'Case of when, not if': Preparing for the big 'megathrust' earthquake from the Hikurangi subduction zone

A leading earthquake geologist says a devastating ‘megathrust’ earthquake linked to the Kaikoura quake is still a possibility.

It's a case of when, not if, New Zealand's largest fault ruptures potentially producing a "megathrust" earthquake with devastating effects.

Evidence suggests the likelihood of a rupture along the Hikurangi subduction zone - which runs from the top of the East Cape to the upper east of the South Island - may be higher than previously understood.

Consequently five Civil Defence Emergency Management (CDEM) Groups from across the North Island East Coast are working together to develop an emergency response plan.

They are using a magnitude 8.9 earthquake and tsunami scenario, which in reality could see many east coast cities and towns inundated, seismic shaking and liquefaction.

A subduction zone is where one tectonic plate subducts (dives) underneath another - the boundary between these two plates forms a large fault.

The Hikurangi subduction zone runs offshore from the east of Gisborne down to the top of the South Island and poses a significant earthquake and tsunami risk to the entire east coast of New Zealand.

GNS scientist Dr Laura Wallace said the chance of a rupture occurring was higher than previously understood.

This was a result of insights gained following the Kaikōura earthquakes, evidence for pressure building on the fault, and geological evidence for prehistoric earthquakes on the subduction zone.

"We have been building up a lot of knowledge over the past 10 to 15 years. GPS data suggests a large portion of the fault is locked up and building pressure, and will be relieved in the future in the form of an earthquake."

Another factor were the "slow slip" events, also known as silent earthquakes.

These events can last from days to years, and can produce up to tens of centimetres of displacements along faults without seismologists even realising it.

Studies have tied them to two 2014 quakes: the 8.1 Iquique earthquake in Chile was preceded by a slow slip event, and a 7.2 shake off the coast of Mexico occurred just two months after one started.

When the Kaikōura Earthquake erupted in November 2016, with the equivalent energy release of 400 atomic bombs, shockwaves travelled hundreds of kilometres through the ground and triggered a series of faults as they spread.

It's believed they also kicked off a slow-slip event along the Hikurangi subduction zone more than 600km from Kaikōura spanning an area of more than 15,000sq km – comparable to the land area of New Caledonia.

New Zealand's largest fault, the Hikurangi subduction zone. Image / East Coast LAB

Silent quakes aren't always stress-builders; along some parts of the plate boundary, they are actually relieving tension.

Wallace said subduction zone faults had been responsible for most of the world's deadliest earthquakes and tsunamis to date, with Japan 2011 being the most recent example.

"We know the Hikurangi subduction zone can produce large earthquakes and tsunamis, and these events have happened in the past.

"While we're carrying out more research to build a clearer picture of the hazard posed by the Hikurangi fault, we know a rupture at some point in the future is certain."

Research has shown there have been about eight major seismic events, potentially subduction earthquakes, along the Hikurangi stretch over the past 7000 years — the last about 500 or 600 years ago.

"We know they are possible, evidence suggests they have happened hundreds of years ago, so the questions are not if, but when, and how large and what the impacts will be," Wallace said.

Led by East Coast Life at the Boundary (LAB), the Hikurangi Response Plan will outline how to respond to a Hikurangi subduction zone earthquake and tsunami, and how to enhance communities' preparedness for such an event.

Project lead Natasha Goldring said building the collaborative response plan was vital in lifting readiness for, and resilience to a future earthquake and tsunami on the Hikurangi fault.

"The scenario we are using to support the development of this response plan is a very realistic example of what we could face in our lifetime, or that of our children and grandchildren," Goldring said.

Although the project is being driven by CDEM groups, people still needed to make sure they understood the risks they faced and take the necessary steps to prepare themselves.

Information on how to prepare for an earthquake or tsunami can be found here.

"Communities are at the centre of all response planning, and we want this project to be a collaborative effort. We are all responsible for ourselves and our families – we are all part of Civil Defence in New Zealand."