TSUNAMI HAZARD ASSESSMENT IN
MOZAMBIQUE COAST
Adérito Celso Félix Aramuge*
MEE08158

Supervisor: Yushiro FUJII**

ABSTRACTThe purpose of this study is to assess tsunami hazard in Mozambique coast, specifically to estimate
tsunami heights as well as tsunami arrival times considering near-field and far-field tsunami source
models. The softwares used for the study were TUNAMI-N2 code and Tsunami Travel Time (TTT) fornear-field tsunami, TUNAMI-F1 for far-field. Tsunami propagation was calculated by using GEBCO
one arc-minute bathymetry grid data. We used 2 tide gauges and 20 assumed stations positioned along
the Mozambique coast as outpoints. For near-field tsunami simulations, we considered normal-fault
type events in the Mozambique channel. The seven source locations were assumed with two
magnitudes (Mw 7.0 and Mw 8.0) and three depths (0, 5 and 10 km), totalizing 42 cases. For far-fieldtsunami simulations, six tsunami sources with the size of Mw 8.0 to Mw 9.3 were adopted. We foundthat for all the source models with Mw 7.0 the calculated tsunami heights do not exceed 0.4 m anddue to that, the coast would not be at high risk if events of this size happened. Regarding the source
models with Mw 8.0, the maximum tsunami heights were of 3 m or 2.5 m for some regions. The
minimum travel times of tsunami to reach the coastal area are less than 10 min for some regions near
the source. If events of this size happened, the coastal area of Mozambique would be in risk ofdamages. For far-field, the coast of Mozambique could be at risk if the event located in south Sumatrawith Mw 9.1 (slip amount of 13 m, fault length and width of 550 km and 175 km, respectively) or with
Mw 9.3 (slip amount of 15 m, fault length and width of 900 km and 175 km, respectively) took place,because the tsunami which its height of 2 m to 3 m could cause some damages to the coastal region.
The time of 9 hours, that the tsunamis took to reach the first region in the coast of Mozambique, canallow people to evacuate.
Keywords: Tsunami hazard, Mozambique coast, Tsunami simulation, Travel time, Fault model.

1. INTRODUCTION
Due to its geographical location, Mozambique is crossed by the rift valley system at the central region.
Most of this region is prone to seismic activities, because a great part of the domestic territory seats in
tectonic faults. It can bring negative impacts on the social and economic development, as hazards forthe country.
The extent of coastline is about 2600 km, allows the country to play a major role intransportation and communication along the southeast African continent due to many natural harbors
along the coast zone. It is necessary to study the tsunami hazard assessment in the coastal area ofMozambique to ensure the risk that the coastal area people and infrastructure are facing. It will be agreat help for developing appropriate hazard mitigation strategies. The possibility of damage causedby tsunamis originating from less-than-optimally oriented regions cannot be excluded without someinvestigation, for near-field as well as for far-field tsunamis.

____________________________*National Institute of Meteorology (INAM), Republic of Mozambique.
**Research Scientist, International Institute of Seismology and Earthquake Engineering, Build ResearchInstitute, Japan.
12. METHODHOLOGY
2.1. Tide Gauge Stations and Bathymetry Data
For this study, 22 tide gauges stations (TG1 to TG22) were usedas output points of tsunami simulation, along the coast ofMozambique. Among those 22 tide gauges stations, 20
represented by red triangles in Figure 1, are assumed in this study
and 2 represented by yellow triangles are operated by National
Institute of Hydrograph and Navigation (INAHINA). Two
different bathymetry data (GEBCO and ETOPO1), which areglobally available, were used for simulations. The eventualcomparison of simulation results was made to decide whichbathymetry would be used and the GEBCO bathymetry data wasselected to be used for this study.
2.2. Tsunami Source Models
To study the hazard assessment in the coastal region of
Figure1. Location of tide
Mozambique it is important to consider two kinds of tsunami
gauges stations.
source, (1) near-field tsunami and (2) far-field tsunami. Two sizesof event were used for near-field tsunami, of which one sourcewith Mw 7.0 and another source were assumed with Mw 8.0. We set three depths of 0, 5 and 10 km foreach magnitude. There are 7 assumed scenarios for each size and the parameters for each source model
were calculated by using scaling law, empirical equations as function of magnitude (Utsu et al., 2001).
The equations of the scaling law, seismic moment (Mo) and moment magnitude (Mw) are expressed asfollows:

log L = 0.5Mw − 1.8
L
W=
2

(1)

log D = 0.5Mw − 3.3(3)

Mo = µDLW

(4)

