Explanation of slow slip earthquakes associated with New Zealand's Hikurangi Subduction Zone. / GNS Science
Scientists have found a massive reservoir lying just a few kilometres beneath the seafloor off the North Island - and it could help explain mysterious “silent” earthquakes thought to play an influential role in our largest fault zone.
Lying off the North Island’s East Coast, it marks the boundary where the Pacific tectonic plate dives underneath the Australian tectonic plate, producing an enormous amount of pent-up energy.
The “mega-thrust” subduction zone earthquakes behind the 2004 Indian Ocean tsunami - and the catastrophic Tōhoku disaster in Japan seven years later - show how, in extremely rare cases, that energy could be released in the worst possible way.
This scenario also remained a big risk in New Zealand: scientists recently 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 made it ever more urgent to understand just how an event like that could occur.
Generally, scientists believe the make-up of the Earth’s 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.
Along our subduction zone, however, a wide variety of quakes are routinely observed: something thought to largely owe to the effects of fluids on the plate boundary.
While previous studies have indicated a mechanism that “hydrated” the subduction zone’s faults and made them weak, it’s only been in recent times that scientists have been shedding light on how this actually happened.
At the same time, they’ve come to recognise the influential role of “slow slip” earthquakes – capable of shifting faults by tens of centimetres over days, weeks and months, and without ever being felt at the surface.
Slow-slip events have been found to occur in an area where the Hikurangi Subduction Zone - marking the boundary of the Pacific and Australian tectonic plates - is transitioning from being "stuck" beneath the southern North Island, to an area where the subduction zone is "creeping" further north, around Gisborne and Hawkes Bay. Image / GeoNet
Many of these slow-burning quakes were thought to be linked to buried water, but, until now, there was no direct geologic evidence to suggest such a large water reservoir existed along the fault zone.
“We can’t yet see deep enough to know exactly the effect on the fault, but we can see that the amount of water that’s going down here is actually much higher than normal,” said the study’s lead author, Andrew Gase, who carried out the work while at the University of Texas.
Now based at Western Washington University, Gase said there was a need to drill deeper to find where this giant pocket ended, so that scientists could determine whether it influenced pressure around the fault.
The reservoir was found amid an ancient volcanic province formed when a massive plume of lava breached the Earth’s surface in the Pacific Ocean.
This event, which took place some 125 million years ago, was one of the planet’s largest known volcanic eruptions and rumbled on for several million years.
Gase used seismic scans to build a 3D picture of the ancient volcanic plateau in which he saw thick, layered sediments surrounding buried volcanoes.
After analysing drill core samples of the volcanic rock, he and colleagues found that water made up nearly half of its volume.
This map shows the Hikurangi plateau: a remnant of a series of epic volcanic eruptions that began 125 million years ago in the Pacific Ocean. A recent seismic survey – carried out in the area marked by the red rectangle - imaged the plateau as it sinks into New Zealand’s Hikurangi Subduction Zone (red line). Image / Andrew Gase
“Normal ocean crust, once it gets to be about seven or 10 million years old should contain much less water,” he said.
The ocean crust in the seismic scans was 10 times as old - but it had remained much wetter.
Gase speculated that the shallow seas where the eruptions took place eroded some of the volcanoes into a porous, broken-up rock that stored water like an aquifer as it was buried.
Over time, the rock and rock fragments transformed into clay, locking in even more water.
That finding was important because scientists think that underground water pressure could be a key ingredient in creating conditions that release tectonic stress through slow-slip earthquakes.
A seismic image of the Hikurangi plateau reveals details about the Earth’s interior and what it’s made of. The blue-green layer under the yellow line shows water buried within rocks. Researchers suspect the water could be dampening earthquakes at the nearby Hikurangi Subduction Zone. Image / Andrew Gase
This usually occurred when water-rich sediments were buried with a fault, trapping the water underground – but this particular fault contained little of this typical material.
Instead, the researchers suspected the ancient volcanoes and the transformed rocks — now clays — were carrying large volumes of water down as they were swallowed by the fault.
University of Texas Institute for Geophysics director and study co-author Demian Saffer, a study coauthor said the findings suggest that other earthquake faults around the globe could be in similar situations.
“It’s a really clear illustration of the correlation between fluids and the style of tectonic fault movement - including earthquake behaviour,” said Saffer, who served as co-chief scientist on the drilling mission.
“This is something that we’ve hypothesised from lab experiments, and is predicted by some computer simulations, but there are very few clear field experiments to test this at the scale of a tectonic plate.”
The study, published in the journal Science Advances, comes weeks after another study shed new light on how fluids trapped and transported within the fault zone influenced its activity.
Scientists have now 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.
This, it’s ultimately hoped, will lead to new physics-based models able to calculate tsunami hazards for all of New Zealand’s local and regional earthquakes.
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.
