New Zealand • Earthquakes • Subduction Zones • New Research
A new high-resolution 3D model of New Zealand’s Hikurangi megathrust reveals a striking contrast beneath the North Island: the southern part of the fault is relatively smooth and strongly locked, while the rougher northern section repeatedly releases stress through slow-slip events. Buried chains of ancient seamounts may help explain why.
Deep beneath and offshore from New Zealand’s North Island, two enormous tectonic plates are locked in a slow-motion collision.
The Pacific Plate is being forced beneath the Australian Plate along the Hikurangi subduction zone, one of the most important plate boundaries in the southwest Pacific.
But this megathrust does not behave the same way everywhere.
Some sections remain locked for long periods, accumulating tectonic stress that can eventually be released in damaging earthquakes. Elsewhere, the fault moves almost silently over days, weeks or months in events so gradual that people never feel them.
Now, a new three-dimensional reconstruction of the Hikurangi plate boundary suggests that part of the answer may be written directly into the shape of the fault itself.
What Is the Hikurangi Megathrust?
The Hikurangi margin runs offshore along the eastern side of New Zealand’s North Island. Here, the Pacific Plate subducts westward beneath the Australian Plate as part of the immense tectonic system surrounding the Pacific Ring of Fire.
Plate convergence varies considerably along the margin, from roughly 20 to 60 millimeters per year from south to north.
This makes Hikurangi particularly complex.
The plate boundary hosts ordinary earthquakes, tectonic tremor, repeating earthquakes and slow-slip events, giving scientists a rare opportunity to study how faults move across the spectrum from almost silent deformation to sudden seismic rupture.
It is also one of New Zealand’s most important earthquake and tsunami sources.
That places the Hikurangi margin firmly within the broader network of earthquake systems that shape the country.
Scientists Built a New 3D Model of the Fault
The new study introduces HSIM 1.0, a three-dimensional model of approximately 700 kilometers of the Hikurangi subduction interface.
Rather than relying on one type of observation, the researchers combined multiple geological and geophysical datasets.
These included:
- 54 seismic-reflection profiles
- a three-dimensional seismic-reflection volume
- seismic tomography
- bathymetric measurements
- relocated earthquake hypocenters
- earthquake focal mechanisms
After filtering the earthquake catalog, the team retained more than 222,000 earthquakes recorded between 1970 and 2020
to help constrain the geometry of the subduction interface.
The resulting model reaches a grid resolution of roughly 3–4 kilometers at depths shallower than 40 kilometers, allowing researchers to examine changes in the shape and curvature of the fault along the entire margin.
The Megathrust Has a Remarkable North–South Split
The clearest result is also the simplest.
The southern and northern Hikurangi megathrust look very different.
In the south, the plate interface is relatively:
- smooth
- low in curvature
- strongly locked between earthquakes
Move northward, however, and the geometry becomes much more irregular.
The northern Hikurangi interface is:
- rougher
- more strongly curved
- crossed by prominent structural bands
- dominated by frequent shallow slow-slip events
Smooth megathrust → strongly locked.
Rough megathrust → frequent slow slip.
According to the researchers, this along-strike transition closely corresponds to changes in the way the fault moves.
That doesn’t mean roughness alone determines whether a megathrust creeps or ruptures. Fault-zone composition, fluid pressure, sediment, temperature and stress conditions also matter.
But the new model shows that geometry is strongly correlated with the segmentation of slow-slip behavior.
What Is a Slow-Slip Event?
A normal earthquake releases accumulated fault stress very rapidly, often within seconds.
A slow-slip event does something very different.
The fault still moves, and potentially by substantial amounts, but the displacement unfolds over days, weeks or even months. Because the movement occurs gradually, it produces little or no strong ground shaking.
That is why slow-slip events are sometimes described as “slow earthquakes.”
Hikurangi is one of the world’s most important natural laboratories for studying these events.
In the northern and central Hikurangi margin, shallow slow-slip events generally occur at depths of less than about 15 kilometers, often recur every one to two years, and typically last less than a month.
