Alpine Fault overdue for magnitude 8+ quake - scientists

The Alpine Fault is overdue for an earthquake that could reshape the South Island, scientists say.

Last year marked the 300th anniversary of the last time it ruptured, moving the south-eastern side about eight metres relative to the north-western side in a matter of seconds.

That quake measured an approximate magnitude 8.1, making it about three times stronger than the Kaikoura quake of 2016.

"Intense shaking caused numerous large landslides, forest damage, and long-term changes to drainage and sedimentation patterns in the Southern Alps, across the coastal plain, and offshore," researchers said on Tuesday.

The New Zealand Journal of Geology and Geophysics has published a special issue on the Alpine Fault, which runs the length of the South Island. There are contributions from scientists at GNS Science, the Universities of Victoria, Canterbury and Otago, and from overseas.

"The intrigue of this earthquake arises from the gradual unveiling of multiple lines of research that have come together to consolidate our understanding of what took place in 1717 AD," the journal said in a statement.

The 1717 rupture moved the Alpine Fault by about eight metres.
The 1717 rupture moved the Alpine Fault by about eight metres. Photo credit: Newshub.

"Although Aotearoa New Zealand had been settled by Māori for about half a millennium when the earthquake occurred, oral histories of the event are rare. The story is told instead by trees, sediments, and the landscape of the South Island's West Coast."

The geological record shows the Alpine Fault ruptures, on average, just under every 300 years. Ten years ago, scientists only had evidence for three separate quakes on the fault - now they have seven recorded at multiple sites along the fault, and 27 in individual stretches.

They were able to narrow down the timing of the last big one to 1717 by looking at tree ring patterns.

"The timing of the disturbance that caused [a] decline [in growth] can be narrowed down to taking place after the 1716 growing season and before the end of the 1717 growing season - ie, the earthquake occurred early in the year 1717 AD."

And being on the boundary of the Pacific and Australia tectonic plates, it will happen again.

"There will be a similar earthquake soon... Whatever we can learn about this fault and how it moves is, to some extent, helping us understand and prepare for the next great earthquake."

The special issue was due to coincide with the 300th anniversary, but research into the Kaikoura quake - one of the most complex ever recorded - took priority.

"The surprises and insights we gained from witnessing a major surface-rupturing earthquake in modern times have made us even more motivated to compile and present current research on the Alpine Fault."