QUICK FORECASTING OF TSUNAMI IN BALI AND
NUSA TENGGARA REGIONS, BASED ON
THE TSUNAMI DATABASE SYSTEM
Titi HANDAYANI∗
MEE08174

Supervisor: Yushiro FUJII∗∗

ABSTRACTTsunami database in Bali and Nusa Tenggara regions, Indonesia has been created in this study. Twotypes of bathymetry grids were applied for tsunami simulations, namely finer mesh grid and coarse
mesh grid. The intervals of finer and coarse mesh grids are 20 arc-second (~616.67 m) and one
arc-minute (~1850 m), respectively. The 1977 Sumbawa earthquake was adopted as a real case study.
We compared the simulation results from the both grids in order to investigate the effective way tocreate a tsunami database. The coarse mesh grid will be used for tsunami simulations of hypothetic
earthquakes to construct the tsunami database. We located 21 source points (SP) along the trench axis,
surrounding the real case of the 1977 Sumbawa earthquake, with three magnitudes (M 7.0, 7.5 and 8.0)
and four depths (0, 20, 40 and 60 km) set for tsunami simulations, in total 252 hypothetic cases. Welocated 38 coastal points (CP) and 38 forecast points (FP) as target points. We estimated maximum
tsunami heights at CPs by applying Green’s law to those at FPs. Inverse refraction diagram was used
to obtain tsunami arrival times at CPs.
Three kinds of method to retrieve values from the database were applied in this study,
namely interpolation methods for epicenter location, magnitude and depth; extrapolation methods formagnitude and depth; and maximum risk methods. Those methods are very useful to pick up data fromthe database, which are used for issuing tsunami warning. The comparison of tsunami heights from thedirect simulation and the retrieved values from the database shows a good agreement for the real case.
Keywords: Tsunami Database, Numerical Tsunami Simulation, Green’s law.

1. INTRODUCTION
Indonesia has a lot of tsunami experiences, almost every 2 years we suffered from an attack of tsunami
generated by earthquake. To save many lives from the tsunami impact, Indonesia has an ultimate goalof establishing Indonesia Tsunami Early Warning System (InaTEWS) issuing warning within 5
minutes after an occurrence of earthquake. In order to quickly issue tsunami warning a tsunami
database system is necessary. In this study, we create a tsunami database and confirm that database
will be applicable to future earthquakes.

2. THEORY AND METHODOLOGY

2.1. Bathymetry Data
We used GEBCO (The General Bathymetric Chart of the Ocean) 1 arc-minute (~1850 m) bathymetry
data to construct the tsunami database. This bathymetry data is processed for calculating tsunami
travel times and tsunami heights.
∗Meteorological, Climatological and Geophysical Agency (BMKG), Indonesia.
International Institute of Seismology and Earthquake Engineering, Building Research Institute, Japan.

∗∗12.2. Scaling Law
The length, width and slip amount of fault plane are determined from empirical formula of scaling law(Utsu et al., 2001), from magnitude of the earthquake, which can be expressed as follows
LogL = 0 .5 M − 1 .8LogW = 0.5M − 2.1(1)
L
W =2LogD = 0.5M − 3.3
where L , W , M and D are fault length (km), width (km), magnitude and slip amount (m) offault, respectively.
2.3. Numerical Simulation of Tsunami Propagation

We performed tsunami simulations by using TUNAMI-N2 (Tohoku University’s Numerical Analysis
Model for Investigation of Near-field tsunamis No.2) with the staggered leap-frog scheme which isapplied to shallow water theory in shallow and deep seas for investigating near field tsunami (Imamuraet al., 2006).
2.4. Tsunami Simulation for Real Case of the 1977 Sumbawa Earthquake

To investigate the effective way to perform tsunami simulation,
we used fine mesh (20 arc-second ~616.67 m) and compared itwith coarse mesh (1 arc-minute ~1850 m) for the real case of
the 1977 Sumbawa earthquake (Mw 8.3) which is located at
11.14° S and 118.23° E with depth of 23.3 km as shown in
Figure 2. First, we set 38 coastal points (CP) along coastal lines
at minimum sea depth of 1 m and 38 forecast points (FP) justseaward of coastal points at sea depth of 50 m based onbathymetry contour. The total number is 76 for output points intsunami numerical simulation.
The setting of fault parameters for the real case isbased on the Global (Harvard) CMT catalog, the strike angle of260° which is parallel to the trench axis, dip angle of 24° andslip (rake) angle of -73°, represented as a normal fault. The
length of the fault of 223.872 km, width of 111.936 km and slip
amount of 7.08 m were determined by the scaling law using
equation (1). We obtained detail of tsunami heights at eachcoastal area (blue rectangle) by pairing 1 coastal point and 1
forecast point.

Figure 1. The location and focal
mechanismofthe1977
Sumbawa earthquake (Mw 8.3).
Red triangles and green circles
denote coastal and forecast
points, respectively.

