NUMERICAL SIMULATIONS FOR TSUNAMIFORECASTING AT PADANG CITY USINGOFFSHORE TSUNAMI SENSORS
Setyoajie Prayoedhie
MEE10518

Supervisor:
Yushiro FUJII
Bunichiro SHIBAZAKIABSTRACT
We conducted numerical simulations to forecast near-field tsunami at ocean bottom tsunami meter(OBTM) and GPS buoy stations along the Sumatra Island. We used modeling codes of TUNAMI-N2(Tohoku University’s Numerical Analysis Model for Investigation of Near-Field Tsunami, No.2) and
TUNAMI-F1(Far-Field Tsunami, No.1) developed by the Disaster Control Research Center (DCRC),
Tohoku University, Japan in order to estimate the tsunami height and arrival time using proposed
scenarios. For numerical simulation, we used three different scenario earthquakes which are consideredalong the Sunda trench. The source parameters for scenario earthquakes are based on the coral-reef
study by Natawidjaja et al. (2006) for the earthquakes of 1797 (Mw 8.7) and 1833 (Mw 8.6 and 8.9).
For the 1797 event (scenario 2), we obtained the tsunami height of 1.547 m with the arrival time of 0.8
min at TM2 offshore output point located in the uplift area, while 0.109 m and 3.9 min for the tsunami
height and arrival time, respectively, at TM3 offshore output point located to the south from the source.
Based on a simulation result for three scenarios using different bathymetry data (1 arc-minute and 30
arc-second), we found that the tsunami waves reach the coastal area in less than 40 min for the output
point located in Padang city (OP2). By using detailed bathymetry data, we can estimate more accurate
on tsunami arrival time and tsunami height.
Keywords: Tsunami forecasting, offshore tsunami sensor, scenario earthquakes, numerical simulation,
Padang city.
1. INTRODUCTION
Since late 2004, a series of earthquakes, has ruptured the plate boundary from the Andaman
Islands to the Sunda Strait, for a distance of nearly 2,500 km. Only 300-km long of the Sunda margin,
the so-called Mentawai segment remained unbroken in the past five years and Padang city lies in frontof on this segment.
Japan Meteorological Agency (JMA) issues the tsunami warnings/advisories based on
hypocentral parameters such as the location, depth and magnitude, and a tsunami simulation databasesystem which stores more than hundred thousand cases of previously conducted tsunami simulation
results. JMA also monitors the changing of sea level data observed at of 172 tide gauges and 12 GPS
buoys located about 10 to 20 km off the coast. By using the tsunami wave observation data of the GPS
buoys, JMA estimated the tsunami heights at the coast deduced from the offshore data. For the 2011
Tohoku earthquake (Ozaki, 2011) JMA classifies the tsunami forecast into two cirteria, tsunami
warning and tsunami advisory.
The purpose of this study is to estimate the arrival time and the height of tsunami waves
which will be observed at OBTM or GPS buoy by adopting the proposed models of historicalearthquakes. For an improvement of the coastal tsunami forecast, we tried to evaluate them by making
Indonesian Agency for Meteorology, Climatology and Geophysics (BMKG), Jakarta, Indonesia.
International Institute of Seismology and Earthquake Engineering (IISEE), BRI, Tsukuba, Japan.

1

discussion about the effectiveness of the current location of the offshore tsunami sensors in relation to
detect the tsunami before arriving at coastal area of Padang city.

