# Offshore Tsunami Hazard for Australia

As part of the Australian Tsunami Warning System Project (2005-09), the Attorney-General’s Department funded Geoscience Australia to develop the national offshore Probabilistic Tsunami Hazard Assessment (PTHA). This assessment could then be used by Australian emergency managers in understanding the tsunami hazard to Australia.

The national offshore PTHA considers the tsunami hazard posed to the entire Australian coast by tsunami caused by subduction zone earthquakes in the Indian and Pacific Oceans. These regions are known to have produced major tsunamigenic events in recorded history and are the most likely sources of future events.

The hazard maps are defined at a bathymetry water depth contour of 100m offshore. This normally falls outside of the Great Barrier Reef or other reef systems. The 100m depth contour is chosen because:

- Estimating the tsunami closer to the coast requires high resolution bathymetric data which does not always exist for the entire coast
- estimating the tsunami closer to the coast is a more computational and time intensive task.

These maps help to identify the areas which are most likely to be at risk to damaging tsunami waves. However, they cannot be used directly to infer how far a tsunami will inundate onshore (inundation extent), how high above sea level they will reach on land (run-up), the extent of damage or any other onshore phenomena. To estimate the onshore tsunami impact, detailed bathymetry and topography of the specific region concerned is required for input to a detailed inundation model. The catalogue of tsunami events used to derive the national offshore PTHA can be used by emergency managers, researchers and individuals however to develop detailed inundation models at any onshore location.

## Process for Developing the PTHA

Broadly, the assessment process was:

1. Detail the source zones included in the assessment.

2. Assign probabilities to different source zones.

3. Determine recurrence relationships for a range of locations around Australia.

4. Estimate the maximum tsunami amplitude at the 100m depth contour for each earthquake source.

The sources included in the assessment are subduction zone earthquakes only, which represents the most dominant source of risk (80-90 per cent overall). Not included are sources from non-seismic tsunami sources (e.g. submarine landslides, volcanoes or asteroids) or earthquakes that do not occur on a subduction zone, such as earthquakes which occur on plate margins or within the plate itself (intra-plate earthquakes).

Assigning probabilities to different source zones involves assessing the likelihood of each earthquake and the resulting tsunami based on the historical record of earthquake events from that source zone, combined with existing knowledge of the rate of tectonic plate movement and other factors controlling earthquake activity at subduction zones. By considering the probabilities of each tsunami which reaches offshore Australia, a recurrence relationship can be calculated for each location offshore. Uncertainties in these recurrence relationships increase with magnitude. Additionally, a maximum earthquake magnitude is assumed to be able to occur on each subduction zone. Details of this methodology are outlined in Burbidge et al 2008.

More than 76 000 earthquakes were included in this assessment to estimate the maximum tsunami amplitude at the 100m water depth contour. Every magnitude between 7.0 and the maximum for each subduction zone has been included at increments of 0.1.

## What the offshore tsunami hazard maps can and cannot be used for

The offshore tsunami hazard maps **CAN** be used to:

- Determine the the maximum tsunami amplitude at a given offshore location for a given annual probability
- determine the annual probability for a given tsunami amplitude at a given offshore location
- determine the relative offshore tsunami hazard around Australia
- prioritised onshore locations for more detailed inundation studies.

In turn, the information can be used to select earthquakes that contribute most to the hazard at each annual probability which can then be used to select specific scenarios or events for detailed inundation modelling. An application to make this selection will be available soon.

The offshore tsunami hazard maps **CANNOT** be used to determine:

- The inundation extent, run-up, damage or other onshore phenomena
- the relative onshore tsunami hazard for Australia
- the tsunami risk for any location in Australia.

## The offshore tsunami hazard maps

*To view these files, it is necessary to download and install the Google Earth application from Google*.

The national offshore PTHA is represented as a series of offshore tsunami hazard maps. These can be viewed as either:

- The maximum tsunami amplitude which could be expected at an offshore location in any given year for a stated probability or chance (for example, what would be the size of a tsunami that has a one per cent chance of occurring at a given location in any given year?)
- the probability or chance that in any given year a tsunami of that amplitude will reach that offshore location (for example, what is the probability that a tsunami 50cm high will reach offshore at a given location in any given year? Note that a probability of zero means there is no chance of that event occurring and a probability of one (or 100 per cent) means it definitely will occur).

Select a map showing the maximum tsunami amplitude at a range of annual probabilities:

- Maximum tsunami amplitude with 1 % chance of occurring per year
- Maximum tsunami amplitude with 0.2 % chance of occurring per year
- Maximum tsunami amplitude with 0.1 % chance of occurring per year
- Maximum tsunami amplitude with 0.05 % chance of occurring per year
- Maximum tsunami amplitude with 0.02 % chance of occurring per year

Select a map showing the annual probability as a number between zero and one for a range of tsunami amplitudes:

Topic contact: hazards@ga.gov.au Last updated: December 5, 2012