Hawkes Bay Civil Defence Emergency Management Group

Tsunami

Hawke's Bay's position on the Pacific Ocean means there are risks of tsunami from both local, regional and distance sources, and it is recognised 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.

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Turihara - Debris left by local tsunami north of Gisborne 26 March 1947
Turihara - Debris left by local tsunami north of Gisborne 26 March 1947

Previous impacts on Hawke's Bay

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 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.

Localised 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. However, the only life known to be lost from tsunami since Europeans settled in New Zealand occurred in the Chatham Islands in the 1868 tsunami from a large earthquake in Peru, South America.

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 only in Hawke Bay:  
  • At Waikokopu Beach, near Mahia Peninsula, three waves deposited fish and shellfish on the beach. 
  • A 3 m surge was reported racing up the Wairoa River shortly after the earthquake. 
  • A large wave was also reported in Waikari River. This appears to have been caused by an earthquake-triggered landslide on the other side of the estuary. The wave destroyed a wool shed and deposited fish on grass about 15 m above high tide level.

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.

Miss Higgins of Napier stands on the front foundation of her parents boat shed which was moved back and wedged between other buildings in 1960.  Below is the remains of the damaged Ahuiriri Estuary footbridge. (Photos courtesy of Russell Spiller)
Miss Higgins of Napier stands on the front foundation of her parents boat shed which was moved back and wedged between other buildings in 1960. Below is the remains of the damaged Ahuiriri Estuary footbridge. (Photos courtesy of Russell Spiller)

Distant tsunami

Tsunami from far off locations have caused significant and 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.

In 1868 a tsunami, generated by a magnitude 9 earthquake in southern Peru, caused a variation of about 1.8-2 m in water levels in the port area of Napier. The water receded far enough to expose the wreck of an old ship, the Montmorency. There was no damage around Napier as the water levels coincided with low tide so reached no higher than a metre above normal high water mark. The tsunami was also observed at Wairoa.

In May 1960, more 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.  They were disappointed and as a result many people still believe there was no tsunami at all in Napier during those two days.  
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.
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.

On Sunday 28 February 2010 at 00:02 hours Hawke's Bay received a National Warning from the Ministry of Civil Defence & Emergency Management advising of a 'Tsunami Threat to New Zealand'.  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.

Tsunami Hazard Maps

The Hawke’s Bay Regional Council has developed tsunami hazard maps to help residents and councils prepare for a large tsunami, along with answers to frequently asked questions about tsunami.

IMPORTANT - These are overview maps only and are not specific enough to predict impact on your individual property.

New Zealand’s entire coast is at risk of tsunami. These maps show scenarios for Hawke’s Bay for tsunami coming from both a very large local earthquake or from across the Pacific Ocean. An event of this scale may happen on average only once in 2,500 years, i.e. this is one of the worst case scenarios.

The maps from Clive to Tangoio were published in local newspapers in August 2011 (NB the key shows the various depths of water inland as a result of both distant & near source waves).
Maps for Wairoa response planning was provided to Wairoa District Council late 2011. Further mapping will be completed following the release of the national tsunami risk review.

The 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.

Self evacuation after a strong earthquake is vital to avoid being caught in a damaging Tsunami.  Hastings Mayor Yule inspects tsunami damage in Thailand 2005.
Self evacuation after a strong earthquake is vital to avoid being caught in a damaging Tsunami. Hastings Mayor Yule inspects tsunami damage in Thailand 2005.

What to do?

In the case of an impending tsunami, warning messages and signals to the public can come from several sources – natural, official or unofficial.

Natural warnings
may be the only warnings possible for local or regional source tsunami.

  • Strong earthquake shaking (i.e. it is hard to stand up or walk steadily, there is significant house contents damage and building damage [classified as Modified Mercalli MM6-MM7])
  • Weak, rolling earthquake shaking of unusually long duration (i.e. a minute or more)
  • Out of ordinary sea behaviour, such as unusual and sudden sea level fall or rise
  • The sea making loud and unusual noises, especially roaring like a jet engine

When experiencing any of the above go immediately to high ground or, if the surrounding area is flat, go as far inland as possible, evacuating all coastal areas or, where present, all evacuation zones. The first wave may arrive within minutes.

Once away from the water, listen to a radio station for information from local civil defence about further action you should take.
Do not wait for an official warning - let the natural signs be your warning and wait for official all clear before returning.

Official warnings

Official warnings are expected to come for sources that are more than three hours of tsunami travel time away from Hawke's Bay.

  • An official warning from Civil Defence Emergency Management may be issued through radio or television broadcasts.
  • Warnings may also be through siren, telephone, loud hailer or other local arrangements.
  • You may receive warnings from one, or several sources. Respond to the first warning, do not wait for more messages before you act.
  • Listen to your radio and follow any official instructions.
  • Evacuate from the areas or zone(s) stated in the warning.
  • Take your `Getaway Kit’ with you.
  • Stay out until the official 'all-clear' is given.

Unofficial/Informal warnings

There are several ways by which people may receive unofficial or informal warnings of an impending tsunami, for example

  • media coverage, following release of a watch/warning bulletin from the Pacific Tsunami Warning Center (PTWC). People may receive unofficial warnings either directly through local or international media, or from friends in New Zealand or overseas that have heard their broadcasts;
  • from people (e.g. by phone) who have already experienced the arrival of the tsunami on coastline closer to the source, or observed a natural warning sign.

Warnings from friends, other members of the public, international media, internet, etc. may be correct; informal communication may be your only warning, especially for tsunami sourced from less than three hours tsunami travel time away from you.

  • If you are in an evaucation zone and you feel the threat is imminent, evacuate to high ground and/or inland immediately, or as directed by officials.
  • Verify the warning only if you can do so quickly (via NZ TV/Radio broadcasts, internet, Civil Defence Emergency Management, Police, Fire).
  • The first or largest wave may not arrive for 6 hours after the forecast arrival time.
  • If New Zealand Civil Defence Emergency Management warnings are available, trust their message over informal warnings.
For more information on tsunami go to http://www.geonet.org.nz/tsunami/  or www.gns.cri.nz
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