Q&A with GNS Science: Megathrust earthquake would produce 'tsunami too fast for official warnings'

NZ Herald·

9 Oct, 2017 09:25 PM4 mins to read

NOW PLAYING • New Zealand's hidden megafault zone

GNS Science explains New Zealands megafault zone, where the entire Eastern North Island is subject to the relationship of the Australian and Pacific plates. Source: Youtube/GNS Science ...

Scientists have warned a "megathrust" earthquake could release 2000 times more energy than the 2011 Christchurch earthquake that killed 185 people.

The risk comes from the Hikurangi subduction zone, a fault that runs along New Zealand's East Coast, where the Pacific and Australian tectonic plates collide.

Scientists say it could trigger a massive 8.4 magnitude quake, which would cripple the lower North Island.

But what do we know about the potential megathrust earthquake? GNS earthquake geologist Kate Clark explains the likelihood of a megathrust quake and what is known about the Hikurangi subduction zone.

How do scientists know about the potential for a "megathrust" earthquake?

The Hikurangi subduction zone is the boundary between the Australian and Pacific plates. This zone has been known about and studied for several decades and we have long known that it could generate a megathrust earthquake. In the past 15 years we have used global positioning satellites (GPS) to track the movement of land above the subduction zone. This has shown us that parts of the zone are "stuck" together and accumulating stress that will eventually be released in a large earthquake. Geologists have also been studying the coastline of the Hikurangi margin and are finding evidence of past large earthquakes and tsunamis, and some will have come from a megathrust earthquake. If a megathrust earthquake were to occur, how widespread could the damage be?

The size of a megathrust earthquake and the extent of the damage depends on how much of the plate boundary ruptures, or slips suddenly, during an earthquake. This is very hard to forecast but realistic scenarios suggest the Hikurangi subduction zone could host an earthquake up to Magnitude 9. Damage would occur from East Cape to Marlborough. Subduction zones can host a wide range of earthquake magnitudes though, so not all will be that large. In 1947, part of the Hikurangi subduction zone offGisborne ruptured in a Magnitude 7.2 earthquake and there was little damage from shaking as the epicentre was far offshore, but a tsunami of up to 10m height was generated, which damaged bridges and cottages along the coast north of Gisborne.

A megathrust earthquake could kill thousands. Photo / GNS

What is the likelihood of a megathrust earthquake along the Hikurangi subduction zone?

"There have been no great Magnitude 8 earthquakes on the Hikurangi subduction zone in the short period [175 years] for which we have a historical record of seismicity. New Zealand's National Seismic Hazard Model combines a range of different datasets to evaluate the likelihood of future earthquakes and that shows different parts of the megathrust could rupture in Magnitude 8.1-8.3 earthquakes every 550-1400 years and the whole margin could rupture in a Magnitude 9 earthquake every 7000 years.

However, these numbers are highly uncertain as we have very little information on the frequency and size of past Hikurangi subduction zone earthquakes. Research to understand when different parts of the margin last had a megathrust earthquake and the size of the earthquake is ongoing.

Why is the Hikurangi subduction zone so hazardous?

All subduction zones are hazardous because they can generate large earthquakes and tsunamis. The Hikurangi subduction zone is hazardous because we know that the plates are stuck together in places and are building up stress that will be relieved in future earthquakes. We also know that it has generated large earthquakes and tsunamis in the past. Large population centres such as Wellington, Napier, Gisborne, Nelson, Blenheim, and Palmerston North are above the Hikurangi subduction zone. Subduction zone earthquakes often produce large tsunamis because there are large and rapid displacements of the seafloor. A tsunami generated at the Hikurangi subduction zone will arrive on coastlines of the east coast about 10 minutes after the earthquake. This is too fast for official warnings to be issued so people living in tsunami zones should heed the "Long or Strong: Get Gone" advice of Civil Defence.

'Megathrust earthquakes' triggered both the 2011 Japanese and 2004 Boxing Day tsunamis.

How is this potential megathrust earthquake different or similar to the earthquakes in Christchurch and Kaikoura in recent years?

A megathrust earthquake on the Hikurangi subduction zone is likely to be quite different from the Christchurch and Kaikoura [earthquakes]. A megathrust earthquake is likely to generate ground shaking for much longer (minutes) and over a wider area (much of New Zealand) than the Christchurch and Kaikoura earthquakes. The intensity of shaking is likely to be different too but this depends on how deep and where the megathrust earthquake occurs. The Kaikoura earthquake generated a tsunami but a megathrust earthquake is likely to generate a larger tsunami that impacts a wider region.

How can people prepare for an earthquake of this potential magnitude?

Websites like http://www.getthru.govt.nz/ and http://www.civildefence.govt.nz/ have a lot of useful information and advice. Having a household emergency plan, emergency survival items, a getaway kit and knowing your tsunami evacuation route are all recommended.