NUMERICAL SIMULATION FOR THE ASSESSMENT
OF TSUNAMI SCENARIOSIN SOUTHERN YOGYAKARTA,CENTRAL JAVA ISLAND, INDONESIA
Tiar Prasetya∗
MEE06012

Supervisor: Fumihiko IMAMURA∗∗

ABSTRACTThe study aims to assess the possible tsunami impact on Yogyakarta, the central coast of Java Island,
Indonesia. Seven tsunami scenarios with different parameters estimated by referring the seismic gap
and tectonics have been applied to calculate the tsunami propagation, inundation, maximum height andtravel time, which are important indices for the assessment. Tsunami numerical modeling was
performed by using TUNAMI-N2 with GEBCO bathymetry data. 38 tide gauge stations are selected as
output points along Yogyakarta coastal area.
Tsunami simulations with two sources of different depths of 0 km and 10 km using similarsources parameters demonstrated that the depth of source affects the tsunami propagation, maximum
tsunami height and inundation area. Through carrying out other simulations with the different
parameters such as length and location, we also confirmed that the location and the length of tsunami
source caused big difference for the tsunami propagation and tsunami height and inundation area. On
the other hand, the catastrophic case (Mw 8.8) adopted from Okal and Synolakis (2007) showed the
maximum tsunami height and the widest inundation area compared to the other tsunami sources.
Tsunamis could reach the most of coastal area in Yogyakarta in less than 20 min to 50 min after theearthquakes according to the seismic gap.
Keywords: Tsunami scenario, Numerical simulation, Java, Tsunami assessment.
1. INTRODUCTION
Yogyakarta is one of tsunami prone areas in Indonesia, located in south of Java Island, face to facewith subduction area. The Java subduction zone, is characterized by an absence of great interplatethrust earthquake (Okal and Synolakis, 2007). From historical earthquakes which occurred in south ofJava Island, it is shown that there is a “Seismic Gap”. Since 1900, despite historical reports, tsunami insouth of Yogyakarta is unclear. NEIC (National Earthquake Information Center)/USGS catalogue lists(http://neic.usgs.gov/neis/epic/epic.html) show six earthquakes with magnitude larger than seven, in
1903, 1921, 1937, 1977, 1994 and 2006. Hence, in regard to the potential of seismic source, thecoastal area of Yogyakarta might be the most prone area of tsunami.

∗

Geophysical Station, Regional Seismic Center VII – Yogyakarta. Meteorological, Climatologicaland Geophysical Agency (BMKG) of Indonesia∗∗
Professor, Disaster Control Research Center, Tohoku University, Japan.

12. DATA AND METHOD OF COMPUTATION
2.1. Bathymetry and Topography Data
The bathymetry topography data are important intsunami simulations. We used 30 arc second grids sizeof bathymetry data from GEBCO (General BathymetricChart of the Ocean) (https://www.bodc.ac.uk/data/
online_delivery/gebco/). Finer spatial grid sizes wereapplied for four regions. The grid sizes used for regions
R1, R2, R3 and R4 are 30, 10, 3.33 and 1.11 arc second
respectively. Figure 1 shows the coverage area ofcomputation. We combined topography data fromSRTM (Shuttle Radar Topography Mission)(http://www2.jpl.nasa.gov/srtm/dataprod.htm)
with
GEBCO data for computation area (R4) of inundation.

Figure 1. Computation area for numericalmodel in R1, R2 (left) and R3, R4 (right).

2.2. Tsunami Sources
A tsunami source plays an important role in tsunami simulation. For this study, seven scenarios havebeen applied to perform tsunami simulations according to the seismic gap at south of Java Island. A
rectangular fault model tsunami sources are applied. Dip angle is assumed to be 100 for all scenarios.
Strike angles are based upon pre-assumed trench alignment. Some scenarios are changed by depth,
position and length. For catastrophic case, tsunami source are adopted similarly from Okal and
Synolakis (2007). All the fault parameters used in this study are shown in Table 1.
Table 1 Parameter of tsunami sources for scenario earthquakes.
Scenario

Mag

No.
1a
1b
2
3
4a
4b
5

Mw
8.3
8.3
7.7
7.7
8.1
8.2
8.8

RigidityµN
3.00E+10
3.00E+10
3.00E+10
3.00E+10
3.00E+10
3.00E+10
3.00E+10

Length
L
(km)
355
355
200
200
300
450
500

Width
W
(km)
135
135
29
29
75
75
150

Depth
h
(km)
10
0
10
10
3
3
10

Strike
φ
(0)
289
289
289
289
280
280
290

Dip
δ
(0)
10
10
10
10
10
10
10

Rake
λ
(0)
102
102
90
90
90
90
90

Slip
u
(m)
2.5
2.5
2.5
2.5
2.5
2.5
10

Longitude

Latitude(0)
111.6888
111.6888
111.6888
110.0000
109.5000
109.5000
112.5000( 0)-11.300-11.300-11.300-11.000-9.900-9.900-10.400

