New Zealand’s largest and longest geological fault line has a “high probability” of rupturing in the next 50 years, producing “one of the biggest earthquakes since European settlement”, according to a new report.

The Alpine Fault runs for 600km along the “spine” of the South Island, marking the confluence of the Australian and Pacific plates.

The government-owned research institute GNS Science says the fault has produced four earthquakes of around magnitude 8 in the past 900 years, most recently in 1717.

In a report to be published in the American Geophysical Union journal Geochemistry, Geophysics, Geosystems, researchers say measurements of past earthquakes and knowledge of the frequencies of earthquakes of different sizes on the fault have allowed them to calculate the probability of a Magnitude 8.1 earthquake in the next 50 years at 30%.

This would release about 30 times more energy that the Darfield earthquake of September 4 2010 – a Magnitude 7.1 event that devastated Christchurch.

“An Alpine Fault earthquake will likely rupture a larger fault length (several hundreds of kilometres rather than several tens of kilometres) over a longer period of time (100s of seconds rather than tens of seconds), and affect a much larger area than the Darfield earthquake,” the report says. 

Aftershocks would be as large as Magnitude 7.

The report’s lead author, Victoria University of Wellington Associate Professor Simon Lamb, says new seismic research has found the landmass of the Pacific Plate “is actually sitting and sliding right on top of the Australian Plate” for some 350km.

The report says a large Alpine Fault earthquake would cause environmental impacts lasting up to 50 years, with major changes to landforms and rivers.