ptha_access

Guide to accessing the 2018 Australian Probabilistic Tsunami Hazard Assessment (PTHA18) results

Note to users -- a security update to the NCI THREDDS Server around 16/07/21 broke the codes here and caused the tests to fail. On 21/07/21 we implemented changes suggested by NCI to work-around the issues. Users will need to update their copy of the codes here in order to use the database.

This guide explains how to access basic tsunami hazard information in easy-to-use csv and shapefile formats.

For access to detailed PTHA18 information, you should instead consult the detailed README, which explains access to:

• scenario earthquakes
• scenario tsunami initial conditions
• scenario wave time-series at 'hazard points'
• additional average-return-interval calculations

The methodologies used in this study, and associated testing, are discussed in the project report and this GJI publication and this PAGEOPH publication and this talk.

For ideas on how to use the PTHA18 results for inundation hazard assessment, see this paper on efficient Monte Carlo methods and this report covering both scenario-based and Monte Carlo approaches.

Code used to conduct the analysis is available open-source in the ptha package. This includes generic software for PTHA, and also a folder with project specific scripts used for the PTHA18. Additional scripts that may be useful in working with the PTHA results are available in example_event_access_scripts.

The study results are provided under a Creative Commons 4.0 International Licence, while the source-code is provided under a BSD3 license.

Geoscience Australia has tried to make the information in this product as accurate as possible. However, it does not guarantee that the information is totally accurate or complete. Therefore, you should not solely rely on this information when making a commercial decision.

Results may be updated if problems are identified. Please report any problems via the github issues page, or send an email to the maintainer (gareth.davies@ga.gov.au) or to hazards@ga.gov.au. See UPDATES.md for notes regarding updates since the 2018 report release and ideas for future updates.

Obtaining basic tsunami hazard information.

Obtaining tsunami maximum-stage exceedance-rates at sites around Australia

The tsunami 'maximum-stage' is the maximum water-level that a particular tsunami attains at a particular location. This gives an idea of how 'big' the tsunami is. In the PTHA18 we ignore tidal variations and assume a constant mean-sea-level (MSL=0), so the 'maximum-stage' is equivalent to the maximum elevation of the tsunami wave above MSL.

The maximum-stage exceedance-rates describe 'how often' tsunamis occur with maximum-stage above a particular threshold value. For example, you could ask how often maximum-stages above 0.5m (or 2.0m) occur, in terms of the average number of events each year at a particular site. At most locations there would be less than one event each year. Thus exceedance-rates are often small numbers. For example, an exceedance-rate of 0.002=1/500 would correspond to one event every 500 years on average. Alternatively, one could say the event has a 500 year Average Recurrence Interval (ARI).

In PTHA18 this information is stored at a set of points in the ocean. These are termed 'hazard points' because they record site-specific hazard information. The wave time-series for every scenario can also be obtained at all hazard points (see here).

Most hazard points are concentrated around Australia and its territories. We also store DART buoy locations (deep-ocean gauges which measure tsunami wave heights), which are useful for model testing. Further, we store a 'thin' layer of hazard points globally at the 100m contour, which is useful for model testing and inter-comparison. If using points far from Austraila, beware that we ignore tsunamigenic source zones which are not considered relevant for Australia (e.g. in the Carribbean, the Mediterrean, the Manila trench, Kaikoura in New Zealand, western Japan). Outside of Australia, you should very carefully consider how the results can be used, noting they may ignore the most relevant source-zones.

The tsunami maximum-stage provides at best a rough characterisation of the tsunami's capacity to produce coastal inundation. The tsunami inundation will be affected by the full details of the tsunami wave train, and how it interacts with the coastal landscape. However, all else being equal, a larger maximum-stage will generally lead to larger inundation.

The simplest way to examine the PTHA18 tsunami maximum-stage exceedance-rates is to download this csv file. It contains the following columns:

• lon, lat give the hazard point location in longitude/latitude (degrees).
• elev is the bathymetry at the hazard point (negative = below MSL)
• gaugeID is a hazard point ID (real number).
• multiple columns with names like STAGE_XXXX where XXXX is a number, and 1/XXXX is the exceedance-rate. These corresponds to the tsunami maximum-stage which has mean exceedance-rate = 1/XXXX. For example, the column STAGE_100 gives the tsunami maximum-stage that is exceeded once every 100 years on average, according to the mean of all the rate models in our logic-tree. The variable-rigidity earthquake model is assumed.
• multiple columns with names like STAGE_upper_ci_XXXX. These values are similar to the above, but describe the upper limit of the 95% credible interval for the stage with the specified exceedance-rate. (i.e. 97.5 percentile)
• multiple columns with names like STAGE_lower_ci_XXXX. These are similar to the above, but describe the lower limit of the 95% credible interval for the stage with the specified exceedance-rate. (i.e. 2.5 percentile)
• multiple columns with names like STAGE_median_XXXX. These are similar to the above, but describe the 'epistemic median' stage with the specified exceedance-rate (i.e. 50th percentile)
• multiple columns with names like STAGE_16pc_XXXX. These are similar to the above, but describe the 16th percentile.
• multiple columns with names like STAGE_84pc_XXXX. These are similar to the above, but describe the 84th percentile.

