TSUNAMI SIMULATION AND HAZARD ASSESSMENT
ALONG THE COASTS OF FIJI
Unaisi Rabetabeta RAWAIKALA*
MEE09196

Supervisor: Bunichiro Shibazaki**
Yushiro Fujii**

ABSTRACTThis study was aimed to assess the tsunami hazard around the coastline of Fiji, in terms of the tsunami
heights and tsunami arrival times. Eleven hypothetical source models were considered along the Tonga
Trench of which six had Mw 8.0 and five had Mw 8.5. The five source earthquakes considered for the
Vanuatu Trench had Mw of 8.0. The fault parameters were determined considering the worst casescenario using Papazacho’s scaling law while the tsunami propagation was computed numericallyusing the TUNAMI- N2 code.
The expected travel time of tsunami from the Tonga trench is about 63 minutes for anearthquake source of Mw 8.0 and 56 minutes for a source of Mw 8.5. Tsunami high risk areas fromany tsunami source along the Tonga Trench were identified to be the Lau Islands, Pacific Harbor andthe Vanua Levu coastlines particularly Udu and Natewa Bay. The earliest tsunami arrival time fromthe New Hebrides Trench is 91 minutes. Tsunami high risk areas were identified to be the coasts ofMalolo Lailai in the Mamanuca Group, Yalobi in the Yasawa Islands, Vunisea Kadavu, Pacific Harborand the coral coast of Viti Levu particularly Korotogo and Natadola.
Generally it can be inferred that a major reason of the relatively high tsunami height is thatsome of these coastlines are within range of maximum directivity of the tsunami, thus, there is little
scattering of tsunami energy. In addition, reflection and refraction phenomenon taking place amplifies
the tsunami heights at some of these locations.
Keywords: Tsunami simulation, Tsunami hazard, Tsunami height, Tsunami arrival time.

1. INTRODUCTION
The nature and severity of tsunami disasters are determined by the conditions related to the attacking
tsunami and the attacked region. The basic parameter of a tsunami is its height; therefore, if a height ofa tsunami attacking a region can be estimated then it is possible to infer how large a tsunami attack
will be and how much damage will occur in that region. As for the region, its basic parameters are its
distance from the coast, height above sea level, coastal disaster prevention facilities and buildings(breakwaters, tsunami control forests, construction type and density) and the state of human activities(sleeping, working, etc) (Murata et al., 2010).
Fiji is located in the southwest Pacific Ocean at the midpoint of the opposing potential
tsunami sources, the Tonga Trench and New Hebrides Trench (Gill 1976), therefore is particularly
vulnerable to tsunamis. In understanding the tsunami hazard and its causes the more we are able toenhance our preparation in reducing the tsunami vulnerability thus reducing the tsunami risks in Fiji.
The main purpose of this study is to assess tsunami threat around the coastlines of Fiji in terms oftsunami heights and arrival times.

*

Mineral Resources Department, Suva, Fiji.
International Institute of Seismology and Earthquake Engineering, Building Research Institute, Japan**− 103 −

2. METHODOLOGY
2.1 Fault Parameters
The worst case scenario was considered for maximum risk analysis; therefore the dip of the fault was
set as 45º, rake as 90º and the depth as 0km (Depth of top edge of the fault). The strikes of the faults
are calculated from the trench alignment at each source location. The longitude and the latitude ofthese source point locations represent the location of the top edge corner of the fault. The length, widthand slip amount of the fault are calculated using the scaling law by Papazachos et al (2004).Tables 1
and 2 show all the fault parameters of the source earthquakes from both the Tonga and New Hebrides
Trench. Figures 1 to 3 illustrates the fault areas of the different sources from the Tonga and New
Hebrides Trenches.
Table 1. Fault Parameters of the eleven tsunami sources with Mw 8 and 8.5 from the Tonga Trench.
S.E.
A1
B1
C1
D1
E1
F1
A2
B2
C2
D2
E2

