Most people think of tsunamis as coming from the ocean, but it turns out they're just as capable of coming from lakes - and New Zealanders are at risk, new research suggests.
After studying Lake Tekapo in the South Island, NIWA marine biologist Joshu Mountjoy discovered it could generate tsunamis more than five metres high.
The research, published by the Geological Society of London, shows other large lakes in New Zealand - including Lake Wakatipu and Lake Wanaka in the South Island, and Lake Taupo in the North Island - are also at risk.
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"Most people think of tsunamis as ocean-based, but they are just as capable of happening in lakes, although little work has been done on this worldwide," says Dr Mountjoy.
"Given New Zealand's geological makeup, this is something New Zealand communities should be more aware of."
Dr Mountjoy said Lake Tekapo was formed by large glaciers that flowed down into the valley until 15,000 years ago. Sediment slowly built up where the lake is today, carving into the valley walls. In recent times, the lake has been dammed and used to generate hydropower.
But over time sediment from large rivers flowing into the lake have created "huge deposits" known as deltas. The combination of these deltas, the steep sides of the lake and the mountains next to it "are prone to collapsing", Dr Mountjoy said.
Researchers were able to model potential tsunamis based on evidence of previous landslides. They found that waves could exceed five metres at many locations around Lake Tekapo's shoreline.
New Zealand is also prone to earthquakes, which could create lake tsunamis. In 2007 an earthquake in Fiordland caused a rock slide that created a tsunami wave, said Dr Mountjoy. The tsunami is said to have destroyed a Department of Conservation jetty.
The Tekapo community have been informed about the risks of lake tsunamis, Dr Mountjoy says, including the damage that could be inflicted on infrastructure from flooding.