STUDY ON TSUNAMI DATABASE FOR TSUNAMIEARLY WARNING SYSTEM IN BANGLADESH
A.K.M.Ruhul KUDDUS*
MEE07173

Supervisor: Yushiro FUJII**ABSTRACT
In this study, numerical tsunami simulations along the Bangladesh coast were conducted in order tomake tsunami database. The numerical model is based on the spatial grid system in Cartesian
coordinate system using a non-linear tsunami theory. Two types of bathymetry grid interval wereconsidered in this study such as (1) one arc minute, that is about 1850 m and (2) twenty arc seconds
that is about 616.67 m. The source points have been set up near the Bangladesh coast and parallel tothe trench. The coastal points, forecast points and one tide gauge station were set up near the coast.
Ten source points were selected with four different magnitudes (Mw 6.5, 7.0, 7.5 and 8.0) and four
different depths (0, 10, 20 and 30 km). Tsunami heights were checked graphically for all cases. The
Green’s Law was applied to obtain the reliable tsunami heights along the coast from the tsunami
heights at forecast points. Results of both grid interval of bathymetry were compared with each other.
The simulation results were stored in MySQL system for a tsunami database.
Keywords: Tsunami arrival time, Tsunami height, Tsunami database.
INTRODUCTION
Bangladesh is surrounded by the regions of high seismicity, which include the Himalayan Arc andShillong plateau in the north, the Burmese Arc, Arakan Yoma anticlinorium in the east, and complex
Naga-Disang-Jaflong thrust zones in the northeast (Hossain, 1989). The northeast side of the IndoAustralian plate forms a subducting boundary with the Eurasian plate on the border of the Indian
Ocean from Bangladesh. The long seismic gap from Bangladesh coast to Andaman Islands of the bayof Bengal could produce a big tsunami in future. Southern part of Bangladesh is a coastal region.
Millions of people are living in this coastal region. Considering the potential seismic source (Cummins,2007), a tsunami in this region poses a very significant threat to a large coastal population. This study
has been addressed a tsunami database for tsunami early warning system, which will prevent human
lives and reduce damage from the tsunami.
METHOD OF ANALYSIS
Bathymetry Data
The bathymetry data were downloaded from the web site of GEBCO. Two types of grid interval ofbathymetry data, one arc minute that is about 1850 m and twenty arc seconds that is about 616.67 m,
were used in this study. GEBCO data have positive values on land grids and its grid format is NetCDF.
It was needed to convert another format by using some GMT commands. The converted bathymetry
grid data became negative values for land (-999) and positive values for sea depth.
*Bangladesh Meteorological Department, Bangladesh.
**International Institute of Seismology and Earthquake Engineering (IISEE), Building ResearchInstitute (BRI), Japan.

1Magnitude and Depth
The four magnitudes and four depths have been set for the computation of tsunami simulation for eachsource point in order to make database. The magnitudes are Mw 6.5, 7.0, 7.5 and 8.0 with the intervalof 0.5. The depths are 0, 10, 20 and 30 km with the interval of 10 km.
Output Points and Source Points
Twenty-two coastal points, twenty-two forecast points and one tide gauge station were considered as
the output points that were set up near the coast. A minimum depth was fixed to 1 m for the coastal
points and one tide gauge station. The forecast points were fixed at 10-minute ≈ 18.5 km away fromthe coast. The output points are shown in Figure 1 with contours of depth.
(a)

(b)

Figure 1. Bathymetry depth contour map near Bangladesh coast. Red circles
and Maroon circles indicate forecast points and coastal points, respectively.
The source points were set up nearand parallel to the trench. It was fixedaccording to grid size (thirty-minuteinterval). Red circles in Figure 2
represent source points. The total numberof source points is ten.

Figure 2. Location of source points.
Fault Parameter and Seafloor Deformation
Fault parameters, length, width, and displacement were calculated by using scaling law (Tatehata,1997) as a function of magnitude. The strike of 320º is set, which is parallel to the trench, consideringthat most of the earthquakes happen along the trench; the dip angle is 45º, slip angle is 90º and numberof fault segment is one. The dip and slip angle are set at these values because it is the worst-casescenario of tsunami. The location of the hypocenter is assumed to be in the middle of the fault plane.