Mw

Depth
(km)

Dip
(º)

Rake
(º)

Strike
(º)

L
(km)

W
(km)

Slip
(m)

Max.
Up.
(m)

Max.
Sub.
(m)

Lat.
(º)

Lon.
(º)

8
8
8
8
8
8
8.5
8.5
8.5
8.5
8.5

0
0
0
0
0
0
0
0
0
0
0

45
45
45
45
45
45
45
45
45
45
45

90
90
90
90
90
90
90
90
90
90
90

210
205
200
190
160
113
210
205
200
193
113

162.2
162.2
162.2
162.2
162.2
162.2
305.49305.49305.49305.49305.49

70.79
70.79
70.79
70.79
70.79
70.79
101.16101.16101.16101.16101.16

2.14
2.14
2.14
2.14
2.14
2.14
4.47
4.47
4.47
4.47
4.47

1.23
1.23
1.23
1.23
1.23
1.23
2.55
2.56
2.56
2.56
2.56

-0.59
-0.59
-0.59
-0.59
-0.59
-0.59
-1.25
-1.25
-1.25
-1.25
-1.25

-23
-21
-19
-17
-15
-14.28
-23
-21
-19
-16.5
-14.28185.4
186.4
187.14187.66187.36185.37185.4
186.4
187.14187.9
185.37

S.E.; Scenario Earthquake, Max. Up. ; Maximum Uplift, Max. Sub. ; Maximum Subsidence,
Lat.; Latitude and Lon. ; Longitude.
Table 2. Fault Parameters of the five tsunami sources with Mw 8 from the New Hebrides Trench.
S.E.

Mw

Depth
(km)

Dip
(º)

Rake
(º)

Strike
(º)

L
(km)

W
(km)

Slip
(m)

Max.
Up.
(m)

Max.
Sub.
(m)

Lat.
(º)

Lon.
(º)

VA1
VA2
VA3
VA4
VA5

8
8
8
8
8

0
0
0
0
0

45
45
45
45
45

90
90
90
90
90

302
323
329
347
344

162.2
162.2
162.2
162.2
162.2

70.79
70.79
70.79
70.79
70.79

2.14
2.14
2.14
2.14
2.14

1.23
1.23
1.23
1.23
1.23

-0.59
-0.59
-0.59
-0.59
-0.59

-23
-22
-20.5
-19
-17.25171
169.5
168.52167.74167.51

2.2 Tsunami Simulation
The TUNAMI-N2 code (Tohoku University Numerical Analysis Model for Investigation- Near field
Tsunami Number 2 code) was used for modeling propagation of tsunami towards the coast of Fiji fromthe proposed tsunami sources along the Tonga and New Hebrides Trenches.

− 104 −

Figure 1. Fault area of the Mw 8 sourcesalong the Tonga Trench.

Figure 2. Fault area of the Mw 8.5 sourcesalong the Tonga Trench.

The computation region used for all scenarios
along the Tonga Trench extends from 176ºW to
190ºE and from -27ºS to -12ºS, with grid points
equal to 840 and 900 points along the longitude
and latitude, respectively. The computation
region used for all scenarios along the New
Hebrides Trench extends from 166ºW to 185ºE
and from -25ºS to -14ºS, with grid points equal
to 1020 and 660 points along the longitude andlatitude, respectively. From the 1 arc-minute
GEBCO bathymetry data, the computational
domain of 1 minute grid spacing is derived foreach of the computation region. For the initialcondition, static deformation of the seafloor(Okada, 1985) was calculated using theparameters of the source models.
The integration time step ¨t is equal to 3.0 s,
computational time is equal to 3 hours, numberof time steps for computational time is 3600, thegrid interval ¨x is 1749.2847 m, ¨y is1844.6793 m and the maximum depth is10568.437 m. This is for the tsunami simulationfrom the Tonga Trench.
As for the tsunami simulation from the
New Hebrides trench, the integration time step
¨t is equal to 3.0 s, computational time is equal
to 4 hours, number of time steps forcomputational time is 4800, the grid interval ¨x
is 1749.2846 m, ¨y is 1844.6793 m and the
maximum depth is 7198.1133 m. With both, theintegration time step of 3.0 s, the condition of
CFL’s stability is satisfied because the value of¨t is less than the quotient between ¨x and2 gd which is equal to 3.84 for the Tonga
Trench simulation region and 4.66 for the New
Hebrides Trench region. Snapshots are taken in
the interval of 1 minute.

