There's a three-in-four chance a large magnitude 7-plus earthquake will strike along the South Island's Alpine Fault within the next 50 years, new research has found - more than double what was previously estimated.
And if an earthquake above magnitude 7 does hit before 2068, it's likely to be destructive - with scientists calculating there's a four-in-five chance that quake will be magnitude 8 or higher thanks to data gleaned from a newly discovered 'quake gate'.
The study, published on Tuesday morning in scientific journal Nature Geoscience, was led by Kiwi scientist Dr Jamie Howarth and included scientists from Victoria University, Otago University, GNS Science, the University of California, and the US Geological Survey.
Researchers analysed data from 20 previous Alpine Fault earthquakes, recorded in four West Coast lakes and two swamps over the last 4000 years, and used it to build "one of the most complete earthquake records of its kind in the world".
"From the record of past earthquakes, we can determine that the probability of a magnitude 7 or higher event is about 75 percent 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," explained Dr Howarth.
The 75 percent figure is significantly higher than previously thought, with scientists until now believing the chance of a massive earthquake along the fault line was about 30 percent.
How 'gate' discovery could forecast earthquake size
One of the major findings from the study was the discovery of what researchers have dubbed the 'earthquake gate' - an area along the 850km-long fault line that could determine the size of future quakes in the region.
The study found earthquakes that stop at the gate - which is located south of Jackson Bay, near Martyr River - produce 'major' earthquakes in the magnitude 7 range. However, those that rupture through the gate tend to become 'great' earthquakes of magnitude 8 and beyond.
Data shows the past three earthquakes ruptured through the gate, producing 'great' earthquakes, magnitude 8 or higher.
"Our modelling shows that if you've had a run of three passing ruptures, then the next one will also likely pass through the gate," said Dr Howarth.
"We are therefore expecting the next earthquake to be similar to the last one in 1717 - an estimated magnitude 8.1, which ruptured about 380km of the fault."
Dr Howarth says their research suggests there is an 82 percent 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, but now we can say the next earthquake will likely happen in most of our lifetimes," he said.
"We need to move beyond planning the immediate response to the next event... to thinking about how we make decisions about future investment to improve our infrastructure and community preparedness."
Researchers applied a physics-based model to the Alpine Fault, which took into account its variations in geometry and slip rate, and the fact it's split up into segments.
Dr Howarth says the research shows earthquake science might now be getting to a point where forecasting could be possible.
"This is the first time there has been a paleoseismic data set that spans multiple large earthquakes and seismic cycles of sufficient quality to allow us to evaluate how such models behave."
Scientists say the areas likely to be most severely affected by an Alpine Fault quake would be the West Coast, inland Otago, Fiordland, inland and low-lying coastal parts of Canterbury, and southern parts of Nelson, Tasman and Marlborough.
