Preliminary results from the first comprehensive survey of the Cook Strait Canyon seabed have begun to reveal tantalising scientific secrets about New Zealand's largest underwater canyon.
Last month marine geology and biology scientists onboard NIWA's research vessel Tangaroa spent a week collecting sediment cores, rock samples and video footage and photographs of the seafloor in the canyon system.
One of the largest underwater canyon systems in the world, the Cook Strait Canyon starts just 10 kilometres off the Wellington coast and plunges to 3 kilometre depths south of Cape Palliser. Initial bathymetric mapping by NIWA showed that the canyon is scarred by more than 150 submarine landslides and many active faults. But, until now, very little has been known about what caused those landslides and the hazards they may pose, or even what lives there.
Three landslides in the upper canyon were sampled on the latest voyage. Initial results show that one of these landslides in particular occurred recently, and may have been caused by the magnitude 8.3 Wairarapa earthquake in 1855. The similarity of this landslide to others in the canyon suggests that many of these occurred during this large earthquake. The Wairarapa earthquake caused widespread landslides on land, uplifted the Wellington waterfront and generated a moderate tsunami.
NIWA Marine Geologist Dr Joshu Mountjoy says "we have long suspected that many of the landslides we see occurred during the 1855 event. We have never had any data to support this so it is really exciting to be getting to the point where we can attribute landslides to this earthquake with certainty."
One of the canyon's three main arms appears to be inactive at the moment and is predominantly filled with mud. However, sediment cores taken from the Nicholson and Cook Strait arms of the system, indicate these canyons are much more active, with repetitive sediment flows occurring. Where these canyons merge at around 1,200 metre depths, towed camera images showed jagged blocks of rock lying on the canyon floor. These may have fallen off the canyon walls, which are more than one kilometre high, or been excavated out of the canyon floor by the current.
"Carbon dating of the sediment cores and rock samples collected will give us more information about how old the sediment is in the canyon floor and the age of the landslides. That helps us assess whether activity in the canyon is caused by earthquakes, tidal currents or other sediment processes," Dr Mountjoy says. "Some landslides in the system are huge, ranging in volume from 2.5 million cubic metres to approximately ten cubic kilometres size, and they certainly have the potential to generate hazardous tsunami. We are now getting a clearer idea of how often the landslides might occur and why.
"Most importantly we will be able to feed this new data on the previous landslides into work NIWA and GNS Science are doing together to quantify the tsunami generation potential of future landslides in the canyon system, ensuring New Zealand is better prepared to respond to natural disasters in the future. "
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