Mw =

log Mo − 9.1
1 .5

(2)

(5)

where L, W , D and µ are fault length, width, slip
amount and rigidity, respectively.
The same locations were considered for
Figure 2. Tsunami source models for Mw 7.0
scenarios 1 and 8, 2 and 9, 3 and 10, 4 and 11, 5 and(green rectangles) and Mw 8.0 (blue,
12, 6 and 13, 7 and 14 (Figure 2). The strike angles
red and black rectangles for depths of 10,
are based on trench alignment of Davie ridge (DR)
5 and 0 km, respectively).
and Mozambique ridge (MR), conjugated with
Quathlamba seismic axis, that connect the two ridges, DR and MR, in north and south region
respectively (Stamps el al., 2008).
The type of focal mechanism occurring in this area is generally pure normal fault (Grimison
and Chen, 1988). For this reason, for source models with Mw 8.0, the fault widths are assumed2

corresponding to the top depths of fault and the bottom ofseismogenic layer. We assumed the thickness ofseismogenic layer as 40 km from Shudofsky et al. (1987)
and CMT solutions. We used equation (4) to calculate thefault length by keeping the values of seismic moment andslip amount.
For far-field tsunami simulation, the source
models parameters and the hypothetical scenarios usedwere based on the study of Okal and Synolakis (2007).
Four events with six scenarios were selected (Figure 3).
We calculated the seismic moment and obtained themoment magnitude for each scenario by using equations(4) and (5), assuming the rigidity of 3.0 x 1010 N/m2.
Those parameters are listed in Table 1.

Figure 3. Tsunami source
models for far field.

Table 1. Source parameters used for far-field tsunami simulations.

Scenario

Mw

Length
L (km)Width
W (km)Depth
d (km)

Strike
ø (º)

Dip
δ (º)

Rake
λ (º)

Slip
D (m)

1
2
3
4
5
6

9.2
9.1
9.3
8.0
8.1
9.2

1100
550
900
200
450
850

150
175
175
100
130
130

10
5
5
5
5
5

359
322
322
290
265
270

8
12
12
10
7
7

110
90
90
102
89
89

15
13
15
6
10
10

2.3. Numerical Simulation of Tsunami Propagation
For near-field tsunami simulation, TUNAMI-N2 software developed by Disaster Control ResearchCenter (DCRC) of Tohoku University in Japan, were used. This software solves the non linear shallow
water long wave equations numerically. The computation region used for all scenarios extends from
31ºE to 45ºE and from -31ºS to -10ºS, with grid points of 840 and 1080 along the longitude andlatitude, respectively. Using the GEBCO bathymetry data, the computational domain of 1 arc minutegrid spacing is adopted for the computation region.
The initial deformation of sea surface is given to thenumerical simulation as an initial value, assumed
identical as the deformation of sea floor. For the initialcondition, static deformation of the sea floor, Okada(1985), was calculated using the parameters of the twosource models. The integration time step ∆t is equal
to 3.0 s, while computation time is equal to 6.5 hours,
the total number of time steps for computational timeis 7800, the maximum depth is 4822.43 m, the gridinterval ∆x and ∆y are 1754.59 and 1844.6 m,
respectively. We confirmed that the condition of CFL’s
stability is satisfied because the value of ∆t is lessthan the quotient between ∆x and 2 gd which is(m)equal to 6.02 s. Snapshoot is taken in interval of 1 min,to make an animation (Figure 4).
Figure 4. Snapshoot of tsunami
propagation for scenario 8 of near fieldsource model.

3For far-field tsunami simulation, we used TUNAMI-F1 (linear theory of tsunami propagationin the spherical coordinate) code developed by DCRC, Imamura et al. (2006). In the governing
equations, the effects of Coriolis force are taken in consideration because the tsunami propagates
through long distance. The computation region is from 30ºE to 120ºE and from -35ºS to 30ºS, withgrid points of 5401 and 3901 along the longitude and latitude, respectively. The integration time step isequal to 3.0 s, while computation time is equal to 17 hours, the total number of time steps is 20400.
We also confirmed that the CFL’s stability condition was satisfied.
2.4. Calculation of Tsunami Travel Time (TTT)The Tsunami Travel Time software, originally developed by Paul Wessel (Georware,
http://www.geoware-online.com), uses Huygens principle to calculate tsunami travel time by applying
it on geographic latitude and longitude grid. By using the software provided by National GeophysicalData Center (NGDC) and International Tsunami Information Center (ITIC), the times required for thewaves to propagate toward the places where the tide gauge stations are located were calculated for all
near field scenarios. Travel times from the tsunami waveforms were also obtained and comparison of
the two methods was made. Results obtained from the comparison shows a negligible difference
between the two methods in the near field simulations.

