 8.2) asrecorded by assumed tide gauge TBN1 and the tsunami waves further propagate and surround theTambisan Island. TBN1 recorded the earliest arrival time of positive wave for all scenarios in
Tambisan Island. In all scenarios, tsunami waves hit Tambisan Island coast in less than 30 min after
the earthquake. In Sandakan however, it takes more than 1 hour 30 min for the first positive wave toreach the assumed tide gauge SDN3b in the Sandakan coast. The waves propagate further to thenorth-westward and south-westward of the Sandakan coast. SDN3b recorded the earliest arrival timeof positive wave for all scenarios in Sandakan.

Arrival Time (min)

135.0
115.0
95.0
75.0

Mw 8.255.0

Mw 8.535.0

Mw 8.815.0

Assumed Tide Gauge Stations

Figure 3. Estimated arrival times of positive tsunami wave at each assumed tide gauge station for all
scenarios.
4.3. Tsunami Run-up Height and Inundation Area
Computed tsunami run-up height is calculated at the coastal points in Tambisan Island and Sandakan.
The computed tsunami run-up heights at these selected coastal points are shown in Figure 4. The
computed tsunami run-up heights in Tambisan Island exceeded 10.0 m at coastal point 3 for the worst
case scenario (M w 8.8). The same location records a run-up height of 5.85 m in Scenario 2 (M w 8.5).
The largest run-up height is recorded in this area among all the selected points in Tambisan Island for
all the scenarios except for Scenario 1 (M w 8.2), where the computed run-up height is 2.06 m. This isdue to the wave concentrated along the coastline which has a characteristic of shallow circular bay andcaused higher run-up. The height of run-up decreases as the wave propagates to the entire island in all
scenarios, as recorded at coastal points 4 and 5.
The largest computed tsunami run-up height in Sandakan is recorded at coastal point 9 inBerhala Island beach for all scenarios. The computed run-up height is 1.36 m in Scenario 1 (M w 8.2),
2.78 m in Scenario 2 (M w 8.5) and 4.75 m in Scenario 3 (M w 8.8). The existence of this island reduced
the impact of tsunami waves that hit Sandakan Bay. Coastal points 7 and 10 represent the water
villages along the coast in Sandakan Bay. Assuming a megathrust earthquake in the Sulu Trench(Scenario 3, M w 8.8), the computed tsunami run-up height is 2.48 m and 2.57 m at points 7 and 10
respectively. In Sandakan city (represented by coastal point 11), the computed tsunami run-up heightis 2.10 m. Coastal point 6 represents the closest location to the Sandakan Airport and recorded atsunami run-up height of 4.04 m in Scenario 3 (M w 8.8). At the other locations such as the Sandakan
Port and the Sandakan Industrial Zone (represented by coastal points 13 and 14) which are locatedadjacent to the port along the Sandakan coast, a computed tsunami run-up height of less than 2.0 m isrecorded for all scenarios.

Figure 4. Computed tsunami run-up heights in Tambisan Island (left) and Sandakan (right) at coastal
points labeled with numbers for all scenarios.

Figure 5 shows the inundation areas for Scenario 3 (M w 8.8) in Regions 4a, 4b and 4c. The
shape of inundated area is similar to those simulated in Scenarios 1 and 2, but the coverage is more
extensive, and the maximum inundation height is higher. In Scenario 3, almost the entire coasts ofTambisan Island are inundated by the tsunami wave. The water villages in Region 4b might beseverely damaged in Scenario 3 with the inundation reaching 2.0 m in height. Sandakan City andSandakan Port are inundated by tsunami wave of more than 1.0 m height in Scenario 3 with extensive
inundated area and the inundation height reaching almost 2.0 m. The Sandakan Industrial Zone which
is located along the coast near Sandakan Port is highly affected by the tsunami wave.

Figure 5. Inundation areas for Scenario 3 (M w 8.8) in Regions 4a, 4b and 4c.
5. CONCLUSIONSThe simulated results show that the Tambisan Island coastline was at risk of tsunami threat withmaximum tsunami wave height of at least 4.7 m in Scenario 3. In the simulation, Sandakanexperienced much smaller tsunami waves of about 2.0 m for all scenarios. Average maximum tsunami
height in water villages along the Sandakan coast is about 2.0 m in Scenario 3. The Sandakan coastnear the airport experienced extensive inundated area, about 800 m distance from the coastline in
Scenario 3. Sandakan city experienced tsunami waves with a maximum height of 1.6 m and maximum
run-up height of 2.1 m. The inundation distance was about 300 m in Scenario 3. It is finally concluded
that the east coast of Sabah faces the risk of tsunami threat from large earthquakes along the Sulu
Trench.
6. RECOMMENDATION
There are still wide-ranging prospects in enhancing tsunami simulation based on the nested grid
system in this study, and we suggest the following:i.
The bathymetry and topography data could be more detailed in grid size in order to improvethe computed result of tsunami wave height, travel time, inundation height and distance,ii.
Sea bottom topography should be taken into consideration as well as the inland vegetation andstructures such as building, etc.
ACKNOWLEDGEMENT
I would like to express my sincere gratitude to Prof. Yoshinobu Tsuji for his valuable suggestions.
REFERENCES
Bautista, B. C., Baulista, M. L. P., Oike K., Wu F. T. and Punongbayan R. S., 2001, Tectonophysics,
279-310.
Dao, M. H, and Tkalich, P., 2007, Nat. Hazards Earth Syst. Sci., 7, 741-754.
Imamura F., 2006, Lecture Notes, IISEE/ BRI.
Mazni A., 2011, Individual studies by participant at IISEE, Japan, 1-50.
Ridzuan M. A., 2010, Individual studies by participant at IISEE, Japan, 1-36.
Salcedo J. C., 2010, Individual studies by participant at IISEE, Japan, 1-38.