New Zealand unlikely to have megaquake, but dangers exist – seismologist

A seismologist says New Zealand is unlikely to have a megaquake such as the one Japan is on high alert for, but a lesser quake could still be more devastating.

Japan’s meteorological agency issued its first-ever megaquake warning around the country’s southern Pacific coast on Friday following a 7.1 magnitude tremor.

A magnitude-5.3 quake has since hit Tokyo and eastern parts of Japan.

Megaquakes have a magnitude of 9.0 or more happen where two plates push against each other.

Seven to 7.9 is classified as a major quake, while quakes of magnitude 8.0 to 8.9 are registered as great.

GNS Science seismologist John Ristau said it was unlikely the North Island’s Hikurangi subduction zone would generate a megaquake.

The Hikurangi subduction zone off the East Coast.
Photo: Supplied / GeoNet

“The big threat [there] though is that this is maybe the only place in the world where a major population centre actually sits right on top of one of those subduction zones.”

John Ristau said the Hikurangi could still cause a quake of 8.5 magnitude, which could be much more damaging than a megaquake given the faultline ran under the Wellington and Wairarapa region.

“But you don’t even need to be talking about a subduction zone – when the Alpine fault or the Wellington fault eventually rupture there will be aftershocks through major population centres and these could be equally devastating.”

He said Japan’s first-ever megaquake warning was likely to be precautionary, although evidence suggested there was a higher chance of a big earthquake.

Japan had a five percent chance of having another large quake, he added.

“What [the Japanese government] is saying now is that the probability of having an earthquake a similar size or larger is increased, which is actually true. The most likely time to have a large earthquake is right after you’ve just had one.”

Ristau said a 7.2 quake happened in Japan two days before the Tohuku 9.1 magnitude megaquake and tsunami that killed nearly 20,000 people in 2011.

“This is probably what has prompted the warning they’ve put out [this week].”

He said the most powerful quake ever recorded was the 1960 Valdivia megaquake off the coast off Chile, estimated to be a 9.5 magnitude.

Ristau said if he was in the area of Japan where the megaquake warning had been issued he would “probably go about my day-to-day life, but just have it in back of my mind that this is a possibility.

“It’s probably a good time to check your earthquake supplies, and make sure you are prepared just on the off-chance this might happen”.

It was sensible for people living in a country like Japan or New Zealand to have supplies anyway, and to know what to do in the event of a large earthquake or tsunami warning, he added.

“Drop cover hold and if there’s strong or long shaking, around the coast that could mean a tsunami. Long or strong, get gone.”