modeling tsunami impacts on the western …...modeling tsunami impacts on the western australian...

Modeling tsunami impacts on the

Western Australian coast

Scott Martin Leggett

Supervisor: Professor Charitha Pattiaratchi

School of Environmental Systems Engineering

This dissertation is presented for the degree of

Bachelor of Applied Ocean Science Engineering

of the University of Western Australia

October 2006

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1 Contents

1 Contents ............................................................................................................. 3

2 Figures ................................................................................................................ 5

3 Tables................................................................................................................ 10

4 Acknowledgements........................................................................................ 11

5 Abstract ............................................................................................................ 12

6 Glossary of terms ............................................................................................ 13

7 Introduction..................................................................................................... 14

8 Literature Review ........................................................................................... 16

8.1 Background to tsunami waves............................................................. 16

8.2 Background to tsunamigenic earthquakes ......................................... 16

8.3 Work done in numerical tsunami modeling ...................................... 18

8.3.1 The MOST model............................................................................... 18

8.3.2 Historic tsunami events modeled using MOST ............................ 21

8.3.2.1 Hokkaido-Nansei-Oki event....................................................... 22

8.3.2.2 Andreanov event.......................................................................... 26

8.3.2.3 Chimbote Event ............................................................................ 30

8.4 Realtime tsunami simulation with MOST .......................................... 31

8.5 Tsunami Risk Assessment for Western Australia ............................. 32

8.5.1 Current recommendations................................................................ 34

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9 Research Motivation and Aim ...................................................................... 35

10 Methodology ................................................................................................... 37

10.1.1 Modeled source location .................................................................. 38

10.1.2 Modeled source parameters ............................................................ 41

10.1.3 Running the MOST model ............................................................... 42

11 Results .............................................................................................................. 44

12 Discussion ........................................................................................................ 74

12.1 Scenario 1................................................................................................. 74

12.2 Scenario 2................................................................................................. 76

12.3 Scenario 3................................................................................................. 77

12.3.1 17th July 2006 Java tsunami .............................................................. 79

12.4 Scenario 4................................................................................................. 79

12.5 Scenario 5................................................................................................. 81

12.6 Scenario 6................................................................................................. 82

12.7 Scenario 7................................................................................................. 83

12.8 Overall ..................................................................................................... 84

13 Conclusions and further work ...................................................................... 87

14 References ........................................................................................................ 89

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2 Figures

Figure 7.3.1: Ocean bottom deformation corresponding to earthquake

parameters used in testing MOST. Ocean surface deformation is assumed equivalent

and instantaneous (Titov and Gonzalez, 1997). ............................................................... 20

Figure 7.3.2: Comparison of the 1993 Okushiri inundation model (crosses),

field observations (circles) and stereo photo data (triangles). Top frame shows aerial

photograph of section of coastline studied. Middle frame shows inundation,

topography and computational nodes. Bottom from shows maximum vertical runup

along the same section. ........................................................................................................ 24

Figure 7.3.3: Modeled maximum tsunami heights and maximum wave

velocities over a cross-section of Aonae Cape (Titov and Synolaksis, 1998). .............. 25

Figure 7.3.4: Comparison between computed (red) and measured (blue) water

levels for various offshore locations in the Andreanov tsunami event (Titov and

Gonzalez, 1997). .................................................................................................................... 27

Figure 7.5.1: Seismicity – Australia, Indonesia and New Zealand 1977 – 1997

(Canterford et al., 2006)........................................................................................................ 33

Figure 7.5.1: Recorded earthquakes of magnitude >4.0 shown as small circles.

Tsunamigenic events on record shown as larger circles (Tsunami Laboratory, 2005).

................................................................................................................................................. 39

Figure 7.5.2: The domain over which modeling took place, with significant

features marked. Location of tsunami sources within the domain used in numerical

modeling indicated by crosses (Tsunami Laboratory, 2005, Robb et al., 2005). .......... 40

Figure 7.5.3: Diagram showing relationship of source parameters to fault. ... 42

Figure 7.5.1: Maximum water heights over the domain for Scenario 1. .......... 45


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Figure 10.8: Water level predicted offshore of Karratha for the duration of

Scenario 1............................................................................................................................... 52

Figure 10.9: Water level predicted offshore of Exmouth for the duration of

Scenario 1............................................................................................................................... 52

Figure 10.10: Water level predicted offshore of Carnarvon for the duration of

Scenario 1............................................................................................................................... 53

Figure 10.11: Water level predicted offshore of Steep Point for the duration of

Scenario 1............................................................................................................................... 53

Figure 10.12: Water level predicted offshore of Geraldton for the duration of

Scenario 1............................................................................................................................... 54

Figure 10.13: Water level predicted offshore of Perth for the duration of

Scenario 1............................................................................................................................... 54


Scenario 2. .............................................................................................................................. 55


Scenario 2. .............................................................................................................................. 55


Scenario 2. .............................................................................................................................. 56

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Scenario 2. .............................................................................................................................. 56


Scenario 2. .............................................................................................................................. 57


Scenario 2. .............................................................................................................................. 57


Scenario 3. .............................................................................................................................. 58


Scenario 3. .............................................................................................................................. 58


Scenario 3. .............................................................................................................................. 59


Scenario 3. .............................................................................................................................. 59


Scenario 3. .............................................................................................................................. 60


Scenario 3. .............................................................................................................................. 60


Scenario 4. .............................................................................................................................. 61


Scenario 4. .............................................................................................................................. 61


Scenario 4. .............................................................................................................................. 62

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Scenario 4. .............................................................................................................................. 62


Scenario 4. .............................................................................................................................. 63


Scenario 4. .............................................................................................................................. 63


Scenario 5. .............................................................................................................................. 64


Scenario 5. .............................................................................................................................. 64


Scenario 5. .............................................................................................................................. 65


Scenario 5. .............................................................................................................................. 65


Scenario 5. .............................................................................................................................. 66


Scenario 5. .............................................................................................................................. 66


Scenario 6. .............................................................................................................................. 67


Scenario 6. .............................................................................................................................. 67


Scenario 6. .............................................................................................................................. 68

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Scenario 6. .............................................................................................................................. 68


Scenario 6. .............................................................................................................................. 69


Scenario 6. .............................................................................................................................. 69


Scenario 7. .............................................................................................................................. 70


Scenario 7. .............................................................................................................................. 70


Scenario 7. .............................................................................................................................. 71


Scenario 7. .............................................................................................................................. 71


Scenario 7. .............................................................................................................................. 72


Scenario 7. .............................................................................................................................. 72

Figure 11.8.1: Scenario water heights after 2.5 hours. Reflective effects of

Wallaby Extension and Wallaby Plateau indicated......................................................... 85

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3 Tables

Table 9.1.1-1: Location of source tsunamigenic earthquakes modeled............ 38

Table 9.1.2-1: Parameters used for all source earthquakes in the MOST

numerical model. .................................................................................................................. 41

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4 Acknowledgements

I would like to thank my supervisor, Professor Charitha Pattiaratchi for his

help and advice on this project. His help, especially with getting the model up and

running and with processing the results, was invaluable.

Thanks also to the high performance computing service provided by the

Interactive Virtual Environments Centre (IVEC). Without the use of this facility, the

numerical modeling which was so crucial to this project would not have been

possible.

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5 Abstract

Using a numerical model over a domain from -5° to -35° North and 100° to

117° east, a series of hypothetical tsunami events were simulated. The sources of

these events were located along the Java Trench, in an area of known tsunamigenic

earthquake activity. These model results were studied in order to determine the areas

of the fault which could produce the most damaging tsunamis in relation to Western

Australia. The results of the tsunami simulation at the coastline of Western Australia

were also studied to determine areas which were particularly susceptible to

tsunamis.

The study found that the bathymetry of the deep ocean had a very strong

effect on the resultant tsunami wave propagation. Areas of relatively shallow water

such as seamounts or plateaus strongly refracted and reflected tsunami wave energy.

Large plateaus adjacent to the north-west coast of Western Australia strongly

refracted tsunami waves towards the coastline at specific locations.

The maximum wave heights predicted along the Western Australian coast

varied with the location of the source, with the closer tsunami sources producing

much larger waves at vulnerable areas. However, the densely populated south-west

coastal areas of Western Australia were not significantly affected in any tsunami

scenarios tested. Again, this was largely due to the offshore bathymetric features of

this area.

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6 Glossary of terms

BoM: Bureau of Meteorology.

IVEC: Interactive Virtual Environments Centre

MOST: Method Of Splitting Tsunami, a numerical tsunami model.

MSL: Mean sea level.

NOAA:

PTWC: Pacific Tsunami Warning Center

Sumatera fault: Also known as the Java trench, this is the earthquake fault to

the north of Western Australia, between the Australian and Eurasian tectonic plates.

TIME: Center for Tsunami Inundation Mapping Efforts

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7 Introduction

The risks of tsunami inundation for Western Australia have been illustrated

clearly by the tsunami of boxing day 2004, in which many countries bordering the

Indian ocean suffered heavy damage and many casualties (Lay et al., 2005). This

event brought tsunamis to the attention of the world and served as a wake-up call for

government organisations, especially in Australia, interested in mitigating the

dangers.

