Will New Zealand’s offshore quakes of 5 March 2021 trigger more?

In the early hours of Friday morning a lot of people in New Zealand’s North Island, including me, were shaken awake by a significant quake about 100 km off Te Araroa, a township on the East Cape – magnitude 7.1, and a depth of 90 km, which is why it was so widely felt.

Quakes of this size are relatively rare, but they happen. As recently as November 2016 a magnitude 7.8 quake struck Kaikoura, sending shock waves rippling north and devastating parts of central Wellington. Usually such shocks occur at relatively long intervals. They’re always followed by aftershocks, which themselves can be very large, but always smaller than the main-shock. But yesterday, 5 March, at 6.41 am, an even larger quake of magnitude 7.4 shook Raoul Island, largest of the Kermadec chain to the northeast of New Zealand, well distant from the first shock. And around 8.30 on Friday morning that was followed by a third quake – magnitude 8.1 – nearby. This last was simply enormous – an energy release equivalent to about 21,000,000 tons of TNT.

For a while the chance of tsunami hitting the northern coasts of the North Island seemed high. Fortunately the underwater disturbances passed without significant problem.

This beach in Napier – which is next to the street where I grew up, way back when – was in the danger zone on 5 March 2021. And no, I didn’t rush down to the beach to take the photo, I’m not Captain Stupid. It’s from my archive

What intrigues me was the fact that there were three big quakes in quick succession. That’s unusual – a quake-apocalypse of unprecedented scale. The two Kermadec events were, almost certainly, two parts of the same seismic event. The possibility of a shock in one area triggering other quakes was not considered plausible until recently. The Kaikoura quake – which involved a complex array of multiple movements across multiple faults – highlighted the possibility. Theoretically, if a quake in one area is large enough, the energy it releases might suffice to trigger a nearby fault that, itself, is already under tension and about to ‘go’. It’s the straw, as it were, that breaks the donkey’s back.

Two things worry me about the 5 March quakes. One is whether they have ‘revved up’ the Hikurangi trench. This is the main subduction zone, east of the North Island, where the Pacific plate is being rammed under the Australian. It has the capacity to produce magnitude 9 quakes and devastating tsunami, all at once. And for New Zealand, that’s bad juju. How bad? Let me put it this way: back in 2013 when I was writing my book on the science of earthquakes, I got in touch with a guy who’d written his PhD thesis on the effects of an Alpine Fault rupture in the South Island. The scenario (later used for a Civil Defence exercise) was horrific. ‘But don’t worry,’ he said. ‘The Hikurangi trench is way worse’.

Um…yay. So what are the chances of the 5 March quakes triggering more? New Zealand’s seismologists at the institute of Geological and Nuclear Sciences in Lower Hutt – who are among the best in the world – are already on to that one. According to GNS, following the 5 March quakes there is a 15 percent of less likelihood of a further quake of magnitude 8.0 occurring within 30 days of 5 March, probably in the Kermadecs but perhaps East Cape. There is also a 1 percent or less chance of a larger quake of magnitude 8.5 occurring in the Kermadec subduction zone, or the Hikurangi trench.

Why the uncertainties? Fault systems atop the mantle – which I liken to broken glass floating on boiling porridge – are highly complex and full data about the specific forces involved isn’t available. Like the weather, quakes can be generally ‘forecast’, but not specifically ‘predicted’ – hence the range of probabilities.

All this puts me in mind of New Zealand’s historical quake-pocalypse year, 1460. Geological analysis has revealed that at least four massive quakes of magnitude 8 or higher ripped across New Zealand around that time. At least one was devastating for early Maori – gaining the name Haowhenua (Land Eater). Historical work I did on the event suggested this name was due to the tsunami it generated. Geological work cannot precisely date old quake events, but the scenario I wonder about is the possibility of all four striking in very quick succession. And the 5 March quakes – all of similar scale, in a six hour time-frame – makes that scenario plausible. The question is whether such a concatenation could happen again – for which the answer is ‘obviously, given enough time’.

I’ve written a book on New Zealand’s earthquakes – explaining the science behind them, and it’s out now in second expanded edition covering the story and history behind all the ‘big ones’ up to 2016. Living On Shaky Ground (Bateman Books 2019). Check it out in all good bookshops, or click through and buy direct from the publisher.

Living on Shaky Ground: The Science and Story Behind New Zealand’s Earthquakes Expanded and Updated Edition: 2019

Copyright © Matthew Wright 2021