The 2004 Sumatra-Andaman Earthquake and Indian Ocean Tsunami shattered the paradigm that guided our understanding of giant subduction zone earthquakes: that massive, magnitude 9+ earthquakes occur only in subduction zones experiencing rapid subduction of young oceanic lithosphere. Although this paradigm forms the basis of discussion of subduction zone earthquakes in earth sciences textbooks, the 2004 earthquake was the final blow in an accumulating body of evidence showing that it was simply an artefact of a sparse and biased dataset (Okal, 2008). This has led to the realization that the only factor known to limit the size of megathrust earthquakes is subduction zone length. This new appreciation of subduction zone earthquake potential has important implications for the southern Asia-Pacific region. This region is transected by many thousands of km of active subduction, including the Tonga-Kermadec, Sunda Arc, and the Makran Subduction zone along the northern margin of the Arabian Sea. Judging from length alone, all of these subduction zones are capable of hosting megathrust earthquakes of magnitude greater than 8.5, and most could host earthquakes as large as the 2004 Sumatra-Andaman earthquake (Mw=9.3). Such events are without historical precedent for many countries bordering the Indian and Pacific Oceans, many of which have large coastal populations immediately proximate to subduction zones. This talk will summarize the current state of knowledge, and lack thereof, of the tsunami hazard in the southern Asia-Pacific region. I will show that 'worst case' scenarios threaten many lives in large coastal communities, but that in most cases the uncertainty in these scenarios is close to 100%. Is the tsunami risk in SE Asia and the SW Pacific really this dire as the worst-case scenarios predict? The answer to this question relies on our ability to extend the record of tsunamis beyond the historical time frame using paleotsunami research.

Since the 2004 Sumatra-Andaman Earthquake, understanding the potential for tsunami impact on coastlines has become a high priority for Australia and other countries in the Asia-Pacific region. Tsunami warning systems have a need to rapidly assess the potential impact of specific events, and hazard assessments require an understanding of all potential events that might be of concern. Both of these needs can be addressed through numerical modelling, but there are often significant uncertainties associated with the three physical properties that culminate in tsunami impact: excitation, propagation and runup. This talk will focus on the first of these, and attempt to establish that seismic models of the tsunami source are adequate for rapidly and accurately establishing initial conditions for forecasting tsunami impacts at regional and teletsunami distances. Specifically, we derive fault slip models via inversion of teleseismic waveform data, and use these slip models to compute seafloor deformation that is used as the initial condition for tsunami propagation. The resulting tsunami waveforms are compared with observed waveforms recorded by ocean bottom pressure recorders (BPRs). We show that, at least for the large megathrust earthquakes that are the most frequent source of damaging tsunami, the open-ocean tsunami recorded by the BPRs are well predicted by the seismic source models. For smaller earthquakes, or those which occur on steeply dipping faults, however, the excitation and propagation of the resulting tsunami can be significantly influenced by 3D hydrodynamics and by dispersion, respectively. This makes it mode difficult to predict the tsunami waveforms.

The major tsunamis of the last few years have dramatically raised awareness of the possibility of potentially damaging tsunami reaching the shores of Australia and to the other countries in the region. Here we present three probabilistic hazard assessments for tsunami generated by megathrust earthquakes in the Indian, Pacific and southern Atlantic Oceans. One of the assessments was done for Australia, one covered the island nations in the Southwest Pacific and one was for all the countries surrounding the Indian Ocean Basin

