NZ: The science of tsunamis and what parts of New Zealand are most at risk

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EXPLAINER: The earthquake struck on a Wednesday in March at 8:32am, 50 kilometres off the coast of Gisborne near the Hikurangi Trough.

Even though it registered magnitude 7.1, it was not widely felt along the coast. The shaking caused no damage.

Thirty minutes later, enormous waves rushed in. There was almost no warning.

The “silent” tsunami hit 115km of coastline on the East Coast, from Mahia Peninsula to Tokomaru Bay.

A 16-metre wooden bridge near Pouawa was flung almost a kilometre inland. Two men were thrown off their feet and deposited on a nearby road.

Two women and another man were trapped in a cottage as the water hurtled inland. The currents rose to head height. Their home disintegrated as the water receded. Only the kitchen was left.

The size of a tsunami is measured by the maximum height it reaches above sea level, or what scientists call its run-up.

The 1947 Gisborne event was one of the largest tsunamis recorded in New Zealand. Run-up heights of about 10 metres were recorded.

Not only was this a colossal tsunami, it was a strange one.

The waves were caused by what’s called a tsunami earthquake. This type of shake involves a rupture of the Earth’s crust at speeds lower than typically observed during a regular earthquake.

A lot of energy is still released, but it’s released over a longer period of time. Therefore, the intensity of the ground-shaking and the magnitude of the subsequent tsunami do not match up.

This kind of earthquake is a big problem. When people feel the ground shake, they’re much more likely to flee inland. When they don’t, they may well stay put.

The curious case of the 1947 Gisborne earthquake illustrates the uncertain risks we face in New Zealand. Last week’s trifecta of earthquakes rammed that point home.

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