The unpredictable and intermittent nature of tsunami events means that

numerical modeling is an essential tool in understanding the characteristics of

tsunami generation and propagation. One group interested in predicting tsunami

impact in Australia was the Australian Bureau of Meteorology. Since the 2004

tsunami, the Bureau have begun using numerical modeling to produce scenarios of

tsunami events, however this is in extremely preliminary stages and the group has

focused mainly on the eastern coast of Australia. There has been very little tsunami

modeling work conducted for Western Australia (Greenslade et al., 2006), and this

leaves the relative dangers for a large section of the coastline unknown.

Current warnings for the region are delivered by the seismic division of

Geoscience Australia, and the Pacific Tsunami Warning Center in Hawaii. These rely

largely on seismic data, so warnings which are passed on the Bureau cannot be

specific with regard to the threat. No modeling of tsunami generation, propagation

or inundation is currently utilised in warning systems (Bureau of Meteorology, 2006).

As a direct result of the 2004 Tsunami, the Australian Government has

allocated 68.9 million dollars to fund the development of a tsunami warning system

for Australia (Downer and Ruddock, 2005). This will take the form of a national

tsunami warning centre jointly managed by the Bureau of Meteorology and

Geoscience Australia. This will form part of the Australian contribution to the

international Indian Ocean Tsunami Warning System.

Modeling of various possible tsunami scenarios with respect to Western

Australia is important in creating an understanding of the wave processes shaping

the impact of tsunamis along the coast. This study seeks to improve the

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understanding of tsunami generation and propagation with respect to Western

Australia.

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8 Literature Review

8.1 Background to tsunami waves

Tsunamis are surface gravity waves with an extremely long wavelength and a

large period, which are generated in a body of water by disturbances of large scale

and short duration. The duration of a tsunami event can be 5-100 min, wavelength

100m-1000km and propagation speed of up to 200m/s, with wave heights in the tens

of metres in coastal areas. Tsunami waves of seismic origin usually have the longest

wavelength; for example the massive tsunami which struck the Indian Ocean on

Boxing Day 2004 had a wavelength of 670km and a height of 12m (Zahibo et al.,

2006).

Due to the extremely long wavelength of tsunami waves, the propagation

model can be treated as simple shallow water propagation, where the square of wave

speed (c) is proportional to gravity acceleration constant (g) and water depth (H):

c gH=

Tsunamis are generated by several mechanisms including seismic, landslides,

and external impact such as an asteroid or comet. However by far the most common

tsunamigenic mechanism is seismic (Kharif and Pelinovsky, 2005). Modeling

research and work has concentrated on this type of tsunami since it is most common,

and because the formation / propagation characteristics are quite different to impact

tsunamis (Kharif and Pelinovsky, 2005).

8.2 Background to tsunamigenic earthquakes

Tsunamigenic earthquakes generally occur along the edges of tectonic plates

at known faults. The parameters of an earthquake dictate the tsunami generation

parameters and three features in particular are most important. These three features,

moment, mechanism, and depth, are in turn described by several important

parameters. (Yalçiner et al., 2005)

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The moment of an earthquake, M0, measures the earthquake strength and is a

product of the rigidity of the earth’s crust in the region (µ), the length (L) and width

(W) of the fault, and the average fault slip (u0). In general, the larger the moment of

an earthquake, the large tsunami generated (Yalçiner et al., 2005). The relationship

between these components is shown in the equation (Titov et al., 1999):

0 0M LWuµ=

This is related to the commonly reported Moment Magnitude (Mw) of the

earthquake as shown:

210 03 (log ( ) 9.1)WM M= −

The mechanism refers to the type of faulting occurring in the earthquake and

the orientation of the fault. When stress is released along a fault in an earthquake

there are some major parameters describing the mechanism. The slip of the fault

describes the amount of movement which has taken place. In general the larger the

slip, then the larger the magnitude of the earthquake, and therefore, tsunami

(Yalçiner et al., 2005). However the slip and magnitude of the earthquake alone

cannot determine tsunamigenesis. There are also the factors of the fault type, rupture

area and speed, and the physical properties of the earth’s crust in the area such as

stiffness which determine tsunamigenic impact of the earthquake (Geist, 2006).

The depth of the earthquake’s epicentre is another important parameter in

determining tsunamigenesis. The closer the epicentre is to the surface of the ocean

floor, the larger the surface offset is likely to be and therefore the larger the tsunami

generated. Also, the orientation of the slip vector is an important consideration.

Work done by (Geist, 2006) shows that the obliquity of the slip vector in the dip of

the fault can have a dramatic effect on the resulting tsunami and its propagation. A

major reason for this is that oblique faulting results in a much different vertical offset

of the seafloor than simple thrust faulting. This oblique movement results in

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secondary tsunamis which move in different directions than otherwise expected

from the orientation of the fault.

Secondary mass failure may also contribute to the generation of a tsunami;

either acting alone or as a result of an initial faulting mechanism. This type of failure

often takes the form of landslides both submarine and coastal. A mass failure can

greatly increase the magnitude of a tsunami as well significantly alter the

characteristics of the resulting wave (Synolakis et al., 2002). A mass failure can also

occur some time after the earthquake has taken place. These effects make the

accurate prediction of tsunami propagation from seismic readings much more

difficult.

8.3 Work done in numerical tsunami modeling

Work on tsunami modeling has been done through various academic

institutions. Since the creation of the Center for Tsunami Inundation Mapping Efforts

(TIME), there has been some collaboration between different teams working on

separate models. TIME monitors advances in tsunami modeling techniques and uses

two major models in its simulations. (NOAA, 2006a)

The TUNAMI-N2 model, originally developed by Professor Fumihiko

Imamura in the Disaster Control Research Center in Tohoku University Japan, is a

key tool in investigating near-shore propagation and coastal amplification of

tsunamis (Yalçiner et al., 2005). It is currently used in the Hawaii branch of TIME

(NOAA, 2006b).

The tool this review will be concentrating on however, and the one which will

be used in the investigation of tsunami risk for Western Australia, is the Method Of

Splitting Tsunamis (MOST) model. This is the standard model used by the TIME

project. MOST uses a finite difference method in order to divide its computational

domain (NOAA, 2006b).

8.3.1 The MOST model

This model was developed and initially tested at the Pacific Marine

Environmental Laboratory (PMEL). At the completion of this initial phase, the model

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was transferred to the Defence Advanced Research Projects Agency’s (DARPA) Early

Detection and Forecast of Tsunami (EDFT) project. In its initial form, the model was

able to simulate a tsunamigenic event near the Alaskan coast, and the impact on the

Hawaiian islands (Titov and Gonzalez, 1997).

Since the late 80’s, the NOAA’s Tsunami Research Program at Pacific Marine

Environmental Lab has been developing numerical models with high spatial

resolution (Mofjeld et al., 2004). The published aims of this programme are to create

accurate inundation maps for US coastal communities bordering the pacific ocean,

and to provide US tsunami warning Centres with improved tsunami forecasting

systems based on real-time reporting through the use of BPG (tsunameters) and on

the use of a database of simulations known as SIFT (Short-tem Inundation Forecast

for Tsunamis) (Mofjeld et al., 2004). A prototype of this database was made available

to Tsunami Warning Centres in February 2004.

The MOST model was the first to show scattering effects of submarine

features on tsunami propagation. It also showed how ridges could act as

waveguides. For example, the 1996 Irian Jaya tsunami was modelled and it was

found that the South Honshu Ridge in the Western Pacific directed wave energy

towards Japan, matching previously unexplained inundation observations (Mofjeld

et al., 2004).

The model calculates the three basic stages of a tsunami event separately. The

stages are generation, propagation and inundation. This gives the model the

capability to simulate an event completely (Titov and Gonzalez, 1997).

The generation process simulation is based on elastic deformation theory. It

uses an initial deformation of the ocean surface due to a seafloor seismic event. This

deformation of the ocean surface evolves to a long gravity wave which then can be

input to the propagation model.

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Figure 8.3.1: Ocean bottom deformation corresponding to earthquake parameters

used in testing MOST. Ocean surface deformation is assumed equivalent and

instantaneous (Titov and Gonzalez, 1997).

The generation model assumes a fault plane model of the earthquake source.

This is based on an elastic half-space overlaid with an incompressible liquid layer –

representing the earth’s crust and ocean respectively (Titov and Gonzalez, 1997).

Linear models are used to study the generation process of the model because the

gravity wave formation due to the initial water disturbance is generally a slow

process driven by hydrostatic forces with negligible non-linear effects (Titov, 1997).

The propagation algorithm of the model is extremely flexible and can

simulate tsunami movement over global scales. Over such large distances, it is

important to take into account properties such as the curvature of the earth, Coriolis

forces and dispersion (Titov and Gonzalez, 1997).

