Earthquake, tsunami and floods…

It’s been a rough week so far in New Zealand. How rough? Well, most of us were woken in the first minutes of Monday by the biggest onshore quake since 2009, and – at a magnitude of 7.5 – the largest to occur near any main population centres since 1931.

[Update: figure officially revised to Mw 7.8 late on 16/11, matching the 2009 Fijordland event and the Hawke’s Bay 1931 quake].

The seismic event began 15 km northeast of the tiny northern South Island town of Culverden (pop. 426). That was the ‘epicentre’ – but that’s not a proper measure, because the tectonic movement occurred along a fault system some 200 km long, meaning that the total energy was delivered in a complex but generally north-easterly pattern that encompassed Kaikoura and the north-eastern coast of the South Island, along with the capital city of Wellington at the bottom of the North Island. Faults that moved included one previously unknown at Waipapa Bay. To portray this (as the media often do) as a bullseye from the epicentre is misleading.

In general it was the first event of its kind on that particular system in a while – one analysis suggests the last similar event was 495 +/- 25 years ago, with another movement maybe 300 years before that.

In this case, the GNS accelerometer in Ward, at the northern end of the seismic movement, apparently recorded ground accelerations of 1.2 gravities. That’s not as high as the devastating February 2011 quake in Christchurch. What was significant was the total energy release of this one event, which was within two percent of the combined energy released by all other New Zealand quakes since 2010, including the whole Christchurch sequence. How does that happen? General energy released – and the detailed energy delivered to a specific place – are two different things. The figure I’ve seen for the general energy of the 14 November 2016 event was 12,000,000,000,000,000 joules. That’s the equivalent energy (unless I’ve dropped a decimal point) of about 2.86 million tonnes of TNT. Woah! [NB: the late 16 November upgrade to Mw 7.8 implies a threefold increase over that figure].

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One of my bookshelves after the quake – which I estimate to have had a ‘felt effect’ at this location of around 5 on the Modified Mercalli scale, as adapted in 2007 to New Zealand conditions.

How does that work? As I generally put it in the book I wrote on the science of these things (Living on Shaky Ground, Penguin Random House 2014), imagine the Earth’s crust, where the plates intersect, as broken glass floating on boiling porridge. The porridge bubbles, the glass fragments move, their edges grinding against each other. And sometimes more than one shard shifts simultaneously.

The quake – which, it appears, was actually two in quick succession on the same system – damaged a wide area across central New Zealand and was perceptible even further afield, even as far as Auckland. In that ‘felt’ sense (a genuine scientific measure, gauged on the Modified Mercalli scale) it was more roll than rock – certainly to me, as I felt it play out for two long minutes.

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State Highway 1 north of Kaikoura in happier times.

The quake was followed by a tsunami warning. Usually those warnings are precautionary, but this time the tsunami waves were an immediate danger, lashing the coastline and doing damage from Banks’ Peninsula up to the North Island. There were evacuations from Lower Hutt, Te Awanga and other places.

Dawn revealed damage from the southern North Island through to Christchurch. Wellington was hard-hit, with severe damage to buildings on reclaimed land near the waterfront and a good deal of general damage and dislocation. Further south, aerial surveys revealed a broken landscape. State Highway One from Blenheim south – usually a lovely coastal drive – had been devastated. Kaikoura, a delightful coastal town popular with tourists for its whale watching, was cut off with 1000 tourists in the district and only a few days’ water.

Inland towns such as Cheviot and Hanmer Springs were hammered, along with their surrounding districts. Two people died; one in the quake-driven collapse of a house, another from a heart attack induced by the quake. Others were injured. This was awful; but there can be no doubt that if the quake had occurred in business hours, casualties and injuries would have been far higher – especially in Wellington, where one floor of the Statistics New Zealand building reportedly pancaked into the one below.

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Hutt River in full flood, 15 November 2016 when torrential rain soused the greater Wellington district. Luckily the quake hadn’t cracked the stop-banks.

As all this unfolded, massive aftershocks – including several over magnitude 6, which were severe quakes in their own right – began pounding the country. And then the rains came, endless rain, pouring down from leaden skies and driven by gale force winds. The capital city, Wellington, whose CBD was still partly cordoned for safety reasons, ground to a standstill. And quake damage was still unfolding. Forty eight hours after the quake, engineering inspections revealed one eight-floor building – which hadn’t been previously listed as quake prone – was in a dangerous state and potentially liable to collapse. I know the building well: I walk past it quite often.