Along the Aceh-Andaman subduction zone, there was no historical precedent for an event the size of the 2004 Sumatra-Andaman tsunami; therefore, neither the countries affected by the tsunami nor their neighbours were adequately prepared for the disaster. By studying the geological signatures of past tsunamis, the record may be extended by thousands of years, leading to a better understanding of tsunami frequency and magnitude. Sedimentary evidence for the 2004 Sumatra-Andaman tsunami and three predecessor great Holocene tsunamis is preserved on a beach ridge plain on Phra Thong Island, Thailand. Optically stimulated luminescence ages were obtained from tsunami-laid sediment sheets and surrounding morphostratigraphic units. Single-grain results from the 2004 sediment sheet show sizable proportions of near-zero grains, suggesting that the majority of sediment was well-bleached prior to tsunami entrainment or that the sediment was bleached during transport. However, a minimum-age model needed to be applied in order to obtain a near-zero luminescence age for the 2004 tsunami deposit as residual ages were found in a small population of grains. This demonstrates the importance of considering partial bleaching in water-transported sediments. The OSL results from the predecessor tsunami deposits and underlying tidal flat sands show good agreement with paired radiocarbon ages and constrain the average recurrence of large late Holocene tsunami on the western Thai coast to between 500 to 1000 years. This is the first large-scale application of luminescence dating to gain recurrence estimates for large Indian Ocean tsunami. These results increase confidence in the use of OSL to date tsunami-laid sediments, providing an additional tool to tsunami geologists when material for radiocarbon dating is unavailable. Through an understanding of the frequency of past tsunami, OSL dating of tsunami deposits can improve our understanding of tsunami hazard and provide a means of assessing fu

Keynote presentation to cover * the background to tsunami modelling in Australia * what the modelling showed * why the modelling is important to emergency managers * the importance of partnerships * future challenges

The high risk of natural disasters in developing nations has considerable implications for international aid programs. Natural disasters can significantly compromise development progress and reduce the effectiveness of aid investments. In order to better understand the threat that natural disasters may pose to its development aid program, AusAID commissioned Geoscience Australia to conduct a broad natural hazard risk assessment of the Asia-Pacific region. The assessment included earthquake, volcanic eruption, tsunami, cyclone, flood, landslide and wildfire hazards, with particular attention given to countries the Australian Government considered to be of high priority to its development aid program. Geoscience Australia's preliminary natural hazard risk assessment of the region aimed to help AusAID identify countries and areas at high risk from one or more natural hazards. The frequency of a range of sudden-onset natural hazards was estimated and, allowing for data constraints, an evaluation was made of potential disaster impact. Extra emphasis was placed on relatively rare but high-impact events, such as the December 2004 tsunami, which might not be well documented in the historical record. While a detailed risk assessment was well beyond the scope of this study, it was recognized that some understanding of the potential impact of natural disasters could be achieved through the simple means of developing appropriate overlays of population and hazard. For example, given an estimate of the frequency and magnitude (VEI) at which volcanic eruptions in a certain region occur, the populations impacted could be roughly estimated by considering the average population close enough to a volcano to receive a significant impact from ash fall.

The report summarises earthquake and tsunami information worldwide in 1997 but with a focus on Australia for use by scientists, engineers and the public. Maps of the seismicity are presented on a state-by-state basis and isoseismal maps are included for the significant earthquakes.

As the tsunami moves across the open ocean it is almost undetectable on the ocean surface. In this example, the tsunami waves are only about half a metre high but have a wavelength of 200 kilometres. Travelling at speeds of up to eight or nine hundred kilometres an hour (the speed of a commercial passenger jet), it will take each wave about 15 minutes to pass a slow moving ship.

As the Australian plate slowly pushes under the Eurasian plate, massive stresses build up in the crust. These stresses also cause the Eurasian plate to be slowly forced upwards - part of the process that builds the mountains and volcanoes of Indonesia, as well as creating the many earthquakes felt in that region of the world each year. When the stresses get too great, the plates will suddenly slip causing massive movements in the seafloor. The part of the crust nearest to the fault zone rapidly moves upwards by a metre or so, lifting the entire body of water above it. A hundred kilometres away the opposite may happen: the seafloor drops and the ocean above it also falls. These two movements (the sudden rise and fall of the seafloor hundreds of kilometres apart), combine to cause a series of tsunami waves which move away from the line of the fault in both directions.

Tsunamis can be produced from volcanoes in a number of ways. During a volcanic eruption, hot fast moving bodies of gas and rock (known as pyroclastic flows) can travel into the ocean, pushing the water outwards and creating a tsunami. In other eruptions, the volcano may collapse inwards or produce large landslides, both of which can cause tsunamis. More than 90 volcanic tsunamis have been recorded worldwide in the last 250 years. The 1883 Krakatau eruption in Indonesia caused tens of thousands of deaths, including 77 about 800 kilometres away from the eruption. The effect of the tsunami was reported up to 10 kilometres inland and one large ship was raised 10m above sea level and carried 3 kilometres inland.