3. RESULTS AND DISCUSSION
3.1 Maximum Tsunami Heights
Figure 4 shows the clarification of the different
regions mentioned in the discussion of results.

Figure 3. Fault area of the Mw 8 sourcesalong the Tonga Trench.

3.1.1 Maximum Tsunami Heights for Mw 8
earthquakes along the Tonga Trench
From Figure 5, source A1 generated
the highest tsunami height of 0.7m at the coastof Onoilau (TG1), Vatoa (TG2), Ogea (TG3)and Waciwaci, Lakeba (TG9). Sources B1, C1− 105 −

3.1.2 Maximum Tsunami Heights for Mw 8.5
earthquakes along the Tonga Trench
Figure 6 shows the graphed maximum
tsunami heights at each of the 53 tide gauge
stations from the Mw 8.5 sources along the
Tonga Trench.
Sources A2 and B2 generated the
maximum tsunami height of 1.9 m and 2.2 m
respectively at the coast of Pacific Harbor (TG21). The directivity of the tsunami from these twosources is particularly towards this coastline. In addition the tsunami height is possibly amplified at
this location because the
1.4
SOURCE_A1
coastline is shaped like a
SOURCE_B1
1.2
bay, therefore there is a
SOURCE_C1
SOURCE_D1
1.0
concentration of tsunami
SOURCE_E1
SOURCE_F1
energy at this coast due to
0.8
repeated reflection and
0.6
refraction as the tsunami
0.4
propagates towards the
0.2
shore. Further more, high0.0
tsunami height at PacificHarbor is indicative of theAssumed Tide Gauge Stations
low gradual slopes of the
Figure 5. Maximum tsunami heights at each assumed tide gauge station forbathymetric profile of this
all the Mw 8. sources along the Tonga Trench.
bay (Vuetibau, 2007).
TG
1
TG
3
TG
5
TG
7
TG
9
TG
11
TG
13
TG
15
TG
17
TG
19
TG
21
TG
23
TG
25
TG
27
TG
29
TG
31
TG
33
TG
35
TG
37
TG
39
TG
41
TG
43
TG
45
TG
47
TG
49
TG
51
TG
53

Tsunami Heights (m)Figure 4. Locations of tide gauges as output points. Region
A -Lau Islands (TG1-TG16). Region B -Lomaiviti Islands(TG30-TG33) Region C -Vanua Levu Group(TG34-TG44). Region D -Southern Islands (TG18-TG20).
Region E -Coral Coast of Viti Levu (TG22-TG24). Region
F -Western Side of Viti Levu (TG25-TG27). Region G Mamanuca Group (TG45-Tg47). Region H -Yasawa
Islands (TG48-TG53). Region I -Central Division of Viti
Levu (TG17) TG28 & TG29). Region J -Pacific Harbor(TG21).

produce the highest tsunami heights of 1.2 m
at the same coast of Lakeba (TG9). Similarly
at the same coast the highest tsunami wave of
1.0 m was calculated for source E1. Source
D1 generated the maximum tsunami heights
of 1.1 m at the coast of Yacata (TG13)
whereas source F1 generated 0.9 m wave
height both at the coast of Ogea (TG3) and
Lakeba (TG9). Most of these coastlines
recording tsunami heights of 0.5 m andhigher are within the Lau islands (Region A).
At the coasts of Udu (TG36) andNatewa Bay (TG38) tsunami heights larger
than 0.5 m were estimated even though theselocations are far away from the sources. Thisis due to the refraction and reflection effect.
For Mw 8.0 sources, the tsunami high risk
areas were identified to be the Lau Islands(Region A), Udu (TG36) and Cicia in Natewa(TG38) in Region C (See Figure 4).

