What is a Tsunami
A tsunami is like a sudden burst of energy created by a sudden deformation of the sea floor usually caused by an earthquake.
Tsunami can travel across the open ocean at around 800km an hour but once the sea depth starts to get shallow, they rapidly slow down and generally come ashore around 30 – 40 km’s an hour. These are the characteristics of distant tsunami generated off the coast of Chile and Peru. Tsunami from Chile take around 11 hours to reach NZ and those from Peru around 15 hours.
New Zealand tsunami facts for students
Local events are generated by the same mechanisms but never reach the speed of the distant events between the earthquake source and the shore. Arrival times will be about 20 minutes on the open coast after the earthquake and in the city about 10 minutes later.
What are the impacts here
Tsunami impacts are lessened considerably in Poverty Bay, at the city end especially, as the ocean depth is shallow for quite a distance off the coast. This means that the surges that reaches our coast have a lot of energy removed and have also slowed considerably. The city is also protected to some extent by the narrow opening to the open ocean, although it is known that the bays shape also causes seiching which can make the surges larger and also because the mouth of the Turanganui River is facing Muriwai not the open ocean. We are also protected by our sand dunes, which a lot of the areas we have seen impacted by recent tsunami are not.
all the images below are from some tsunami modelling done in 2009 and 2012 and it shows the extent of inundation from a 9.4 earthquake off the coast of Peru except for Wainui and Tokomaru Bay which from earthquakes associated with the plate boundary off our coast. The model has also simulated the impact of the biggest surge occurring at high tide. This is our worst case scenario for a distant event and has a predicted return period in excess of 500 years. The modelling system has shown that the worst credible case local events are all similar to the level of inundation for distant events except for Wainui where local events are worse. Our evacuation zones are based on this modelling and if you do not live within those areas you have no need to evacuate for either a local or distant event.
|Peru - City||Peru - Wainui||Wainui worst case|
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|Tokomaru Bay||Peru - Hicks Bay||Peru - Te Araroa||Peru - Tolaga Bay|
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What to do
The evacuation maps define areas in coastal communities that are part of the planned evacuation areas for a distant and local tsunami. The only real credible threat from a distant tsunami to Gisborne communities is from Chile/Peru with around 11 - 15 hours warning.
Note: The community of Makorori will be totally evacuated so does not need a map.
The primary means of warning people to evacuate will be by door knocking in the areas identified in the maps. Authorities will have between 10 - 12 hours to do this. Those at risk will be advised by civil defence volunteers, Police and NZ Fire Service. There will also be information on all of our radio stations and on the civil defence website.
It is possible that evacuations for a distant event could last for 10 -12 hours, as there are many surges in a distant tsunami event. If inundation does occur then some areas may be closed for sometime.
For a locally generated event, which will be preceded by a very strong felt earthquake or a slow earthquake (one that goes for over one minute), self evacuation is the only option. The earthquake will be your only warning and you need to make the decision, if you are in the danger area, whether to go or stay. All of the evacuation maps have both local and distant evacuation zones on them. You only need to get clear of the evacuation areas not miles inland or on top of the highest peak. If you are on a part of the coast that isn't covered by one of the maps and you are evacuating you should get as far inland or as high as you can within a 20 -30 minute time frame (that's walking quickly not driving). If you can take a transistor radio you can listen for the all clear or otherwise you should wait 2 hours before returning home - if no tsunami has been generated.