Farther south, deeper slow-slip events occur around 15–50 kilometers depth and can last more than a year, with recurrence intervals of several years.
You can explore the mechanics behind these unusual fault movements in our guide to earthquake science and slow-slip events.
Ancient Seamounts May Be Distorting the Megathrust
The northern Hikurangi margin contains another major clue.
The new model reveals two prominent northeast-trending bands of unusually strong curvature beneath the region.
The researchers interpret these structures as possible signatures of subducted seamount chains.
These ancient underwater volcanic mountains formed on the Hikurangi Plateau before being carried toward New Zealand on the Pacific Plate. As the plate descends, the seamounts can be dragged into the subduction zone and deform the megathrust above them.
That can deform the plate interface above them.
Imagine pushing a row of buried mountains underneath a giant sheet.
The sheet would bend, buckle and warp around them.
Something broadly comparable may be happening beneath northern Hikurangi.
New Zealand’s tectonic environment is also closely connected to the country’s active volcanism, which forms part of the same larger Pacific plate-boundary system. You can explore that side of the story in New Zealand Volcanoes Explained.
Then Scientists Found a Strange Corridor Behind a Buried Seamount Chain
One of the most intriguing features appears within the rough northern margin.
The largest cumulative shallow slow-slip patches recorded between 2002 and 2014 are concentrated inside a northeast-trending corridor where the fault surface is actually relatively flat compared with the surrounding rough terrain.
The researchers interpret this feature as a possible stress shadow created behind a subducted seamount chain.
As a large seamount moves beneath the overriding plate, stress is not distributed evenly around it.
The region behind the buried topographic high may experience reduced stress and accumulate sediments, forming what the authors describe as a possible sediment lens.
That combination could produce conditions favorable for shallow slow slip and tectonic tremor.
That is a far more interesting result than simply saying that “rough faults produce slow earthquakes.”
The new model suggests that the interaction between buried topography, fault curvature, sediment and stress may create specific zones where slow slip becomes particularly favorable.
The Fault Changes Shape With Depth Too
HSIM 1.0 also shows major differences in how the plate bends as it sinks beneath New Zealand.
In northern Hikurangi, maximum bending of the subducting plate occurs at depths of roughly 70–80 kilometers.
Toward the south, that maximum bending deepens substantially to around 120–130 kilometers.
The southern interface also steepens much more abruptly with depth.
These variations reflect the extraordinary structural complexity of the subduction zone beneath New Zealand.
Why the Smooth Locked Section Matters
The discovery has obvious implications for earthquake hazard — but it needs to be interpreted carefully.
The southern Hikurangi interface is currently more strongly coupled, meaning the two plates are partially locked together while tectonic motion continues.
When a fault is locked, strain can accumulate rather than being continuously released through creep.
That does not mean the new study predicts an imminent earthquake.
It does mean that understanding the geometry of the locked portion of the megathrust is important for modeling how a future rupture might behave.
Smooth fault surfaces may provide fewer geometric barriers capable of stopping a propagating rupture, whereas strongly irregular interfaces can sometimes segment fault behavior.
The researchers therefore argue that the improved geometry provided by HSIM 1.0 could eventually help refine physics-based earthquake simulations, strong-ground-motion models and tsunami hazard assessments.
That matters because large megathrust earthquakes are capable of displacing enormous sections of the seafloor and generating destructive tsunamis.
But again, this study is about fault structure and behavior, not a prediction that a large Hikurangi earthquake is about to occur.
Hikurangi is not the only subduction zone where scientists are trying to understand how locking, slow slip and fault geometry interact.
Across the Pacific, the Cascadia megathrust beneath the Pacific Northwest also combines a locked offshore fault with episodes of slow slip and tectonic tremor, making it another major natural laboratory for understanding how megathrusts store and release strain.
New Zealand Sits Above One of Earth’s Most Complicated Plate Boundaries
The Hikurangi system highlights why New Zealand experiences such an extraordinary concentration of earthquakes, volcanoes and tectonic deformation.