2The depth of top left corner (TLC) of each fault was used as the input depth in order to computetsunami simulation.
The seafloor deformation due to fault wascalculated by Okada’s formulas (Okada, 1985), using thefault parameters. The vertical deformation of the sea
bottom was used as the initial condition for the numericaltsunami simulation. The initial size of tsunami depends
on the amount of vertical seafloor displacement. It isdetermined by the magnitude of earthquake, depth andfault plane mechanism. Figure 3 represents deformation
area of source point F for magnitude of 8.0. The red
contours denote uplift with the contour interval of 0.1 m,
while the blue contours denote subsidence with thecontour interval of 0.1 m.
Figure 3. Vertical displacement of the seafloor.
Numerical Tsunami Simulation
TUNAMI-N2 code is applicable in order to compute a tsunami propagation. The simulation program(e.g. Fujii, 2008a) is carried out considering the bottom friction and using the non-linear term. Thearea of computation is determined by considering the source points and coastal points. The area is 15º
N to 23ºN and 85ºE to 95ºE. The number of grid points is 601 and 481 in x and y direction,respectively. The calculation time is set to12 hours for the area. The time step is set to 3 s. Number oftime steps for snapshots is 200 (10 min). Tsunami heights at the coast are calculated by using Green’s
law from tsunami heights at the forecast points.
Inverse Refraction Diagram
Refraction diagrams were used to calculate the tsunami travel times by using TTT (e.g. Fujii, 2008b).
TTT originally developed by Paul Wessel, Geoware and modified by UNESCO (TTT SoftwarePackage).
Refraction diagram can be drawn backwards
from the coast. Such a diagram is called inverse
refraction diagram and is sometimes used to estimate
the tsunami source area. It is prepared to calculate
inverse travel times of tsunami from the coastal points
to the source. The grid points that have absolute values
greater than 0.04 m are considered as the source points
inside the seafloor deformation area. The red color ofFigure 4 indicates the grid points. Light black curves
are the travel time arcs computed for three-outputpoints. Red area indicates assumed tsunami source. Thecontour interval is ten minutes. The difference between
any two contour lines of inverse refraction diagram
indicates that tsunami wave velocity is dependent onwater depth.
Figure 4. Inverse refraction diagram.
RESULT AND DISCUSSION
The results of tsunami simulation (tsunami heights and tsunami travel times at the output points) arestored first into Excel sheets in order to make a database.

3Tsunami Heights
The simulation results are represented in Figure 5 for checking. One is tsunami height versus
magnitude with constant depth and another one is tsunami height versus depth with fixed magnitude.
(a)

(b)

A_T g_D0km

0.30

0.020
A_T g_M6.5
0.015

0.20

Height (m)Height (m)A_T g_D30km

0.10

0.010
0.005
0.000

0.00
6.5

77.5

80

8.5

10203040

Depth (km)Magnitude

Figure 5. (a) Magnitude versus tsunami height with constant depth.
(b) Depth of fault center versus tsunami height with fixed magnitude.
A: source point, Tg: tide gauge station, D: depth, and M: magnitude.
Figure 5(a) shows that as the magnitude increases the tsunami height also increases. Figure5(b) shows that as the depth increases the tsunami height decreases. Simulation results are checked for
all cases and all coastal points. Then we found that big magnitude together with shallow earthquake
produces large tsunami.
According to the simulation results,
2.50
tsunami amplitudes become the maximum at2.00
the coastal points that are closer to the source
1.50
and tsunami amplitudes become the minimumat the coastal points, which are far from the1.00
source. Depending upon magnitude, depth and0.50
location of source, Figure 6 shows that the0.00
tsunami heights along the coast are different.
A_ M8 .0 _ D0 km_ M TH
B_ M8 .0 _ D0 km_ M TH