2. THEORY AND METHODOLOGY
2.1

Methodology for near-field tsunami forecast

2.1.1 Basic equations
A non linear long wave approximation for the equation of motion, and equations of continuity withbottom friction without Coriolis force (Satake, 1995), are needed in order to estimate tsunami height at
the offshore output points using a numerical simulation. The equations are below :,whereandare the averagevelocity in the directions of x (east) andy (south) components, respectively. is
sea surface displacement,is seadepth,
is bottom friction coefficientandis gravity acceleration (9.8 m/s2).
2.1.2 Numerical simulation
The tsunami propagation and wave height distributions at output points are calculated byusing TUNAMI-N2 code based on uniform grid systems in Cartesian coordinate with a single domain ofcomputation area. Meanwhile, TUNAMI-F1 is used to calculate Green’s functions at offshore output
points (GPS Bouy/OBTMs) and coastal output points (OPs) for all subfaults and to estimate the tsunami
height for each output point. An initial sea-surface displacement model is needed for the computationof Green’s functions. The slip amount on the subfault is related directly with the crustal deformation.
Hence, the tsunami height/amplitude is also linearly related to the slip amount. The tsunami amplitude
for different slip can be estimated once the tsunami waveform for a certain amount of slip is computed(Satake, 1995). The observed tsunami waveform is expressed as a linear superposition of Green’s
functions as follows :(3): Green’s function/computed waveform at the ith station from the jth subfault: Slip amount on the jth subfault: Observed tsunami waveform at the ith station
The dimensions of the computation area are set to 540 grid points for longitude and 540 grid points forlatitude in the region between 94°E-103°E in longitude and 1°N-8°S in latitude. In a wave equation, theCFL (Courant Friedrics Lewy) condition should be satisfied for stability of numerical computation
given by the following formula (Imamura et al., 2006):,where Δt and Δx are the temporal and spatial grid lengths, and hmax
is the maximum still water depth in a computation region. In order tostabilize the numerical computation, the temporal grid sizes (Δt)
were set to 3.0 s for a uniform grid of 1 arc-minute bathymetry data computations and 2.0 s for a
uniform grid of 30 arc-second bathymetry data.
3. DATA
3.1

Ocean bottom tsunami meters (OBTMs) and GPS buoy

There is an OBTM observation system located around Mentawai islands and the off coastWest Sumatra, which is operated by BPPT. Within the stations, three GPS buoy systems along the2

Sunda trench subduction zone are covered in this study (Table 1). The OBTM and GPS buoy data arerecorded and transmitted in real-time to the Real-time Data Server (RDS) at BPPT office.
Table 1.

Location of output points and water depth in bathymetry data from GEBCO

No

Code

Location

Lat.

Lon.

1
2
3
4
5
6
7
8
9
10

TM1
TM2
TM3
TM4
OP1
OP2
OP3
OP4
OP5
OP6

North East Sipora island
South West Sipora island
South West Sipora island
South of Pagai island
Pariaman city
Padang city
Bayang city
Jurai city
Sutera city
Lengayang city

-1:56
-2:47
-0:35
-4:58
-0:37
-0:57
-1:18
-1:21
-1:35
-1:41

100:02
98:54
97:23
100:33
100:06
100:20100:29100:33
100:38
100:41

3.2

Depth(30 Arc-Second)
1714
5579
5348
6173
3
11
24
14
29
33

Type

Remarks

OBTM
GPS Buoy
GPS Buoy
GPS BuoyTide Gauge
Tide Gauge
Tide Gauge
Tide Gauge
Tide Gauge
Tide Gauge

ActiveNo data to RDS
No data to RDS
No data to RDS
AssumedAssumedAssumedAssumedAssumedAssumedScenario earthquakes

We adopt three scenarios (Natawidjaja et al., 2006) of tsunamigenic earthquakes for tsunami
simulation. These scenarios consist of multisegments with different slip models, referring to the source
parameters of the 1797 and 1833 Sumatra earthquakes (Table 2). For all scenarios, we consider
different sizes and slip of each segment as shown in Table 3. The top depth of the fault for scenario 1
is set to be 5000 m and for scenarios 2 set to be 1000 m while for scenario 3 set to be zero since thefault planes are located near the trench. We used bathymetry data from the General Bathymetric Chartof the Ocean (GEBCO) with 1 arc-minute and 30 arc-second resolution. We tried to obtain andcompare the synthetic result of the tsunami heights at the OBTMs and buoys using modeling codes
called TUNAMI-N2 and TUNAMI-F1.
Table 2. Fault parameters for all scenarios
No
1
2
3
4
5
6
7
8
9
10

Scenario

Mw
(yr)

1

8.6
(1833)

2

8.7(1797)

3

8.9(1833)

Segment

Length
(km)

Width
(km)

Strike
(°)

Dip
(°)

Rake
(°)

Slip
(°)

A
B
C
A
B
C
D
A
B
C

180.55
70
26.73
18.13
67.42
75.19
212.78
165
71.66
31.30

79.01
116.1788.83
147.58139.44130.11
147
135
180
180

325
325
325
325
325
325
325
325
325
325

10
10
10
10
10
10
10
10
10
10

100
100
100
100
100
100
100
100
100
100

10
9
7
4
6
8
6
18
11
9

Top
Depth
(m)
5000
5000
5000
1000
1000
1000
1000
0
0
0

Latitude(°)

