Megathrust earthquakes: Scientists drill for the truth

Between eight and 10 quakes measuring 7.5 or above have struck the Hikurangi subduction zone in the past 7000 years. Credit: International Ocean Discovery Program

Earlier this week, a UK tabloid warned "12 metres waves will CRUSH New Zealand after rumblings in Pacific Ocean".

As much as the Brits might like that to happen, the truth is we really don't know when a megathrust quake off the North Island's east coast might strike.

"A large Hikurangi earthquake similar to the ones found in the geological record might on average occur once in several hundred years," GNS Science senior geophysicist William Power told Newshub.

"It is very hard to say if we are overdue because the pattern of earthquakes is not regular, the intervals between them vary considerably. We should be prepared for the possibility that one could occur at any time, while also keeping the perspective that these are not frequent events."

Between eight and 10 quakes measuring 7.5 or above have struck the Hikurangi subduction zone in the past 7000 years, scientists estimate. The strongest may have been as strong as 9.0 - releasing 22,000 times more energy than the 2011 Christchurch quake.

If another one was to hit today, 12m waves are a real possibility. This weekend, scientists from New Zealand and the United States will be heading out to the Hikurangi subduction zone to learn more about how the quakes occur, and if they can be predicted in advance.

Subduction zones are where one continental plate is slipping beneath another, and the cause of the world's most powerful quakes - known as megathrust, or subduction zone earthquakes. They're also the site of slow-slip earthquakes - less destructive, but in GNS Science's own words, "enigmatic".

"We don't yet understand the physical processes that cause faults to behave in such a way, and we don't know very much about their relationship to large subduction zone earthquakes," said expedition co-leader Dr Demian Saffer of Pennsylvania State University.

Damage caused by the Kaikoura quake. Photo credit: Getty

Slow-slip events happen once every year or two. One happened after the Kaikoura quake of 2016, right in the region where the expedition plans to investigate.

They'll be installing high-tech measuring and monitoring equipment 500m below the seabed in two pre-drilled holes, where they'll sit for up to 10 years collecting information. One observatory will go in the Australian plate, and other in the fault zone on the border with the Pacific plate.

"This expedition will yield information that's key to understanding why destructive tsunamis happen after shallow earthquakes at plate subduction zones and underwater landslides," said James Allan of the National Science Foundation's Division of Ocean Sciences.

The scientists hope the research will allow authorities to better prepare for destructive tsunamis.

Dr Power says how far water reaches inland depends on many factors, including the size and duration of the wave, and the shape and contours of the coast.

Tsunami-prone zones are "based on computer modelling, and there are a few different approaches that are used".

"For several cities, computer models of tsunami inundation have been used to model the physics of potential tsunamis as they run inland, although this requires a lot of computer power. In more rural areas it is more typical to estimate potential tsunami heights at the coast and then use a formula to estimate the maximum distance that they can travel inland."

Despite regular false alarms, Dr Power says tsunami warnings still need to be taken seriously.

"A future earthquake similar to the largest of the geological events (about 7000 years ago) could cause earthquake shaking damage that strongly affects the whole eastern side of the North Island - most strongly from the East Cape southwards - and the northern South Island, causing a lot of earthquake shaking damage.

"[It could be] accompanied by a tsunami that would potentially affect the whole east coast of the North Island and the South Island at least as far as Canterbury."

The expedition, aboard scientific drilling ship JOIDES Resolution, is being funded by the International Ocean Discovery Program and the US National Science Foundation.

Civil Defence tips on how to stay safe in a tsunami are available on its website.