Subduction zone mystery: do mountains beneath the sea fuel silent earthquakes?

Scientists lower into the ocean an electromagnetic instrument that's enabled them to peer into natural processes unfolding within New Zealand's Hikurangi Subduction Zone. Photo / GNS Science

State-of-the-art technology is helping scientists unravel the secrets of silent, slow-burning earthquakes that play out within New Zealand's largest geological hazard.

What are called slow-slip earthquakes – a phenomenon observed only in the last two decades - can last from days to years and produce up to tens of centimetres of displacements along faults.

But because they happened too slowly to be picked up by seismometers – or to be felt by humans – they could only be recorded using special GPS equipment measuring the slow movement of land.

In some cases, they've been suspected to have triggered massive earthquakes and tsunamis, but, in nearly all instances, these episodes unfold with little major consequence.

In the past few weeks, scientists happened to detect two separate events near Porangahau and Gisborne, both of which were linked to flurries of local quakes, some reaching magnitude 4.2.

Over time, scientists have come to understand these events as normal, relatively regular occurrences within the Hikurangi Subduction Zone – the largely-offshore margin where the Pacific Plate dives, or subducts, westward beneath the North Island.

Specifically, they tended to happen within areas where the subduction zone was transitioning from being "stuck" beneath the southern North Island, to an area where the subduction zone was "creeping" further north, around Gisborne and Hawke's Bay.

New Zealand slow-slip quakes play out in an area where the Hikurangi Subduction Zone 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

Just what causes them has remained less clear. But, in a just-published study, US and Kiwi researchers have pointed to one potential driver.

That was fluid stored deep within large mountains beneath the sea, and being fed down into the subduction zone, where it could prime the environment for slow-slip quakes.

In their study, the researchers – Dr Christine Chesley and Associate Professor Kerry Key of Columbia University, Dr Samer Naif of the Georgia Institute of Technology and Dr Dan Bassett of GNS Science – drew on a new tool that allowed them to explore the complex processes at play.

Called marine electromagnetic methods, the technology essentially enabled them to take an MRI scan of the Earth at different locations of the seafloor off the coast of Gisborne.

That meant they could analyse a specific natural property, electrical conductivity, at depths they'd otherwise not be able to access.

"Electrical conductivity tells us how well a rock or other material can transmit an electric current," the researchers explained in a joint statement.

"Because seawater is conductive, electromagnetic methods are highly sensitive to the presence of seawater trapped in the pore spaces of rocks."

Natural variations in Earth's magnetic field provided one source for this method, and the team towed an instrument that could generate an electromagnetic source just above the seafloor.

"Other instruments that we place on the seafloor allow us to record how these electromagnetic waves travel through the underlying rock of the seafloor," they said.

"With these data, we are able to create a geophysical picture of the material beneath the seafloor."

They soon discovered that local seamounts could store large amounts of water, which was transported back down into the Earth at subduction zones.

It was already generally thought fluids could play a role in modulating different types of earthquakes, and in this particular part of the subduction zone, scientists had happened to have documented many slow-slip events.

"It turns out there are a number of large seamounts on the seafloor in this region, so part of the major finding of our paper was that these seamounts might be locally supplying the subduction zone with water that could lead to conditions that favour silent earthquakes."

Elsewhere in their research, the scientists used their imaging method to paint an electrical picture of what was going on a closer to shore in the "forearc", where the subduction zone began.

"In that section of our study area, we see a few very conductive features above a seamount that is subducting as we speak," they said.

"This suggests that the subducting seamount that is descending back into the Earth has damaged the plate above it.

"The high conductivity in the broken pockets of rock indicate that fluids were transferred into these damage zones, possibly from the subducting seamount or from subducting sediments."

Source / GeoNet

One startling finding was that a class of tiny, repeating earthquakes were linked with recent silent earthquake clusters around the damage zone.

"That realisation seems to point to a link, if still an enigmatic one, between silent earthquakes and subducting seamounts, which will undoubtedly be a relevant consideration in natural hazard assessments for coastal regions."

Another mystery surrounding slow-slip quakes was why some happened with almost predictable frequency.

Near Porangahau, for instance, scientists had observed them playing out every five years – and when one kicked off in May, an array of instruments had already been set up to capture it.

"Probably part of the answer is that the slow slip area is being loaded steadily by motion between the tectonic plates, which is pretty constant," GNS scientist Dr Laura Wallace told the Herald last month.

"So there may be some kind of threshold - dependent on the strength of the fault - that is reached every five years that causes them to occur so regularly."

It was also possible that the build-up of fluids could be influencing the timing – an idea GNS Science seismologist Dr Emily Warren-Smith had already been exploring.