Scientists aboard Niwa research vessel Tangaroa handle an ocean bottom seismometer deployed near subantarctic Macquarie Island. Photo / Joshu Mountjoy
Scientists aboard Niwa research vessel Tangaroa handle an ocean bottom seismometer deployed near subantarctic Macquarie Island. Photo / Joshu Mountjoy
Scientists have ventured to one of the wildest corners of the planet in the hope of unlocking secrets about a process known to trigger enormous underwater earthquakes and tsunamis.
Niwa marine geoscientist Dr Joshu Mountjoy recently returned from an instrument recovery mission to Macquarie Island, roughly halfway between New Zealand and Antarctica, where an international team had deployed 27 seismometers on the rugged ocean floor in 2020.
Essentially forming a giant telescope pointing to the Earth's centre, the instruments had been continuously recording ground motion over more than a year, picking up everything from near and distant earthquakes to storms and whales.
Mountjoy said this region, lying some 1100km from Invercargill, was renowned for its dramatic climate and undersea geology.
Macquarie Island lies more than 1100km south of New Zealand. Image / Google
The Macquarie Ridge, hidden kilometres below the ocean surface, featured underwater mountain ranges with slopes steeper than those in the Himalayas.
Above the water, ships had to contend with gale-strength winds and huge swells that were typical of the Southern Ocean's "Furious Fifties" latitudes, and had forced previous expeditions to seek shelter in the lee of the island itself.
Mountjoy said the latest voyage, aboard Niwa's RV Tangaroa, met mostly favourable weather before encountering a severe storm.
"We had waves as big as I'd ever seen ... there were 10m swells and 70-knot (129km/h) winds, which was pretty hairy."
Amid the wild Southern Ocean, the RV Tangaroa encountered high seas - with swells in one storm reaching 10m high. Photo / Joshu Mountjoy
But for geologists like Mountjoy, there was a compelling reason to travel there.
Along with being the only place on the planet where rocks from the Earth's mantle were being actively exposed above sea level, the submarine ridge marked the boundary between the Australian and Pacific tectonic plates.
"It's near the southern end of the plate boundary that runs through New Zealand and carries on further – and it's a place where it's hypothesised that we might be able to see the very beginnings of subduction."
Subduction occurs where one plate dives or is "subducted" beneath another – sometimes resulting in tensions and faulting within the Earth's crust that kicked off large earthquakes and eruptions.
Recent subduction zone cataclysms include the 9.1 quake that caused Japan's Tohoku disaster in March 2011, killing nearly 20,000 people, and the similar-sized "megathrust" Indian Ocean quake behind 2004's Boxing Day tsunami, claiming an estimated 227,898 lives across 14 countries.
While it's not clear whether New Zealand's Hikurangi Subduction Zone could unleash earthquakes of such scale, scientists say ruptures measuring above 8.0 are certainly plausible and recently ran a hazard planning scenario based on an 8.9 event.
"So, while we have subduction of the Pacific plate under the North Island of New Zealand, down at Macquarie, there's no subduction taking place, but there are indications it might be developing," he said.
"Ultimately, it could become a new subduction zone in the future, which means there's an opportunity to get new insights into how the initial stages of it takes place."
And by better understanding how subduction worked, Mountjoy said, scientists could build a clearer picture of how these devastating quakes occurred.
"One part of it is understanding how subduction systems actually develop over time and whether, in the early stages, they can make these thrust earthquakes," he said.
Macquarie Island lies halfway between New Zealand and Antarctica. Photo / Christo Baars
"Another part is looking at the hazard to Macquarie Island itself, where there's a research base. This is a very tectonically-active area, so it's a great place to understand all of these processes."
Mountjoy is working on this project with an international team led by Australian based scientists.
The project's chief scientist, Professor Hrvoje Tkalčić of the Australian National University, said the origins of subduction was one of the biggest puzzles in plate tectonics.
"Researchers have grappled with this question for decades, probing active and extinct subduction zones around the world for hints, though the picture remains murky."
Meanwhile, in a separate research programme funded by the EQC, more ocean-bottom seismometers will be dropped off along New Zealand's coast to investigate subduction processes here.
Study leader and Victoria University geophysicist Professor Martha Savage said a large portion of the Hikurangi Subduction Zone was "locked" - and in this part of the fault, sea floor seismometers had only been deployed for short periods.
"The GeoNet network does an excellent job of monitoring earthquakes on land. Yet, we are effectively blind to small earthquakes offshore," Savage said.
"The behaviour of these more frequent small earthquakes can tell us more about the larger earthquakes that occur less often. We're expecting to see 10 times more earthquakes than are currently reported."
She said the programme would ultimately improve our understanding of why the locked and slipping sections of the zone behaved differently.
"This is a big hazard for us because if it goes, it's going to go fast. The result would be a large and sudden earthquake that could also trigger a tsunami."