While Coriolis can be accounted for by the use of explicit terms in the

governing algorithm, dispersion can be taken into effect by exploiting the numerical

dispersion inherent in finite-difference algorithms. This allows the use of non-

dispersive linear or non-linear equations in the wave propagation model. This

numerical dispersion scheme is used in MOST, along with spherical co-ordinates and

Coriolis terms. The non-linear shallow water wave equations are solved numerically

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using a splitting method similar to that outlined in (Titov, 1997). It has been shown,

through the use of, and comparison with, historical tsunami data, that the splitting

method is highly accurate in modeling both the propagation and the runup of

tsunami waves over complex topography when compared with similar models. This

is despite the fact that the dynamics of the breaking wave front cannot be resolved

(Titov, 1997, Titov and Gonzalez, 1997, Titov and Synolaksis, 1998).

The major difficulties in the modeling of inundation are a lack of detailed

field measurements used for testing the models, and also lack of fine resolution

bathymetry and topography data required as input. The lack of field measurements

has been addressed to some extent through large-scale runup experiments conducted

by the Coastal Engineering Research Center (CERC) of the US Corps of Engineers, as

well as several surveys of tsunami effected areas which yielded detailed field data

(Titov, 1997, Titov and Synolaksis, 1998).

A lack of high resolution bathymetric and topographic data is harder to

overcome, however it has been made available for some areas historically subject to

tsunami inundation, such as Okushiri Island, Japan, which was the site of a tsunami

in 1993.

The MOST model, unlike other models based on the shallow wave

approximation, does not use explicit dissipation terms. Though bottom friction

undoubtedly has an effect on wave evolution, it has been shown that inundation

results are largely insensitive of the roughness coefficient (Titov and Synolaksis,

1998). The neglect of arbitrary friction factors and artificial viscosity values in MOST

makes it relatively simple when compared to similar existing models (Kobayashi et

al., 1987, Zelt, 1991), and removes the risk of introducing additional numerical error

(Titov and Synolakis, 1995).

8.3.2 Historic tsunami events modeled using MOST

All modeling conducted using the MOST model, to date, has had a distinct

concentration on the Pacific. Little effort has been directed at the Atlantic Ocean and

even less at the Indian or Southern oceans. This is reflective of the major funding

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contributor of the research being the US Department of Defence, as well as the lack of

many governmental interests in areas other than the Pacific.

MOST modeling has found that accurate realism of the model relies much

more on accurate and high-resolution bathymetric and topographical data rather

than on numerical grid resolution within the model. Bathymetric grids of 150m

resolution have been found to be adequate for reproducing overall runup heights,

while a 50m resolution grid is required to reproduce the extreme runup heights and

flow velocities observed in the field. While the studies have tended to show the

accuracy of the MOST model, there is still a lot of work to be done in order to

understand the effects of friction, wave breaking, and wave forces on structures,

ground deformation over time and the performance of seafloor displacement models.

8.3.2.1 Hokkaido-Nansei-Oki event

An early published use of the MOST model algorithm was in simulating the

tsunami produced at Okushiri Island as a result of the Hokkaido-Nansei-Oki

earthquake. This earthquake had a moment magnitude of 7.8, and occurred on July

12, 1993. The resultant tsunami had an extreme runup of 30m and velocity of 20m/s

inferred from structural damage. (Titov and Synolaksis, 1997)

Other models used to reproduce this tsunami event, based on the shallow

water wave approximation (SW) have been successful to some extent. These models

predicted correctly the first order runup heights along the coast of Hokkaido and to a

lesser extent around Okushiri. However, these models were unable to reproduce

either the extreme runup values, or the extreme velocities observed around the south

end of Okushiri; at best, the predictions were a factor of two from field

measurements of the extreme runup heights. These models applied the previously

ubiquitous (Imamura et al., 1993, Satake et al., 1993, Yeh et al., 1993, Synolakis et al.,

1995, Satake and Tanioka, 1995) method of a 10m depth computational threshold.

That is, the model calculations were stopped at the 10m depth and the height of the

wave at this point was used to infer the runup heights along the coast. For this event,

and using the identical grid resolutions, the models underestimated inundation by a

factor of two on average. This method also significantly underestimated the height at

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the 10m contour compared to the full inundation calculations used by MOST,

suggesting that this method may have significant inaccuracies in tsunami modeling

(Titov and Synolaksis, 1998). It is clear, in any case, that there is considerable wave

evolution between the 10m contour and maximum runup height.

The MOST model, on the other hand, successfully predicted runup heights

along the entire coastline of Okushiri Island. A small canyon in the south-west area

of Okushiri served to amplify the effect of the tsunami, with field observers

recording a runup height of 31.7 metres. This was one area where previous models

had failed to accurately reproduce effects of small scale local bathymetry, predicting

only ~15 metre runup. The MOST model predicted a runup of 29.7 metres,

remarkably close to the measured field data, especially considering the resolution of

the bathymetry being 50 metres. It is suspected that the inclusion of dissipation

factors in the other models meant that bottom friction was taken to be much more

significant than in reality. In general, the MOST model predicted runup heights

slightly above those measured in field data. It is suspected that the non-inclusion of

dissipation factors is responsible for this. (Titov and Synolaksis, 1997)

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Figure 8.3.2: Comparison of the 1993 Okushiri inundation model (crosses), field

observations (circles) and stereo photo data (triangles). Top frame shows aerial photograph

of section of coastline studied. Middle frame shows inundation, topography and

computational nodes. Bottom from shows maximum vertical runup along the same section.

This event was ideal for use with MOST because of the availability of high-

resolution bathymetric data. The entire computational area was covered by nested

grids, with the coarsest resolution being 450m. This covered a large portion of the Sea

of Japan. Areas around Okushiri Island and west Hokkaido Island had a resolution

of 150m and areas near Aonae and Monai had a 50m resolution grid.

Another interesting result to come from this study was the effect of

bathymetric resolution on the numerical model. Computations performed using the

150m grid were able to produce the correct distribution of heights along the coast,

but failed to predict the extreme runup height. Calculations done using the 450m

grid failed, by a significant margin, to correctly predict the distribution of runup

heights – most predictions seriously underestimated the correct distribution. This

suggested to the researchers that the smallest scale features along the coast, and thus

high-resolution bathymetry data, was essential in accurate prediction of local

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inundation patterns. This is important because while wavelength measurements can

be interpolated to produce higher resolution data, coarse topographical

measurements, when interpolated, retain the same resolution (Titov and Synolaksis,

1997).

Tsunami waves overtopping a peninsula or other narrow strip of land, is

known as overland flow. It is a more challenging scenario to model when using the

shallow water theory, and has been identified as a leading cause of casualties and

physical damage associated with tsunami inundation (Synolakis et al., 1995). Though

runup measurements may not suggest an extreme event, high flow velocities can

cause extensive damage. This usually supercritical flow occurs because while the

slope of a shoreline converts the initial kinetic energy of the tsunami into potential

energy, the kinetic energy of overland flow is reduced only by dissipation. In the

Hokkaido-Nansei-Oki tsunami event, the peninsula at Anoae Cape exhibited such

overland flow in the model, and observations taken after the event also suggested

low-level, high velocity flow (Titov and Synolaksis, 1998).

Figure 8.3.3: Modeled maximum tsunami heights and maximum wave velocities

over a cross-section of Aonae Cape (Titov and Synolaksis, 1998).

Figure 8.3.3: Modeled maximum tsunami heights and maximum wave

velocities over a cross-section of Aonae Cape shows the computed maximum

velocities and wave amplitudes for a typical section of the Aonae Cape. It illustrates

that while the highest amplitudes occurred on the western side of the peninsula,

maximum flow velocities were much higher for the eastern side of the peninsula, and

coincided with the lowest amplitudes. This area, however, was the scene of most

devastation, suggesting that inundation heights alone are not the best indicator of

tsunami danger.

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A hydraulic shock, or standing bore, was another phenomenon predicted by

the MOST model for this tsunami. The bore was formed on the west side of the

Anoae peninsula from the rundown of the first wave of the tsunami. This resulted in

the retreat of the waterline to the 10m contour, approximately 500m from the

shoreline. While it was not reported in any eyewitness accounts (possibly due to the

fact that the tsunami occurred at night), numerous accounts of distant water

withdrawal for other tsunami events can be found, indirectly supporting the

calculated estimates (Titov and Synolaksis, 1998). In addition, field estimates of

overland flow for the peninsula of 10-18m/s (Shimamato et al., 1995) are consistent

with computed values of 10-19m/s (Titov and Synolaksis, 1997), further illustrating

the usefulness of the MOST model, especially in examining inundation.

8.3.2.2 Andreanov event

The Andreanov tsunami, which occurred on June 10 1996, provided an early

opportunity to test the generation and propagation algorithms of the model. This

was the example used in the announcement of the MOST algorithms (Titov and

Gonzalez, 1997). Several bottom pressure records (BPRs) from the Andreanov event

were compared to the time series generated by the MOST model, demonstrating the

model’s accuracy and usefulness. The generation and propagation sections of the

model were used to simulate the 1996 Andreanov tsunami, with results in good

agreement to field measurements taken (Titov and Gonzalez, 1997).

