If you feel a long, strong earthquake and you’re near one of New Zealand’s big lakes, head for higher ground.
And if you think that sounds like advice for avoiding a tsunami, it is.
Tsunamis that put towns at risk are possible in lakes like Taupō, Rotorua, Tekapo, Wakatipu, Wanaka and, potentially, many more.
Niwa marine geologist Dr Joshu Mountjoy has studied Lake Tekapo – where a tsunami wave caused by a landslide could be bigger than five metres – and says people need to be aware of the risk.
“If you’re on [a] lake shore and you feel a long and strong earthquake, exactly as the coastal procedure, use that warning to get away from the lake shore,” he said.
“Most people go ‘tsunami in a lake – really?’ It’s not something that people think about … but they can be significant and we need to understand the hazard in New Zealand.”
The towns on those big lakes were vulnerable because they were low-lying, had a lot of people in them, and some had hydro power generation connected to them.
How bad can a lake tsunami be?
If you’re like those people Mountjoy has referred to and doubt the veracity of a lake tsunami occurring in NZ, there are plenty of examples of how damaging one could be.
In Italy in 1963, a “vast chunk of the mountainside, the size of a small town and 400m deep, sheared off” above a lake created by the construction of a hydroelectricity dam, according to the BBC.
It plunged into the lake, causing a 200m high tsunami that smashed into the town below the dam, killing almost 2000 people – 80 per cent of the population.
The wall of water pushed a pocket of air before it with such force that most of the victims were found with their clothes blown off.
It’s not quite in a lake but the biggest recorded tsunami in the world was instead in a bay – landslides can have similar effects in bays and fjords – in Lituya Bay, Alaska in 1958.
“That didn’t kill anyone but it was 500m high, which it totally extreme, and it kind of went over a big hill and stripped all the trees off it – incomprehensibly large,” Mountjoy said.
“There have been other examples. There’s a good one in Canada called Chehalis Lake in the ’90s, I think. Nobody was there, it was in the middle of winter and they found the evidence after and again it was a rock fall into the lake and it [the tsunami it caused] stripped all the trees off around the margin of the lake up to a large level and … it shows the scale of things that can happen.”
Chehalis Lake has seen more than one tsunami. Another in 2007 had a similar effect.
The New Zealand setting
Back at home, lake tsunamis could also be caused by earthquakes on faults or volcanic eruption.
“Between those three, we cover all of the major lakes in New Zealand, really,” Mountjoy said.
“The fiords are another thing and the hazard from rock avalanches into Milford Sound especially has been clearly identified as a hazard.
“There was an earthquake in 2007 in Fiordland in George Sound that triggered a rock fall that did create a tsunami that destroyed a Doc (Department of Conservation) jetty … and ran up by a few metres on the opposing bank.”
Other than that, there haven’t been examples of it happening – which makes it hard to assess how damaging a lake tsunami would be.
Knowing the risk level to towns would require lake tsunamis to be happening on a human time scale, Mountjoy said.
“We really need to understand how often they’re happening,” the Niwa marine geologist said.
“What we can see in Tekapō is these things are happening every thousand-ish years or maybe more regularly.”
“Another aspect of this we haven’t had a chance to study is the paleotsunami …when you have a big tsunami you have big deposits related to that and nobody’s really looked for those around lakes and it’s going to be challenging … but that’s something that should be done in the future.”
Tsunamis would behave differently depending on the lake and the way the wave was generated.
Without specific modelling on each lake it would be hard to tell how big waves could get, Mountjoy said.
Understanding previous landslides into lakes could indicate what was possible in the future.
“That was out approach in Tekapo – a pilot study for how we can approach this in other lakes in New Zealand. At the moment we’ve completed that and we’re looking for a way to carry on the work.”
Researchers hoped that the study could be used as a basis for similar work on lakes like Wakatipu, Wanaka and Taupō.
Despite all that, Mountjoy said no-one should be losing any sleep over the issue.
For now, Mountjoy is hoping to alert people to the risk.
“Our main driver at the moment is just to make people aware that it is something that could happen,” he said.
“So if there’s a big earthquake, you know, this is something you should consider.”
The Tekapo study
A tsunami wave on Lake Tekapo could damage infrastructure and cause flooding in an area heavily used for tourism.
So Mountjoy and other researchers mapped the lake floor.
There were large rivers moving sediment into the lake and creating huge deposits known as deltas, he said.
“Both these deltas, and the steep sides of both the lake bed and the mountains next to it, are prone to collapsing.”
Researches modelled tsunami from slope failures within and into the lake, based on the evidence for the size and location of past landslides.
Results showed that waves could exceed 5m at many locations around the lake shoreline.
“A really interesting aspect of the results from this study is that there is clear evidence that many landslides have occurred at the same time and we believe this has happened during large earthquakes,” Mountjoy said.
“We cannot be sure if this is an Alpine Fault earthquake or something local, but it does mean there may be a natural warning if such an event happens again.”
The study has just been published by the Geological Society of London.
This article was originally published by Stuff.co.nz. Read the original here.
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