Big quake likely within 50 years

NEW Zealand's bigrisk Alpine Fault has a 75% chance of causing a major quake — most likely a powerful magnitude8 event — within the next 50 years.

A new study has found the probability of an event within coming decades is much higher than the earlierestimated 30%.

Scientists have also calculated an 82% chance the quake will be greater than 8.0.

The Alpine Fault, which runs about 600km up the western side of the South Island between Milford Sound and Marlborough, poses one of the biggest natural threats to New Zealand.

It has a clear geologic record of rupturing around every three centuries — and 2017 marked the 300th anniversary of what is thought to have been a magnitude8 quake that moved one side of the fault by about 8m in a matter of seconds.

Recent research has suggested a big quake could block South Island highways in more than 120 places, leave 10,000 people cut off, and cost the economy about $10 billion.

‘‘Detailed analysis and careful dating of the sediments in lakes that lie adjacent to the Alpine Fault have led to new insights into the distribution of earthquakes along the fault," study coauthor Prof Sean Fitzsimons of the University of Otago.

The research pointed to a curious ‘‘earthquake gate’’ on the fault south of Jackson Bay, near Martyr River, which appeared to determine how large an Alpine Fault earthquake could become.

Some ruptures stopped at the gate, producing ‘‘major’’ earthquakes in the magnitude7 range, but the ruptures the gate let through grew even larger ones.

Study lead author Dr Jamie Howarth, of Victoria University, said the gate may help with forecasting the size of future Alpine Fault earthquakes.

‘‘From the record of past earthquakes, we can determine that the probability of a magnitude 7 or higher event is about 75% in the next 50 years,’’ he said. ‘‘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.’’

The record showed the past three earthquakes ruptured through the gate, producing plus8 earthquakes.

‘‘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,’’ he said.

‘‘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.’’

‘‘This finding doesn't change the fact the Alpine Fault has always been hazardous,’’ Dr Howarth said.

‘‘But now we can say the next earthquake will likely happen in most of our lifetimes.’’

He said New Zealand needed to move beyond planning the immediate response to the next event, and to make decisions to improve the area’s infrastructure and community preparedness.’’

❛ the next earthquake will likely happen in most

of our lifetimes