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.

Copyright © Matthew Wright 2021

12 thoughts on “Will New Zealand’s offshore quakes of 5 March 2021 trigger more?

    1. The hazards of the Pacific Rim! And, of course, there is no way of getting around the uncertainties of the forecast. There was a guy here in NZ who insisted he could ‘predict’ quakes down to time and place, apparently due to spiritual moon arglegarble fairy or the higher crystal vibrations of wooble-ooble – which was both dangerous and irresponsible of him, because some people actually believed his garbage. Ouch.

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      1. A network of sensors on the ocean floor a couple of hundred kilometres offshore can apparently give a 90-second warning now. That might be enough time for people to take cover. Or maybe panic and run outside, which is not recommended. The best advice is still to be prepared with survival supplies, which is not terribly reassuring.

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  1. Thanks Matthew, for your book….I bought a copy for my grandson’s “library.” That Hikurangi trench is a biggie alright…it was good that the people in that area reacted positively to it. That pleased me…I used to live in that area so got used to the ongoing earthquake possibility…. When this 5th March one woke me in Coromandel I knew it was bad news for somewhere else in NZ.

    Cheers Bev Thatcher

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    1. Thanks for buying my book! Much appreciated. Yes, that issue of feeling a quake and not knowing whether it’s local or distant is interesting. Back in 2010, when the Darfield quake struck near Christchurch, my wife and I felt it in Wellington as a long rolling rumble that set the house creaking. It could have been local, who knew just then? A few minutes later my wife began fielding texts from relatives in Christchurch – they’d just felt a huge quake and figured that as Christchurch wasn’t supposed to be on a major fault line, Wellington must have been totally flattened.


  2. Ugh, that scenario is not at all reassuring. Have the building regs been updated after Christchurch? Over here, all our planning seems to be for ‘best case’ scenarios. 😦
    The house in the foreground of that photo looks a long way above sea level. Would a big tsunami be capable of reaching that high???

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    1. I haven’t caught up with the building regs yet, but something will have to be done I suspect. They were changed some years ago as a result of the neo-liberal ‘revolution’ from a specific prescription to a ‘hands off’ guideline that has had disastrous effects. When the November 2016 Kaikoura quake hit Wellington, all but a few buildings pre- that change survived with minimal damage. Those after the change – one after another – all stood up but were condemned as unsafe thereafter, with the result that a lot of buildings put up in the past 10 years have since been knocked down again. It’s because they were designed that way: it’s cheaper than building something that can withstand quake after quake.

      The building in the foreground is New Zealand’s National Aquarium, which used to be just the Napier aquarium until they got the bigger building. It’s on the foreshore, only a couple of metres above sea level. Most of the beach area in the picture is earthquake uplift from a magnitude 7.8 quake of February 1931, which created a much wider foreshore. Not that this is great protection from tsunami – there’s a shingle bank to the left of the picture (where the row of trees, road and houses are) beyond which the ground slopes away again in the Napier South residential district. A big enough tsunami would roll straight over it and flood that residential district, with not much to stop it.

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      1. Strange economics to build something only to knock it down again. I guess I’d be wondering just how safe ‘safe enough’ actually is.
        The Napier South residential district is just plain terrifying. Do the residents know? Are they drilled on getting out? Do they listen?
        Sorry, the thought of a massive wall of water just waiting to fall is the stuff of my nightmares.

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        1. A friend of mine who still lives there said they evacuated at 3.00 am when the news came, and a lot of other people did the same. Mind you, I lived in that district as a teenager and didn’t have the slightest concern that anything might happen. The story I still remember hearing as a kid was how a tsunami from a 1960 Chilean quake had crossed the Pacific with sufficient force to spur an alarm in NZ. A lot of people in Napier rushed down to the foreshore to have a look. And it had enough force to break a bridge in the harbour, when it arrived. Attitudes have changed a bit, I suspect…

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        2. I suspect it’s the strategy that produces maximum profit for the builder, and anything else is the insurance company’s problem… The same neo-liberal revolution also produced a ‘leaky building’ crisis, via relaxation of the building regulations that allowed some billions of dollars worth of housing stock to be built without proper attention to basic matters such as waterproofing and using properly tanalised timber. The cost has been apocalyptic for home-owners caught by the problem.

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          1. I haven’t come across that ‘leaky house’ thing personally – I was very lucky with my builder – but the news often features stories about people who have been diddled by their builder and left with really shoddy workmanship. 😦

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