The present study reports on recent developments of the Indonesia Tsunami Early Warning System (InaTEWS), especially with respect to the tsunami modeling components used in that system. It is a dual system: firstly, InaTEWS operates a high-resolution scenario database pre-computed with the finite element model TsunAWI; running in parallel, the system also contains a supra real-time modeling component based on the GPU-parallelized linear long-wave model easyWave capable of dealing with events outside the database coverage. The evolution of the tsunami scenario database over time is covered in the first sections. Starting from the mere coverage of the Sunda Arc region, the current state contains scenarios in 15 fault zones. The study is augmented by an investigation of warning products used for early warning like the estimated wave height (EWH) and the estimated time of arrival (ETA). These quantities are determined by easyWave and TsunAWI with model specific approaches. Since the numerical setup of the models is very different, the extent of variations in warning products is investigated for a number of scenarios, where both pure database scenarios and applications to real events are considered.

Numerical codes for probabilistic tsunami hazard assessment, available for download in github: https://github.com/GeoscienceAustralia/ptha

PTHA18 estimates the frequency with which tsunamis of any given size occur in deep waters around the Australian coastline. To do this it simulates hundreds of thousands of possible tsunami scenarios from key earthquake sources in the Pacific and Indian Oceans, and models the frequency with which these occur.

The 2018 Probabilistic Tsunami Hazard Assessmetn (PTHA18) outputs are can be accessed following the README instructions here: https://github.com/GeoscienceAustralia/ptha/tree/master/ptha_access

<p>The 2018 Australian Probabilistic Tsunami Hazard Assessment (PTHA18) was developed by Geoscience Australia to better understand Australia’s tsunami hazard due to earthquakes in the Pacific and Indian Oceans. The PTHA18 contains over a million hypothetical earthquake-tsunami scenarios, with associated return periods which are constrained using historical earthquake data and long-term plate tectonic motions. The tsunami propagation is modelled globally for 36 hours, and results are stored at thousands of sites in deep waters offshore of Australia. Average Return Interval (ARI) estimates are also provided, along with a representation of the associated uncertainties. ARI uncertainties tend to be large because of fundamental limitations in current scientific knowledge regarding the frequency of large earthquakes on global subduction zones. <p>The PTHA18 provides a nationally consistent basis for earthquake-tsunami scenario design, as required for inundation hazard assessments. The results and source-code are also freely available. The current paper aims to provide a short and accessible introduction to the PTHA18 methodology and results, while deliberately limiting technical details which are covered extensively in the associated technical report and code repository.

This report describes the 2018 Probabilistic Tsunami Hazard Assessment for Australia (henceforth PTHA18). The PTHA18 estimates the frequency with which tsunamis of any given size occur in deep waters around the Australian coastline. To do this it simulates hundreds of thousands of possible tsunami scenarios from key earthquake sources in the Pacific and Indian Oceans, and models the frequency with which these occur. To justify the PTHA18 methodologies a significant fraction of the report is devoted to testing the tsunami scenarios against historical observations, and comparing the modelled earthquake rates against alternative estimates. Although these test provide significant justification for the PTHA18 results, there remain large uncertainties in “how often” tsunamis occur at many sites. This is due to fundamental limitations in present-day scientific knowledge of how often large earthquakes occur.

A mini-poster on GA's capability in tsunami hazard modelling.

