Wanaka focus of landslip tsunami study

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A UK-based student is studying the potential for a landslip-generated tsunami in Lake Wanaka as part of his PhD.

Ryan Dick, of Newcastle University, will travel to Wanaka in the coming months to survey the lake bed for more information on past landslips and to determine whether any of these would have generated a tsunami.

The potential for a landslip-generated tsunami in the lake was highlighted in 2016 by Wanaka-based engineering geologist Graeme Halliday, who presented a paper at an international conference in Italy.

Mr Halliday’s report found that the high probability of a major M8 earthquake as a result of the Alpine Fault was likely to trigger many landslips around the Southern Alps, including the shoreline of Lake Wanaka.

Speaking via email from the UK, Mr Dick said although the Otago Regional Council and GNS Science had databases that included landslips around the lake, these contained known historical and recent landslips, or landslips that had identifiable on-land deposits.

“It is difficult to understand the potential risk without the data from the lake bed that could preserve the evidence of past landslides for longer periods of time – lakes are good at holding archives of past events.

“Until we can determine how many landslides are in Wanaka, and, if they likely triggered tsunamis, it is hard to determine the exact risk posed by an Alpine Fault rupture.”

His studies would involve attaching a sonar system to a small boat, which would allow him to map the surface of the lake bed to determine any deposits on the bed surface.

“I’m focusing mostly on large bedrock landslides that originated on the hill slopes around Wanaka and flowed into the lake, but I’m also interested in finding any landslides that occurred fully underwater.”

One of the challenges was the sheer size of the lake, he said.

By using satellite and aerial imagery, he had identified target areas where the slopes seemed to exhibit evidence of past landslips, but his survey area might have to be adapted.

Establishing exactly when each landslip occurred was another potential challenge, he said.

“If we had a better understanding of when the landslides might have occurred, we could then try to link to possible triggers like past Alpine Fault ruptures, as there is a good record of past ruptures.

“Understanding whether these landslides generated tsunamis is another challenge.

“If possible, I would like to try to look for tsunami deposits on shore as that would give clues as to the run-up of any tsunami that occurred, but preservation of tsunami deposits can be poor.”

He will be collaborating with Prof Tim Davies, of the University of Canterbury, along with Geosolve’s Graeme Halliday.

Globally, there were a lot of studies that would help him with his work, including the National Institute of Water and Atmospheric Research’s recent survey of Lake Tekapo, he said.

“All these different works emphasise the importance of this research so we have a good understanding of where these events might happen again.

“The important part is data sharing. My data and thesis will become available to the NZ authorities and research organisations and can hopefully feed into ongoing, collaborative risk-reduction strategies.”