Sources C2 and D2 generate maximum tsunami heights of 1.7 m at Ogea (TG3) and 2.1 m at
Udu (TG36), respectively. Udu is along the path of the tsunami ray and is also a cape whereby thewave rays are liable to cluster, therefore amplifying the tsunami height. The highest tsunami height of
2.3 m is recorded at the coast of Lakeba (TG9) for source E2.

− 106 −

2.5

SOURCE_A2
SOURCE_B2
SOURCE_C2
SOURCE_D2

Tsunami Heights (m)2.0

SOURCE_E2
1.5

1.0High tsunami risk
areas are identified to be the
Lau Islands (Region A), PacificHarbor (TG21) in Region J,
Udu (TG36) and Natewa(TG38) both in Region C (seeFigure 4).

0.5

3.1.3 Maximum Tsunami
HeightsforMw
8.0
earthquakes
along
the
New
Assumed Tide Gauge Stations
Hebrides Trench
Figure 6. Maximum tsunami heights at each assumed tide gauge
Figure 7 shows thestation for all the Mw 8.5. sources along the Tonga Trench.
maximum tsunami
1.2
heights at each of
VA1
VA2
1.0
the 53 assumed tide
VA3
VA4
gauge stations from
0.8
VA5
each of the New
0.6
Hebrides Mw 8.0
0.4
source.The highest
0.2
tsunamiiscalculated
as
1.0
m
0.0
at the coast ofAssumed Tide Gauge Stations
Natadola(TG24)
Figure 7. Maximum tsunami heights at each assumed tide gauge station for all
generated by source
the Mw 8 sources along the New Hebrides Trench.
VA2. This wasfollowed by source
VA1 generating 0.6 m tsunami height at the coast of Yalobi (TG48). Source VA3 generate tsunami
height of 0.3 m at the coast of Ono I Lau (TG1) in Region A, Vunisea Kadavu (TG20) in Region D,
Pacific Harbor (TG21) in Region J, Korotogo (TG22), Natadola (TG24) both located in Region E, andTavewa (TG53) in Region H. For source VA4, the highest tsunami height of 0.4 m is recorded at thecoasts of Kadavu (TG20), Malolo Lailai (TG46) in the Mamanuca Group (Region G), Yalobi (TG48)
and Tavewa (TG53) both in the Yasawa Group (Region H). Lastly, VA5 generate 0.2 m of tsunami
height at the coasts of Suva (TG17) in Region I, Vunisea Kadavu (TG20) in Region D, Lautoka(TG25) in Region F, Malolo Lailai (TG46) in Region G and Yalobi (TG48) in Region H (see Figure
4).
Sources VA3, VA4 and VA5 are located at the rear of the Vanuatu Islands so most of thetsunami energy is dissipated along the coastlines of these islands before it propagates towards Fiji. Theenergy of tsunami spreads and weakens with distance resulting in the decrease of tsunami heights as it
approaches the coastlines of Fiji.
Tsunami high risk areas from the New Hebrides Trench include Region D specifically thecoast of Vunisea Kadavu (TG20), Pacific Harbor (TG21) in Region J, Coral Coast of Viti Levu
(Region E) especially the coast of Korotogo (TG22) and Natadola (TG24), Malolo Lailai (TG46) in
the Mamanuca Group (Region G) and the Yasawa Group in Region H (Yalobi (TG48)).
TG
53

