Ancient waves: Scientists find evidence of massive tsunami in South Island Lake

While such events are rare, they can have devastating consequences: if a tsunami triggered by a landslide hit Milford Sound during the peak tourist season, the powerful waves could kill up to 3,500 people, a new study suggests.

In November 2022, a 5.6 magnitude earthquake was linked to the Lake Taupo unrest TowardsThe storm battered the town’s waterfront, overturning bollards and damaging two boats.

In the latest study, a team of researchers mapped four South Island lakes – Rotoroa, Rotoiti, Brunna and Mapourika – for signs of group movement caused by past underwater landslides.

The scientists used a ship-mounted multibeam echosounder to create high-resolution 3D models of each lake bed.

The results catalogued 86 previously undetected large-scale motions and showed that 16 of them were strong enough to generate tsunamis, suggesting that the intensity of these events may have exceeded initial expectations.

These included the massive collapse of the Lake Rotorua delta, which displaced nearly 50 million cubic metres of sediment.

It was the largest flood ever recorded on New Zealand lakes and dwarfed similar floods recorded in other parts of the world.

The study also showed that river deltas, rather than surrounding mountains, are the most common source of these massive landslides, challenging scientists’ long-held assumptions.

Lead author Katie Hughes, a graduate student at the University of Victoria, said the findings suggest that our famously active tectonic environments come with a heightened danger of lake tsunamis.

“The results also suggest that lake tsunamis have the potential to occur in a range of different lakes across New Zealand.

“Given the high number of tourists to the country’s picturesque lakes and fjords, an in-depth hazard assessment is vital.”

To do this, “we need more quantitative and detailed studies of past lake tsunamis, particularly relating the magnitude and frequency of past lake tsunamis to seismic activity,” said study co-author Professor Sean Fitzsimmons of the University of Otago.

The research is the first part of a wider project led by Victoria University and the University of Otago, with scientists currently reconstructing the timing and magnitude of the tsunami they found.

Another co-author, Victoria University associate professor Jamie Howarth, said the limited number of studies to date may have contributed to lake tsunamis being under-recognized in disaster planning, not to mention low public awareness of the threat.

“We also hope the findings will increase awareness of lake tsunamis in the community, particularly those who spend time around New Zealand’s many beautiful lakes,” Mr Horwath said.

“When you are relaxing on the lake shore and encounter a strong earthquake, you should move away from the shore and go to higher ground.

“The message of ‘No matter how long or strong: get out of the way’ applies to lakes as well as shorelines.”

Jamie Morton is a science and environment reporter. He joins The Herald Founded in 2011, he writes about conservation, climate change, natural disasters, and new technologies.