Note 'max stage' values below 2cm (or above 20m) are treated as missing data (NA). Such values are unlikely to be of interest, but if necessary they can be reconstructed from the detailed information. The latter also shows how to access exceedance-rates for the "constant rigidity model" (rather than the "variable rigidity model" used for the above CSV file).

Similar data is available in shapefile format here. You must unzip the file after download. Shapefiles have a weakness; attribute names must not exceed 10 characters. Therefore the attributes are renamed in some instances, as compared with the above csv:

• lon, lat give the location in longitude/latitude (degrees).
• elev is the bathymetry at the hazard point (negative = below MSL)
• gaugeID is a real hazard point ID
• ST_XXXX is the same as STAGE_XXXX described above
• STu_XXXX is the same as STAGE_upper_ci_XXXX described above
• STl_XXXX is the same as STAGE_lower_ci_XXXX described above
• ST50_XXXX is the same as STAGE_median_XXXX described above
• ST16_XXXX is the same as STAGE_16pc_XXXX described above
• ST84_XXXX is the same as STAGE_84pc_XXXX described above

At most hazard points there is large uncertainty in the maximum-stage for a given exceedance-rate. This is largely due to uncertainty in the frequencies of high-magnitude subduction zone earthquakes. A more detailed discussion of these topics can be found in the Australian Tsunami Hazard Modelling Guidelines.

Note: The above results follow the methodology in this PAGEOPH publication to compute exceedance-rate percentiles, which is slightly different to the methodology in the original PTHA18 report (see discussion in Section 3.5 of the PAGEOPH paper). Differences are generally small and unlikely to be important, but for reference the older results can still be obtained in csv form and shapefile form.

Obtaining site-specific hazard information (including source deaggregation)

For each hazard point, the PTHA18 includes a standard pdf plot which shows:

1. The maximum-stage vs exceedance-rate
2. A convergence check on the above
3. The hazard deaggregation information for a range of return periods
4. Information on the maximum-stage for each unit-source tsunami.

An example plot can be downloaded here. Because there are thousands of hazard points, the plots at other sites are provided in a set of zip folders. Each zip folder contains around 200 sites in a particular longitude range. The zip folders can be accessed here. Follow the link to the http download to get the file. The zip folder names are of the form revised1_station_summary_plots_longitudes_LOWER_UPPER.zip where LOWER is the lower longitude limit, and UPPER is the upper longitude limit.

For example if I were searching for a hazard point at (lon,lat)= (151.408, -34.08), then by inspection of the LOWER and UPPER longitudes in files here, it should be contained in the file revised1_station_summary_plots_151.38_152.zip (because the LOWER and UPPER longitudes bracket the value 151.408, which is the one I want).

Note: The exceedance-rate percentile calculation has been revised follow the methodology in this PAGEOPH publication, as discussed above for the csv and shapefile outputs. The updates are relatively minor, but the older results are still available here in case you need to check them for some reason.

Interpreting exceedance-rate information

Please read the PTHA18 report and the PAGEOPH Paper for further information on interpreting the exceedance-rate information.

The maximum-stage exceedance-rates vary from site to site, depending on exposure to earthquake-generated tsunamis. For a given exceedance-rate, there is also a general tendency for the tsunami size to increase in shallower water. Such 'shoaling' is a well known property of ocean waves.

The model results are not expected to be accurate everywhere, but in general results far offshore and in deep water are expected to be higher quality than nearshore results. The reasons are:

• The PTHA18 tsunami model has a spatial grid size of 1 arc minute (around 1.8 km), and is run on relatively coarse elevation data (a combination of the Australian Bathymetry and Topography Grid 2009 product, and the global GEBCO 2014 bathymetry grid). While appropriate for modelling oceanic-scale tsunami propagation, it is not expected to accurately model tsunamis near the coast and in shallow waters.
• In shallower waters, where wave heights become an appreciable fraction of the water depth, the assumptions underlying our linear tsunami model are violated. This is most likely to be a problem in shallow waters, and for larger tsunamis.

Because of this, for modelling purposes we strongly encourage the use of points well offshore in deep water. If you can, use sites where wave heights of interest do not exceed a few percent of the water depth. For tsunami propagation modelling, it may be preferable to simulate the tsunami from source (using initial conditions provided here), which circumvents these issues. Modelling from source also also facilitates the use of alternative hydrodynamic models (e.g. with dispersion and/or friction, which are important in some situations), and using other bathymetric data when simulating these large scales. Nearshore points should only be used as a rough guide to possible tsunami wave heights, NOT FOR FORCING INUNDATION MODELS, and should be refined in future using higher resolution models and data.

The PTHA18 can help enable national consistency in site-specific tsunami inundation hazard studies. At any particular coastal site, the tsunami inundation hazard will reflect both the frequency with which significant "offshore" tsunamis occur, as well as how these tsunamis interact with the local coastal topography. The PTHA18 was created to provide a nationally consistent view of the "offshore" part of the problem. To determine the onshore hazard at a particular site, the offshore results can be used for force high-resolution inundation models (which also require detailed site-specific elevation data). This is analogous to how national scale extreme-rainfall information provides a consistent basis for flood hazard assessments.

Obtaining detailed information on earthquake scenarios, tsunami initial conditions, and wave time-series

Please see the detailed README for information on extracting this kind of information from the output files.