Height (m)C_ M8 .0 _ D0 km_ M TH

D_ M8 .0 _ D0 km_ M TH
E_ M8 .0 _ D0 km_ MTH

F_ M8 .0 _ D0 km_ MTH

G_ M8 .0 _ D0 km_ M TH
H_ M8 .0 _ D0 km_ MTH
I_ M 8 .0 _ D0 km_ M TH

Tg

Br-12
Br-13
Br-14
Br-15
Ku-16
Ku-17
Ku-18
Ku-19
Ku-20
Ku-21
Ku-22

Ch-1
Ch-2
Ch-3
Ch-4
Ch-5
Ch-6
Ch-7
Ch-8
Ch-9
Br-10
Br-11

J _ M 8 .0 _ D0 km_ MTH

Coastal point

Figure 6. Tsunami heights against the coastal points for different
tsunami sources with the same magnitude and depth.
Comparison between Tsunami Travel Times from Tsunami Waveforms and TTT
Depending upon different source points with the same magnitude Mw 8.0 and depths (0 and 30 km),Figure 7 shows the comparison between tsunami travel times from tsunami waveform’s times andtsunami travel times calculated by using TTT software.
The tsunami travel times arealmost similar for the same case.
400.00
Tsunami travel times are difficult to300.00
detect from the first small change ofwaveforms at the coastal points.
200.00
C_ M 8.0 _ D3 0 km_ TTT

C_ M 8.0 _ D3 0 km_ TWT
D_ M 8.0 _ D0 km_TTTTime (min)D_ M 8.0 _ D0 km_TWT
I_ M8 .0 _ D3 0 km_ TTT

I_ M8 .0 _ D3 0 km_ TWT

100.00

Br-12
Br-13
Br-14
Br-15
Ku-1 6
Ku-1 7
Ku-18
Ku-19
Ku-20
Ku-2 1
Ku-2 2Tg

Ch-1
Ch-2
Ch-3
Ch-4
Ch-5
Ch-6
Ch-7
Ch-8
Ch-9
Br-10
Br-11

0.00

Coastal point

Figure 7. Comparison between tsunami travel times using waveforms
and TTT against 22 coastal points and one tide gauge station.

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Tsunami Heights with Green’s Law
GEBCO, one arc minute data, is not good enough to calculate accurate tsunami heights along the coast.
The Green’s Law, energy conservation law along the ray (Satake, 2008), is needed to obtain thereliable tsunami heights along the coast from the forecast points at the sea. Tsunami heights at thecoast (H1) are calculated by tsunami amplitudes at the forecast points (H) multiplied by the fourth rootof the sea depth ratios at the forecast points (h) and at the coastal points (h1). The sea depth at the coast(h1) is assumed 1 m. Hence, tsunami heights along the coast becomes,
H1 = 4 h H

Height (m)(1)
Depending upon different source points with the same magnitude and depth, the tsunami heights
calculated by applying Green’s Law are shown in Figure 8.
The maximum tsunami height with
F_ M8 .0 _ D0 km_ GH
Green’s Law against the coastal point (chG_ M8 .0 _ D0 km_ GH
5.00
1) is estimated 4.8 m. This coastal point
H_ M8 .0 _ D0 km_ GH
shows the highest tsunami height with
4.00
I_ M 8 .0 _ D0 km_ GH
Green’s Law as it is closest to the tsunami
J_M
8.0_D0km_GH
3.00
source.
2.00
1.00

Ku- 22

Ku- 20Ku- 21

Ku- 17
Ku- 18Ku-1 9Br-13
Br-14Br- 15
Ku-1 6

Ch- 9
Br-10
Br-11
Br-12

Ch- 6
Ch-7
Ch-8

Ch-4
Ch-5

Ch- 1
Ch-2
Ch- 30.00

Coastal point

Figure 8. Tsunami heights with Green’s Law for different
tsunami sources with the same magnitude and depth.
Finer Grid Computation

