Undersea tsunami threat

SEAFLOOR SLIP: The steep 300-metre-deep walls of Nicholson Canyon show signs of a massive tsunami-causing rockslide.

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Evidence of a massive undersea landslide has been found less than 15 kilometres from Wellington airport, raising fears another slip could trigger a devastating tsunami.

Niwa scientists found signs of a huge submarine rockslide in the Nicholson Canyon, which lies in Cook Strait, south of Wellington Heads.

Using underwater scanning equipment, they found a giant scar on one of the canyon's 300-metre- high walls.

Niwa ocean geologist Joshu Mountjoy said the amount of rock that had collapsed into the canyon was huge - a billion cubic metres.

That's four times the size of Wellington's Mt Victoria.

There was no way of telling how old the landslide was, as the debris had now washed away.

Evidence of smaller rockslides was also found in the nearby Cook Strait canyon.

An underwater landslide could cause a tsunami, depending how fast the slip moves and the ocean depth.

"It's just the same as moving a large piece of wood under the water in a bath."

Mr Mountjoy said scientists would try to work out the risk that could be posed by the slip zone.

"Obviously Wellington airport is only a few metres above sea level, so it's vulnerable, but I couldn't say what the damage would be."

Greater Wellington regional council hazard analyst Iain Dawe said it was thought a tsunami triggered by the 1855 Wairarapa earthquake might have been helped along by an underwater landslide.

That tsunami sent waves between Lyall Bay and Evans Bay.

He said submarine landslides tended to cause more localised tsunamis than those sparked by earthquakes.

A tsunami from the Nicholson Canyon would hit within Wellington "within minutes", he said.

"It's potentially one of the most devastating tsunami events you can have."

- © Fairfax NZ News