2.5. Green’s Law Application

When tsunami reaches the coast, the calculation is not applicable if we are still using a coarse meshgrid. Therefore, to estimate tsunami heights at the coast, Green’s law (energy conservation law) isapplied. The equation of Green’s law is as follows

H =4h1H1h(2)

where H and H 1 are tsunami height at the coastal and forecast point and h and h1 are sea
depth at coastal and forecast point, respectively (Kamigaichi, 2009).

22.6. Construction of Tsunami Database

We assume that an earthquake is a pure reverse fault for
database creation. The reverse fault could generate severetsunami with slip angle of 90°, dip angle of 45° and strikeangle parallel to the trench axis. We used coarse meshgrid (1 arc-minute ~1850 m) for tsunami simulations. We
divided the region into 19 coastal blocks for issuing
tsunami warning purpose (Figure 2). Each coastal blockis defined by taking into account the administrative
district of local government, in which one coastal blockcorresponds to one prefecture. Especially for two coastal
blocks of the Sumba Island whose the locations areopposite with each other, that condition could be aproblem for issuing tsunami warning. Because locationof coastal blocks facing the source points will be affectedby tsunami with higher amplitude than the opposite side.
Therefore in this case we divided them into 2 sub coastal
blocks.
We set 21 Source Points (SP) along the trenchaxis, surrounding the real case of the 1977 Sumbawa
earthquake with three magnitudes (M 7.0, 7.5 and 8.0)
and four depths (0, 20, 40 and 60 km) set for tsunami
simulations (see Figure 2). Each source point is locatedon the grid with distance interval of 30 arc-minutes(~55.5 km). The dimension of computation area is 570
grid-points for longitude and 390 grid-points for latitude
which covers the region from 113°E – 122.5°E inlongitude and 7.5°S – 14°S in latitude. The time step was
set to 3 s which satisfy the Courant Friedrics Lewy (CFL)
stability condition. The calculation time was set to 3hours. The deformation area on the ocean bottom iscomputed for each fault by using the elastic dislocation
theory (Okada, 1985). Deformation area of source point
C with magnitude 8.0, depth of 0 km, fault length of
158.489 km, width of 79.245 km and slip amount of 5.01
m, which were determined by the scaling law using
equation (1), is shown in Figure 3.

Figure 2. Source points (red circles) andcoastal blocks (rectangles). Yellow stardenotes the 1977 Sumbawa earthquake(Mw 8.3).

Figure 3. Deformation area of source
point C with magnitude 8.0, depth of 0km. The red contours show uplift areawith contour interval of 0.1 m, blue
contours show subsidence area with
contour interval of 0.05 m.

2.7. Inverse Refraction Diagram to obtain Tsunami Travel Time (TTT)

Figure 4. Inverse refraction
diagram from a coastal point tothe source points.

We used TTT Software Package (ver. 2.21) which isprovided by NGDC (National Geophysical Data Center) and
ITIC (International Tsunami Information Center) to calculate
inverse refraction diagrams. In this software we can calculate
tsunami travel times on all the geographic latitude andlongitude grid points using Huygen’s principle. Figure 4
shows inverse refraction diagram from a coastal point to thesource points. The tsunami arrival time is obtained as aminimum value within the travel times from the grid points
inside the deformation area to the coastal point. All resultsfrom inverse refraction diagrams will be stored into adatabase as tsunami arrival times at coastal points.

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2.8. Retrieve Values from the Tsunami Database

Three kinds of method to retrieve values from the database were applied in this study, namelyinterpolation methods for epicenter location, magnitude and depth; extrapolation methods for
magnitude and depth; and maximum risk methods (JMA, 2007). Those methods are very useful topick up data which is already prepared in the database and will give recommendation for issuing
tsunami warning at each coastal block. The data points surrounding the supposed hypocenter are usedwith interpolation methods of epicenter location, magnitude and depth. Extrapolation method isapplied if no data is available in database such as lower and upper magnitude. The maximum risk
methods 1 and 2 are applied by using circle and rectangle area, respectively, for target searching as thedatabase output. In order to test these methods, we used the magnitude and depth of the real case, the1977 Sumbawa earthquake.

3. RESULTS AND DISCUSSION
3.1. Tsunami Simulation for Real Case of the 1977 Sumbawa Earthquake using Finer Mesh

In tsunami simulation of the 1977 Sumbawa earthquake, we compared the result of tsunami heights by
the finer mesh and the coarse mesh and found that mean and median of tsunami heights with finermesh are higher than ones with coarse mesh at most areas. The tsunami heights at forecast points
applying Green’s law generally agree with the range of minimum and maximum tsunami heights offiner mesh at coastal areas (Figure 5). In some areas, however, tsunami heights at coastal points using
the finer mesh were lower than using the coarse mesh. These may be because the complexity ofbathymetry data caused difficulty of selecting coastal and forecast points as output points. Based onthese results and examined computation times for both grids, we decided to use coarse mesh grid andapply Green’s law for constructing a tsunami database.