Longitude
(°)

-4.42
-3.083-2.561-3.66-3.528
-3.03
-2.47
-4.70
-3.47
-2.94

100.9
99.97
99.6050
99.62
99.52599.17
98.785100.3899.52
99.15

4. RESULTS AND DISCUSSION
4.1

Seafloor deformation for all scenarios

The initial conditions for all scenarios are calculated based on the input parameters in Table 2.
Referring to research conducted by Natawidjaja et al. (2006), there are three segments with different
slip values for scenario 1 (Figure 1a) and four different segments for scenario 2 (Figure 1b), and threesegments for scenario 3 (Figure 1c). Along the cross section from 98°E to 101.5°E, the uplift andsubsidence are ranging from 5 m to -4 m depend on the slip amount of each scenario. For scenario 1,
we obtained the maximum subsidence of 2 m and the uplift of 3 m. For scenario 2, the top edge depthof this fault is set to be zero which makes the fault area is close to the trench. The size of the fault in3

scenario 2 is about 370 km x 577 km which covers almost all the area of the Mentawai islands, fromsouth Hibala island in the north until the southern part of South Pagai island. The maximum
subsidence obtained for scenario 2 was 1.6 m and 1.4 m for the uplift. For scenario 2, 1 offshore
output point (TM1) and 6 coastal output points are located in subsidence area. For scenario 3, The
fault area has slip varying between 9 and 18 m. The size of the fault plane is approximately 267 km inwidth and 495 km in length. The maximum subsidence of 3.6 m and the uplift of 3.2 m are estimatedin this scenario.

Figure 1. (a) Seafloor deformation for scenario 1. Blue dash lines and red lines show the subsidence
and uplift area respectively. The contour intervals are 0.2 m. Green diamonds show the location of theoffshore tsunami meters while pink diamonds are the coastal output points. The thick green line
shows the cross section (b) Seafloor deformation for scenario 2. the contour intervals are 0.1 m. (c)Seafloor deformation for scenario 3. The contour intervals are 0.5 m.
4.2

Maximum tsunami height

Maximum tsunami heights for scenario 2 were obtained from the west coast area of Mentawai island,
Sipora island to the west coast of North Pagai island (Figure 2b), while the maximum tsunami heights
for scenario 1(Figure 2a) were obtained at the southwest area of Sipora island to the south of Pagai
island ocean side. From Figure 2b, we can identify that the most of tsunami energy are heading to theIndian ocean. The pattern of the maximum tsunami height for scenario 3 (Figure 2c) is almost similar
to other scenarios. However, for scenario 3, the maximum tsunami height involves much wider area,
this could be due to the large amount of slip distribution.

Figure 2. Estimated maximum tsunami height for (a) scenario 1, (b) scenario 2 and (c) scenario
3. Based on the scale, dark color shows lower tsunami height while brighter color shows higher
tsunami waves.

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4.3

Comparison of the initial tsunami height for scenario 2
In this study, scenario 2 was selected
as the scenario representing the unbroken
segment. To calculate Green’s function in for all
output points we made 27 subfaults with thefault size of 50 km x 50 km and slip of 1 m
covering the scenario fault area (Figure 3). We
also compare the scenario 2 results obtained byusing different source codes (TUNAMI-N2 and
TUNAMI-F1) using the 1 arc-minutebathymetry data. A numerical simulation to
obtain tsunami waveforms was performed using
the output points data in Table 2. To find therelationship between offshore output points(TM1-TM4) and coastal output points(OP1-OP6), we compare the arrival time and theinitial height of tsunami waveform. Using
TUNAMI-F1 code for the bathymetry data of 1
arc-minute, the calculated tsunami height at OP2(Padang city) are in the range of 3-4 m. At mostof the offshore output points (TMs) the first
positive waveforms arrived in less than 13 min(Table 3).

Figure 3. Subfault model for scenario 2. The green rectangles shows the different sizes of fault
segments for scenario 2 with slip varying between 4 and 8 m. Yellow boxes within the fault areas
show the assumed subfaults with 50 km x 50 km size and 1 m slip for each. The number withinevery subfault is a slip amount. The green diamonds are the locations of the offshore tsunami
meters while pink diamonds are the coastal output points.