27

Figure 8.3.4: Comparison between computed (red) and measured (blue) water

levels for various offshore locations in the Andreanov tsunami event (Titov and Gonzalez,

1997).

Through the inversion of seismic data, the initial parameters of the

earthquake were estimated and entered into the model. The strike, dip, slip and

moment were estimated using this method and the Harvard CMT solution (Nettles,

28

2006) for this event. The distribution of aftershocks gave an estimate of the fault

length, and the relationship between the moment, the shear modulus, area of the slip

gave the magnitude of the slip.

The parameters gained from this rough estimation were input to the elastic

earthquake model. This gave an initial deformation which was then assumed to be

identical to the ocean surface initial deformation. This was the input for the MOST

model. The window of the tsunami model was 50° North-South, and 60° East-West

with a bathymetric resolution of 4 minutes (Titov and Gonzalez, 1997).

Despite being a relatively small tsunami, with amplitudes in the range of

10mm, the model performed well at matching the measured waveforms. It also

successfully predicted measured anomalies such as the interactions between the

continental shelf and the Aleutian Trench. Titov and Gonzalez (1997) also discuss the

runup model testing using the example of Okushiri Island.

In particular they conclude that the MOST model is well suited to creating

tsunami hazard and forecasting services. Modeling in this study was carried out at

6.5 times real-time, and Titov and Gonzalez suggest that advances in computer

architecture could see speed improvement factors in the range of 10-100 times. This

means that if input parameter collection becomes more streamlined, real-time

forecasting / prediction could become useful. The study also emphasises the modular

nature of tsunami forecasting, matching the modular nature of the MOST model.

However, the study also outlines problems with this idea. The acquisition of

initial earthquake parameters is all-important for modeling tsunamigenic events and

currently there is a lot of estimation involved with this. The simulations themselves

can only be assumed reliable after many iterative computations conducted by an

experienced tsunami modeller (Titov and Gonzalez, 1997).

A later study used the MOST model to simulate the same event, aiming to

find the sensitivity of tsunami propagation to various parameters describing the

fault. These included length, width, dip, strike and slip angle. With this information,

a database of pre-computed scenarios varying the most sensitive parameters could

be produced. Since tsunami propagation is a linear process in the deep ocean, this

29

would allow addition of sources multiplied by factors to reproduce complex faulting

along known fault lines (Titov et al., 1999).

An important choice for this particular study was of stiffness, µ. This

parameter varies between 1e+10 and 6e+10, and was chosen as 4.5e+10, an average

for this area. By eliminating the variation of parameters to which the tsunami was

not particularly sensitive, the database was greatly simplified while still producing

useful results (Titov et al., 1999).

Numerical analyses conducted in this study revealed that only the first wave

in the train carried significant information about the source. Later waves were

strongly affected by reflection/refraction.

Analytical methods were first used to provide a constraint to numerical

parameters and guide the modeling stricture. Small scale features not having a great

spatial extent nor extending very far into the water column were shown to be

negligible in tsunami attenuation. Larger scale features such as the Emperor

Seamount chain / Hawaiian ridge did have the effect of wave ‘channelling’ or

‘guiding’. Thus the propagation section of the model did not require greatly detailed

bathymetry (Titov et al., 1999), unlike inundation. Large fields of seamounts did

provide some attenuation of the tsunami, but since these are resolved in the large

scale bathymetry, it is taken into account by the model. Thus the primary attenuation

method of tsunamis in the deep ocean was through spreading refraction from a finite

source (Titov et al., 1999).

Since the vast majority of historical tsunamigenic earthquakes in the AASZ

were both shallow and aligned with the fault, varied parameters were limited to

length, width, dip, slip and location.

Location proved to be the parameter tsunamigenesis was most sensitive to.

Variations in the location (along the existing fault) of the generating earthquake

affected arrival time and amplitude as well as the period and shape of the wave train.

Period and shape of the leading wave was effected far less.

With a constant magnitude, the tsunami proved to be very insensitive to

initial fault dimensions. Variations in rake within the common range also had very

30

limited impact on the tsunami characteristics. However lowering the dip from 20° to

10° increased the leading wave amplitude by 30%.

The outcome of these results was that tsunami waves are characterised

primarily by the source earthquake’s magnitude and location. The linear nature of

the propagation equations used in the model also lends itself to using combinations

of closely spaced models to simulate multiple-fault sources. (Titov et al., 1999)

suggest that such a database could be used by an experienced modeller to adjust

factors of each earthquake model in order to find a solution which matched real-time

data and thus produce forecasts which are adjusted as data becomes available.

The study goes on to suggest that the next step is to produce site specific

inundations forecasts as a function of offshore tsunami characteristics. This would

lead to an inundation scenario database for specific areas which could be used in

conjunction with forecasts of propagation (Titov et al., 1999).

8.3.2.3 Chimbote Event

The earthquake off the coast of Peru, on the February 21, 1996, was a good

example of a highly tsunamigenic event, despite the magnitude of the earthquake

being relatively small. An earthquake of this type can be more damaging to coastal

communities, many of whom may not even be aware that a seismic event has

occurred (BOURGEOIS et al., 1999). Bathymetry and topography data for this area

was relatively poor, and consisted of four on-land profiles, as well as a 9 km grid of

bathymetry data. These were merged and interpolated down to a 600m resolution in

the near-shore area using data from nautical charts. This level of resolution was too

poor for accurate inundation calculations, and so the method of 10m depth threshold

computations was used instead (Titov and Synolaksis, 1997), except in areas with

high resolution 1-D transact information available.

The Harvard CMT solution (Nettles, 2006) was used extensively when

working on this model, providing the fault parameters such as strike, dip and rake.

Dimensions of the fault were determined by looking at the distribution of aftershocks

also listed in the Harvard CMT solutions. Accurate reproduction of the tsunami itself

required several trials with slight adjustments to the various parameters. This helps

31

to illustrate the inherent problems with rapid and accurate modeling of tsunamis.

The use of the CMT solution in this example, though criticised to some extent,

produced results not noticeably different to that calculated using an inverted seismic

source (BOURGEOIS et al., 1999). The advantage of this is that CMT solutions are

available almost in real-time, giving a distinct advantage to tsunami forecasting.

Several areas of the coastline had a local maximum of inundation which

wasn’t reproduced in any of the models. One of these areas was at the deepest point

of a small cove, while another was near the city of Trujillo, Peru. These served to

illustrate the need for higher resolution bathymetry in order to produce accurate

calculations of runup. Bourgeois (1999) noted that runup discrepancies noted near

Trujillo could equally be a result of multiple source faults, and it would require

another study using higher resolution local bathymetry to resolve this.

The one of the four areas of higher bathymetric resolution was at an area of

sandbar. This allowed the modeling of the overtopping, which matched the Okushiri

overtopping model (Titov and Synolaksis, 1997).

8.4 Real-time tsunami simulation with MOST

The “Holy Grail” of tsunami forecasting would be the ability to perform

greater than real-time tsunami simulation, assimilating real-time data into the model

as the information became available (Titov et al., 1999). However this is difficult for

various reasons including:

• Insufficient real-time data.

• Lack of accuracy in data and in model. For example fault parameters,

especially preliminary ones, are estimated using empirical formulas

which attempt to relate magnitude to size and slip amount.

Though it is quite simple to monitor areas at risk for seismic disturbances, it is

very difficult to predict the type of faulting which has led to the disturbances. In fact,

even many months after the event, further investigations continue to adjust the

precise nature of the fault model in a seismic event.

Another problem is with the modeling itself. MOST still requires a

considerable amount of quality control, judgement, and iterative, exploratory

32

computations to be conducted on a scenario before it is considered to be a reliable

simulation. Titov and Gonzales (1997) recommend the creation of a database of pre-

computed and carefully analysed scenarios which can be referred to in order to

construct a robust tsunami forecasting and hazard assessment capability.

In fact, such a database has already been created for the Pacific Ocean region,

and is planned for the Indian Ocean (Canterford et al., 2006).

8.5 Tsunami Risk Assessment for Western Australia

For Australia in general, the historical threat of tsunamis comes from distant

sources (Canterford et al., 2006). While it is true that Australia’s intraplate location

leads to a low siesmicity compared to interplate locations, it remains one of the most

active intraplate locations in the world, especially in Western and Central Australia.

This is due largely to the northward shift of the Indo-Australasian tectonic plate and

its resultant compression (Brown and Gibson, 2004). In fact, while earthquakes with a

magnitude greater than Mw 6.0 do occur, they are relatively infrequent and seem to

be mostly non-tsunamigenic.

Underwater slippage of sediments off the continental shelf is a concern in

some areas (Canterford et al., 2006), and this effect is suspected to have contributed

to the tsunamigenic properties of some recent earthquakes such as the 1998 Papua

New Guinea tsunami event in the Indian Ocean (Synolakis et al., 2002).