Figure 5. Comparison of tsunami heights by using finer mesh and coarse mesh.
3.2. Construction of Tsunami Database

After we completed the tsunami simulations for 252 cases, we calculated the maximum tsunami
heights of forecast points applying Green’s law, equation (2). Figure 6 shows comparison of tsunami
heights at coastal points and those from Green’s law application for source point E with magnitude 8.0
and different depths.

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We found thattsunami heights at thecoastal points get higher as
depth becomes shallower.
Tsunami heights at coastalpoints
which
wereobtained from applicationofGreen’s
lawcalculations
showedhigher values than theresults obtained directlyfrom tsunami numericalsimulation for most cases.

Figure 6. Tsunami heights of Green’s law calculation at coastal points.

3.3. Retrieving Values from Tsunami Database

The results of maximum risk methods 1 and 2 for the real case are shown in Figure 7. Before weapplied the maximum risk methods, first we interpolated the location and depth as shown in blue line.
Then we extrapolated of the magnitude as shown in purple line. All source points inside the target area
will be searched to find the highest value as the database outputs.
For maximum risk method 1, we calculated half length of the fault (~111.936 km) which isdetermined from the scaling law, equation (1), and then the maximum tsunami height at coastal point
was searched within the circle with the radius, the half length of fault of the supposed hypocenter, as
shown in orange line. It was the combination of all maximum values at source points inside the circle.
In the maximum risk method 2, the maximum tsunami height at coastal point was searched inside themaximum extent of the possible fault of the supposed hypocenter. The maximum risk method 2 showshighest tsunami heights (red line) because it was the combination of all maximum values at source
points inside the rectangle area.

Figure 7. Tsunami heights at coastal points retrieved by maximum risk methods 1 and 2.
Comparison of tsunami heights for the real case with the retrieved values from the databaseis shown in Figure 8. Red line is the result of retrieved values from the database which were obtained
from interpolation of location and depth (M 8.0 and depths of 20 and 40 km), then extrapolation ofmagnitude (from M 8.0 to M 8.3) by multiplying the coefficient value of 2.0. Green line was obtained
directly from numerical simulation by the coarse mesh with Green’s law application. Blue line wasobtained directly from numerical simulation with finer mesh. Red line and green line are close eachother and show similar pattern. It was confirmed that the retrieved values from the database has goodagreement with the simulation by Green’s law application and the tsunami database would beapplicable to future earthquakes.

5Figure 8. Comparison results from real case with the retrieved values from database.

4. CONCLUSIONSThe tsunami database in Bali and Nusa Tenggara regions was successfully constructed for quicktsunami forecasting. We performed the tsunami simulations by using TUNAMI-N2 from Tohoku
University and TTT Software Package (ver. 2.21). The maximum tsunami heights and arrival time arecalculated and the results are stored in the database for issuing tsunami warning.
In tsunami simulation of the 1977 Sumbawa earthquake as a real case, we compared theresult of tsunami heights by finer mesh and coarse mesh and found that mean and median of tsunami
heights with finer mesh are higher than ones with coarse mesh at most areas. The tsunami heights at
forecast points applying Green’s law generally agree with the range of minimum and maximum
tsunami heights of finer mesh at coastal areas. In some areas, however, tsunami heights at coastal
points using finer mesh were lower than using coarse mesh. These may be because the complexity ofbathymetry data caused difficulty of selecting coastal and forecast points as output points. In tsunami
database creation, we applied Green’s law for the tsunami heights at forecast points to estimate theones at coastal points. In order to obtain the tsunami arrival time, inverse refraction diagram was usedfrom each coastal point to the deformation area of the source point.
In retrieving values from the database, the data points surrounding the supposed hypocenterare used with interpolation methods of epicenter location, magnitude and depth. Extrapolation method
is applied if no data is available in database such as lower and upper magnitude. Comparison of thetsunami heights for the real case with the retrieved values from the database showed good agreement.
Maximum risk methods could be applied to search the most severe case of a tsunami event. Throughthe test cases, we confirmed that the tsunami database system would be applicable to futureearthquakes. The retrieved values from database will give us recommendation for issuing tsunami
warning at each coastal block.
AKNOWLEDGEMENT

I would like to express my deepest gratitude to Dr. Bun’ichiro Shibazaki as tsunami course leader ofIISEE who helped me a lot, guided and supported me during my individual study in IISEE/BRI.
REFERENCES

Imamura, F., et al., 2006 Revision, Tsunami Modeling Manual, Asian Inst. Tech, 1-72.
JMA, 2007, Presentation of Database Searching Process, Japan.
Kamigaichi, O., 2009, Encyclopedia of Complexity and Systems Science, JMA.
Okada, Y., 1985, Bulletin of the Seismological Society of America (BSSA), 75, 1135-1154.
Utsu, T., et al., 2001, Encyclopedia of Earthquakes (the second edition), Asakura, pp.657.

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