Why there is a major tsunami risk to the east coast of New Zealand.
A new study highlights the tsunami threat facing Southland, where a nearby earthquake zone is thought capable of unleashing 12m-high waves
The risk faces communities including Invercargill, Bluff and Riverton - but also major transport hubs like Southland’s main port and airport
Scientists see an urgent need to better understand the hazard, and also ensure vulnerable residents are aware and prepared
Tsunami waves as high as four-storey buildings could be suddenly sent toward a part of New Zealand where many of us might least expect them.
Scientists behind a just-published study say there’s an urgent need to better understand Southland’s tsunami hazard, which sits behind only that of the Chatham Islands, the North Island’s east coast and northeastern Northland.
They report that, over an average “return period” of 2500 years, much of the region’s southern coast sits exposed to tsunamis 8m to 12m high - and waves of 4m to 8m over 500-year periods.
That put at risk communities including Invercargill, Bluff and Riverton, along with major transport hubs like Southland’s main port and airport.
The study’s lead author, Dr Caroline Orchiston of the University of Otago, said much of that threat came from the Puysegur Subduction Zone, a tectonic plate boundary stretching hundreds of kilometres into the Southern Ocean.
Considered one of the world’s youngest and smallest subduction zones, a major rupture within it could still bring “damaging water levels and speeds”, she said.
Compared with the 14 hours of warning Invercargill might get from a tsunami-generating earthquake off the west coast of Peru, the timeframe was much shorter for an Puysegur scenario.
Two large earthquakes measuring 5.6 and 6.3 struck on August 4, 2019, in a plate boundary area called the Puysegur Subduction Zone. Scientists think it’s capable of generating tsunamis measuring up to 12m high. Image / GeoNet
The region’s seismic potential has already been shown by 2009′s 7.8 Dusky Sound earthquake – powerful enough to yank the South Island’s southwestern tip 30cm closer to Australia - and another 7.2 event that struck in the Puysegur Trench five years earlier.
“While tsunamis from these earthquakes were small, larger earthquakes triggering bigger tsunamis are a realistic scenario for this part of the plate boundary.”
Studying the hazard wasn’t simple.
“The Fiordland region is famous for its remote and steep coastlines, so whether by sea or by air, the area is expensive to undertake science in,” co-author Dr Ursula Cochran said.
“But there are some good opportunities to improve understanding of the region’s vulnerability.”
Past evidence of giant events along our two largest geohazards – the Hikurangi Subduction Zone and the Alpine Fault – had helped prompt Kiwis to prepare for future ones.
“We would like to see this kind of awareness and preparedness developed for the Southland region,” Orchiston said.
That meant detailed new inundation models and evacuation plans, she said, but also a general understanding among Southlanders.
“They should know to respond to natural warning signs - long or strong earthquake shaking - by evacuating quickly inland or uphill as soon as the ground stops shaking.”
Emergency Management Southland group controller Lucy Hicks said the Southland Civil Defence Emergency Management Group Plan recognised tsunami as a high risk natural hazard for Southland.
“This paper will further inform Southand’s response planning and feed into the ongoing reviews of our plan.”
Hicks said Invercargill did not have tsunami warning sirens and so people needed to know what to do.
“Damaging tsunamis don’t happen very often, but when they do, we don’t want people waiting for official evacuation advice after an earthquake. The life-saving message is, ‘if it’s long or strong, get gone’.”
Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.
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