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Source: GNS Science

New research shows the chances of the Alpine Fault in the South Island producing a damaging earthquake in the next 50 years are higher than previously thought.

The study, led by Jamie Howarth of Victoria University of Wellington with support from earthquake geologists at GNS Science, puts the probability of an earthquake on the central section of the fault at 75 percent in the next 50 years. This compares to 29 percent based on previous Alpine Fault studies.

Study co-author Rob Langridge from GNS Science says this research does not change the likely impacts and consequences of a quake on the Alpine Fault, including the likely scale and extent of damage.

The Alpine Fault runs 600km up the spine of the South Island between Milford Sound and Marlborough. It last ruptured in 1717 producing an earthquake of about magnitude 8.

“Scientists have been studying the Alpine Fault for decades, and this study gives us a more precise picture of the likelihood of an earthquake – and how the fault might behave.

“Communities throughout the South Island, and especially on the West Coast, have been preparing for an Alpine Fault earthquake through initiatives such as the EQC-funded AF8 programme.

“These programmes are building collective resilience and preparedness for the next Alpine Fault rupture.

“As the next rupture will almost certainly happen within many of our lifetimes, these education and awareness initiatives are now more important than ever.”

A 14-centre AF8 roadshow is currently under way in the South Island, where residents can talk directly to scientists and access information that is targeted to their region.

The “earthquake gate”

The research presented by Howarth and his team combines field studies from lakes along the Alpine Fault, with sites previously described from Fiordland by GNS Science, and a physics-based 4-D model of the Alpine Fault along its length. The model simulates up to 100,000 years of fault activity, comparing with the last 4000 years of known earthquakes on the fault.  Dr Langridge says the simulations [RL1] show that the fault has two primary rupture modes through time.

“One mode is where the central section of the fault ruptures independently of the southern section in Fiordland.

“In the other mode, the two sections rupture together and pass through an ‘earthquake gate’ on the fault south of Jackson Bay – growing to a magnitude of 8 or more.

In 2014 scientists drilled nearly 1km into the Alpine Fault in the Whataroa Valley north of Franz Josef to learn more about its inner workings. Image – Julian Thomson.

“The last three earthquakes on the Alpine Fault passed through this gate – increasing the likelihood the next one will too.

“Modelling indicates that there is an 82 percent chance the next major earthquake will be of magnitude 8 or higher.”

Previous studies have identified the last 27 ruptures going back 8000 years with the last major earthquake on the fault occurring around 1717.

This study, funded by EQC and the Rutherford Foundation, was published in Nature Geoscience this week. It built a physics-based model of the fault that simulates earthquake ruptures over tens of thousands of years.