Q&A: Hikurangi Subduction Zone's big quake threat

The natural threat posed by New Zealand's largest fault, the Hikurangi Subduction Zone, has again been in the headlines. Although the system is capable of generating earthquakes larger than 8.0 - and perhaps 9.0 - GNS Science earthquake geologist Dr Ursula Cochran says we shouldn't feel frightened. Instead, we should be prepared. She discusses the threat with the Herald.

What do we know about the Hikurangi Subduction Zone, and what it has done in the past?

The Hikurangi Subduction Zone is New Zealand's largest fault because of the area that could move in a large earthquake.

Geological and geophysical evidence shows that the zone can produce magnitude 8.0 and larger earthquakes.

It is possible it could produce magnitude 9 earthquakes but we don't yet have strong evidence for this.

Geological evidence suggests the last magnitude 8 earthquake happened about 500 years ago and there were two magnitude 7+ earthquakes offshore of Gisborne in 1947.

We are doing a number of studies to better understand the past behaviour of the subduction zone — ultimately, we hope to develop earthquake records similar to what is known for the Alpine Fault.

What threat could it pose to coastal communities today — and how quickly?

One of main hazards of a large Hikurangi subduction zone earthquake is the tsunami it would trigger.

Scientists aboard the Joides Resolution want to learn more about the risks the Hikurangi Subduction Zone poses. Photo / File

The size and extent of the tsunami would depend on how big the earthquake is and how the fault moves.

Modelling shows that parts of the East Coast close to the subduction zone - like eastern Wairarapa - could receive the tsunami about seven minutes after the earthquake.

Most locations will have longer gaps between the earthquake and the tsunami.

However, these short arrival times illustrate why it is so important to evacuate without waiting for an official warning — there will be no time to give one.

If you feel shaking that is long or strong, move away from the coast.

Know your evacuation zones so you know where to get to safety — your council website is the best place to find these maps.

Why has it been a challenge to understand how the zone behaves and how are scientists now attempting to gain deeper insights?

The Hikurangi subduction zone lies underneath the North Island and only comes to the surface offshore of the East Coast so we can't easily see it or study it.

There are now several new research programmes, including an MBie Endeavour project, aimed at understanding the Hikurangi subduction zone from different angles.

Overseas interest in the characteristics of our subduction zone means we have big research vessels such as the scientific drilling vessel Joides Resolution visiting our shores to help gather data.

We are deploying instruments onto the seafloor to measure plate movement, drilling into the fault to look at its properties, seismic surveying to get an image of the physical conditions on the plate boundary, investigating more of the onshore and offshore geology to find evidence of past earthquakes.

Many communities and research institutions are involved in the effort to better understand likely impacts on people if this fault moves in a big earthquake.

How does the threat it poses differ from the South Island's Alpine Fault?

Both faults are capable of big earthquakes but because of their locations and geometry, they will have quite different impacts for people.

The most obvious difference is that the Alpine Fault is onshore so one of the hazards will be the surface rupture itself where the ground will break and shift the land by metres.

There will also be extensive landsliding because of the mountainous country the fault crosses.

The Alpine Fault may cause a tsunami if it ruptures offshore or triggers undersea landslides.

However, the tsunami from a Hikurangi Subduction Zone earthquake is likely to be a much greater hazard because much of the fault is underwater.

It is great news as of yesterday that there is continued and new funding for efforts by the East Coast Lab and AF8 to be better prepared for earthquakes from these faults.

What, ultimately, do you feel is the take-away for people around this natural hazard?

Don't be scared, be prepared.

We have an amazing country to live in, we just need to be aware of the hazards of living in New Zealand and keep working to improve our ability to live safely in this active landscape.