5Ku-21
Ku-22

Ku-16
Ku-17
Ku-18
Ku-19
Ku-20

Br-12
Br-13
Br-14
Br-15

Ch-8
Ch-9
Br-10
Br-11

Ch-3
Ch-4
Ch-5
Ch-6
Ch-7

Ch-1
Ch-2

Height (m)Twenty arc second grid intervals (~ 616.67 m) of bathymetry data are applicable to compute thetsunami simulation for the same area. The twenty arc second grid intervals of bathymetry data are resampled from GEBCO one arc minute grid data. The number of grid points is 1801 and 1441 in x andy direction, respectively. The time step is set to 2 s. Number of time steps for snapshots is 300 (10min). Figure 9 shows that tsunami heights for the twenty arc second grid interval are little higher thanthose for the one arc minute grid interval, because the grid size is smaller. Finer grid computation canexpress shorter wavelength of tsunami than coarser one. The black curve denotes tsunami heights
against coastal points for the twenty arc second grid interval; the green curve denotes tsunami
heights against coastal points for the one arc minute grid interval.
The simulation time required
for the twenty arc second grid
B_8.0_D0km_1min
interval is four hours for each case.
2.5
B_8.0_D0km_20c
2.0
Meanwhile the simulation time
1.5
required for the one arc minute grid
1.0
interval is twenty-one minutes for
0.5
each case. The computation with the0.0
one arc minute grid interval ofbathymetry data and Green’s Law
were useful to save time of about 474
Coastal Pointhours.
Figure 9. Comparison of tsunami heights for twentyarc second grid interval and one arc minute grid interval.

DATABASEMySQL Database
Simulations data is put into a MySQL database by using some MySQL commands. The simulation
result is divided into two tables under the name of “BMD_tsunami” main table, that is, “Hyp_Ruhul”
table and “Simudata”. “Hyp_Ruhul” table contains all information regarding hypocenter parameters aslatitude (º), longitude (º), depth (km), magnitude, slip (cm), length (km), width (km), strike (º), dip (º),
rake (º) and vertical displacement (m). Output points together with simulation results are stored at the“Simudata” table. “Simudata” table contains the number of output points, name of output points,
longitude (º), latitude (º), depth (m), tsunami travel time (min), travel time of maximum tsunami
heights, maximum tsunami heights (m), Green’s heights (m), FILE_NAME.
The method of database system used in this study is important in order to issue tsunami warning.
The numerical simulation takes a long time, if we ran the simulation after the occurrence of anearthquake, tsunami would arrive at the coasts before tsunami warning is announced.
CONCLUSION
In this study, a prototype tsunami database in Bangladesh is created. Simulation results are stored into
the MySQL database. When an earthquake occurs, the tsunami heights and tsunami travel times along
the coastal area will be obtained by interpolation of the data using the location of epicenter, magnitude
and depth from the database for the information of tsunami warning.
According to the analysis of simulation results, it is found that big magnitude together withshallow earthquake produces large tsunami. The coastal points, which are more far in distance fromthe source, have the smallest tsunami heights and take the longest tsunami travel times. On the other
hand, the tsunami sources nearest to the coastal area cause higher tsunami heights. Tsunami heights
computed for the finer grids (20 arc second grid) are slightly higher than that for the one arc minutegrids against the same coastal points because the grid size is smaller. Finer grid computation canexpress shorter wavelength of tsunami than coarser one.
ACKNOWLEDGEMENT
I would like to articulate my heartfelt gratitude advisor Dr. B. Shibazaki, for his heartiest andspontaneous support, valuable comments and guidance during my study.
REFERENCES
Cummins, Phil R., 2007, Nature 449, 75-78 doi: 10.1038/nature06088.
Fujii, Y., 2008a Tsunami Simulation, 2007-2008 IISEE Lecture Note, BRI.
Fujii, Y., 2008b Tsunami Source, 2007-2008 IISEE Lecture Note, BRI.
Hossain, KM., 1989, 7th Geological Conference, Bangladesh Geological Society.
Okada, Y., 1985, Bulletin Seismological Society of America, 75, 1135-1154.
Satake, K., 2008, Tsunami Generation and Propagation, 2007-2008 IISEE Lecture Note, BRI.
Tatehata, H., 1997, The New Tsunami Warning System of The Japan Meteorological Agency, JMA,Japan.

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