• The giant plate boundary that is the Hikurangi Subduction Zone has the potential to cause magnitude-8.0 “megathrust” earthquakes and tsunamis that could cause tens of thousands of casualties in New Zealand.
• Scientists have come to understand how tectonic energy built up within the zone can also be quietly dissipated through months-long “slow-slip” earthquakes that are never felt at the surface.
• Now, scientists have used seismic waves sent from earthquakes as far away as Mexico and Japan, much like an ultrasound scan, to image our subduction zone and back up a central theory about how this natural pressure-release valve works.

By Jamie Morton

Scientists have used signals sent from distant overseas earthquakes, thousands of kilometres from New Zealand, to shed crucial new light on the mysterious inner workings of our largest natural hazard.

That’s the sprawling system that is the Hikurangi Subduction Zone, marking the boundary where the Pacific plate plunges under the Australian plate, beneath the North Island.

As seen in the Indian Ocean in 2004, and in Japan in 2011, subduction zones are known to produce some of the largest earthquakes on the planet: “megathrust” events that can quickly send tsunamis toward the shore.

Scientists recently calculated a one-in-four chance of such a rupture – measuring greater than 8.0 – striking beneath the North Island within the next 50 years, posing the potential for tens of thousands of casualties.

These mammoth quakes came with the sudden, violent release of enormous amounts of energy pent up by tectonic plates colliding in an endless geological scrum.

But scientists have come to find how it can be discharged in gentler ways: namely “slow-slip” earthquakes that can gradually displace faults over days to months, without ever being felt at the surface.

These enigmatic events frequently unfold off the North Island’s east coast, but also beneath the Manawatū region, where they can roll on for up to two years at a time.

Over the past few years, studies have been telling us more about what drives these deep-seated cycles, which have sometimes been linked to swarms of local quakes in places like Pōrangahau.

One recent paper described how water trapped within the subduction zone’s rocks helped hydrate and weaken its faults, meaning piled-up energy could be quietly released through regular slow-slips.

It also hypothesised those buried fluids were being tightly sealed by a layer of impermeable rock at the top of the diving Pacific tectonic plate.

As fluid pressure built up below the seal, it was eventually released during slow-slip quakes by temporarily fracturing rocks, allowing the water to escape upwards.

Within a few months, the seal rocks healed and the cycle restarted: a process that could explain why slow-slip quakes played out in such predictable patterns.

Now, another groundbreaking study has offered some of the first physical evidence to support this release-valve theory.

And remarkably, it’s come with scientists using seismic waves from earthquakes as far as Mexico and Japan – and picked up GeoNet seismograph stations in Manawatū – to image the geological environment beneath.

Much like an ultrasound scan, the waves illuminated patterns in the Earth’s crust, with telltale signals linked to high-fluid pressures changing in step with observed local slow-slip events.

“While we had expected to see some connection, it was exciting to detect it from our data,” said the study’s lead author, Dr Pasan Herath of GNS Science.

The work, also confirmed the presence of that layer of sealing rock that had been hypothesised to build up fluid pressure – but Herath said there were still many more questions to answer.

While scientists now had a clearer picture of this natural pressure-valving, there was still a big need to understand how the process influenced what happened in the more brittle, “locked” part of the fault zone – where deadly quakes were most likely to be triggered.

One of the world’s leading geophysicists working on the subduction zone, Dr Laura Wallace of Germany’s Geomar, nevertheless said the new findings marked a significant step in learning more about Hikurangi’s megathrust potential.

“This is an important result, as it may also be possible that such spatial and temporal changes in water distribution within subduction zones could influence the timing of damaging, seismic earthquakes,” she said.

“This highlights the great value of routinely tracking time-varying changes in the properties of major plate boundaries like the Hikurangi Subduction Zone, as it provides an important tool for monitoring such faults in the lead-up to the next big earthquake.”

Earlier this year, scientists warned a quick-fire Hikurangi tsunami could kill more than 22,000 and injure nearly 26,000 – even assuming three-quarters of people could evacuate in time.

In the event of a sustained, strong earthquake with potential to cause a tsunami, people in vulnerable coastal areas were urged to heed the “long, strong, get gone” message – and move immediately to the nearest point of high ground, or as far inland as possible.

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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