For scientists trying to understand the complex mechanics of New Zealand’s largest natural hazard, it’s perhaps the biggest question of all to answer.
What is it that drives the “slow-slip” earthquakes that we never feel at the surface, yet which appear to cast so much influence on activity at our plate boundary?
Now, an international study has taken us much closer to solving the enigma of these deep-seated, slow-motion processes, which often act as pressure-release valves in a fault zone capable of unleashing monster earthquakes and tsunamis.
If we drained the ocean, this margin, called the Hikuarangi Subduction Zone, would appear as a vast mountain range rising up from the sea floor off the North Island’s East Coast.
On a map, it appears as a long line curving from waters well north of the East Cape to the northeastern South Island, beneath which the Pacific plate dives – or subducts – beneath the Australian plate.
The constant mashing together of these two vast chunks of the planet’s crust produces an enormous amount of pent-up energy that must be somehow released.
The “mega-thrust” subduction zone earthquakes behind the 2004 Indian Ocean tsunami - and the catastrophic Tōhoku disaster in Japan seven years later - show how this can happen in the worst possible way.
Along our subduction zone, scientists have estimated a 26 per cent chance of an event with a magnitude of 8.0 or larger striking beneath the lower North Island within the next 50 years.
That’s underscored the importance of a major research focus on the role of slow-slip earthquakes, which unfold along the boundary silently, yet pack the power to shift faults by tens of centimetres over days, weeks or months.
They’re known to occur at shallow depths off the East Coast, but also at deeper levels off Manawatū and Kāpiti regions, where one ongoing event recently released pent-up energy equivalent to a 7.0 quake.
The new study, published in the journal Nature GeoScience, sheds fresh light on how the right conditions for these mysterious events may be created.
Generally, scientists believe the make-up of the crust is a major factor in how tectonic energy is released, with softer, wetter rocks allowing plates to slip slowly, and drier, brittle rocks storing energy until they fail in violent and deadly mega-quakes.
Study co-author and GNS Science seismologist Dr Bill Fry said that along our subduction zone, scientists routinely observed a wide variety of earthquakes.
“This wealth of diversity can be explained, to a large extent, by the effects of fluids on the plate boundary fault that we call the ‘mega-thrust’,” he said.
“When there is a lot of fluid, the fault tends to be weak and move slowly. When the fault is relatively dry, it tends to be strong and break in normal ‘fast’ earthquakes.
“In New Zealand, we see both phenomena and everything in between.”
The 1947 earthquakes that drove tsunamis into the coast of Gisborne, for instance, were believed to have been “faster” than creeping ones, but slower than normal quakes.
Previously, scientists had singled out a mechanism that hydrated the subduction zone’s faults and made them weak.
But they still hadn’t pinpointed what kept the fluids in place over periods of tens of thousands of years.
“The big previous challenge was to show where the water was being sourced, as we thought some of it would be lost during every earthquake cycle and would rapidly drain, making the fault stronger with every earthquake,” Fry said.
“That’s not what we see on the Hikurangi.”
There, a 2018 ocean seismic survey identified a potential answer to the riddle in seamounts - huge underwater mountains that stretch from the ocean floor without reaching the surface.
In capturing the first 3D scan of one ever, they also found evidence to suggest that these fluids remain trapped in a trough made as seamounts pass their way through the subduction zone, allowing the fault to be weakened over many earthquake cycles.
Their images showed the Pāpaku Seamount – a long-extinct volcano lying kilometres beneath the sea floor, east of the North Island – colliding with the subduction zone, amid a pattern of stresses, fluids and sediments.
While earlier models suggested sediments were pushed down the subduction zone ahead of the seamount, the scan revealed something different: an enormous sediment trail in Pāpaku’s wake.
In another surprise, the scientists spotted the fading trail of a much larger seamount that had long since sunk beneath New Zealand’s North Island.
The discovery suggested that sinking seamounts drag down enough water-rich sediment to create conditions in the crust suitable for slow-slip earthquakes, at least in New Zealand.
“That older one seems to be very much linked to an uplifted ridge that’s really in the bullseye of where recent slow-slip activity has been,” said the study’s leader, Dr Nathan Bangs of the University of Texas at Austin in the US.
Ultimately, Fry said the findings allowed scientists to better understand “why” slow-slip earthquakes worked.
“It also extends beyond improving our understanding of slow-slip earthquakes to improving our knowledge of tsunami earthquakes and even the way fast, giant earthquakes might pass through or around these weak regions,” he said.
“At the end of the day, we undertake work like this to try to improve our societal resilience to earthquakes and tsunamis.
“This result represents another piece in the puzzle that we can start using in our large-scale earthquake cycle simulations.”
It was possible the discovery had implications for subduction zones elsewhere on the planet.
Bangs said there could be other areas like Cascadia, in the US Pacific Northwest, that had subducting seamounts and a lot of sediment, “but because the subducting crust there typically has less water than Hikurangi, they may be less likely to have the same kind of shallow slow-slip activity”.
Meanwhile, in New Zealand, Fry and colleagues have begun modelling the effects of trapped fluids on mega-thrust earthquakes all the way through to the Kermadec subduction zone, with plans to extend the work to the entire southwest Pacific.
“We believe this overcomes one of the key last hurdles prior to using large computer models to calculate the physics-based probabilistic tsunami hazard for all of our local and regional earthquakes,” Fry said.
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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