Tsunami in Hawke's Bay

What is a tsunami?

A tsunami is a series of fast traveling waves. These are 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 shallow coastal waters, they change dramatically - a wave 1-2 metres at sea grows into waves that can be over 30 metres in height.

Hawke's Bay's position on the Pacific Ocean means there are risks of tsunami more distance sources, such as from Peru.

What warning will there be?

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, if there is:

• Strong earthquake shaking , making it hard to stand up or walk steadily, and there’s significant household contents damage and building damage
• Weak, rolling earthquake shaking which is unusually long, a minute or more
• Unusual sea behaviour, such a sudden sea level fall or rise, or the sea making loud and unusual noises, especially roaring like a jet engine.

How do I get prepared?

First Check if your home/school/work is in a tsunami evacuation zone

Plan your evacuation route – walking/running or biking can be better than driving, because roads may be damaged in the earthquake.  The aim is for you and your family to be safe.

If you think you cannot get inland in time, there may be a multi-level, sturdy building nearby where you can get access to go up - look for reinforced concrete or structural steel buildings.

Remember you may have to take this route at night. Know the route and have a torch.

Boat owners – check out Boat Tsunami Advice on the appropriate safe distance for boats to evacuate in a near source tsunami, remembering in a local tsunami there is only 10 minutes on the water to take the correct action.

What to do when it happens and after?

If you are in a coastal tsunami evacuation zone, don’t wait for an official warning as you should get to safety before a warning can be issued. Go immediately to high ground or, if the surrounding area is flat, go as far inland as possible. 

If you get caught in water, get out as soon as you can – staying in the water is dangerous because of all the debris carried along in it.

Remember, if a tsunami hits the coast, the first wave is may not be the biggest. Wait for official 'all clear' before returning to a coastal hazard zone.

Tsunami – vertical evacuation advice

We get a lot of queries about vertical evacuations during a tsunami. ‘Vertical evacuation’ means evacuating to a high floor of a building, instead of leaving the tsunami evacuation zone.

Japan has structures specifically designed for vertical evacuations, and the US has building standards for it, including foundations that are very deep and reinforced to a higher standard than most buildings, and an open ground-floor level to allow water to flow through.

New Zealand doesn’t have building standards for vertical evacuation structures, so we can’t say that any building in an evacuation zone is suitable as being a safe place in a tsunami. The Ministry of Business, Innovation and Employment is investigating a New Zealand building standard for vertical evacuation, and we may find that some buildings already meet the standard once it is determined.

We advise you to leave evacuation zones, making sure you carefully exit any building and are aware there may be failing debris, and follow your route to a safe location. But we understand people may need to consider other options in some situations:

• You are not sure you will be able to get out of the tsunami evacuation zone in time to a safe location. If there is a multi-level, sturdy building on your evacuation route that you can access, you could choose to go up – look for reinforced concrete or structural steel buildings.
• Those who are in a high-rise building may decide to stay there. They may feel uncomfortable leaving their building and making their way through streets – possibly in the dark, and with damage from the earthquake. This is something you and your household or workplace will need to discuss and decide.

Vertical evacuation could be a better option in some circumstances, but you need to be aware the building might not be built to withstand the impact of a tsunami. You could also be isolated in a building for days before help can get to you, and there is the risk of fire. If you choose to evacuate vertically, you should go to at least the third floor. If your building doesn’t have a third floor, staying there is not a good option.

Each option has risks and we cannot advise one over the other. You need to be comfortable with your own choice.

Whatever you decide works for you, we strongly recommend you spend some time putting together a grab bag, and practice the route you would take if you do leave your building. Knowledge is power: the more you can prepare for this now, the better.

Our message for tsunamis remains – If you feel an earthquake that is either longer than a minute OR strong enough that it’s hard to stand up, then get to high ground, out of all zones, as soon as the shaking stops. If it’s long or strong, get gone.

Why are we at risk?

New Zealand’s entire coast is at risk of tsunami but the East Coast of New Zealand has been identified as having the highest tsunami risk because of the subduction zone marked by the Hikurangi Trough.

Scientists have confirmed the subduction zone could generate severe tsunami from earthquake sizes of Magnitude 8-9.  This means in the future we could see tsunami like those in the Indian Ocean in 2004 and in Japan in 2011.

Everyone should be familiar with the Hawke’s Bay tsunami hazard maps which can help residents and councils prepare for the impact of a large tsunami. These maps show the worst case scenarios – a tsunami from a very large 2,500 year return period local earthquake or one coming across the Pacific Ocean.   Our risks include destruction of homes, businesses, productive land and infrastructure in inundation zones, along with injuries and loss of life, environmental devastation and the slow process of recovery.

In Hawke’s Bay we have been assessing our coastal hazards from Clifton to Tangoio. Find out more on this website. 

In 2015 a project also began studying the Hikurangi plate boundary as the source of earthquakes and tsunami. Find out more on this website.

Local-source tsunami

Several moderate-size tsunami have been observed along the Hawke’s Bay coast in the 160 years or so of written historical record. On several occasions, the lives of Hawke’s Bay people have been threatened.

3 February 1931: the largest earthquake in the Hawke’s Bay’s history, the magnitude 7.8 Hawke's Bay earthquake, initiated a moderate tsunami:

• At Waikokopu Beach, near Mahia Peninsula, three waves deposited fish and shellfish on the beach.
• A three-metre surge was reported racing up the Wairoa River shortly after the earthquake.
• A large wave was 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 metre above high tide level.

26 March 1947: the worst effects of a tsunami were experienced on the coast north of Gisborne, where the waves were 10 metres high. In Hawke’s Bay, Mahia Peninsula was 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.

February 2010 – Chile Tsunami

Just after midnight on Sunday 28 February 2010 a magnitude 8.8 earthquake in Chile generated a tsunami. Napier Port evacuated all vessels and some coastal residents self-evacuated. At 09:37 the first wave arrived at Napier with a height of 0.2 metres, followed by a surge of waves around the Ahuriri Harbour. Along the coast the tsunami was around 1 metre high. In Waimarama a local fisherman was swamped by the 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 on the floating pontoon in the Inner Harbour at Napier 28 February 2010 were measured by Jeff Lynex as show

ing a 1.42 metre drop after 18 minutes.

Changes in water levels on the floating pontoon in the Inner Harbour at Napier 28 February 2010 were measured by Jeff Lynex as showing a 1.42 metre drop after 18 minutes.

May 1960 – Chile Tsunami

Serious damage at Ahuriri Estuary in Napier and at Te Awanga resulted from 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 thousand people in Chile and several hundreds in total in Hawaii, Japan and the Philippines.

The first wave struck Napier without warning after 1am on 24 May 1960. 50 metres of a footbridge across Ahuriri Estuary was torn away, breaking the power and gas lines along it. A number of boats were damaged, some 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.

Around 17000 cubic metres of sand was scoured from the boat harbour. At Te Awanga, 8 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 metres above high tide mark was damaged. H

Descriptive accounts suggest water levels reached at least 3 metres and possibly over 4 metres above high tide mark. 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). Not the right thing to do!