The East Coast of New Zealand has been identified with a significant tsunami risk because of our subduction zone marked by the Hikurangi Trough. The massive tsunami in 2004 in the Indian Ocean, in 2009 in the South Pacific and in 2011 in Japan overturned many assumptions regarding the potential for severe tsunami to be generated on subduction zones throughout the Pacific. As new research into New Zealand’s subduction tsunami sources will take time, it is best to assume that our subduction zone could generate severe tsunami from earthquake sizes of MW 8-9.
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Hawke's Bay's position on the Pacific Ocean means there are risks of tsunami from both local, regional and distance sources, and the East Coast of NZ has the highest risk in the country. Tsunami (pronounced tsu - nam - ee) is a Japanese word meaning 'harbour wave'. It describes a series of fast travelling waves caused by large disturbances on the ocean floor, such as earthquakes, landslides or volcanic eruptions. In the deep ocean tsunami pass almost unnoticed, but as they approach land and therefore shallower coastal waters, they change dramatically - a wave 1-2 metres at sea grows into waves that can be over 30 metres in height.
As New Zealand’s entire coast is at risk of tsunami, GNS Science and the Hawke’s Bay Regional Council have developed a series of tsunami hazard maps in to help residents and councils prepare for a large tsunami impacting our region.
These maps show some of the worst case scenarios, up to a 2,500 year return period for Hawke’s Bay for tsunami coming from both a very large local earthquake or from across the Pacific Ocean. Check out the Hazard Information Portal, to see the possible inundation extents along our coast line, along with our Tsunami Evacuation Zones.
Our risks include destruction of homes, businesses and infrastructure in inundation zones, along with injuries and loss of life, with environmental devastation and the slow process of recovery.
Tsunami modelled results are being used by District and City Councils working with local people to prepare community response plans and evacuation maps. If you are keen to be involved, please contact your local Council for information.
WHAT CAN YOU DO?
If you feel a long or strong earthquake, and you are in a tsunami evacuation zone, drop, cover and hold for the earthquake. The first wave may arrive within 15 minutes so quickly get to a safe location - by foot or by bike. Leave immediately, every step counts. Go as fast as you can.
Sometimes natural warnings, like any of those below, will be the only warning of a tsunami, so don’t wait for an official warning, go immediately to high ground or, if the surrounding area is flat, go as far inland as possible, evacuating all coastal evacuation zones.
Plan your evacuate route now – remembering walking/running or biking might be better than driving, as roads may be damaged in the earthquake.
If you cannot escape the tsunami in time, go to an upper storey of a sturdy building (look for reinforced concrete or structural steel buildings). If you get caught in water, the best advice is to climb onto a floating object – staying in the water is deadly because of all the debris carried along.
Remember, in the event of a tsunami the first wave is may not be the biggest. Wait for official 'All Clear' before returning.
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Although there are only a few written records of tsunami striking the Hawke's Bay coastline, the geological record shows that the area has been impacted by large tsunami in the past, on average approximately one every 900 years. The risk of damage and financial loss from future events is becoming greater with increasing coastal development and use.
Local Source Tsunami
Several moderate-size tsunami have been observed along Hawke’s Bay coasts in the 160 years or so of written historical record, and on several occasions, the lives of Hawke’s Bay people have been threatened.
The largest earthquake in the Hawke’s Bay’s history, the magnitude 7.8 Hawke's Bay earthquake on 3 February 1931, initiated a moderate tsunami. It was reported at a few locations in Hawke Bay:
Although the worst effects of the 26 March 1947 tsunami were experienced on the coast north of Gisborne, where the waves reached 10 m vertically above sea-level, a small part of the Hawke’s Bay region north of Mahia Peninsula was also affected.
Distant Source tsunami
Tsunami from far off locations have caused damaging tsunami surges in Hawke's Bay. The 1868, 1877 and 1960 tsunami generated by large earthquakes in South America have had the greatest impact. The surges lasted several days in each case, the largest of the surges generally occurring within the first 24 hours.
Serious damage was reported at the Ahuriri Estuary in Napier and at Te Awanga as a result of the tsunami generated by the 20th century’s largest earthquake, a massive magnitude 9.5 in southern Chile. The tsunami was responsible for the deaths of several thousands in Chile and several hundreds in total in Hawaii, Japan and the Philippines.
The first wave struck in Napier after 1 o’clock in the morning on 24 May 1960 – there was no warning. 50 metres of a footbridge across Ahuriri Estuary was torn away and the power and gas lines along it were broken. A number of pleasure boats were damaged, and some were swept out to sea. Buildings and a caravan were flooded and moved, endangering the lives of the Napier Sailing Club’s caretaker and his family.
17000 m3 of sand was scoured from the boat harbour. Descriptive accounts suggest water levels reached at least 3 m and possibly over 4 m above high tide mark. At Te Awanga, eight people at the campground were swept from their tents and across the road, while seaside cabins were battered. At Clifton Domain, a sea wall 3 m above high tide mark was damaged. High seas were noted at Porangahau and Pourerere, but people were unaware a tsunami had occurred and no damage was done.
Two days later on 26 May 1960 a large aftershock in Chile resulted in the broadcast of a nationwide warning on radio in New Zealand. Although some communities were evacuated on the east coast in the North and South Islands, in Napier it was reported ‘the effect of the warning, which was broadcast at frequent intervals during the afternoon, was to drive people towards the beaches rather than away from them. All afternoon seafronts at Marine Parade and Westshore were thronged with larger numbers of people than usual at this time of year” (The Times 27/5/60). Foolishly people hoped to see a tsunami instead of escape from one.
Hawke's Bay received a National Warning from the Ministry of Civil Defence & Emergency Management advising of a 'Tsunami Threat to New Zealand' on Sunday 28 February 2010 at 00:02 hours. This had been generated by a magnitude 8.8 earthquake in Chile and the tsunami was confirmed by 01:11. The Port of Napier evacuated all vessels and some coastal residents self-evacuated. At 09:37 the first wave had arrived at Napier at a height of 0.2 metres, followed by a surge of waves around the Ahuriri Harbour. The tsunami was observed in Hawke's Bay with vertical changes in water levels around the coast and in harbours of around 1 metre. In Waimarama a local fisherman was swamped by a metre-high surge of water, followed by two more waves and was sucked 20 metres out into the ocean. He managed to swim ashore and suffered cuts and bruises.
Changes in water levels in the Inner Harbour at Napier 28 February 2010 on a floating pontoon. Jeff Lynex measured the rise and fall, one showing a 1.42 metre drop after 18 minutes.