Where New Zealand's next big earthquake is going to strike

by Sally Blundell / 21 October, 2018
Volcanos such as Mount Tarawera generally erupt independently of earthquakes. Photo/Alamy

Volcanos such as Mount Tarawera generally erupt independently of earthquakes. Photo/Alamy

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The country is about due for a big quake, and it may not be along the Alpine Fault.

In Wellington, we waited for the big one. In 2011, in Christchurch, we thought we’d had it. Five years later, Kaikōura surprised us all in producing the strongest ground shaking ever recorded in New Zealand, shunting the South Island 5m closer to Wellington and prompting massive onshore and offshore landslides. Now the Hikurangi subduction zone, arcing down the east coast of the North Island to the top of the South Island, is promising to be the big one.

All are related to the long boundary between the Australian and Pacific plates. In the central South Island this boundary is marked by the Alpine Fault, stretching some 500km from Marlborough, down through the main street of Franz Josef and heading out to sea at Milford Sound, the result of the continental crust of the Pacific Plate sliding past the continental crust of the Australian plate. Research into the past 27 large earthquakes on this fault shows a turnaround of about 290 years. The last rupture was in 1717AD when an estimated 8.1 magnitude earthquake, according to a recent special edition of the New Zealand Journal of Geology and Geophysics, ruptured along 380km of the South Island, jolting the fault’s south-eastern side 8m further south in just seconds. So yes, we are at the end of the last seismic cycle, but these figures are based on averages – intervals between earthquakes in the seismic record range from 100 to 500 years.

At its northern end, the Alpine Fault splays out into the Marlborough faults, some onshore, some offshore, some connected but with different names. How these interact in a single earthquake is not fully understood. Before the 2016 Kaikōura earthquake, scientists estimated an earthquake of magnitude 7.5 was possible in the region, but they did not anticipate 21 faults rupturing, nor the ability of seismic energy to “jump” distances between faults of up to 20km.

Further north again, the Hikurangi subduction zone promises even larger surprises. Subduction zones – when one plate dives under the other – are responsible for the largest and most powerful earthquakes and tsunamis in the world, as seen in recent years in Sumatra (2004), Chile (2010) and Japan (2011).

The geological record tells us that a large megathrust earthquake of magnitude 8 along the Hikurangi subduction zone, triggering a tsunami of up to 10m, will happen at some time.

Whether the whole fault will break at once, potentially causing a massive magnitude 9 (that is 63 times more powerful, in terms of energy release, than Kaikōura’s 7.8 earthquake), is uncertain.

Smaller earthquakes of magnitude 7 or 8, including two off Gisborne in 1947, two around Wairau Bar in the past 1000 years and at least eight around Napier in the past 8000 years, indicate shorter areas of rupture. But the record is short, the number of earthquakes too few to establish a pattern, and smaller faults and slow-slip events – Kaikōura triggered slow-slip events beneath Hawke’s Bay and the Kāpiti Coast and Marlborough Sounds – do release some of this pent-up energy.

Earlier this year, as part of an ongoing study into the earthquake and tsunami potential of the large undersea fault system that marks the Hikurangi plate boundary, a team of national and international scientists on board the scientific research vessel Joides Resolution sailed off the coast of Tolaga Bay to drill under the ocean floor, inserting two “observatories” to investigate the processes that underlie slow-slip events.

Though tectonic plates are all connected, the impact of one earthquake on another fault is hard to ascertain. Geological stresses in the rock can be transferred, making one earthquake follow another, says GNS Science’s Kelvin Berryman, “but we don’t know if an earthquake on the Alpine Fault will cause a subduction earthquake, or vice versa”.

On the skewed horseshoe that is the Pacific “ring of fire”, understanding those interactions is hugely important for increasing our knowledge of earthquakes and volcanoes.

Off New Zealand’s east coast, as the Pacific plate descends into the hot interior of the earth, hot water from the descending crust rises up and mixes with the melting rock to form magma that feeds the volcanoes in the Taupō volcanic zone.

But evidence of cause and effect between subduction-zone earthquakes and eruptions, says Victoria University of Wellington geologist and recipient of last year’s Rutherford Medal, Colin Wilson, has been hard to gauge from the geological record. In Japan, for example, the only geological evidence of the 2011 earthquake was the tsunami. “If that evidence is destroyed,” he says, “which is likely, what evidence do you have that such an event occurred?”

In New Zealand, the last three sizeable events – Rangitoto, Tarawera and Taupō – appear to have no precedents we are aware of in the geological record.

Internationally, he says, there are only two cases where big earthquakes most likely triggered eruptions – and in both cases the eruptions were small.

“The volcano has to be in a mood to want to erupt,” says Wilson. “It’s like prodding a horse – if the horse is dead no amount of shaking will wake it up.”

Even if half asleep, it may slumber on, but if it is close to waking it could take just a small nudge to swing into action. But a volcano’s state of sleep is hard to gauge. Even if it begins to puff and rumble, as the Taupō volcanic zone has done in the past, that does not mean an eruption is going to happen or, if it is, how big it is going to be. Comparisons suggest unrest events are one to three times more frequent than eruptions but as Wilson says, “Volcanos march to their own beat.”

“We know, within reasonable bounds, what to expect when any one of New Zealand’s volcanoes erupts again. What we don’t know is how many episodes of unrest we will see before the volcano decides to get its act into gear; how we can tell unrest from eruption before the magma hits the surface.”

As with earthquakes, there is some evidence of clusters of volcanic activity but whether these are triggering each other or responding to the same changes in the crust we don’t know.

The Auckland Volcanic Field, involving 53 volcanoes in the most densely populated volcanic field in the world, is intraplate, the result of “some kind of anomaly in the mantle”, says Wilson, rather than two plates interacting. Most are monogenetic, meaning they erupt only once, so it is likely that the next volcanic vent in Auckland will erupt in an entirely new location. Again, the where and when is complex. The oldest eruption, Pupuke, was about 200,000 years ago, while the most recent, Rangitoto, was around 600 years ago. There have been periods of intense activity, with six to 10 volcanoes erupting within a 4000-year timeframe, followed by thousands of years of silence.

This article was first published in the September 29, 2018 issue of the New Zealand Listener.

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