TG
51

TG
49

TG
47

TG
45

TG
43

TG
41

TG
39

TG
37

TG
35

TG
33

TG
31

TG
29

TG
27

TG
25

TG
23

TG
21

TG
19

TG
17

TG
15

TG
13

TG
9
TG
11

TG
7

TG
5

TG
3

TG
1

Tsunami Heights (m)

TG
1
TG
3
TG
5
TG
7
TG
9
TG
11
TG
13
TG
15
TG
17
TG
19
TG
21
TG
23
TG
25
TG
27
TG
29
TG
31
TG
33
TG
35
TG
37
TG
39
TG
41
TG
43
TG
45
TG
47
TG
49
TG
51
TG
53

0.0

3.2 Tsunami Travel Time
Tsunami travel times were obtained from the waveforms of the tsunami simulation computed from the
TUNAMI-N2 code.
For the first five sources (A1-E1) their first tsunami waves are recorded at tide gauges within
the Lau Group in Region A whereas for source F1 the first tsunami wave is recorded at a tide gauge
station within the Vanua Levu Islands in Region C. This is because source F1 is much closer to the− 107 −

Vanua Levu Islands than to the Lau Group. For any tsunami sources of Mw 8.0 from the Tonga
Trench, the estimated tsunami travel times are around 63 minutes.
For source A2, the first tsunami wave reaches the coast of Ono I Lau (TG1) within 52
minutes from the origin time of the earthquake. For sources B2, C2 and D2 the waves arrive first at thecoast of Vatoa (TG2) in 52, 53 and 64 minutes, respectively. As for source E2 the expected arrival
time is 58 minutes at Cikobia (TG35). For any source models of Mw 8.5 from the Tonga Trench the
expected arrival times are within 56 minutes.
For sources VA1, VA2 and VA3 their first tsunami waves are recorded at the coast of
Natadola (TG24) at the time of 79, 87, and 94 minutes, respectively. As for sources VA4 and VA5 thetsunami waves will arrive first at the coast of Viwa (TG49) in Region H at a time of 99 and 97 minutes,
respectively. For any tsunami sources of Mw 8.0 along the New Hebrides Trench, the expectedtsunami travel times to reach the coast of Fiji are within 91 minutes (about 1 hour 30 minutes).

4. CONCLUSIONSFor source models of Mw 8.0 along the Tonga Trench, tsunami high risk areas were identified to be thecoasts of the Lau Islands, Natewa and Udu. The tsunami arrival time calculated for an earthquake
source of Mw8.0 from the Tonga Trench is generally 63 minutes. As for source models of Mw 8.5
along the Tonga Trench, tsunami high risk areas include the Lau Islands, Pacific Harbor and the
Vanua Levu coastlines of Udu and Natewa Bay. Tsunami arrival time was computed to be within 56
minutes after the earthquake occurrence. Tsunami high risk areas for an earthquake source from the
New Hebrides Trench of Mw 8 include the coast of Malolo Lailai in the Mamanuca Group, Yalobi in
the Yasawa Islands, Kadavu, Pacific Harbor and the Coral Coast of Viti Levu particularly Korotogo
and Natadola. The travel time of tsunami from the New Hebrides Trench was computed to begenerally 91 minutes. In view of these tsunami simulation results, the residents of the tsunami highrisk areas would have adequate time to evacuate to higher grounds in time of a tsunami.

REFERENCES
Gill J.B., 1976, Geological Society of America Bulletin, 87, 1384–1395.
Murata S., Imamura F., Katoh K., Kawata Y., Takahashi S., Takayama T., 2010, Advanced Series on
Ocean Engineering, Vol. 32.
Okada Y., 1985, Bulletin, Seismological Society of America, 75, 1135-1154.
Papazachos B. C., Scordilis E. M., Panagiotopoulos D. G., Karakaisis G. F., 2004, Bulletin of theGeological Society of Greece, Vol. XXXVI.
Vuetibau L.P., 2007, IISEE/BRI, Tsukuba, Japan.

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