2.3. Tsunami Simulation
We used TUNAMI-N2 (Tohoku University Numerical Analysis Method for Inundation), developed by
Disaster Control Research Center (DCRC), Tohoku University of Japan (Imamura et al., 2006) for
modeling propagation and inundation of tsunami in coastal area of Yogyakarta. In this modeling code
non linear shallow water equations with bottom friction terms are discretized by the leap-frog finite
difference scheme. This model is widely used to simulate tsunami propagation and inundation on dry
land (Koshimura et al., 2006).
Tsunami waveforms were calculated at 38 tide gauge stations along Yogyakarta coast as
target points to obtain tsunami heights and arrival times. All stations are assumed at the front face ofsea. Some stations are determined because of the high population in that areas. In this study the time
step (Δ t) was set to 0.5 s to satisfy the stability condition. The number of time steps for computation is21,600 (three hours).

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3. RESULT AND DISCUSSION
3.1. Tsunami Wave Propagation into the coast
Figure 2 shows that the snapshots of tsunami propagations in R3 from Scenario 1a (top) and Scenario
1b (bottom) are slightly different. Tsunami wave length in Scenario 1a is longer than that in Scenario
1b. The wave propagation in Scenario 1a is also wider than Scenario 1a. The first positive tsunami
could reach the coast more than 30 min after the earthquake.
Figure 3 shows that the snapshots tsunami propagation from Scenario 2 (top) and Scenario 3(bottom) are similar. Before tsunami waves reach the coastal area, some disturbances occur. The
disturbance in Scenario 3 is stronger than Scenario 2. In contrast with Scenario 3 which focused on thewest part of Java Island, the area affected by tsunami in Scenario 2 is mostly concentrated at the east
part. Yogyakarta coastal area will be more affected by tsunami in Scenario 3 compared to Scenario 2.
40 min53 min70 min45 min70 min90 min40 min53 min70 min45 min70 min90 minFigure 2. Snapshots of tsunami propagation in R3 Figure 3. Snapshots of tsunami propagation in R3
at 40, 53 and 70 min after the earthquakes for at 45, 70 and 90 min after the earthquakes for
Scenario 2 (top) and Scenario 3 (bottom).
Scenario 1a (top) and Scenario 1b (bottom).
Multi-faults cases were applied in Scenario 4. Scenario 4a consists of two faults as thetsunami source, in which the length of the each fault is 150 km. Scenario 4b consists of three faults.
Because the time of deformation for each fault is quite short, it can be assumed that the initialcondition for deformation of sea floor occurs at the same time. Figure 4 shows the propagation oftsunami wave in R3 at 25, 41, and 55 min after the earthquake. Even though the propagation is similar,
the Scenario 4b produces wider and higher tsunami. The positive part of tsunami might reach the
Yogyakarta coast in less than 30 min after the earthquake.
We applied the tsunami source for the most catastrophic case (Mw 8.8) which has been
adopted from Okal and Synolakis (2007). Scenario 5 refers the full rupture of Java segment of the
subduction zone. Figure 5 shows the tsunami propagation in R3 at 10, 20, 30, 60, 120 and 180 min
after the earthquake.
25 min41 min55 min10 min20 min30 min25 min41 min55 min60 min

120 min180 minFigure 4. Snapshot of tsunami propagation in R3 at
25, 41 and 55 min after the earthquake for
Scenario 4a (top) and Scenario 4b (bottom).

Figure 5. Snapshot of tsunami propagation in
R3 at 10, 20, 30, 60, 120 and 180 min after theearthquake for Scenario 5.

From all scenarios we found that there are some disturbances of tsunami propagation which
occur before it reaches Yogyakarta coast. The disturbances are caused by some “small sea mountains”

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at sea floors which changes bathymetry. Those disturbances will be shown in tsunami waveform withincreasing high frequency signal at each target point. Figure 6 shows the disturbance of tsunami (a),the small sea mountains (b) and high frequency waveforms (c).

(a)

(b)

(c)

