This weekend’s tragedy on Japan’s Mount Ontake reminds us that life around the Pacific ‘rim of fire’ is often risky.
That string of tectonic plate collisions stretches around the whole circumference of the Pacific and has shaped life in many ways. It was cause of the 2011 tsunami that devastated eastern Japan. It gave the US Yellowstone. It provokes earthquakes. It has also shaped my home country, New Zealand – and has been doing so for at least the past ten million years. The obvious question is ‘what next’ – something that has exercised seismologists and vulcanologists for generations. One way of finding out is to look back into the past, figuring out where fault lines are and how often they move.
That’s certainly been a focus of ongoing work in New Zealand, which straddles the collision between the Australian and Pacific plates and is prone to massive earthquakes. And of all the historical quakes, it seems few were as spectacular as the series that ripped through the country around 1460, as an indigenous Maori culture began to emerge from its Polynesian settler origins. All of them were around magnitude 8 or higher. They began, it seems, in the south as the Alpine Fault moved. Then there was a quake off what is now Wellington. And another in the Wairarapa. And another at Ahuriri, creating the Te Whanganui-a-Orotu lagoon. Wham! Tsunami followed, 10 metres or more high.
Maori refer to the 1460 Wellington quake as Haowhenua – the ‘land swallower’. Superficially that’s a paradox; the quake created land, raising the channel between Miramar, then an island. But the quake also triggered tsunami, washing far around the coasts and inundating settlements and gardens on the south coast of the Wairarapa. For Maori, the key issue was the loss of food-stuffs by a disaster that had, literally, swallowed their land.
A succession of quakes of this magnitude remains unprecedented. Seismology, to date, has usually treated quakes as independent events. And yet it’s clear that earthquakes occur in clusters, and seismologists have been asking questions of late that point to connections. One of those is interactions between fault lines. A quake on one fault might deliver enough energy to a nearby fault to trigger it, providing that fault was already under stress. There is also the effect of ‘slow quakes’. This only emerged in the early twenty-first century when GPS measurements revealed that, at certain points where the Pacific plate dives under the Australian – usually east or west of the New Zealand land mass itself – there are areas where the two slip slowly, but not smoothly. Huge earthquakes follow, but the energy released is spread out over months and not detectable by conventional instruments.
What these quakes seem to do is stress shallower fault lines, east in the plate interface. Current analysis indicates that a slow-slip quake under Kapiti island in early 2013 was likely cause of the succession of conventional quakes that struck in a semi-circular arc around Kapiti from mid-2013 – the Cook Strait and Grassmere quakes of July and August; the Eketahuna quake of January 2014; and the Waipukurau quake of April 2014.
All were severe quakes, but not in the league of the 1460 series. As yet the jury’s still out on the linkages. If the hypothesis is right though, the issue is obvious. Slow quakes might provoke successions of conventional shallow quakes in New Zealand. And if the 1460 sequence was one of those, it’s clear these quakes can be large indeed.
That begs a question: what would happen were New Zealand to suffer a similar quick-fire succession of huge quakes? That’s something I’ve tackled in my book Living on Shaky Ground (Penguin Random House). I won’t repeat the details here – suffice to say, it’s spectacular and I can’t help thinking that Mars looks appealing about this time of year.
Copyright © Matthew Wright 2014