33

Figure 8.5.1: Seismicity – Australia, Indonesia and New Zealand 1977 – 1997

(Canterford et al., 2006)

The current Australian Tsunami Alert System (ATAS) relies on co-operation

between the Bureau of Meteorology (BoM), Geosciences Australia (GA) and

Emergency Management Australia (EMA). In the event of an earthquake, GA notifies

the National Meteorological and Oceanographic operations Centre (NMOC) of the

BoM with the details of the event. Together, the organisations try to predict the

likelihood of a tsunami and its probable characteristics. As well as its own sea level

data, the NMOC currently relies heavily on warnings and other information from the

Pacific Tsunami Warning Centre (PTWC) in Hawaii and the Japan Meteorological

Agency (JMA) about possible tsunamis in the Indian ocean (Canterford et al., 2006).

These warnings are then disseminated through current meteorological warning

system infrastructure by the BoM.

In response to the 2004 Sumatra tsunami, the Australian government

recognised the need for a tsunami warning system similar to that deployed in the

Pacific. This system is to be known as the Australian Tsunami Warning System

(ATWS). In the mean time, the ATAS will be gradually upgraded and eventually

integrated with the ATWS.

34

8.5.1 Current recommendations

Previously published studies of tsunami events have produced

recommendations which can be generally applied. These recommendations form the

basis for a real-time tsunami forecasting capability (Titov et al., 2004).

Modeling of a range of tsunami scenarios and maintaining a database of such

scenarios is an important part of ensuring preparedness for tsunami events, as is

remote sensing (Titov et al., 2004). Both this technology and a network of water level

meters need to be applied together in order to provide effective warnings to coastal

communities in the event of a tsunami. Deep ocean tsunami wave gauges, or

tsunameters, are especially important as they provide several advantages over the

traditionally used coastal water level meters.

One important advantage of deep ocean tsunameters is the response time to a

tsunami event. Since tsunami waves travel much more quickly in deep water than in

shallow coastal areas, these tsunameters allow rapid tsunami observation. A

strategically placed network of such gauges would allow early detection of tsunamis

threatening a relatively large area of coastline compared to coastal water level

meters.

Harbour response, or seiching, can contaminate recorded tsunami

frequencies. This is especially true of water level meters located outside harbour and

along the coast (Synolakis, 2003). Because tsunameters are located in deep water far

from the coast, they are unaffected by these local effects and are able to more

accurately record the full spectrum of the tsunami wave (Titov et al., 2004).

The strong linearity of the tsunami wave in the deep ocean, along with well

understood and accurate models of wave propagation allow the use of tsunameter

data to relatively accurately predict tsunami size and direction. The accuracy of this

prediction is certainly much greater than that relying on relatively complex coastal

wave data (Titov et al., 2004).

35

9 Research Motivation and Aim

The danger of tsunami inundation to coastal communities, outlined in the

introduction, represents a clear threat to Western Australia. Concern about this threat

provided most of the motivation for this study. However simple curiosity about the

nature of tsunamis in the Indian Ocean was also a strong motivation. As well as

being a terrible natural disaster, tsunamis are a fascinating, powerful, and rarely

documented force of nature. By their nature, tsunamigenic earthquakes are

impossible to accurately predict.

Historically, there is relatively little information available about tsunami

events and inundation in relation to Australia, especially the WA coast. A dearth of

field data related to tsunamis in the Western Australian context meant that in order

to investigate tsunami impacts on the WA coast, it was required to carry out

modeling of tsunami events.

My research involves the use of a well-tested numerical model (MOST) to

perform the following investigations:

• Predict areas of high risk in relation to Western Australia along the

section of the Sumatran fault closest to Australia.

By selecting a large area of the Sumatran/Australian subduction zone in

which to model source earthquakes, I aim to be able to predict the areas of the fault

which produce tsunamigenic earthquakes having the most significant impact on

Western Australia. The position of the earthquake and orientation of the fault, as

discussed in the literature review, has a major effect on the resultant areas affected by

a tsunami. Source location and orientation were chosen on the basis of being close to

WA, having fault orientation which directed energy towards WA, and being in line

with historic tsunamigenic earthquakes.

• Predict areas of high risk along WA coast.

36

As previous modeling studies as well as previous tsunami events have

shown, the coastal areas of maximum inundation in a tsunami event are difficult to

predict. Patterns of inundation are not intuitive and are strongly related to the

bathymetry of the offshore zone adjacent to the coastline. Even post event modeling

has difficulty in reproducing the observed runup patterns in some cases. For this

reason, it is important to investigate whether there are areas of the Western

Australian coastline which are subject to a concentration of tsunami energy.

• Investigate bathymetric / topographic features responsible for

observed and future tsunami propagation patterns.

Previous modeling, especially in the Pacific and areas around Japan, has

shown that offshore bathymetric features can have a strong effect on the distribution

of tsunami energy (Mofjeld et al., 2004, Titov and Synolaksis, 1997). Various effects

have been observed, such as waves scattering, wave guidance and refraction, as well

as focusing of tsunami energy. This can cause specific areas of coastline to be either

relatively protected from or relatively susceptible to tsunami impact in a majority of

scenarios. By modeling a series of different tsunami events along the fault area, it

should be possible to determine these areas along the coast of Western Australia.

This will allow the identification of susceptible areas along the Western Australian

Coast.

37

10 Methodology

As mentioned previously, the lack of field data for tsunamis is a major

constraint on studies in the area. Accurate collection of field data is important in

confirming the accuracy of models. Western Australia has extremely sparse

population over much of its coast. This, combined with a historically low incidence

of tsunami inundation in populated areas has meant that in the past there has been

little interest in conducting surveys of tsunami inundation along the Western

Australian coast.

This attitude has been changed in light of the Sumatran tsunami event of

2004. For example, Geoscience Australia conducted a survey of inundation at Steep

Point following the reports of tsunami impact in the area following the Javanese

earthquake of 17 July 2006 (Prendergast, 2006). This work is yet to be published.

Due to the lack of repeatable field measurement inherent in tsunami study,

the major research area of this study was in numerical modeling. It was important to

initially decide on the boundaries of the area used for modeling. Since we were

mostly interested in measuring impacts on the Western Australian coast, the area

had to include both the length of the source fault along which we wanted to test, and

the area of the Western Australian coast we were examining.

One major constraint on modeling was the memory of the computer which

the model was run on. Modeling was conducted using the IVEC facility – specifically

the Carlin computer. Limitations on memory meant that the scenarios had to be run

in pairs. It also meant that the entire Indian Ocean and Western Australian coast

could not be used in the model. Instead, spatial extent of the model was limited to

the area 100° to 117° E, and -5° to -35° N. This enclosed both a large extent of the fault

and the south-west of Western Australia, as well as a large area of ocean between the

two and west of Australia, important for investigating the projected impacts of

bathymetry on tsunami propagation.

The bathymetric data used in this study was sourced from the General

Bathymetric Chart of the Oceans (GEBCO). This data has a one minute resolution

and was suitable for use as input to the MOST model in the propagation phase

38

(British Oceanographic Data Centre, 2006, International Hydrographic Organisation

et al., 2006).

10.1.1 Modeled source location

Seven hypothetical source earthquakes were chosen along the Sumatran fault,

to the south of the active deformation zone of the Sunda trench. The precise locations

were every two degrees between 103° and 115° E inclusive. The latitude of the

sources was chosen to be equivalent to the position just north of the deformation

zone at the Sumatran fault, to an accuracy of one degree. This area is historically the

most earthquake prone, as well as being most likely to produce tsunamigenic

earthquakes. The precise locations used are tabulated below in Table 10.1.1-1, as well

as shown in the plot in Figure 8.5.2. Figure 8.5.1 also shows historical earthquakes in

the area as well as tsunamigenic events within the modeled domain.

1 2 3 4 5 6 7

Latitude (°) -6 -7 -8 -9 -9 -10 -11

Longitude (°) 103 105 107 109 111 113 115

Table 10.1.1-1: Location of source tsunamigenic earthquakes modeled.

39

Figure 8.5.1: Recorded earthquakes of magnitude >4.0 shown as small circles.

Tsunamigenic events on record shown as larger circles (Tsunami Laboratory, 2005).

N

40

Figure 8.5.2: The domain over which modeling took place, with significant features

marked. Location of tsunami sources within the domain used in numerical modeling

indicated by crosses (Tsunami Laboratory, 2005, Robb et al., 2005).

N

Exmouth

Plateau

Cuvier

Abyssal Plain

Wallaby

Plateau

Christmas

Island

Karratha

Exmouth

Carnarvon

Steep Point

Geraldton

Perth

Wallaby

Extension

41

10.1.2 Modeled source parameters

The MOST model accepts several source parameters describing the

orientation, magnitude and type of fault occurring. These parameters include the

length and width of the source, the dip, rake, strike and slip amount, as well as the

depth of the fault. The parameters used were kept constant between sources

modelled, and were chosen based on typical values for that section of fault, using

data from the USGS historical tsunami database (USGS Earthquake Hazards

Program, 2006) and Harvard CMT Catalog (Nettles, 2006) as a basis. The parameters

used are tabulated below in Table 10.1.1-1.

