Something slightly scary occurred to me the other day. Analysis of the Mw 7.8 quake that ripped through central New Zealand last November suggests it was awesomely complex.
We usually imagine quakes being caused when one fault line moves. Or maybe two or three faults, because faults tend to exist in connected systems. And often, that is precisely the case. But with the Kaikoura quake, you know how many faults moved? It was (wait for it) – twenty one. It was, in effect, a multi-epicentre event.
What’s more, that movement bridged several otherwise unconnected fault systems. Seismologists hadn’t known that earthquakes, anywhere in the world, could involve such a widespread systemic movement. Now we know they can, and that’s where research is going to start looking.
This discovery was made, I should add, because the dedicated seismologists and geologists working at New Zealand’s Geological and Nuclear Sciences walked the ground, checking out every fault and measuring what had happened.
The discovery doesn’t surprise me too much. Imagine a sheet of broken glass sitting on a large vat of boiling porridge. Small bubbles in the porridge will move one or two fragments, but a really big roil will set a fair number grinding against each other. It’s not an exact analogy, of course, but you get the picture.
There’s something else that came out of it for me, though. A few years ago, when I wrote a science book on New Zealand’s earthquakes (now out of print) I covered all the main prehistoric quakes we know about in the country.
Prehistoric, here in New Zealand, means anything that happened before the advent of locally written records – which in a practical historical sense is the late eighteenth century when explorers began nosing into our waters in numbers.
Identifying and dating prehistoric quakes has been a challenge: only the very largest movements leave traces in the rocks, which is a relatively coarse-grained record. The quake can certainly be pinned down to a specific decade, sometimes even a particular year – especially if tree-growth evidence can be added to the mix – but beyond that it’s hard to pin it to a specific month, week or day. And smaller quakes leave no traces at all.
What worries me is this. Around 1460, New Zealand was ripped apart by a rapid succession of magnitude 8+ quakes that apparently began in the far south and continued along the main fault lines into Hawke’s Bay. They included a major movement of the Alpine Fault, a huge earthquake in Wellington that triggered huge tsunami which devastated Maori gardens and settlements – going down in the oral record as Haowhenua (‘the land eater’) – and another massive shock in Hawke’s Bay where the down-thrust created the Ahuriri lagoon, Te Whanganui-a-Orotu.
The usual consensus, given the vagaries of dating, is that these quakes occurred in quick succession, but just how quick isn’t clear – they might have been months or even a year or two apart. We can’t quite tell. There is nothing unusual about this: the way in which one quake can set up another, which then happens months or even years later, is well established.
What struck me, though, is that maybe they weren’t a succession of separate quakes, causally driven when one quake increased tensions in the fault lines further along the system. Maybe they all happened at the same time. That’s perfectly possible within the dating uncertainties (which are normal to science – it doesn’t mean they don’t know, it means they are being careful and scientific).
The scenario that follows is deeply scary. Imagine it: New Zealand – ripped up by a compound-fault quake so huge it appears geologically as a succession of multi-epicentre Mw 8+ shocks. Ouch. Of course we don’t know – it’s equally possible they were at intervals.
But still, because underlying tectonic pressures don’t stop, the obvious question is whether such a combination (if it occurred at all) could happen again.
Copyright © Matthew Wright 2017