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 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 Natural Hazard Impacts Project (NHIP) at Geoscience Australia has developed modelling techniques that enable coastal inundation to be predicted during a tsunami. A Collaborative Research Agreement between Geoscience Australia and the Fire and Emergency Services Authority (FESA) was formed in 2005 to understand tsunami risk and inform emergency management in WA. Through this partnership a significant tsunami risk was identified in NW Western Australia, leading to the development of inundation models for several coastal communities in this region, including Onslow and Exmouth. Recognising the importance of this research to Geoscience Australia, FESA and the communities of Onslow and Exmouth, this year's graduate project was designed to assist the NHIP and to further strengthen ties with FESA and community organisations. The project had several distinct outcomes which can be divided into data acquisition and community interaction. High quality elevation data was gathered by GPS surveying in order to ascertain the quality of the Digital Elevation Model (DEM) that is currently used in inundation models. Improved accuracy in the elevation data allows the capture of subtle changes in topography that may not be present in the existing DEM and so may improve model accuracy. Secondly, ground-truthing of predicted inundation areas supplements the survey data, provides critical assistance in the production of accurate inundation models and potentially aids in the production of emergency plans. Prior to fieldwork a community-specific tsunami awareness brochure was designed and produced for Onslow. This brochure was presented to Onslow local emergency managers and FESA personnel, and subsequently to Emergency Management Australia and the Bureau of Meteorology. It has received widespread positive feedback, and consequently may provide a template for other community brochures in similarly vulnerable regions of Australia. Finally, graduates represented Geoscience Australia at several community meetings in Onslow where NHIP research was presented. These meetings provided insight into specific community concerns in the event of a tsunami and provided an opportunity for the attendees to ask questions about tsunamis and their impacts. Fortuitously this community interaction also led to the discovery of anecdotal evidence of past tsunami events in Onslow, including the tsunami triggered by the 1883 Krakatau eruption, a 1937 tsunami that may be attributed to an earthquake near Java, and the 1994 and 2004 tsunamis.

The Great Sumatra-Andaman Earthquake and Indian Ocean Tsunami of 2004 came as a surprise to most of the earth science community. Few were aware of the potential for the subduction zone off Sumatra to generate giant (Mw>= 8.5) earthquakes, or that such an earthquake might generate a large tsunami. In retrospect, important indicators that such an event might occur appear to have not been well appreciated: (1) the tectonic environment of Sumatra was typical of those in which giant earthquakes occur; (2) GPS campaigns, as well as paleogeodetic studies indicated extensive locking of the interplate contact; (3) giant earthquakes were known to have occurred historically. While it is now widely recognised that the risk of another giant earthquake is high off central Sumatra, just east of the 2004 earthquake, there seems to be relatively little concern about the subduction zone to the north, in the northern Bay of Bengal along the coast of Myanmar. It is shown here that similar indicators suggest the potential for giant earthquake activity is high: (1) the tectonic environment is similar to other subduction zones that experience giant megathrust earthquakes; (2) stress and crustal strain observations indicate the seismogenic zone is locked; and, (3) historical earthquake activity indicates that giant tsunamigenic earthquakes have occurred in the past. These are all consistent with active subduction in the Myanmar subduction zone, and it is hypothesized here that the seismogenic zone there extends beneath the Bengal Fan. The results suggest that giant earthquakes do occur off the coast of Myanmar, and that a very large and vulnerable population is thereby exposed to a significant earthquake and tsunami hazard.

The tragic events of the 2004 Indian Ocean Tsunami highlighted the real threat posed by tsunamis to coastal communities worldwide. With subduction zones to the north and east of Australia, tsunamis pose a real threat to the Australian coast. Geoscience Australia has been developing methodologies for quantifying the severity of tsunami impacts to assist emergency management authorities in planning for this threat. Tsunami inundation modelling is computationally intensive and is often restricted to a small number of discrete communities. As a result, communities must be prioritised for this detailed modelling. One method for prioritisation is the Probabilistic Tsunami Hazard Assessment (PTHA) of Australia. In the PTHA, tsunamis were modelled from all likely earthquake sources across the deep ocean using a computationally faster linear solution and coarser model domain. Results are considered valid only to the 100 m depth contour, where we calculate return periods for tsunami wave height around Australia and generate a database of tsunami wave forms. However, tsunami waves are shaped considerably by the bathymetry between the 100 m depth contour and the coast, and tsunami behaviour near the coast is therefore highly non-linear, dependent on elevation, coastline shape, wave height, period and momentum. This non-linear reality of the near shore environment raises a number of questions. Is offshore wave height alone the best predictor of onshore tsunami hazard? Analytical solutions to the 1-D shallow water equations exist for predicting wave run-up on plane beaches. Can these be applied to offshore waves to measure inundation potential? Or are other metrics, such as wave energy, more appropriate? Comparisons with results from detailed inundation models will explore the utility of these measures for prioritising communitie

To follow

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 tsunami leaves the deep water of the open ocean and approaches the shallower waters near the coast, it slows down and may grow in height depending on the shape of the seafloor. A tsunami that is unnoticeable by ships at sea may grow to be several metres or more in height near the coast. Our example tsunami is now 1.5 metres high with a wavelength of 100 kilometres and is moving at about 400 kilometres an hour.

Landslides can happen on the seafloor, just like on land. Areas of the seafloor that are steep and loaded with sediment are more prone to undersea landslides, such as the edge of the continental slope. When an undersea landslide occurs (perhaps after a nearby earthquake) a large mass of sand, mud and gravel can move down the slope. This movement will draw the water down and may cause a tsunami that will travel across the ocean.

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.