Length (km) 100

Width (km) 50

Dip (°) 25

Rake (°) 90

Strike (°) 299

Slip amount (m) 10

Depth (km) 10

Resultant Moment Magnitude 8.2

Table 10.1.2-1: Parameters used for all source earthquakes in the MOST numerical

model.

As shown in Figure 8.5.3, the parameters above completely describe the

simple faulting mechanism. The length and width describe the horizontal area of the

fault, and are an estimate of the extent of the fault. The depth of the fault describes

the distance to the top of the fault area from the top of the earths crust. The slip

amount describes the average amount of movement between the opposing sides of

the fault.

42

Figure 8.5.3: Diagram showing relationship of source parameters to fault.

10.1.3 Running the MOST model

The MOST model was run using the facilities of the Interactive Virtual

Environments Centre’s (IVEC) Carlin server. Each scenario was run as a separate

process, with two scenarios being run simultaneously. Constraints on memory

allocation for the model programme limited the number of simultaneous scenarios

which could be run. Each scenario took several hours to complete.

Each scenario was run to simulate a period of 300 minutes, or 5 hours. This

meant that each wave had propagated completely through the domain by the end of

the model. This allowed the construction of a series of snapshots of water heights, as

well as a plot of maximum heights throughout the event for each scenario. This

showed clearly where tsunami energy was being directed against the Western

Australian coast and throughout the rest of the domain. These plots are listed in the

Results section.

Various locations just offshore of population centres were analysed for wave

heights predicted over the course of the event. This gives a fairly good indication of

potential tsunami inundation. For reasons discussed in the Literature Review, the

inundation component of the MOST model could not be used in this study, due to

the lack of detailed topographic information for large sections of the Western

North

Strike Direction ΦS

δ

Slip Direction

λ δ: Dip (°)

ΦS: Strike Azimuth (°)

λ: Rake (°)

Ocean floor

Fault

Line

43

Australian coastline. These plots of water heights over the course of the event are

also listed in the Results section.

11 Results

The results of the numerical modeling are presented in two sections. The first

section consists of a plot of maximum wave heights over the domain of the model for

each of the seven scenarios. The second section shows plots of water heights for

locations just offshore of population centres on the Western Australian coast. These

locations are Karratha, Exmouth, Carnarvon, Steep Point, Geraldton and Perth.

45

Figure 8.5.1: Maximum water heights over the domain for Scenario 1.

46


47


48


49


50


51


52

Figure 11.8: Water level predicted offshore of Karratha for the duration of Scenario 1

Figure 11.9: Water level predicted offshore of Exmouth for the duration of Scenario 1

53

Figure 11.10: Water level predicted offshore of Carnarvon for the duration of Scenario 1

Figure 11.11: Water level predicted offshore of Steep Point for the duration of Scenario 1

54

Figure 11.12: Water level predicted offshore of Geraldton for the duration of Scenario 1

Figure 11.13: Water level predicted offshore of Perth for the duration of Scenario 1

55

Figure 11.14: Water level predicted offshore of Karratha for the duration of Scenario 2.

Figure 11.15: Water level predicted offshore of Exmouth for the duration of Scenario 2.

56

Figure 11.16: Water level predicted offshore of Carnarvon for the duration of Scenario 2.

Figure 11.17: Water level predicted offshore of Steep Point for the duration of Scenario 2.

57

Figure 11.18: Water level predicted offshore of Geraldton for the duration of Scenario 2.

Figure 11.19: Water level predicted offshore of Perth for the duration of Scenario 2.

58



59



60



61



62



63



64



65



66



67



68



69



70



71



72



74

12 Discussion

The maximum wave height plots over the entire domain are extremely

informative in describing the offshore characteristics of the tsunami propagation.

There are four specific effects of bathymetric features on tsunami waves which have

been described to some extent previously for the Pacific Ocean, but to a much lesser

extent for the Indian Ocean, especially not in relation to Western Australia. These

effects are:

• Wave scattering / reflection

• Structures producing wave ‘shadow’

• Wave refraction

• Shallow wave guides

Each particular scenario will be discussed, describing the most important

features, and then the characteristics shown overall will be discussed. Finally, the

implications for the coastline of Western Australia will be considered.

12.1 Scenario 1

Model location: 103 E, -6 N.

The first scenario modelled was the greatest distance from the Western

Australian coast. This source earthquake in this scenario was located just south of the

island of Sumatera. In this location, the scattering and shadowing effects of offshore

bathymetry is readily apparent. The location of Christmas Island and its surrounding

areas of shallow bathymetry directly between the source of the tsunami and

population centres on the Western Australian Coast, as well as lack of bathymetric

features to direct energy towards the coast, means that there is relatively little

tsunami impact on the Western Australian coast.

However, there is significant refraction and concentration, of what tsunami

energy reaches the coast, by offshore bathymetry. The southern edge of the Exmouth

Plateau causes significant refraction of tsunami energy, which is concentrated

towards the area of coast around Exmouth. This refractive effect also affects the

water levels predicted offshore of Karratha, as the refracted waves arrive less than 15

75

minutes after the direct tsunami wave, with a height almost a large. Exmouth,

however, experiences an initial tsunami wave followed by a significantly larger

refracted wave.

In the plot of wave heights for Carnarvon a similar pattern is observed. The

initial tsunami wave height is matched 15 minutes later by a second wave. This is

then followed almost 50 minutes later by a third, even larger, wave. By studying the

animation of the tsunami event, the source of the second wave is recognised to be the

complex bathymetry to the west of Christmas Island. Being directly in the path of the

tsunami, even the reduced reflection caused by this feature is significant at the

Western Australian Coast.

The third wave recorded much later is a result of scattering and reflection of

tsunami energy by the Wallaby Extension. This is the relatively shallow area of ocean

to the west of the Wallaby Plateau scatters and reflects a significant amount of

tsunami energy. Since it is also close to the direct path of the tsunami, it scatters

enough energy to be responsible for the maximum wave height offshore of

Carnarvon.

In this scenario, offshore of Steep Point records relatively small maximum

water levels of less than 10 cm above MSL. The angle of incidence of the tsunami

wave leads to a focusing of energy further north, more towards Carnarvon. For this

reason, while the basic pattern of wave heights is similar between Steep Point and

Carnarvon, the effects at Steep Point are on a much smaller scale.

The Wallaby Plateau causes significant refraction as well as focusing of

energy through the ‘bridge’ of shallow water between the large shallow offshore

plateau, and the coastline. Despite the relatively small amount of tsunami energy

which reached the coast at Western Australia, there was significant wave heights

predicted for areas affected by the bathymetry. Thus, for these areas, water level

meters which detect tsunamis by water level changes may not show a significant

tsunami passing while some areas of the coast are still devastated.

Geraldton and Perth exhibit a similar pattern of wave heights to each other,

and for largely similar reasons. The initial tsunami impact is followed by a peak

attributable to the reflection from the Wallaby Extension. The difference between

76

these locations and those further north is the spike in wave heights almost 5 hours

after the tsunami event. By further examination of the animation of the event, this

can be attributed to reflection from the large plateau to the west of cape Leeuwin.

12.2 Scenario 2


The second scenario modelled was interesting because of the greater heights

recorded along the coast than scenario 1, despite the apparent ‘shadowing’ of the

tsunami energy by Christmas Island. The energy here was scattered by features

around Christmas Island, especially the shallow areas to the southwest. Since the

majority of the initial tsunami energy was not directed towards the Western

Australian coast, this meant that the scattering effects actually directed more energy

towards the coast than would have occurred otherwise. This is visible in the

increased maximum heights offshore of Perth. The same refractive and focusing

effects of the Exmouth Plateau and The Wallaby Plateau seen in scenario 1 are

visible. The heights are increased in the area of the coast affected by the Wallaby

Plateau, as in the metropolitan area.

An interesting effect is that the coastline south of Karratha experiences lower

maximum water heights than in the previous scenario. The reason for this is the

scattering effect of the bathymetry southwest of Christmas Island. Much of the

energy is deflected, though not to a great enough extent to reach the Exmouth

Plateau. The tsunami energy is deflected more to the south than to the west.

The pattern of wave heights for Karratha and Exmouth show a similar pattern

of three maximum wave heights. After the initial tsunami impact, the second wave is

a result of scattering near the tsunami source. The third wave, just over 30 minutes

later at Exmouth and almost an hour later at Karratha, is again a result of the

scattering effect of the Wallaby Extension.

While the water level offshore of Carnarvon shows the impact of the initial

wave followed by the reflection of the Wallaby Extension, the water levels at Steep

Point exhibits two smaller waves followed by a single, very large wave. This clearly

shows the effect of the refraction and focusing around the Wallaby Plateau. In the

77

position offshore of Steep Point, there is a distinct series of three waves which have a

period of approximately 10 minutes. The first two waves which reached the position

here are smaller, with a maximum excursion from MSL of less than 10 centimetres.

This represents the wave travelling directly from the earthquake source, and the

wave reflected from the island of Java.

The third wave peak is preceded by a trough of almost 30 centimetres below

MSL, and then reaches a height of almost 50 centimetres above MSL. This represents

the concentration of wave energy by the Wallaby plateau, which is refracted from its

original direction towards the coast at Steep Point. The extent of the concentration of

wave energy is significant because it is collected from over the area of the plateau.