3. RESULTS AND DISCUSSION
3.1. Tsunami Heights
For source models of Mw 7.0 with depth of 5 km,
the following tsunami heights were obtained in
22 tide gauges for the 7 different scenarios. The
highest tsunami of 0.35 m was calculated at
Nampula (TG5) for scenario 2, followed by 0.26
m at Inhambane (TG15) and Maputo (TG22) for
scenario 4 and 6, respectively (Figure 5). For the
rest of scenarios the calculated tsunami heights
were less than 0.2 m. The results by using otherdepths (0 km, 10 km) show small difference
among them. For the case of source models of
Mw 8.0 (Figure 5), at depth of 5 km, the tsunami
height is of 3.8 m at Cabo Delgado (TG1), 2.9 m
at Inhambane (TG15) and 2.3 m at Nampula(TG5). For the rest of scenarios the tsunamis
reached less than 2 m. For depth of zero km, thetsunami reached 3 m at Cabo Delgado (TG1) for
scenario 10 and more than 2.5 m at Nampula
province (TG5 and TG8) for scenarios 9 and 14
respectively. For depth of 10 km the obtained
results show 3 m at Inhambane (TG17), 2.7 m at
Cabo Delgado (TG1) and 2.4 m at Nampula(TG5).
The results of far-field tsunami
simulation (Figure 6), show that the maximum
heights calculated in different scenarios were
3.02 m at Gaza (TG19), 2.07 m at Gaza andMaximun tsunami heights in 14 scenarios for depth equal to 5 km.
2.884

Scenario 14

1.986

Scenario 13

1.495

Scenario 12

0.789

Scenario 11

S ce n ario s

Scenario 10

3.778

Scenario 9

2.324
1.504

Scenario 8
Scenario 7
Scenario 6
Scenario 5
Scenario 4
Scenario 3
Scenario 2
Scenario 1

0.168
0.247

TG22
TG17

0.12
0.26

TG15
TG8

0.187
0.354
0.162

TG6
TG5
Tsunami heights (m)TG1Figure 5. Near field maximum tsunami heights
in 14 scenarios for depth of 5 km.

Maximum tsunami heights of 6 scenarios for far-field simulation.

Scenario 6

0.30
0.30

Scenario 5

0.34

Scenarios

0.39
Scenario 4

3.02

Scenario 3

TG22
2.07
2.07

Scenario 2

Scenario 1

TG19
TG14
TG3

1.61
Tsunami heights (m)

TG2
TG1

Figure 6. Far field maximum tsunami heights in
6 scenarios for depth of 5 km.

4Sofala (TG19 and TG14) and 1.61 m at Cabo Delgado (TG2), for scenarios 3, 2 and 1 respectively.
The rest of scenarios (4, 5 and 6) the heights were less than 0.4 m. Despite the length of scenario 6larger than the one of scenario 5, the tsunami heights for these two scenarios are nearly same, and the
maximum reached only 0.3 m at Cabo Delgado (TG1, TG2 and TG3). Scenario 4 has the highest
tsunami of 0.4 m at Maputo (TG22).
3.2. Tsunami Travel Times
Tsunami arrival times for source model with Mw 7.0 (Figure 7) for the depth of 5 km, indicate that thetsunami arrived first in the coasts of Nampula (TG6) and Cabo Delgado (TG1) for scenario 2 and 3,
respectively in 0.2 hours (11.7 min). For depths of zero and 10 km, there is no big difference in thecalculated travel times.
Tsunami travel time for source model Mw7 at depth of 5 km
7.0

Travel time (hour)

6.0
Scenario 1

5.0

Scenario 2
Scenario 3

4.0

Scenario 4
3.0

Scenario 5
Scenario 6

2.0

Scenario 7

1.0

TG
1
TG
2
TG
3
TG
4
TG
5
TG
6
TG
7
TG
8
TG
9
TG
10
TG
11
TG
12
TG
13
TG
14
TG
15
TG
16
TG
17
TG
18
TG
19
TG
20
TG
21
TG
22

0.0

Tide gauge stations

Figure 7. Tsunami travel times in 22 tide gauges for near field source models with
Mw 7.0, for depth of 5 km.