As I write this, the full scale of the damage and events has yet to unfold, and I’ll be blogging on some of that later. There will unquestionably be a long aftershock sequence – and our seismologists warn that there is a small chance of the quake ‘revving up’ nearby fault systems in the near future, with potential to generate other quakes of up to magnitude 7 or more. We’ll have to see.

For now – well, it’s been an adventurous few days, one way or another. More soon.

Copyright © Matthew Wright 2016

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20 thoughts on “Earthquake, tsunami and floods…

  1. Been in the US for 5 weeks. Heard about the quake but not had much Internet access. Have it briefly tonight. I thought of you when I heard about it and am glad to hear you are okay. Susan

    1. The main shock hit just after midnight, luckily – I think it would have been much worse had it been in business hours, certainly in central Wellington. I had a look around today and took some photos, which I’ll post in a day or two.

  2. Thank God you and your family are safe. The books can handle it. Praying for you and New Zealand that there will not be any more occurrences. Take care!

    1. Thank you! And I am – it’s become a matter of conscious decision now as to which route I take through central Wellington on foot, for instance. Parts of the city are still cordoned off because of the risk of falling glass, and one building has been condemned and will be demolished. I am not keen to have any of this around me in another shake – which, alas, will happen. The current sequence is going to generate a few more strong ones of itself, luckily diminishing as time goes on but potentially enough to dislodge anything already weakened. We have to live with it – but it WILL pass.

  3. Here on Canada’s west coast, we’re waiting for the Big One — some say a magnitude 9 “megathrust.” The last one was almost 317 years ago, and they apparently happen every 200-500 years. There have been big quakes all around the “Ring of Fire” in recent years, so it could be any minute — or in a couple of hundred years. If one thinks about this too much, one begins to think about moving to the centre of the continent, like back to Saskatchewan (where I lived for 12 years, always dreaming of returning here). Good to hear there were so few fatalities in the latest NZ quake.

    1. Yes, the toll here was mercifully light. It would likely have been worse in business hours. The damage continues to unfold today with various buildings in Wellington being evacuated with structural issues, including one that has been condemned. Like Canada we have a couple of potential armageddon scale faults – the Alpine Fault, which is due to rupture at Mw 8+ , and the Hikurangi Trench off the east coast which has potential to generate a Mw 9+ event with tsunami. All part of that same ‘Ring of Fire’. I guess all we can do is live with it and be prepared.

  4. Glad that you got through it okay. The amount of damage in Wellington is a real worry, I’d hate to think what might’ve happened if it struck during the day, or any closer to a more populated area. I felt it down here in Dunedin, but not nearly as much of a shake as the first big one in the Canterbury sequence. It went on for a long time though which I knew was a bad sign.

    I hope the aftershocks settle down quickly.

    1. It’s a sign of what will likely happen when one of the local faults ruptures. What intrigues and worries me has been the way modern buildings have been failing under just a third of their designed quake loadings. Doubtless it will be looked into but I can see an endless round of remedial work ahead. Curiously all the quake condemned buildings in Cuba Street stayed up…

      1. Yes, that really is a concern. I hope that this recent earthquake is used as a ‘second chance’ to get things right before one of the local faults goes. The tsunami drills could use some practice too, we might not be so lucky next time.

        1. Me too. There was definitely something lacking in the tsunami warnings. My sister was tipped off by a message from the Netherlands, where the news apparently arrived before it did in her area.

  5. I hope all continues to go well for you and yours, and for you Kiwis in general.

    I’ve been through a few quakes myself, but I’ve never had to worry about tsunamis in conjunction with them, let alone have the tap turned on for rain after it all.

    1. It’s sunny today after more torrential rain last night. Cleanup continues! At least one building will be demolished starting today. It’s been intriguing: quake proofing has meant we don’t get the catastrophic structural failures of earlier decades but the damage is still enough to ko the building. Plus side is they stay up and don’t shed debris, so occupants and passers by are safe.

    1. We get that here too, only more slanted on the back of gales. Brollies have a service life measured in minutes… things coming back to normal but there are worries about the structural integrity of some downtown Wellington buildings and questions being asked about their seismic performance.

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