The angle of incidence of the tsunami wave to the plateau is responsible for the

difference between this scenario and the previous one. Here, the energy is refracted

almost directly towards Steep Point, while in scenario 1, it was directed further

north.

The water depth at this position is 106 metres and so a deviation from MSL of

almost 40 centimetres is significant. Though it is not possible to accurately predict

inundation heights above MSL at the coast without more detailed bathymetric

information, a wave of this size is certainly large enough to cause damage to coastal

structures and property.

Again, Geraldton and Perth are relatively unaffected, with deviation from

MSL of less than 10 cm offshore.

12.3 Scenario 3

Model location: 107 E, -8 N

In the third scenario, there are similar scattering effects near the source to that

seen in scenario 2, though to a much greater extent. The majority of the tsunami

energy is directed towards Christmas Island, perpendicular to the Sumatran fault.

This leads to a much greater scattering effect than that which was observed in the

previous two scenarios. The reflective scattering effect of the shallow areas to the

southeast of Christmas Island is also significant.

78

The tsunami energy is directed towards the Wallaby Plateau, which strongly

refracts the wave towards the coast. The reflective effect of the Wallaby Extension is

lessened in this case because it is shielded by the bathymetry surrounding Christmas

Island.

The Exmouth Plateau refracts and focuses tsunami energy around the

southern edge of the Exmouth plateau, towards the coastline between Karratha and

Carnarvon. In this scenario there is a strong reflection, from the island of Java, of the

tsunami heading north from the source. This causes a secondary wave of similar

amplitude to the initial tsunami wave at the Western Australian coast.

The complexity of tsunami waves is also illustrated in the comparison

between the water heights for the duration of the event at Exmouth and at Steep

Point. Exmouth experiences a series of waves of similar amplitude over a period of

about 100 minutes. Steep Point, on the other hand, experiences a series of small

amplitude waves, followed by one having an amplitude four times that of the other

waves.

Once again, the water height data from Steep Point indicates that there is a

strong refractive effect occurring in the offshore area of Wallaby Plateau. Though less

pronounced than in scenario 2, the maximum heights of the series of waves remains

at about the magnitude of 10 cm, before the third wave in the train reaches a

maximum of almost 40 cm above MSL.

This tsunami energy refracted towards the coast by the Wallaby Plateau is

represented in both the Carnarvon and the Steep Point wave profiles. In the

Carnarvon profile, the refracted wave arrives at almost the same time as the source

wave reflected by Java; hence the small spike in water height immediately before the

large refracted wave. At Steep point, due to the greater distance the refracted wave

has to travel over shallow bathymetry, it arrives slightly after the two direct tsunami

waves. The effect of the initial scattering of tsunami energy near the source is

exhibited in the smaller maximum wave height recorded here compared to the

previous scenario.

Once again, the areas offshore of Geraldton and Perth are largely unaffected

by the tsunami, recording a maximum excursion from MSL of less than 6 cm.

79

12.3.1 17th July 2006 Java tsunami

On the 17th July 2006, a tsunami devastated the southern coastline of the

island of Java, killing over 500 people (USGS Earthquake Hazards Program, 2006).

The location of the earthquake source of this tsunami was extremely close to the

source modeled in Scenario 3. A relatively large tsunami was reported at Steep Point.

According to surveys carried out by Geoscience Australia, the tsunami inundation

reached a vertical height of roughly 10 metres above MSL, making this tsunami event

the largest recorded in Australia (Prendergast, 2006).

Anecdotal evidence collected from eyewitnesses by the Geoscience Australia

survey team suggested that there was a series of waves similar to that predicted in

Scenario 3. Witnesses reported a series of three waves with the second wave being

much larger than the others (Prendergast, 2006). This matches quite well with the

water levels predicted for Steep Point in Scenario 3. Because the initial tsunami wave

was small compared to the others and the water level rose over the course of about

15 minutes, it may not have been as noticeable as the following three significant

sharp rises in water level. This tsunami is one of the first to be surveyed in Western

Australia.

12.4 Scenario 4


The dispersal of energy both offshore and along the coastline of Western

Australia is significantly different in Scenario 4 compared to any of the previous

scenarios. The most obvious aspect of this is the direction of the initial tsunami

energy. Since the source of the tsunami along the Sumatran fault is several hundred

kilometres east of Christmas island, the scattering effects of this obstacle and the

bathymetry surrounding it are minimised. This means that the majority of the

tsunami energy is directed in a perpendicular direction to the fault. This direction

runs almost parallel to the coast and straight out into the Indian Ocean, with very

little dispersal effects. For this reason, though the source of the tsunami is very much

closer to the Western Australian coastline than in Scenario 3, there is not a massive

increase in wave heights along the coast. The bathymetry around the source causes

80

the tsunami energy heading south to be split into two main branches, one of which is

directed almost parallel to the Western Australian coast.

The refractive effects of the Exmouth plateau and the Wallaby plateau still

dominate the patterns of wave heights along the coast.

Offshore of Karratha experiences a very different pattern of waves compared

to the previous scenarios. The initial tsunami wave is followed by two much larger

waves, and then one which is slightly smaller. Inspection of a series of stages of the

model again allows the derivation of the wave’s source. The first of the larger waves

is result of reflection from the island of Java, back towards Australia. It is larger

because the reflection angle directs the energy towards the Exmouth Plateau. The

second wave is a result of the scattering caused by the bathymetry in the direct path

of the initial wave. It is larger again due to the reflection of the energy in the main

path of the tsunami towards the ‘wave guide’ of the Exmouth Plateau, which focuses

tsunami energy.

The third wave is a result of the very strong refractive properties of the

southern edge of the Exmouth Plateau. The path of the wave here is so strongly

refracted that it is directed back up towards Karratha, running almost parallel to the

coastline. Part of this wavefront is visible in the plot of wave heights for Exmouth,

where the initial tsunami wave, already increased in magnitude by the refractive

effects of the Exmouth Plateau, is followed by a second larger wave.

The wave heights offshore of Carnarvon are difficult to attribute to the effects

of reflection and refraction by the plateaus in this scenario. The large secondary wave

is possibly a result of small scale refraction or reflection from the northern area,

however the scale of the model makes this difficult to determine.

The area at Steep Point remains a focus of wave energy, recording a large

maximum wave height compared with the surrounding coast. However, the wave

train over the course of the event at this point is quite different to the previous

scenarios. While the previous scenario had two small waves, followed by a much

larger refracted wave, scenario 4 has a single small wave followed by two larger

waves. The two secondary waves can be attributed to reflection of first the initial

81

tsunami wave and then the reflection from Java by the Wallaby Plateau which then

focuses this energy towards Steep Point.

Again, the deviation from MSL in the area of Geraldton and Perth is less than

8 cm. The maximum water level for Geraldton is higher than in the previous scenario

due to reflection from Wallaby Plateau.

12.5 Scenario 5


Scenario 5 clearly illustrates the scattering and wave guiding effects of

bathymetry. The source is extremely close to an area of relatively shallow water

(200m deep) south of Java. The shallow ridge running parallel to the fault in this area

serves to guide the wave north-east, back towards Sumatera. What little energy is

directed south is split into three main components.

The direction of the main path of tsunami energy is perpendicular to the fault

as might be expected. A small shallow area southwest of the source causes wave

energy to be refracted towards it, thus drawing energy in a southwest direction to an

area south of Christmas Island. However a significant amount of energy is refracted

by the bathymetry southwest of the source and is concentrated and guided by a

‘bridge’ of shallow water between java and Western Australia. This joins onto the

northern part of the Exmouth Plateau, and then propagates towards the coast, again

in the area between Karratha and Carnarvon. However due to much of the wave

energy being dissipated by the initial scattering effects near the source, the maximum

wave heights predicted along the coast in this scenario are less than in the previous

two scenarios. This is despite the fact that these two scenarios were significantly

further west (2° longitude at this latitude = 220 km horizontal distance, and similarly

4° = 440 km).

Offshore of Karratha experiences a series of tsunami waves at a period of

around 20 minutes. The relatively large number of similarly sized waves for Karratha

and Exmouth in this scenario reflects the strong scattering of the tsunami at its

source. Another effect of the strong scattering is the relatively small excursion of the

water level from MSL, despite the proximity of the source.

82

The Carnarvon plot of wave heights once again shows the effect of the

refraction around the Wallaby Plateau, which is represented by the second, much

larger wave in the train. The maximum water level for Steep Point is only

Water levels offshore Geraldton and Perth again show a very small response

to the tsunami. In fact, the response is considerably smaller than that for previous

scenarios, even though the source of the tsunami is closer. The strong scattering of

the tsunami energy at the source, as well as the guiding effect of the two plateaus

serve to shield the southwest of Western Australia from much of the tsunami energy.

