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Natural Hazards: Tsunami

Tsunami is a Japanese word meaning 'harbour wave'

Tsunami

What is a Tsunami?

Tsunami is a Japanese word meaning 'harbour wave'. Tsunami are large and powerful ocean waves that can grow in size as they reach the shore. These waves can move very fast and for very long distances. Some tsunami have been known to travel for thousands of kilometres across the ocean and travel at speeds of up to 900 km per hour.

How are they created?

Giant slabs of the Earth's crust, called tectonic plates, grind together. Sometimes, though, the plates get stuck, the pressure builds up and they suddenly move into a new position and this causes an earthquake. If an earthquake lifts or drops part of the ocean floor, the water above rises and starts spreading across the ocean, causing a tsunami. Underwater landslides or volcanic eruptions can also displace water (cause water to spread across the ocean) and may lead to a tsunami.

What do they look like?

Tsunami waves appear either as a strong and quick moving tide that can wash people and objects out to sea, or as a series of large breaking waves that can cause significant damage at the shoreline.

How big are they?

Out in the open ocean, tsunami waves are only about one-metre high because the water is deep. However, as the water becomes shallow, the waves slow down and begin to grow. Tsunami waves come up higher than normal waves and can rise 35m or higher. However, the scariest thing about a tsunami is its wavelength, as this determines how far inland it can travel. Whereas a large wave caused by a storm might have a wavelength of up to 150m, a tsunami could reach up to 1,000km.

 

Tsunami on the East Coast 

Many communities along the East Coast are at risk from tsunami. If the tsunami is triggered close to the shore, the sea gets sucked back very quickly past the normal low tide mark and then quickly comes in higher than before. Then a wave blasts onto the shore minutes later, then another and another for two hours or more. There may also be up to one hour between each wave. If this happens, there won't be enough time for Civil Defence to issue a warning and it is important that you know what to do, and that you act quickly.

 

Past tsunami events on the East Coast 

26 MARCH 1947

It was March 26, 1947 and a tsunami swept on to the East Coast after what the Gisborne Herald reported as a “fairly severe” earthquake just after 8.30 a.m.  Honeymooners Don Tunnicliffe and his wife Novena were visiting Tatapouri Point near Gisborne, staying at the home of Albert and Annie Hall, who were partly deaf and didn’t heard the roar “it sounded like a powerful motorbike”.  Mr Tunnicliffe went outside.

“Approaching the shore, and us, at breakneck speed and roaring like an express train was a wall of dirty coloured water towering a good 30 feet, boiling and curling as it picked up acres of beach sand on its way to engulf us” said Mr Tunnicliffe

“I saw a young man squatting on his haunches, oblivious to the huge wave towering above him.  I only had time to yell, “look out” before the tsunami hit.”

“Novena must have been flung back into the kitchen by a wave top breaking sideways as it hit the wall.  The young fellow shot past me like a spinning top as I also disappeared under the swirling rolling and now seething mass of water, sand and seaweed”

The young man, Roger Winkfield, a nephew of the Halls, was carried through a gap in the fence, over the road and was slammed against a two-metre bank on the far side.

“I tumbled along and stopped when I became entangled in the top strands of the barbed-wire fence. As I was a good six feet under in a world of blackness, time was obviously important.  I was a strong swimmer and used to holding my breath, so perhaps that experience saved my life,” he said.

Flattened by a second wave, Mr Tunnicliffe stood up to see the initial wave careering up the hills for hundreds of metres, then coming back “like a wave in reverse, carrying posts, firewood, logs and even old doors”.

“We could only gaze in stupefied wonder as sheds were picked up as though by a giant hand and smashed down on to the several feet of water.  Then the massive waters, still receding with the load of debris, churned the seawater around the house into a maelstrom of battering ram intensity.  The roof peak of the house was bobbing up and down, in and out of the water at the whim of the forces trying to move both ways,” he said.

“There was no sign of life at the house.  The kitchen where the three were last seen was the last to succumb to this relentless pressure as weatherboards were loosened, wrenched free and dashed away.  It was like watching murder.” 

“Finally some gaping holes appeared in the kitchen wall facing us and our hypnotised eyes saw an arm appear from within.  Then the quavering voice of Mr Hall said ‘we’re alright’”.

“It was like a prayer answered, a miracle to outshine miracles.”

Mrs Tunnicliffe had floated on the rising water inside the kitchen, maintaining a grip on the mantelpiece to keep her head clear, while Mr Hall supported his wife by clinging to the door.  Miraculously everyone escaped with bruises.

23 MAY 1960

On 23 May 1960 a large earthquake off the coast of Chile generated a tsunami that travelled across the Pacific to hit New Zealand at night.  If Grant Anderson and Maurice Leech had known then that a tsunami was funnelling its way down Lyttelton Harbour they would have headed for the hills instead of into the water to save a dog swept away by “freak tides”.

“I thought the water was just a bizarre tide and a dog needed to be saved.  I took a dinghy and Maurice walked along the foreshore towards the Charteris Bay jetty.  I was taken out by the water and then the dinghy was sitting in the mud.  Then the water would come back in and I’d start rowing again,” said Mr Anderson.

The water was flooding low-lying farmland and the Wheatsheaf Tavern about 400m inland at Teddington, near Charteris Bay.  On the other side of the harbour, the port electrical cables and the dry dock took a pounding.

“I remember there was an old chap living in the Wheatsheaft at the time.  During his lifetime he’d apparently been shipwrecked twice.  He got a fright of his life when he woke up to see his sea chest floating and his room full of water,” recalled Mr Anderson.

“Another chap in the areas was apparently having a bath.  The water went up the drains and pushed the plug out of the bath and filled it with seawater.  The next day people were picking fish up off the ground.  It was strange stuff.  But I remember my grandmother saying there’d been a similar thing 100 years before.  Then they were picking up fish stuck in the farm fences.”

Maurice Leech said “ I was awestruck when I got the call about the dog.  It had been swept out with the kennel it was tied to.  The water was only about two foot high.  There was a series of these waves; they all came in on top of each other.  They came in at a rush”.

There was a seven-metre variation of water in the harbour, and at its peak the Harbour swelled two metres above the high water.  There was no fear.  More wonderment than anything else.  And the swimming dog?  He apparently reached shore without any help.