Figure 6. Snapshot from Scenario 1a at 19 min after the earthquake (a), small sea mountains atbathymetry (b), and high frequency signal shown in tsunami waveforms (c).
3.2. Maximum tsunami heights along the shore
Maximum tsunami height is one of the components obtained from tsunami simulation. The maximum
tsunami height in Scenario 1a reached 3.0 m, near the Parang Tritis area. The maximum tsunami
height higher than 2 m occurred at 10 tide gauge stations, mean while in Scenario 1b only at 3 tidegauge stations. Maximum tsunami height between 1.0 and 2.0 m occurred in Scenario 1a and 1b are:
13 and 15 tide gauge stations respectively. In Scenario 1b, the highest tsunami was calculated at TG18,
which located not so far from highly populated Banaran area.
In Scenario 2 and 3, only at one tide gauge station tsunami maximum height Scenario 2 morethan in 1 m. In contrast with Scenario 2, according to the waveform data, the maximum tsunami heightin Scenario 3 is higher than 2 m at 3 tide gauge stations. The highest tsunami of 2.2 m is found at
TG20. Another tsunami occurs at height less than 1 m in most areas of the east part of Java Island. The
position of seismic source in Scenario 3 is located in the direction almost perpendicular to Yogyakarta
coast and might affect the maximum height of tsunami.
Figure 7 shows the maximum tsunami heights for all stations all scenarios. In Scenario 4acompared with 4b, maximum tsunami will be affected by the length of tsunami sources. Tsunami
higher than 2 m can occur at 2 tide gauge stations in Scenario 4b, but the maximum tsunami height is
not exceeding 2 m in Scenario 4a. However, maximum tsunami height between 1 and 2 m are still
dominant for both scenarios. The east part of Yogyakarta still obtains maximum tsunami height lessthan 1 m.
The maximum tsunami heights in Scenario 5 exceed 4 m at all tide gauge stations. The
maximum height of 11.2 m occurs at Teluk Pucung tide gauge station. According to the other four
Scenarios before, at the east part of Yogyakarta coast, tsunami wave is smaller than at the west part.
However in Scenario 5, the highest tsunami wave occurs at the east part.

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Figure 7. Maximum tsunami heights for all scenarios.
3.3. Inundation Area around Yogyakarta
Figure 8 show the inundation area for all scenarios. First column shows the inundated area in Scenario
1a and 1b are less than 1 km. The shape of inundation area is similar only limited of area, but the
maximum inundation height is higher in Scenario 1a compared to Scenario 1b. The inundated area inthe west part of Opak River is wider than in the east part.
Scenarios 2 and 3 illustrated at the second column in Figure 8 show that the inundation area
Scenario 2 is smaller than in Scenario 3. The inundated area exceeds only a few hundred meters,
making small inundated area. The shape of inundated area in Scenario 3 is similar to the two
Scenarios of Scenario 1a and 1b, but the area smaller and steeper. The maximum inundation high in
Scenario 3 is higher than in Scenario 2, but lowers than Scenarios 1a and 1b.

Figure 8. Inundation areas for all scenarios.
Since the source length used in Scenario 4b is longer than Scenario 4a, the maximum
inundation height in Scenario 4b is higher than that in Scenario 4a. Figure 8 at the third column
illustrated Scenario 4a and 4b. In Scenario 5 which is tsunami will reach more than 2 km inland withmaximum height of more than ten meters. All coastal areas will be affected by the tsunami.
All scenarios show that some areas are always inundated compared to the other areas. It
consists of three regions, namely, the southern of Plered (7056’S, 105007’E), near the mouth of OpakRiver (8000’S, 110016’E) and Parang Tritis area (8001’S, 110019’E).
3.4. Tsunami Travel Time of tsunami to the Shore
Figure 9 shows the tsunami travel times for all scenarios. In Scenarios 1a and 1b, the tsunami reaches
the coastal area in less than 30 min after the earthquakes. In Scenarios 2 and 3, tsunami travel times

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obtained at all tide gauge stations are more than 30 min after the earthquakes. Scenarios 4a and 4b are
similar with Scenarios 1a and 1b, the tsunamis arrived in less than 30 min after the earthquakes at
Pacitan Bay and propagated to the west gradually. As for Scenario 5 the tsunami travel time was lessthan 30 min after the earthquake. Tsunami travel time recorded at the Pacitan Bay showed asignificant difference with the station located next to it.

Figure 9. Maximum tsunami heights for all scenarios.
4. CONCLUSIONS
The maximum heights, travel times and inundation areas of tsunamis are derived from the tsunami
simulations. The depth of tsunami source will affect the height, inundation area and travel time oftsunami. Also, the shape of the coastal area plays an important role in increasing the tsunami height.
We found that the different positions of sources have affected the different propagation and height oftsunami. Although the fault parameters used are same, the source located almost perpendicular to thecoast of Yogyakarta might be more dangerous for coast.
Based on the tsunami simulation result, we conclude that tsunamis could reach the coast of
Yogyakarta in around 20 to 40 min after the earthquakes on the seismic gap at south of Java island.
Tsunami heights would exceed up to 3 m and the inundated area could cover more than 500 m on land.
In the catastrophic case, tsunami height could reach more than 10 m and the inundated area could
cover 2 km on land.
AKNOWLEDGEMENT
I would like to express my sincere gratitude to my advisor Dr. Y. Fujii, IISEE for continuous
supports and for providing all of information, always helping and guiding me from the beginning tocomplete this research.
REFERENCES
Imamura, F., et al., 2006, Tsunami Modeling Manual, Tohuku University, Japan.
Okal, Emile A. and Costas E. Synolakis, 2007, Geophysics Journal International,172,995-1015.
Papazachos, B.C., et al., 2004, Bulletin of the Geological Society of Greece Vol. XXXVI Proceedingsof the 10th International Congress, Thessaloniki.
Koshimura, S., et al., 2006, Nat Hazards 39:265–274.

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