5

6

7

TG

TG

TG

8
TG
9
TG
10
TG
11
TG
12
TG
13
TG
14
TG
15
TG
16
TG
17
TG
18
TG
19
TG
20
TG
21
TG
22

4

TG

5

TG

2

3

TG

1

TG

TG

Travel tim e (hour)

We recognize here that for all scenarios the tsunami waves reaches the coast of Sofala (TG13)
and Maputo (TG21) very late. For TG13, this behavior must be related with the wide area of shallower
bathymetry from the coast toward ocean in this region. For TG21, this must be because of a process ofinterference that takes place in this region during the propagation of tsunami due to its geographicallocation inside the bay, whichcause delay of the tsunami
Tsunami travel time of 6 scenarios for far-field simulation.
arrival. For event scenarios in
14.0
the central part of channel
12.0(scenario 14), the wave arrivein 20 min at the coast of
10.0
Scenario 1
Scenario 2
Nampula (TG8) but for the
8.0
Scenario 3
rest of tide gauges it reaches in
Scenario 4
6.0
Scenario 5
1 hour and later.
Scenario 6
4.0
For far field (Figure 8),
2.0
scenarios 5 and 6, located in
0.0
Makran, took 6.5 hours toreach the north coastal area of
Tide gauge stations
Mozambique channel and 9.5hours to reach the south coast.
Figure 8. Tsunami travel times of 6 scenarios for
Tsunami due to scenarios 1, 2
far-field tsunami simulation.
and 3 located off the coast of
Sumatra, took 9 hours to reachthe coastal area of north part of Mozambique channel and 10 to 12 hours to reach the south to central
part. Scenario 4 took 10 to 11 hours to arrive at the north part and 12 to 13 hours to arrive at south part

4. CONCLUSION
We have assessed the tsunami hazard in the coast of Mozambique. For near field tsunami simulations,
42 normal-fault events were considered, with event sizes of Mw 7.0 and Mw 8.0, distributed along theMozambique channel. For far-field tsunami simulation, six source models capable for generatingtransoceanic tsunamis were considered.
For near-field tsunami simulation, the coastal regions which are far from the source, in thechannel of Mozambique, have the smallest tsunami height and the longest tsunami travel timecompared with the coastal regions near the sources, which were affected by higher tsunami height inshort time. For the cases in which the source is located in northern part of the Mozambique Channel,
the regions in the southern part can have time for evacuation, if the warning system is efficient. By
varying the depths of fault (0, 5 and 10 km), we notice that no significant difference between thecomputed tsunami heights. For source model with Mw 7.0 the coastal area would not be in risk while
for source model with Mw 8.0 the coastal area would be in risk of suffering damages because thetsunami heights can reach 3 m.
For far-field tsunami, the coast of Mozambique could be at risk if the event located in south
Sumatra with Mw 9.1 (slip amount of 13 m, fault length and width of 550 km and 175 km,
respectively) and the other event located in south Sumatra with Mw 9.3 (slip amount of 15 m, fault
length and width of 900 km and 175 km, respectively) took place because the tsunami heights canreach 2 to 3 m and it can cause some damages to the coastal region. The time of 9 hours that thetsunami takes to reach the first region, in the coast of Mozambique, can allow people to evacuate. For
the event located in Makran zone with Mw 8.1 (slip amount of 10 m, fault length and width of 450 km
and 130 km, respectively) and the larger event with Mw 8.9 (slip amount of 10 m, fault length andwidth of 850 km and 130 km, respectively), the coast would not have risk of damages because thetsunamis have small height of 0.4 m or less.

ACKNOWLEDGEMENTS
I am very much grateful to Dr. Bunichiro Shibazaki, IISEE and Professor Emile Okal, Northwestern
University, U.S.A for their supports to provide some of relevant literatures and suggestion for much
improvement in this study.

REFERENCES
Grimison, N. L .and W. P. Chen, 1988. J. Geophys. Res.,vol. 93, Nº B9, pages 10,439-10,450.
Imamura, F. et al., 2006. Tsunami Modeling Manual, 1-43.
Okada, Y., 1985. Bulletin of the Seismological Society of America, 75, 1135-1154.
Okal, E. A. and C. E. Synolakis, 2007. Geophys. J. Int., 172, 995-1015.
Shudofsky, G. N. et al., 1987. Geophys. Res. Letts. Vol. 14, No. 7, pages 741-744.
Stamps, D. S. et al., 2008. Geophys. Res. Letts. Vol. 35, L05304.
Utsu, T., E. Shima, T. Yoshii and K. Yamashina, 2001. Asakura, pp.657 (in Japanese).
Web site: Georware online store. Retrieved on April 2009. http://www.geoware-online.com

6