The Hikurangi subduction zone is potentially the largest source of earthquake and tsunami hazard in New Zealand.

GNS Science is researching how often this zone can produce earthquake events, and how large these events can be.

Understanding the Hikurangi subduction zone

Over the past five years, a large team of national and international scientists have been studying the Hikurangi plate boundary to find out what risk it poses to New Zealand. The Hikurangi plate boundary, located off the East Coast of the North Island, is where the Pacific tectonic plate subducts (or dives underneath) the Australian tectonic plate. This is called a subduction zone. 

Subduction zones are responsible for some of the largest and most powerful earthquakes and tsunamis in the world.

The Hikurangi subduction zone is potentially the largest source of earthquake and tsunami hazard in New Zealand. At GNS, we are leading a majority of the research on the Hikurangi subduction zone, in a five-year, $6 million MBIE-funded Endeavour project, with major collaboration and contributions from international partners.

Model of Magnitude 8.9 Hikurangi Earthquake and Tsunami transcript

Leading New Zealand scientists have designed a credible scenario for an earthquake and tsunami on the Hikurangi Subduction Zone.

The scenario, further developed by GNS Science experts, helps emergency responders to plan and prepare for a magnitude 8.9 earthquake and tsunami on the Hikurangi Subduction Zone which is off the east coast of the North Island.

The scenario shows impacts from the earthquake and tsunami for the most effected areas across the North Island's East Coast in New Zealand.

It is one of many possible scenarios and does not predict the future. It is unlikely that a Hikurangi Subduction Zone earthquake and tsunami will happen exactly like this.

This scenario is based on a magnitude 8.9 earthquake and shows the potential levels of ground shaking. The earthquake's rupture begins 70 kilometres off the coast Porangahau, Hawke's Bay at a shallow depth of nine kilometres.

An earthquake like this would release around 45 times more energy than the 2016 magnitude 7.8 Kaikoura Hurunui earthquake.

The scenario is run at double the actual speed and the scale bar shows the level of shaking. White shows extreme shaking and red shows strong to severe shaking.

The earthquake rupture continues north and south creating long and strong shaking along the east coast of the North Island.

Gisborne would experience particularly strong and long shaking in this scenario due to the different rock types in the area that increase the shaking's intensity and duration. 

Wellington would feel ground shaking 90 seconds after the initial earthquake rupture.

Areas further away from the earthquake rupture would experience strong shaking.  For example Auckland though a good distance away would experience around 30 seconds of severe shaking.

As well as the shaking the earthquake would potentially cause liquefaction, landslides and fires in some areas.

The sudden movement of the sea floor during the earthquake causes a lot of water to move creating a tsunami.

The scenario is now showing the tsunami created by this large earthquake. Within the first few minutes there are rapid changes in sea level along the east coast of the North Island.

The tsunami moves in all directions.

After the earthquake rupture some parts of the coast may see a rise in sea level shown in red. In others such as Hawke's Bay in this scenario the sea-level would rapidly decrease, pulling away from the coast and returning with enormous speed and force when the tsunami waves arrive.  This is shown in blue.

This is because the ocean first draws down and sucks water away from the coastlines to later return.

Close to the shore along the East Coast waves could reach about ten metres above normal sea level in some places. In a few locations where the tsunami is funnelled into steep valleys on shore it might reach even higher, possibly up to 20 metres above sea level. 

In some other earthquake scenarios run-up heights could be even greater than 20 metres at some locations. Tsunami evacuation zones account for multiple possible earthquake scenarios.

In this scenario the speed the tsunami waves travel means there would not be enough time for an official evacuation warning.

The long or strong earthquake is your signal to self-evacuate all tsunami evacuation zones.

Check your tsunami evacuation zones on your local civil defense group website.

Make sure that you and your family and others who rely on you know your evacuation route to higher ground or inland whether you're at home, at work, at school or out and about.

In this scenario around 13 minutes after shaking begins in Wellington large tsunami waves arrive at the harbour entrance and extend into the tsunami evacuation zones. It's important to evacuate all tsunami evacuation zones in a long or strong earthquake.

The scenario is sped up to show how the tsunami moves in the first two hours of the earthquake.

The tsunami would reach cities as far away as Auckland, Christchurch and even the West Coast of the North Island.

While this and other science-based scenarios do not predict the future and a single scenario

cannot provide all the answers on its own, they help emergency managers to plan and prepare for events like a Hikurangi subduction zone earthquake and tsunami.

To get prepared visit getready.govt nz

To check your tsunami evacuation zones visit your local civil defense emergency management website.

The best place to study slow slip events

Also referred to as “slow earthquakes”, as they happen slowly over a period of weeks to months, rather than suddenly in one large earthquake. The world’s shallowest slow slip events occur just offshore Gisborne, and offer a globally unique opportunity to understand why slow slip events happen.

The ongoing subduction of the Pacific plate under the Australian plate in this off-shore Wellington region causes the continent's crust to move in both slow-slips and historically strong ruptures.

More recently, a new study has given scientists a better understanding of the way plate tectonic stresses build up and are dissipated on the Hikurangi subduction zone. We are, in effect, “taking the pulse” of Hikurangi to see if we spot the signals that herald an earthquake.

Earthquake Monitoring – Taking the PULSE of the Hikurangi subduction zone transcript

Over a couple of weeks, a team of around 10 scientists have deployed more than 50 instruments to record signals related to slow slip earthquakes.

The PULSE project is an important step towards better understanding earthquake hazard in New Zealand, and the subduction processes that are happening along the Hikurangi subduction zone along the East Coast.

The Porangahau region is particularly interesting because these slow slip events happen approximately every five years or so in this region. 

The last event was triggered in 2016 and November after the Kaikoura earthquake.

And so we've been expecting that there would have been a slow slip sometime in 2021, about five years after 2016. The slow step events themselves happen so slowly that they don't produce vibrations and ground shaking, and that means that we can't record them or know that they're happening with normal seismic instrumentation.

The only way that we've been able to observe them and map out where and when they happen is by deploying GPS equipment. So these are sensors that sit at the ground surface and record their position, and that can tell us how points on the earth surface are moving really slowly through time.

So while these sorts of events don't produce any noticeable ground shaking in themselves, they can produce and trigger small earthquakes.

So the other type of instruments that we can install are these seismometers, these earthquake recorders. We bury the sensors in the ground, and we leave them there for a few months, and they basically record any kinds of vibrations that travel through the crust.

When we record these small earthquake signals, we're able to do a much better job of telling where these earthquakes are happening. And that can tell us something about how these slow slip events are changing the state of stress in the crust and how they're triggering the seismicity.

These slow slip events do happen on other subduction zones around the world, but New Zealand and the Hikurangi in particular has really been a magnet for world leading science in terms of slow slip event understanding in recent years.

And this is because the Hikurangi is in a really unique setting to be so close and so shallow beneath our feet underneath the North Island.

And the observations that we make are much clearer then if we were trying to  observe things much deeper down in the earth.

The type of fault motion that we see in these slow slip events is exactly the same as what we see in these large damaging tsunamigenic subduction zone earthquakes.

And so we want to try and understand that if the driving forces behind these two different slip modes, the slow slip and the damaging slip, -  if the driving forces are the same, then can we use the much more frequent slow slip earthquakes to better understand how the fault is driven to failure.

And this deployment wouldn't have been possible without the generous help of many of the landowners along New Zealand's east coast. Thank you for letting us put one of these instruments on your land.