12.6 Scenario 6


Scenario 6 has entirely different characteristics to any of the previous

scenarios. The source of the tsunami in this scenario is at a position where two ridges

of shallow bathymetry branch towards the south-west and south-east. The

bathymetry directly perpendicular to the fault in this location is deep and flat – an

abyssal plain. This effectively splits the tsunami energy in half, with one portion

headed towards the deep ocean to the southwest, and the other along the ‘bridge’ of

shallow water towards the Exmouth plateau offshore of northwest Western

Australia.

This ‘bridge’ acts as an extremely strong wave-guide, which is clearly

illustrated by the path of maximum wave heights all the way along it. This energy is

concentrated along the wave guide and is directed along the coastline inshore of the

Exmouth plateau – once again the area between Karratha and Carnarvon, and

offshore of Exmouth. The guidance of a significant fraction of the tsunami energy

towards this area of coast combined with the physically closer location of the source

means that the wave heights recorded here are much greater than in any of the

previous scenarios. In fact, the area offshore of Exmouth recorded the highest

excursion above MSL with this scenario. The wave guide from the tsunami source to

the top of the Exmouth Plateau provides a direct path for the tsunami energy to the

area offshore of Exmouth. However a significant proportion of the tsunami energy

travels perpendicular to the fault and travels through deep ocean to the southern

83

edge of the Exmouth Plateau. Here the effect of energy refracted around the

Exmouth Plateau is visible, in the plot of maximum wave heights. The large initial

wave in the area offshore of Exmouth is a result of this refracted wave and the wave

travelling directly from the tsunami source crossing and reinforcing at this location.

Carnarvon and Steep Point again experience initial peaks in wave height

followed by a higher peak provided by the refracted wave energy of the Wallaby

Plateau. Perth and Geraldton are again shielded from the majority of the tsunami

energy by the guiding effects of the two plateaus, and the scattering effects of

bathymetry at the tsunami’s source.

12.7 Scenario 7


This scenario, as well as being physically closest to Western Australia, has

very little scattering effects close to the source. The majority of the energy released is

directed perpendicularly to the fault, parallel to the north-western edge of the

Exmouth Plateau. As in previous scenarios, the shallow area concentrates the wave

energy as it is refracted towards the coast.

While most of the tsunami energy is directed away from the Western

Australian coast, a significant portion of the energy of the tsunami is guided by the

Exmouth plateau and directed once again to the coastline between Karratha and

Carnarvon, offshore of Exmouth.

The strong refractive effect of the Exmouth plateau is visible on the southern

edge in this scenario. Wave energy is strongly refracted towards the coast, which

results in the maximum water level in this area of almost 30 cm. Some of the energy

also propagates across the edge of the Exmouth plateau and hits the Wallaby plateau

which once again directs energy towards Steep Point. This effect is responsible for

the maximum wave height at this location. The wave energy is again concentrated in

the Steep Point area in comparison to the immediately adjacent coastline. However

the tsunami impact here is insignificant compared to that inshore of the Exmouth

Plateau.

84

Geraldton and Perth, largely shielded from the tsunami waves by the north-

western coastline of Western Australia record relatively insignificant wave heights

less than 2.5 cm in height.

12.8 Overall

Overall effects of the bathymetric features offshore of Western Australia on

tsunami wave train can be summarised into general effects for all the scenarios

discussed above. Two of the most important bathymetric features are directly

offshore of the Western Australian Coastline. These are the Exmouth Plateau, to the

west of Karratha, and the Wallaby Plateau, including the Wallaby Extension, to the

west of Carnarvon. These two relatively shallow areas of offshore bathymetry have a

strong effect on the pattern of maximum wave heights along the Western Australian

Coastline in a tsunami event.

In every scenario the Wallaby Plateau, and the ‘bridge’ of shallow water

between the plateau and the shallow coastal zone, significantly refracted wave

energy towards the coast, especially the area around Steep Point. This effect, while

visible in every scenario, more significantly affected the pattern of maximum wave

heights along the coast in some. The strength of the effect relates strongly to the

angle of incidence of the tsunami energy – and thus the location of the tsunami.

The Wallaby Plateau, and its extension, also reflects tsunami energy. This

effect is responsible for the delayed maximum wave height along the coast and can

be clearly seen in Figure 12.8.1.

85

Figure 12.8.1: Scenario water heights after 2.5 hours. Reflective effects of Wallaby

Extension and Wallaby Plateau indicated.

The southwest of Western Australia, including Geraldton and the major

population centres around Perth, are not affected to the same extent as the areas to

the north which have been previously discussed. The reasons for this, as has been

shown in the modeling, are twofold. Firstly, Perth is simply at a much greater

distance from the source fault. The dissipation of energy over this distance is much

greater than that for Karratha, Carnarvon or Steep Point.

Secondly, and perhaps more importantly, is the bathymetry offshore of Perth.

As can be seen in the maps of bathymetry shown in Figure 8.5.2, the continental shelf

is at one of its narrowest points near Perth. There is no significant offshore structure

similar to the Exmouth and Wallaby Plateaus to the Northwest, and for this reason

the tsunami energy from the Sumatran fault is not refracted in the same way as it is

in coastal areas to the north. Much of the wave energy travelling directly from the

source fault is uninterrupted and continues travelling parallel to the coast and

towards the southern ocean.

Wallaby

Extension

Wallaby

Plateau

86

The wave heights for different positions along the coast show that the impact

of the tsunami cannot be measured by the limited gauge of the maximum water level

over the entire course of the event. In fact, for the offshore positions in some of the

scenarios the maximum water level change from Mean Sea Level was negative.

While this obviously limits the coastal damage due to inundation, the water

velocities involved in such a sudden drawdown or raising of the water levels at

coastal locations are considerable. This effect can amplify the damage to coastal

structures beyond that predicted by maximum water heights alone. Specific coastal

effects are beyond the scope of this study.

It is interesting to note that in most locations measured, the pattern of water

level change over the course of the event differs considerably between scenarios. In

every case, the initial movement of the water level from MSL is to rise. This is due to

the orientation of the fault; the coastline of java would experience an initial

drawdown. This means that along the Western Australian coastline there is no local

warning of impending inundation, and reinforces the requirement for an accurate

and rapid warning system for population centres along the coast.

Tsunamis are likely to excite seiching, which would explain the oscillations

following the initial tsunami impacts (Synolakis, 2003). Interestingly, the initial wave

in these areas is followed by a series of similar magnitude or greater oscillations in

water level. Though this could be due to seiching, in combination with wave

reflection, this would require a smaller scale study of these areas.

87

13 Conclusions and further work

Tsunamis originating from the area of the Sumatran fault studied represent a

significant threat to some coastal areas of Western Australia. The Western Australian

coastline from Steep Point northwards is particularly susceptible to tsunami impact,

and possible inundation. The tsunami of 17th July 2006 gave an indication of the

possible impact of tsunamis along the coast.

It is particularly difficult to estimate tsunami impacts along the coast without

detailed bathymetric information. The results of modeling various tsunami scenarios

show that source earthquake location, magnitude and orientation have a strong effect

on coastal tsunami impact, but must be used along with detailed bathymetric data to

produce an accurate prediction of tsunami propagation.

The area of Steep Point is particularly susceptible to tsunamis from a wide

range of source locations along the fault. The Wallaby Plateau collects, focuses and

directs wave energy at this area of the coast. Further north, the Exmouth Plateau also

collects and focuses tsunami energy towards the coast. Especially in the case of a

tsunamigenic earthquake in the area of the source of Scenario 6 and Scenario 7, the

coastline north of Steep Point would experience a large tsunami impact.

Due largely to the bathymetry offshore of Western Australia, the areas of the

coast around Geraldton and Perth are relatively unaffected tsunamis from the section

of the fault investigated in this study. There is no plateau offshore in these areas and

this means that the effect of refracting wave energy towards the coast is missing.

While previous use of the MOST model has shown it to be reasonably

accurate, predictions of tsunami propagation must be compared to field data in order

to be trusted as accurate for this domain. One factor which may influence the

accuracy of the models is the limited size of the domain, which means that any

refractive or reflective effects of bathymetric features outside it are not taken into

account. These bathymetric features could have a significant impact on maximum

water heights along the Western Australian coast.

Lack of reliable field data was a significant constraint on this study. Limited

resolution topographical data for large areas of the Western Australian Coast meant

88

that it was not possible to perform inundation modeling. Lack of field data for

historical tsunamis within the domain also meant that the accuracy of the modeling

results could not be verified. To assist in future modeling work, it would be desirable

to amend these shortcomings.

Since much of this area of coast is extremely sparsely populated, the coastal

topographical data would only need to be made available for areas around

population centres. This would greatly assist in providing accurate inundation maps

and preparing for evacuation from affected areas in the case of a tsunami.

Any future tsunami inundation along the coastline must also be surveyed in

order to compare with predictions made by modeling and to confirm predictions

made about tsunami propagation through the domain.

As mentioned in the Literature Review, in order to provide accurate early

warning of a tsunami, a network of tsunameters must be deployed offshore. Ideally,

these tsunameters would provide feedback about water levels in real time, allowing

comparison against a database of scenarios such as those produced in this study.

This would allow the prediction of coastal areas which should be evacuated.

89

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