Lake Tekapo landslide could cause 25m tsunami

CHRIS HYDE

Last updated 17:39, October 12 2015

SUPPLIED/Fraser Gunn

A landslide at Mt John is one of the ways a tsunami could be created in Lake Tekapo, a new GNS Science study says.

Mass landslides at Mt John have the potential to create a 25-metre tsunami in Lake Tekapo.

That is the worst case scenario identified in a new Environment Canterbury-commissioned GNS Science report which will be presented to the Tekapo Community Board on Tuesday night.

The Tsunami and Seiche Hazard Scoping Study focuses on the possibility of seismic activity creating tsunamis in five Mackenzie District lakes.

It concludes that landslides from the steep slopes around Tekapo, as well as the more sparsely populated Lake Pukaki and Lake Ohau, have the potential to create tsunamis as low as 0.5m and as high as 25m.

Flatter terrain around Lake Ruataniwha and Lake Alexandrina means there is little or no chance of landslides, but waves of at least 2 to 3m are likely if the Ostler Fault at Ruataniwha and the Irishman Creek Fault at Alexandrina rupture.

The report said a wave with a height of 25m would have "significant impacts" but whether it was even possible was subject to "considerable uncertainty".

"Further field based study would be required to improve understanding of the stability of the steep slopes surrounding the lakes," the report said.

There are four areas around Lake Tekapo that are at risk of sliding into the lake – steep slopes on the northwestern lake shore below Mistake Peak, the western face of Mt Hay, a rockface at the mouth of Boundary Stream and the eastern face of Mt John, it said.

Other tsunami threats to Tekapo included a tsunami at least 2m tall being created by an earthquake that extended from the Coal River fault or the Irishman Creek fault into the lake itself.

Seiches (shockwaves caused by earthquakes on faults outside the lake area such as the Alpine Fault) and landslides under the surface of the lakes were also threats but not enough data was available to predict a possible wave height.

The only known example of a landslide generating a tsunami in Lake Tekapo occurred in 1992.

A large rock avalanche fell from Mt Fletcher and travelled down the Maud Glacier valley and into the small glacier lake.

Eight million cubic litres of water were displaced generating a flood wave that travelled 45 kilometres down the Godley River to Lake Tekapo. The lake rose by 98 millimetres.

Tekapo Community Board chairman Peter Munro said he recalled the 1992 tsunami but it was not generally something that people living in the town thought about.

Munro said he did not think tsunamis were a major threat to the Tekapo township but they were in the process of putting together a civil defence plan and it was good to be informed about all hazards.

"Certainly a whole lot of events would have to line up for it to cause serious problems," Munro said.

"The lake would probably have to be at its highest point and it's been down about five or six metres for a fair bit. It's also pretty hard to imagine a big landslide at Mt John. It was created by glaciation so the reason it still exists is because it's pretty solid rock."

TSUNAMI THREATS

Lake Tekapo: Rupture of known faults under the lake (at least 3m high), landslides from four areas (up to 25m high), seiches (unknown), underwater landslides (unknown).

Lake Pukaki: Landslides from one area (up to 25m high), seiches (unknown).

Lake Ohau: Landslides from three areas (up to 25m high), seiches (unknown) underwater landslides (unknown).

Lake Ruataniwha: Rupture of known fault under lake (at least 3m high), seiches (unknown), underwater landslides, (unknown).

Lake Alexandrina: Rupture of known fault under lake (at least 2m high), seiches (unknown), underwater landslides (unknown).

http://www.stuff.co.nz/timaru-herald/news/72940566/lake-tekapo-landslide-could-cause-25m-tsunami