The Hikurangi subduction zone: what is it and how can we all prepare? / East Coast LAB
The Hikurangi Subduction Zone represents New Zealand’s largest natural threat, with one-in-four odds of generating an 8.0 magnitude event in the next 50 years
In a major new study, GNS Science researchers will use new imaging techniques to build an unprecedented picture of the giant fault zone’s mechanics
The project will also help to compare the system with other subduction zones that have unleashed monster quakes: including those off Japan, Indonesia and Chile.
It’s thought capable of unleashing an earthquake and tsunami large enough to kill tens of thousands and cause nearly $150 billion in damage.
Now, scientists aim to get their clearest picture yet of the complex mechanics of our big-risk Hikurangi Subduction Zone – and what sets it apart from similar systems around the planet.
Running alongside New Zealand’s East Coast, the sprawling fault zone marks the meeting of the Pacific and Australian tectonic plates.
This endless geological scrum creates an enormous amount of pent-up energy. Recent subduction zone cataclysms like the 2011 Tōhoku, Japan, and 2004 Indian Ocean disasters show how it can be suddenly and violently released in “mega-thrust” earthquakes.
It’s a scenario that could happen here, and it’s more likely than we might think. Projections give one-in-four odds of a magnitude-8.0 event occurring in the southern section of the Hikurangi margin within the next 50 years.
The Hikurangi Subduction Zone is where the Pacific Plate dives beneath the Australian Plate on the east coast of the North Island. Images / East Coast Labs
When it comes to understanding the potential drivers of such an event, one of the biggest challenges facing scientists is untangling the striking variability in the giant fault zone’s slip behaviour.
In its northern section, the subduction fault is known for producing slow-slip events – mysterious, slow-motion earthquakes that unfold over days or months and don’t cause the violent ground shaking of traditional quakes.
But further south, the picture looks very different, with the plates appearing to be more strongly locked and building up large amounts of strain – raising the risk of a local mega-thrust event.
Slow-slip events occur 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 Hawke's Bay. Image / GeoNet
This intriguing north-south contrast, and the relatively shallow depth of the plate boundary, made Hikurangi a “globally unique” place to directly test theories about what processes cause these systems to slip, said GNS Science geophysicist Dr Brook Tozer.
In a new study, just awarded a $360,000 Marsden Fund grant, Tozer and colleagues plan to build a rich new picture of what’s happening beneath the seafloor.
“Our study will produce the most detailed images yet of the shallow Hikurangi subduction zone, setting a new benchmark for understanding its physical condition,” he said.
To do it, the researchers will use a powerful imaging technique called Full Waveform Inversion, allowing them to extract finer details from decades of seismic reflection data collected off the East Coast.
“First, we aim to image the physical conditions within the subduction zone and along the subduction fault,” Tozer explained.
“These conditions are believed to influence where different types of fault slip occur, such as slow-slip events or fault locking.”
Secondly, he said, these same conditions were thought critical in determining how a large quake might “unclamp” the subduction zone.
“For instance, variations in physical properties may either inhibit or promote fault slip, control how close to the surface the fault ruptures, and - most importantly for tsunami generation - impact the extent of seafloor displacement.”
By mapping out these variations, the team ultimately hoped to improve the accuracy of tsunami hazard assessments in New Zealand.
Tozer said the findings could also be compared with data from other subduction zones to better understand what made some faults more likely to generate big tsunamis than others.
Phuket's Phi Phi Island, 2004. The Indian Ocean earthquake and tsunami - caused by a subduction zone "megathrust" rupture - caused nearly 230,000 deaths. Photo / Brett Phibbs
For instance, some deep-seated processes appear to have limited the tsunami-making power of a monster 8.1 subduction zone quake off the coast of Chile in 2014 – while others made for a larger-than-expected tsunami in the event of Indonesia’s 9.2 quake in 2004, resulting in 230,000 deaths.
“Megathrust earthquakes are rare events, and investigating their largely offshore structures presents significant challenges, leaving much to uncover about the physical processes driving fault behaviour,” Tozer said.
“Our study, grounded in exploration and discovery, aims to bridge these knowledge gaps and contribute meaningfully to earthquake and tsunami preparedness.”
The project is running alongside two other new Marsden-funded GNS studies exploring New Zealand’s major earthquake potential.
One will combine mātauranga (knowledge) from Māori settlers with cutting-edge modelling to reconstruct 1460AD’s powerful Haowhenua earthquake in Wellington – and simulate what would happen if a similar event struck the city today.
The other will investigate what could happen if a big Hikurangi quake struck simultaneously with another on a major fault like the Wellington Fault.
“Our physics-based earthquake simulations will help understand where and how often such multi-fault earthquakes can occur, and what their ground shaking and tsunami impacts could be,” study co-leader Dr Andrew Howell 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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