The country lies near the boundary between the Pacific and Australian plates, within the broader Ring of Fire.
But even along one plate boundary, fault behavior can change dramatically over just a few hundred kilometers.
One section can remain locked.
Another can creep.
Another can release stress through slow earthquakes.
And buried mountains carried into the planet by a sinking tectonic plate may help decide where those different behaviors occur.
A Fault Is Not Just a Flat Crack in the Earth
One of the most useful lessons from this study is also one of the easiest to overlook.
Subduction zones are often shown in textbook diagrams as smooth plates sliding neatly beneath one another.
Real megathrusts look nothing like that.
They contain ridges, depressions, buried seamounts, sediment pockets, changes in curvature and zones with very different frictional properties.
The new Hikurangi model makes that complexity visible in three dimensions.
And that geometry appears to correspond remarkably well with where the fault remains locked — and where it moves almost silently.
The Study
The study, “3D Subduction Interface Model for the Hikurangi Margin, New Zealand: Implications for Slow Slip Events,” was published in Geophysical Research Letters on August 28, 2026.
The research was led by Haoran Ma and Maomao Wang together with Andreas Plesch, Fuyuan Wang and Renqi Lu.
Their HSIM 1.0 model integrates seismic reflection data, earthquake locations, focal mechanisms, seismic tomography and bathymetry to produce a new three-dimensional representation of the Hikurangi plate interface.
The authors conclude that variations in megathrust geometry and curvature closely align with the segmentation of slow-slip events and that subducted seamount chains may play an important role in creating some of those conditions.
Primary source:
Ma et al. (2026), 3D Subduction Interface Model for the Hikurangi Margin, New Zealand: Implications for Slow Slip Events, Geophysical Research Letters.
Bottom Line
A new 3D model of New Zealand’s Hikurangi megathrust reveals a striking geological divide: the southern section is relatively smooth and locked, while the rougher northern margin hosts frequent shallow slow-slip events.
Buried chains of ancient seamounts appear to distort the northern plate boundary, while some of the largest slow-slip patches occur inside a relatively flat, sediment-rich corridor that may have formed in the stress shadow behind one of those submerged mountain chains.
The discovery does not predict the next major New Zealand earthquake.
But it gives scientists a much clearer picture of the hidden geometry controlling how one of the world’s most fascinating —and potentially dangerous—megathrusts moves.
Hikurangi Megathrust: FAQ
What did scientists discover beneath New Zealand’s Hikurangi megathrust?
A new high-resolution 3D model reveals a striking north-south divide. The southern locked section is relatively smooth and has low curvature, while the northern region where shallow slow-slip events are common is much rougher and more strongly curved.
What is the Hikurangi megathrust?
The Hikurangi megathrust is the plate-boundary fault east of New Zealand’s North Island, where the Pacific Plate descends beneath the Australian Plate along the Hikurangi subduction zone.
What is a slow-slip event?
A slow-slip event is fault movement that releases tectonic strain gradually over days, weeks or months instead of within seconds like a conventional earthquake. These events can therefore move a fault without producing strong shaking.
Why is the northern Hikurangi megathrust so rough?
The new model identifies prominent curvature bands that researchers associate with chains of seamounts carried beneath New Zealand on the subducting Pacific Plate. These buried volcanic mountains can warp and deform the plate boundary.
How could buried seamounts influence slow slip?
Some of the largest cumulative slow-slip patches occur within a relatively flat corridor interpreted as a stress shadow and sediment lens behind a subducted seamount chain. The researchers suggest this environment may favor shallow slow slip and tectonic tremor.
Does the study predict a major New Zealand earthquake?
No. The study does not predict when an earthquake will occur. It improves understanding of the megathrust’s geometry and could help future earthquake, ground-motion and tsunami hazard modeling.
Why is the locked southern Hikurangi megathrust important?
The southern section is relatively smooth and strongly locked, allowing tectonic strain to accumulate. Understanding its shape is important for modeling how future megathrust ruptures could behave, but the new study does not indicate that a major earthquake is imminent.