The complexity of coseismic slip distributions influences the tsunami hazard posed by local and, to a certain extent, distant tsunami sources. Large slip concentrated in shallow patches was observed in recent tsunamigenic earthquakes, possibly due to dynamic amplification near the free surface, variable frictional conditions or other factors. We propose a method for incorporating enhanced shallow slip for subduction earthquakes while preventing systematic slip excess at shallow depths over one or more seismic cycles. The method uses the classic k-2 stochastic slip distributions, augmented by shallow slip amplification. It is necessary for deep events with lower slip to occur more often than shallow ones with amplified slip to balance the long-term cumulative slip. We evaluate the impact of this approach on tsunami hazard in the central and eastern Mediterranean Sea adopting a realistic 3D geometry for three subduction zones, by using it to model * 150,000 earthquakes with Mw from 6.0 to 9.0. We combine earthquake rates, depth-dependent slip distributions, tsunami modeling, and epistemic uncertainty through an ensemble modeling technique. We found that the mean hazard curves obtained with our method show enhanced probabilities for larger inundation heights as compared to the curves derived from depth-independent slip distributions. Our approach is completely general and can be applied to any subduction zone in the world.

Hazardous tsunamis are rare in Australia but could be generated by several mechanisms, including large plate-boundary earthquakes in locations that efficiently direct wave energy to our coast. With few hours between detection and tsunami arrival, prior planning is important to guide emergency response and risk mitigation. This drives interest in tsunami hazard information; which areas could be inundated, how likely, and how confident can we be? In practice the hazard is uncertain because historical records are short relative to tsunami frequencies, while long-term sedimentary records are sparse. Hazard assessments thus often follow a probabilistic approach where many alternative tsunami scenarios are simulated and assigned uncertain occurrence rates. This relies on models of stochastic earthquakes and their occurrence rates, which are not standardised, but depend on the scenario earthquake magnitude and other information from the source region. In this study we test three different stochastic tsunami models from the 2018 Australian Probabilistic Tsunami Hazard Assessment (PTHA18), an open-source database of earthquake-tsunami scenarios and return periods. The three models are tested against observations from twelve historical tsunamis at multiple tide gauges in Australia. For each historical tsunami, and each of the three models, sixty scenarios with similar earthquake location and magnitude are sampled from the PTHA18 database. A nonlinear shallow water model is used to simulate their effects at tide gauges in NSW, Victoria and Western Australia. The performance and statistical biases of the three models are assessed by comparing observations with the 60 modelled scenarios, over twelve separate tsunamis. Presented at the 30th Conference of the Australian Meteorological and Oceanographic Society (AMOS) 2024.

In November, 2018 a workshop of experts sponsored by UNESCO’s Intergovernmental Oceanographic Commission was convened in Wellington, New Zealand. The meeting was organized by Working Group (WG) 1 of the Pacific Tsunami Warning System (PTWS). The meeting brought together fourteen experts from various disciplines and four different countries (New Zealand, Australia, USA and French Polynesia) and four observers from Pacific Island countries (Tonga, Fiji), with the objective of understanding the tsunami hazard posed by the Tonga-Kermadec trench, evaluating the current state of seismic and tsunami instrumentation in the region and assessing the level of readiness of at-risk populations. The meeting took place in the “Beehive” Annex to New Zealand’s Parliament building nearby the offices of the Ministry of Civil Defence and Emergency Management. The meeting was co-chaired by Mrs. Sarah-Jayne McCurrach (New Zealand) from the Ministry of Civil Defence and Emergency Management and Dr. Diego Arcas (USA) from NOAA’s Pacific Marine Environmental Laboratory. As one of the meeting objectives, the experts used their state-of-the-science knowledge of local tectonics to identify some of the potential, worst-case seismic scenarios for the Tonga-Kermadec trench. These scenarios were ranked as low, medium and high probability events by the same experts. While other non-seismic tsunamigenic scenarios were acknowledged, the level of uncertainty in the region, associated with the lack of instrumentation prevented the experts from identifying worse case scenarios for non-seismic sources. The present report synthesizes some of the findings of, and presents the seismic sources identified by the experts to pose the largest tsunami risk to nearby coastlines. In addition, workshop participants discussed existing gaps in scientific knowledge of local tectonics, including seismic and tsunami instrumentation of the trench and current level of tsunami readiness for at-risk populations, including real-time tsunami warnings. The results and conclusions of the meeting are presented in this report and some recommendations are summarized in the final section.