Know Your Risk - Tsunami

Australia has been affected by over 50 Tsunami in the past 200 years.

The largest Tsunami to have affected the NSW coast in recent times occurred in May 1960 after a 9.5 magnitude earthquake in Chile resulted in a 1 metre tidal fluctuation at Fort Denison in Sydney Harbour. This caused widespread damage to marine infrastructure along the NSW Coast including damage to boats, wharves, jetties and beaches.

Most tsunami occur in the Pacific and Indian Oceans. The boundary of the Pacific Ocean experiences frequent earthquakes. This boundary is commonly known as the Ring of Fire. There are two major subduction zones in the Indian Ocean that can also generate tsunami.

Tsunami can be generated by a number of causes including, volcanic eruptions, undersea landslides and meteor impacts. However, undersea earthquakes are the more likely source of tsunami that could affect NSW.

Earthquakes from subduction zones are the most common source of destructive tsunami. These earthquakes are generated when two tectonic plates meet and one goes under the other, usually at a rate measured in centimetres. The sinking (subducting) plate drags against the upper plate, causing flexing. Continued stress on the plate boundary results in the upper plate rebounding to its original position, displacing the sea water above. This can lead to the generation of tsunami waves.

There is a moderate likelihood of tsunami in NSW compared with other parts of the world. Tsunami do have an effect on our coastline, Even with relatively small tsunami, there could be strong waves, rips and currents and unusual sea levels that can be dangerous.

NSW SES has developed evacuation areas across most coastal areas of NSW.

It is important to remember that these zones are evacuation areas only, NOT inundation maps.

NSW SES Tsunami Evacuation Areas

The Joint Australian Tsunami Warning Centre monitors earthquakes that could potentially cause tsunami that affect Australia by operating an enhanced network of seismic stations nationally and has access to data from international monitoring networks. It determines the magnitude, location and characteristics of a seismic event which has the potential to generate a Tsunami.

Based on this seismic information, the Joint Australian Tsunami Warning Centre runs a tsunami model to generate a first estimate of the tsunami size, arrival time and potential impact locations.

The Joint Australian Tsunami Warning Centre verifies the existence of a tsunami using information from an enhanced sea level monitoring network of buoys situated strategically between the key earthquake zones and the NSW coastline.

The Joint Australian Tsunami Warning Centre then sends advice and warnings on any possible tsunami threat to State & Territory emergency management services, media and the public.

Tsunami poses the following key risks:

Risk to life:

There is a significant risk of drowning during tsunami with either marine and land inundation threat.  This is due to the potentially dangerous rips, waves, water born debris and strong ocean currents.

• During a Marine and Immediate Foreshore threat, the risk to life includes drowning,

• During land inundation tsunami inundation is widespread and extends to areas not normally submerged by water.  Tsunami waves can run up well beyond the high tide mark, in extreme cases up to a kilometre inland or up to 10 metres above sea level. Tsunami can also extend into harbours, rivers and estuaries.  This is why tsunami safety advice for a tsunami with land inundation threat states that you should move away from low lying coastal areas, harbours and estuaries as soon as a Tsunami Warning is issued

• Tsunami are a series of multiple waves that can last several hours. Tsunami are fast moving and very powerful and the first wave may not always be the biggest. Therefore it is important not to return to low lying coastal areas until advised by emergency services that it is safe to do so

Risk of property damage

• Boats and other vessels can be at risk from damage due to tsunami.  Boats are most at-risk in shallow water where the surge associated with tsunami waves can capsize vessels, push them into other boats and obstacles, or wash debris into the boat. The skipper and any passengers are still at risk if they are still on-board or within the tsunami impact area when the tsunami arrives

• Marinas, moorings, ports and other marine based infrastructure can be at risk of damage by the sudden surge of water caused by tsunami. Much of this damage is unavoidable and the best advice is to securely tether boats to moorings. You should always prioritise life over property damage by leaving the area as soon as a Tsunami Warning is issued, and ensuring you are outside the area at-risk well before the first wave arrives

• Homes, businesses and other property such as caravans and cars situated in low-lying coastal areas can also be affected by tsunami.  Much of the damage to this property will be caused by land based tsunami and in these rare cases is often unavoidable.  You should prioritise your life and the lives of others above any damage to property.  However planning for natural disasters may assist you to make good early decisions that could potentially help you mitigate some of this damage.

The NSW State Tsunami Emergency sub plan covers arrangements for the emergency management of tsunami in New South Wales. This plan deals only with preparedness for and response to tsunami and the initiation of recovery coordination arrangements following the impact of a tsunami. The plan accounts for all tsunami magnitudes and generation mechanisms.