Natural hazards

Tsunami

Tsunami may be tens of metres in height in shallow water, however most tsunami are less than 1m in height at the shore. Historical information suggests that the most likely hazardous tsunami event for the Auckland region is likely to be caused by an earthquake off the west coast of Chile.

For example, a magnitude (Mw) 9 earthquake occurring at 15 km depth below the earth's surface, off the west coast of northern Chile, could generate tsunami waves in the Waitemata Harbour as large as 3.6 m above mean sea level (not including local amplification effects). A far field event from this source would affect both coasts, with historical evidence suggesting the east coast response to be twice that of the west coast.

What is a tsunami?

A tsunami is a series of waves that have long periods (usually 15-60 minutes), that are generated in a body of water by an impulsive disturbance that vertically displaces the water column. Tsunamis usually result from large-scale movement of the sea floor, from a volcanic eruption, submarine earthquake, or landslide.

Although usually barely noticeable at sea, in deep water they can travel as fast as 700 km/hr (~430 knots) and have small wave heights, so are barely noticeable. They can travel great distances (e.g. across oceans) in a matter of hours. In shallow water, they travel more slowly but can grow to tens of metres high through shoaling.

In 1996, a tsunami hit Paupa New Guinea. An earthquake triggered a large underwater landslide, which in turn created a large local tsunami. This tsunami had wave heights of up to 15m and killed more than 3 000 people.  A simulation of the tsunami can be viewed below.

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Hawaiian Beach engulfed by 1957 tsunami from Aleutians

Tsunami generated by earthquake on 9 March 1957, Aleutian Islands, Alaska. Third photo in a series of three sequential photos that show the arrival of a major wave at Laie Point on the Island of Oahu, Hawaii, about 3,600 km from the source. The 8.3 earthquake occurred south of the Andreanof Islands, in the Aleutian Islands. It generated an 8m tsunami that did great damage on Adak Island, especially to the fuel and oil docks. However, the Hawaiian Islands incurred the greatest damage (about $5,000,000 in 1957 dollars). The highest wave was 16m at Haena, Kauai. Photograph credit: Henry Helbush

Consequences of tsunami

Tsunami waves reaching 3.6m above mean sea level could cause:

• injury or drowning of people in low-lying coastal areas due to inundation and impact by floating debris
• damage to moorings and coastal structures due to fast wave velocities and scour
• coastal erosion and potential loss of support to structures in the coastal area
• sudden inundation of coastal infrastructure such as roads, airport and ports resulting in flotation of vehicles and accidents, temporary obstruction or failure of roads (up to 2 days)
• disruption to water networks
• short and long term economic losses due to clean-up, and construction of repair and remedial structures
• negative impacts on the tourism industry
• major social and psychological disruption.

Aftermath of 1960 Chilean tsunami in Hilo, Hawaii

Tsunami generated by earthquake of 22 May 1960, Coast of Chile. Aftermath of the Chilean tsunami in the Waiakea area of Hilo, Hawaii, 10,000 km from the generating area. Parking meters were bent by the force of the debris-filled waves. Note the scattered debris and the gutted foundation. The earthquake off the coast of central Chile generated this tsunami that affected the entire Pacific Basin. This was one of the most seriously affected areas, where 61 deaths and $23 million in damage occurred. In the area of maximum destruction, only buildings of reinforced concrete or structural steel, and a few others sheltered by these buildings remained standing, and even these were generally gutted. Frame buildings either were crushed or floated nearly to the limits of flooding. Photograph credit: U.S. Navy

Locally generated tsunami

Near-field tsunamis from earthquakes along local faults (e.g. Kerepehi Fault) or from a volcanic eruption are also possible, but are much less likely. Wave heights around the region will not be uniform and will be dependent on local coastal geometry.

Main hazards are inundation, penetration of waves up some estuaries and strong currents causing erosion and damage to moorings and coastal structures. The effects from a locally generated tsunami of this scale will be similar to the effects of a distantly generated tsunami, although less severe (see above).

What is the likelihood of tsunami in Auckland?

Distantly generated tsunami

The most likely source of a damaging tsunami event is one generated off the west coast of South America. In any one year, it is estimated that there is a 1.33% chance that Auckland could be affected by a damaging tsunami from this location.

Locally generated tsunami

Part of the Kerepehi Fault is located in the Firth of Thames, Hauraki Gulf, New Zealand. If sufficient movement occurs along the submarine section of this fault, a tsunami could be generated. Fault movement is thought this is likely to occur every 4,500-9,000 years or so. A tsunami generated by this mechanism could reach up to 3 m above mean sea level.