Alpine Fault quake risk higher than experts thought

Monday, 19 April 2021

New research reveals the chances of the South Island’s Alpine Fault generating a damaging earthquake in the next 50 years are much higher than previously thought.

New research led by Te Herenga Waka–Victoria University of Wellington senior lecturer Dr Jamie Howarth shows the probability of that earthquake occurring before 2068 is about 75 per cent.

Until now, it had been thought to be about 30 per cent, based on sequences of sediment deposited adjacent to the Alpine Fault in northern Fiordland.

Scientists from Victoria University, the University of Otago, GNS Science, the University of California, and the United States Geological Survey also calculated there is about an 82 per cent chance the earthquake will be of magnitude-8 or higher.

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“From the record of past earthquakes, we can determine that the probability of a magnitude-7 or higher event is about 75 per cent in the next 50 years. So we now know the chances of seeing a large Alpine Fault earthquake in our lifetime are better than a coin toss. That is a really significant result, but we can’t forecast the magnitude of the next event from these data alone,’' Howarth said.

One of the researchers’ findings is that a curious “earthquake gate” on the fault south of Jackson Bay, near Martyr River, appears to determine how large an Alpine Fault earthquake gets.

Howarth, from Victoria University’s School of Geography, Environment and Earth Sciences, says this earthquake gate may help with forecasting the size of future Alpine Fault earthquakes.

The new work showed that not every earthquake seen at Milford Sound made its way all along the fault. Some seemed to stop at the gate.

The Alpine Fault goes onto the land at Milford Sound, travels up the base of the Southern Alps, then does a dogleg through northern Marlborough to go offshore at Cloudy Bay.

'Sometimes the gate is open and earthquakes rupture through that geometric complexity of the fault.'

When that happened, the earthquakes were magnitude-8 or higher.

'Other times the gate is closed and earthquakes stop there.'

These passing or stopping ruptures tend to occur in sequences, producing phases of major or great earthquakes through time.

It was not known for sure where along the fault earthquakes had started in the past, he said.

'Currently we have a hypothesis that many of the earthquakes start in the south and rupture to the north, but that's certainly not always the case.'

If there was a rupture in the Fiordland and south Westland section of the fault, the result would be an earthquake of somewhere in the magnitude-7 range, with ground motion in a part of the country that did not have a high population density.

GNS Science earthquake geologists studying layers in the trench across the Alpine Fault at Springs Junction.

'We would all get woken up by that earthquake … but the immediate impact probably wouldn't be that severe.'

But if an earthquake broke through the gate and ruptured the southern and central section, the rupture would go from Fiordland to inland of Hokitika, or even further north. Then there could be a magnitude-8-plus event, with the rupture extending much closer to areas where more people lived, Howarth said.

Previous research from the site near Milford Sound showed large earthquakes happened there about every 300 years. The new research showed that further north the time between earthquakes was about 250 years.

'The Alpine Fault is like a metronome, it produces earthquakes extremely regularly … It's that behaviour that allows us to forecast when the next event is likely to occur.

'If we get a great earthquake on the fault that ruptures from Milford to Hokitika, or more, and has a south to north direction, then you end up casting strong damaging ground motions north of where the fault has ruptured.'

In that instance there would be damaging ground motions in Nelson and Marlborough and potentially in Wellington and Christchurch.

'One of the things we don't know particularly well is what the ground motions are going to be for the next Alpine Fault earthquake.'

The Alpine Fault is 850km long, and how much of that breaks in any given earthquake determines the magnitude and impact.

It extends from offshore at the bottom of Fiordland all the way to Cook Strait. It goes onto the land at Milford Sound, travels up the base of the Southern Alps, then does a dogleg through northern Marlborough to go offshore at Cloudy Bay.

Until this work, most of the information on the pattern of past earthquake behaviour was from a single site near Milford Sound.

The new research provided long earthquake records from multiple sites along about 350km of the fault, from Milford Sound to around the Hokitika area – the part of the fault that produces the largest earthquakes most often, compared to anywhere else on the fault.

The best forecast was there was an 82 per cent probability the next such earthquake will pass through the gate and cause an earthquake of magnitude-8 or higher.

“This finding doesn’t change the fact the Alpine Fault has always been hazardous,” Howarth said.

“But now we can say the next earthquake will likely happen in most of our lifetimes. We need to move beyond planning the immediate response to the next event, which has been done well through the AF8 programme, to thinking about how we make decisions about future investment to improve our infrastructure and community preparedness.”