Tsunami 

A tsunami is a natural phenomenon consisting of a series of waves caused when a large mass of earth on the bottom of the ocean drops or rises, rapidly displacing the water above it.

Large earthquakes which cause a vertical displacement of the seabed along a fault line are the most common source of damaging tsunamis worldwide.

This type of displacement commonly occurs in large subduction zones, where the collision of two tectonic plates causes the oceanic plate to dip beneath the continental plate to form deep ocean trenches. Volcanoes and underwater landslides are other potential causes of tsunami.

A large tsunami may feature a series of waves arriving over a period of hours, with significant time between the wave crests. The first wave to reach the shore may not be the largest.

Our coastline is also at risk of storm surges, ocurring as a result of low atmospheric pressure and persistent wind stress (or wind set-up). Low atmospheric pressure may result in higher than predicted sea levels, while strong onshore winds result in the sea “piling up” against the coast.

We have modelled inundation zones for tsumani and storm surges which are are available on the Otago Natural Hazards Database for 17 localities along the Otago coast. These include:

• Tautuku
• Papatowai
• Catlins
• Kaka Point
• Lower Clutha
• Toko Mouth
• Taieri Mouth
• Brighton
• Kaikorai
• South Dunedin
• Long Beach and Purakaunui
• Blueskin Bay
• Karitane and Waikouaiti
• Moeraki
• Hampden
• Kakanui and Taranui
• Oamaru
  
  

The hazard areas shown in the database were created by NIWA in 2007 to help identify the level of inundation associated with credible tsunami events for specific communities along the Otago coast. The modelled tsunami scenarios available on the database are:

• A tsunami originating from an earthquake in South America, with an estimated return period of 500 years.
• A tsunami originating from an earthquake on the Puysegur Trench to the south of New Zealand, with an estimated return period of 600 years.
  
  

The NIWA investigation also modelled the extent of inundation due to an event with a higher likelihood of occurring (a tsunami originating from an earthquake in South America, with an estimated return period of 100 years).

Although the Database does not map the maximum speed at which tsumani waves are likely to travel, this information can be accessed via the NIWA source report (Otago Regional Council Tsunami Modelling Study – Sept 2007).

Other settlements and unpopulated stretches of the Otago coast are also subject to tsunami hazard. The lack of modelled tsunami information in these areas does not indicate that this hazard does not exist, rather that we do not currently hold the same level of information about inundation extent and wave speed for these localities.

Visit the Otago Civil Defence and Emergency Management website for information on what you can do to prepare for a tsunami. 

For technical reports on tsunami and storm surges risk go to our technical reports section