Tsunami

What is a tsunami?

Tsunami (pron: 'soo-nar-me') is a Japanese word: 'tsu' meaning harbour and 'nami' meaning wave. Tsunami are waves caused by sudden movement of the ocean surface due to earthquakes, landslides on the sea floor, land slumping into the ocean, large volcanic eruptions or meteorite impact in the ocean.

Until recently, tsunami were called tidal waves, but this term is generally discouraged because tsunami generation has nothing to do with tides (which are driven by the gravity of the Earth, Moon and Sun). Although some tsunami may appear like a rapidly rising or falling tide at the coast, in other situations they can also feature one or more turbulent breaking waves.

How do tsunami differ from regular waves?

A tsunami is different from a wind-generated surface wave on the ocean. While wind-generated waves in deep water only cause water movement near the surface, the passage of a tsunami involves the movement of water from the surface to the seafloor. Interestingly, this causes the speed of a tsunami to be controlled by the water depth, with faster speeds in deeper water, unlike wind-generated waves. Consequently, a tsunami slows as it approaches land and reaches increasingly shallow water, with the distance between successive wave peaks decreasing. Because the total energy within the wave does not change, the energy is transferred to increasing the wave height (or amplitude). This is called wave shoaling.

A tsunami is often a series of waves and the first may not necessarily have the greatest amplitude. In the open ocean, even the largest tsunami are relatively small, with wave heights typically tens of centimetres or less away from the initial tsunami generation zone. Higher oceanic wave heights are sometimes observed very close to the tsunami generation zone (e.g., oceanic waves near two metres were measured close to the source of the 2011 Japan tsunami). In any case, the shoaling effect can greatly increase open ocean wave heights upon reaching the coast, with some tsunami reaching an onshore height more than ten metres above sea level. Such extreme inundation is more likely to occur nearer to the tsunami generation location (where oceanic wave heights are larger), and at locations where the coastline shape is particularly favourable to amplification of the tsunami. Most tsunami do not cause such extreme coastal inundation and the effect of small events may not be noticeable to without careful analysis of tide gauge measurements.

What causes tsunami?

Earthquakes

Most tsunami are caused by large earthquakes on the sea floor when slabs of rock move past each other suddenly, causing the overlying water to move. The resulting waves move away from the source of the earthquake event.

Landslides

Landslides can happen on the seafloor, just like on land. Areas of the seafloor that are steep and loaded with sediment, such as the edge of the continental slope, are more prone to undersea landslides.

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.

Volcanic eruptions

Tsunami initiated by volcanic eruptions are less common. They occur in several ways:

  • destructive collapse of coastal, island and underwater volcanoes which result in massive landslides
  • pyroclastic flows, which are dense mixtures of hot blocks, pumice, ash and gas, plunging down volcanic slopes into the ocean and pushing water outwards
  • a caldera volcano collapsing after an eruption causing overlying water to drop suddenly.

Where do tsunami occur in Australia?

There is evidence that the Australian coast may have experienced large tsunami during the past few thousand years. This evidence is revealed through anomalous sedimentary deposits (such as those containing shell or coral) or other geomorphic features (Dominey Howes, 2007; Goff and Chauge-Goff, 2014). More recently, tsunami continue to be recorded in Australia with most presenting little threat to coastal communities. The significant tsunamis recorded in recent times have all been recorded at tide gauges around the country with some causing damage in the marine environment.

The tsunami hazard faced by Australia ranges from relatively low along the southern coasts of Australia to moderate along the west coast of Western Australia. This area is more susceptible because of its proximity to large subduction zones along the south-coast of Indonesia, which is a region of significant earthquake and volcanic activity.

Several significant tsunami have impacted Australia's north west coast region. The largest run-up (measured as elevation about sea level) was recorded as 7.9m (Australian Height Datum (AHD)) at Steep Point in Western Australia from the July 2006 Java tsunami. The largest reported offshore wave height was six metres near Cape Leveque from the August 1977 Sunda tsunami.

References:

Dominey-Howes D. (2007) Geological and historical records of tsunami in Australia. Marine Geology 239: 99¿123 doi:10.1016/j.margeo.2007.01.010

Goff, J. and Chauge-Goff, C. (2014) The Australian Tsunami Database: A Review. Progress in Physical Geography 38(2): 218-240. DOI: 10.1177/0309133314522282

  • Tsunami can travel at speeds up to 950km/h in deep water, which is equivalent to the speed of a passenger jet.
  • Several significant tsunami have impacted Australia's north west coast region. The largest run-up resulted from the 2006 Java tsunami that was recorded at 7.9m AHD at Steep Point Western Australia. The largest reported offshore wave height was six metres near Cape Leveque from the August 1977 Sunda tsunami.
  • The tsunami that reach the Australia coast at Steep Point on 17 July 2006 was generated by a magnitude 7.7 earthquake south of Java. The tsunami caused widespread erosion of roads and sand dunes, extensive vegetation damage and destroyed several campsites up to 200 metres inland. The tsunami also transported a 4WD vehicle ten metres. Fish, starfish, corals and sea urchins were deposited on roads and sand dunes well above the regular high-tide mark.
  • Further north in the Onslow-Exmouth region in June 1994, tsunami waves travelled inland to a point four metres above sea level and washed 300 metres inland after appearing out of a calm sea. Both tsunami were generated by earthquakes in Indonesia.
  • In May 1960, a magnitude 9.5 earthquake in Chile generated the largest tsunami recorded along the east coast of Australia. The event generated tsunami waves of just under one metre at the Fort Denison tide gauge in Sydney Harbour. Slight to moderate damage was caused to boats in harbours at Lord Howe Island, Evans Head, Newcastle, Sydney and Eden.
  • The 1998 tsunami in northern Papua New Guinea was caused by an earthquake that is believed to have triggered an undersea landslide.
  • The 1883 eruption of Krakatau volcano in Indonesia unleashed a series of devastating tsunami that resulted in the loss of tens of thousands of lives.