Stations
TM1
TM2
TM3
TM4
OP1
OP2
OP3
OP4
OP5
OP6

Table 3. Arrival times and heights of initial tsunami waveform
Bathymetry 30
Bathymetry 1 Arc-Minute
Arc-Second
TUNAMI-N2
TUNAMI-F1
TUNAMI-N2
Time (min)Heights (m)Time (min)Heights (m)Time (min) Heights (m)6.3-0.1146.7-0.0027
6.3-0.145
0.9
0.404
0.8
1.48
0.8
1.547
4.2
0.139
1.9
0.0347
3.9
0.109
13
0.078
12.8
0.0279
12.6
0.068
37.5
0.782
39.3
2.7482
33.1
1.791
35.5
0.277
36.8
0.291435.3-0.168
67.5
0.229
52.3
0.0432
60.7
0.197
78.2
0.199
67.4
0.183
61.6
0.037
63.8
0.709
60.2
0.269
65.4
0.423
55.6
0.078
53.1
0.1723
49.3
-0.1

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4.4

Tsunami travel time to coastal points

We estimated the initial tsunami arrival time within the range 0.8-12.8 min at the offshore
output points (TM1-TM4) by using TUNAMI-N2 with 1 arc-minute bathymetry data. Based ontsunami arrival time data at TM4, It took about 35 min before the tsunami wave reachs OP2 which isthe coastal output point of Padang city. As comparison, we also calculated the initial tsunami arrival
time using TUNAMI-F1 with 1 arc-minute bathymetry data. The earliest arrival time was 0.8 min at
TM2 offshore output point. We found that the initial tsunami arrival time for the offshore output point(TM1-TM4) are not so much different if we compare the result obtained by using TUNAMI-N2 and
TUNAMI-F1. Moreover, the computation using the 30 arc-second grid produces relatively fasterarrival time of tsunami. This might be caused by the accuracy of bathymetry data.
4.5

Real-time tsunami forecast

The concept of a real-time tsunami forecast in this study is based on a high precision of theinitial tsunami height at nearby coastal output points (OP1-OP6) when the offshore output points(TM1-TM4) detect a change of the sea level as the initial tsunami height on real time. The difficulties
to issue tsunami forecasting occur when the epicenter of the earthquakes is close to the target area.
Therefore, this problem might be solved if we can introduce information about the tsunami source
into the forecasting scheme by conducting a tsunami waveform inversion for slip distribution on anassumed fault plane (Tsushima et al., 2009). The other way to overcame the problem of inaccurate
tsunami forecasts is to increase the density of OBTM stations with appropriate spatial distribution.

5. CONCLUSIONSIn this study, we made a simple forecasting of tsunami height and also estimated the arrival
times of tsunami waves in the coastal area of Padang city, using the 1797 and 1833 earthquakes thattook place in the west coast of Sumatra island. We chose the 1797 earthquake (scenario 2) since it
fulfills the criteria of a tsunamigenic earthquake and the epicenter is located in the unbroken segment
for the past 200 year. Using scenario 2, we compared all the results with different bathymetry data andfound that the differences of initial tsunami arrival time for the offshore output points (TM1-TM4) arenot so significant. Contrarily, based on the numerical simulation with the finer resolution ofbathymetry data, we obtained faster initial tsunami arrival times at the most of coastal output points.
The accuracy of coastal tsunami height can be affected by a spatial relationship between thetsunami source and OBTMs. Considering the effect of the radiation pattern from the tsunami source,
an unaccuracy of tsunami forecasting would be reduced by deploying more OBTMs at the offshore of
Padang.

REFERENCES
Imamura, et al., 2006, Tsunami Modelling Manual (TUNAMI model), DCRC (Disaster Control
Research Center, Tohoku University, Sendai, Japan.
Natawidjaja, et al., 2006, Journal of Geophysical Research, Vol. 111, B06403,
doi:10.1029/2005JB004025.
Ozaki T, 2011, Outline of the 2011 off the Pacific coast of Tohoku earthquake (Mw 9.0) – Tsunami
warnings/advisories and observations, Earth Planet Space.
Satake, K., 1995, PAGEOPH, 144, 455-470
Tsushima, et al., 2009, Vol. 114, B06309, doi:10.1029/2008JB005988, 2009.
Website : General Bathymetric Chart of the Ocean (GEBCO)
http://www.bodc.ac.uk/data/online_delivery/gebco/.

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