New seismic study shows risk of tsunami

Niwa researchers on the water at Lake Wanaka in December 2019, using equipment to map sediments...
Niwa researchers on the water at Lake Wanaka in December 2019, using equipment to map sediments beneath the lake bed. PHOTO: SUPPLIED
Results of a seismic mapping project on the bed of Lake Wanaka have shown an increased risk of a lake tsunami occurring in an earthquake.

Otago Regional Council commissioned Niwa to undertake the mapping in late 2019, after a desk-based review by GNS Science identified a likely fault under the Otago lake earlier that year.

The northwest Cardrona fault was previously thought to have run northeast from the Cardrona Valley, through Albert Town to Hawea.

GNS scientists have now proposed the fault runs northwest past the foot of Mt Alpha, beneath part of the Wanaka township, and out under the lake.

Niwa project leader and marine geologist Dr Joshu Mountjoy said the new data indicated there were features that could be related to faulting.

“We have identified large-scale folding of lake sediments, which suggests some deformation or compression of the ground surface immediately past the lakeshore in Roy’s Bay.

‘‘The data do not unequivocally show there is a major fault here, but the features identified mean it cannot be ruled out,” Dr Mountjoy said.

ORC natural hazards manager Dr Jean-Luc Payan said the research was not conclusive, but the results of the lake bed mapping provided further supporting evidence of an active fault beneath Lake Wanaka.

‘‘This raises the possibility, in the event of an earthquake, that a lake tsunami could be among the hazards faced by the township,’’ Dr Payan said.

It was also an important reminder of the tsunami risk associated with earthquakes was not just in coastal areas but also inland around alpine lakes, he said.

The ORC has planned a range of work over the coming years concerning lake tsunamis in Otago.

The work included modelling a Lake Wanaka tsunami scenario to better understand how such an event could affect the township and to raise public awareness, Dr Payan said.

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