This week’s news that a previously unsuspected magnitude 8+ mega-quake could hit central New Zealand and then douse the place with tsunami isn’t too surprising to me. I wrote the most recent pop-sci book on our earthquakes. It was published by Penguin Random House last year.
While I was writing the book I had a chat with a seismologist at the University of Canterbury, who pointed out that New Zealand is staring down the barrel of some fairly large tectonic guns. The big one on land is the Alpine Fault, which ruptures with 8+ intensity every few hundred years. The last big rupture was in the 1770s, meaning another is due about now – the probability of it happening before 2100 is around 92 percent.
Another risk factor is the Taupo volcano – another product of tectonic plate collision. This is one of the biggest volcanoes on the planet, and evidence is that a monster eruption about 27,000 years ago threw the world into an ice age. It’s got every potential to wreak similar havoc again – check out Piper Bayard’s awesome novel Firelands for her take on what might happen in the US when Taupo next ‘blows’ the world climate. We won’t mention New Zealand’s likely fate in that scenario…
But New Zealand also faces another major tectonic challenge, the Hikurangi Trench, a subduction zone where the Pacific plate plunges under the Australian, off the coast of the North Island. My contact at Canterbury pointed out that this is the other big gun – a potential 8+ quake followed by tsunami that could wipe out the east coast of the North Island.
That’s where the new study comes in. It’s already known that the Southern Hikurangi Margin – the plate collision between Cook Strait and Cape Turnagain – is locked, meaning strains are building up. When they break, it’s going to be devastating – a quake of magnitude 8.4 – 8.7, triggering massive onshore destruction from Napier to Blenheim, followed by tsunami. Now, it seems, this region generates such quakes a couple of times a millennium. Two have been identified; one 880-800 years ago, a second 520-470 years ago.
Uh – yay. On the other hand, it doesn’t really change the risk factors. New Zealand shakes. The end. The issue isn’t worrying – it’s quantifying the risk, which is why work to explore past quakes is so important.
The report also highlights something for me. The discovery that a mega-thrust quake hit central New Zealand somewhere between 1495 and 1545 – seems to unravel one mystery that has long puzzled me. At a date usually put down to roughly around 1460, plus or minus, New Zealand was riven by a rapid-fire succession of great earthquakes, all thought to be over magnitude 7.5 and most over magnitude 8. They included movement on the Alpine fault, another movement in Wellington that turned Miramar into a peninsula, and another in Hawke’s Bay where a dramatic down-thrust created the Ahuriri lagoon.
Things get a bit vague when sorting out timing because the traces of past quakes are difficult to date beyond a broad range of possible dates.
The Wellington event was so huge it went down in Maori oral tradition – Haowhenua, the Land Swallower. Why swallower? That was odd, given the quake was an upthrust – but actually, it DID eat land that counted to Maori. Massive tsunami flooded the southern North Island coasts, inundating important gardens near Lake Onoke on the south of the Wairarapa. In short, swallowing the land. I was, I believe, the first one to publish that explanation, not that anybody noticed. But I digress.
The point is that the date-range for the “1460” series overlaps the date range for the newly discovered mega-thrust quake – which included tsunami. And it explains why New Zealand was, apparently, hit by so many large quakes in quick succession. Even if they were not the same event – and, seismologically, they probably weren’t – the way strains and stresses redistribute after a major quake is well known to be liable to trigger another. Is that what actually happened? Research is ongoing. We’ll see.
Copyright © Matthew Wright 2015