What is Geoscience Australia's role in reducing risk to Australians from tsunami?

Geoscience Australia is committed to support Australia's capability to manage the impact of natural hazards, including tsunami. Geoscience Australia:

  • develops an understanding of natural hazards and community exposure to support risk mitigation and community resilience
  • provides authoritative, independent information and advice to the Australian Government and other stakeholders to support risk mitigation and community resilience
  • maintains and improves systems for effective natural disaster preparedness, response and recovery
  • contributes to Australia's overseas development program.

In particular, Geoscience Australia:

  • jointly operates the Joint Australian Tsunami Warning Centre (JATWC) with the Bureau of Meteorology. Geoscience Australia identifies and characterises the triggering source for a tsunami to initiate the tsunami warning chain
  • supports international efforts for the Indian Ocean tsunami warning and mitigation system (IOTWMS). In particular, the JATWC is one of three official Regional Tsunami Service Providers (TSP) for IOTWMS
  • contributes data to the Pacific Tsunami Warning System for tsunami warnings in the South West Pacific
  • develops national-scale offshore hazard maps as a fundamental input to assessing the local tsunami hazard and impact
  • assesses the potential impact of tsunami on coastal communities in collaboration with state and territory emergency services
  • supports national efforts to manage the potential impacts of tsunami, for example through participation in the Australian Tsunami Advisory Group (ATAG)
  • collaborates nationally and internationally to enhance the tsunami hazard risk modelling methods.

Roles in the JATWC

Geoscience Australia's role in the JATWC is two-fold:

  • to detect earthquakes that have the potential to generate tsunami that can impact Australia's coastline, and advise the Bureau of Meteorology of this potential within 10 minutes of the earthquake occurring
  • to undertake tsunami risk studies to assist local and state organisations in planning for tsunami events.

The Bureau of Meteorology's role is also two-fold:

  • to use its network of sea level monitoring equipment, including coastal tide gauges and tsunameters (deep ocean tsunami sensors), and tsunami propagation models to confirm the existence of a tsunami and estimate its likely impact at the Australian coast
  • to issue the relevant tsunami warnings and bulletins for Australia and external territories as required.

What does the JATWC do?

Geoscience Australia receives real-time data from over 60 seismic stations in Australia and more than 130 international seismic stations. The seismic information is automatically analysed by Geoscience Australia's seismic monitoring and analysis systems that form part of the 24 hours a day, seven days a week operations centre. When an earthquake occurs, this system automatically computes preliminary information on the earthquake's origin time (time at which the earthquake happened), location, depth and magnitude. The Duty Seismologist assesses this information and then calculates a moment magnitude, Mwp, (similar to a Richter magnitude) to assist in determining the potential for the earthquake to cause a tsunami. If Duty Seismologist considers that the earthquake has the potential to generate a tsunami that may impact Australia, the seismologist sends the information to the JATWC office in the Bureau of Meteorology in Melbourne via a dedicated data link. This process is completed within 10 minutes of the earthquake's origin time.

The JATWC also receives data from the Bureau of Meteorology's sea level observations and other international sea level stations. These instruments provide real-time sea level observations that can verify whether an earthquake has generated a tsunami and, if so, monitor its path. The data are provided by coastal sea level stations and deep ocean tsunami detection sensors. Equipped with these sea level data and the Bureau of Meteorology's tsunami modeling, specially trained JATWC staff then issue a warning that is in keeping with the determined threat level. These warnings identify affected coastal regions, initial tsunami arrival times and whether the tsunami threat is to land or marine areas. Upon receiving the earthquake alert from Geoscience Australia, the Bureau of Meteorology issues a tsunami bulletin within 10 minutes of receiving the alert. The JATWC is thus able to issue tsunami bulletins within 20 minutes of the origin time of the earthquake.

The Bureau of Meteorology issues advice and warnings on identified tsunami threat to emergency management agencies and the public using procedures similar to those used for warnings of other severe weather or hazardous events. Procedures include:

  • distributing tsunami bulletins and warnings to the media, key agencies such as the state and territory emergency services, local councils, port authorities, police and the public
  • working with media organisations across Australia to inform the public in the case of a tsunami event
  • maintaining tsunami bulletin and warning distribution lists at each of the Bureau of Meteorology's state and territory Regional Forecasting Centres. These distribution lists are used for both national JATWC bulletins and regional warnings. The bulletin and warning messages are also automatically uploaded to the JATWC website.

The JATWC is also part of a network of international tsunami watch centres that cooperate under arrangements coordinated by the UN's Intergovernmental Oceanographic Commission (IOC) within UNESCO.

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

Copyright Google, modified by Geoscience Australia

Copyright Google, modified by
Geoscience Australia

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:

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