Only small quake needed to trigger landslide, tsunami

Monday, 19 June 2017

More than half of Lake Tekapo's 83 square kilometre lake floor is covered in landslide deposits, some spreading more than 1km across the lake bed.

A tsunami like the one caused by a small quake in Greenland could happen in New Zealand but the level of risk is not yet known.

Four people are reportedly feared dead in Greenland after a magnitude-4 earthquake off the Arctic island's west coast triggered a tsunami that flooded a village on Saturday.

Experts said the quake likely triggered a landslide into the sea, resulting in the tsunami and flooding.

New Zealand frequently experiences earthquakes of M4 or stronger. Since the start of June, there have been five quakes between M4 and M4.2.

* More than half of Tekapo lake bed covered in large landslides

* Lake Tekapo landslide could cause 25m tsunami**

Niwa marine geologist Joshu Mountjoy said this was not a rare occurrence in places like Greenland because of the 'dynamic landscape'. Eroding glaciers and weak material could come away during a relatively small quake and cause a landslide into the sea, or a slide on the seafloor.

'It's a very small earthquake to trigger a landslide. Something must have been ready to go,' Mountjoy said.

A similar event happened in Greenland in 2001, leading to the abandonment of a local village.

And in 1958, Alaska's largest tsunami was caused by a rockslide of 30 million cubic metres, following a M7.8 quake.

Mountjoy said similar events had led to tsunamis in New Zealand in the past and could continue to do so, especially in areas where mountains rose up from small bodies of water.

In 2003, a M7.2 quake caused a landslide and subsequent tsunami in Fiordland.

If the land was loose, like in Greenland, the earthquake did not need to be particularly large to lead to a landslide and possible tsunami.

Mountjoy said Lake Wanaka, Lake Wakatipu, Lake Tekapo and Fiordland were all at-risk areas.

Sub-marine landslides in underwater canyons could also lead to tsunamis, he said.

There was evidence of this generating tsunami waves of up to five metres in the Cook Strait every 1000-5000 years.

'So it's pretty infrequent but that would be quite significant for the south coast.'

Mountjoy and other New Zealand scientists were currently working to better understand where was at-risk and the magnitude of the hazard.

Work was currently underway looking at landslides on the bed of Lake Tekapo.

So far, data had revealed an unexpectedly dynamic lake bed. More than half of the 83 square kilometre lake floor was covered in landslide deposits, some spreading more than 1km across the lake bed.

Mountjoy said significant tsunamis were rare in New Zealand but that didn't mean there wasn't a risk.

The wave generated by the Kaikoura quake was the largest in New Zealand in 50 years.

It was important to create methodology that allowed then to assess the hazards across the country and better understand the level of risk they posed, he said.