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].
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.
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.
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