Scientists have revealed new insights into what's piling pressure on New Zealand's big-risk Alpine Fault – and whether the past decade's quakes have added to its squeeze.
The monster fault, which runs up the western side of the South Island between Milford Sound and Marlborough, poses one of the biggest natural threats to the country and is expected to rupture over coming decades.
A large rupture could be catastrophic, stranding about 10,000 and killing and injuring many.
Victoria University and GNS Science researchers set out to better understand what stresses were being placed on the fault – especially as it neared tipping point.
Using nearly a decade of earthquake data from the Southern Alps and South Westland, they compiled the largest catalogue of characteristics – so-called "focal mechanisms" - of quakes in those regions.
Gaining a clearer picture was all the more pressing because the fault was considered to be "late" for a big rupture, which tended to occur every 300 years. The last one was about 1717AD.
Most of the data used in the study was collected by sensors that have been constantly recording since 2008.
They have picked up ab out 7700 earthquakes, of which 845 were selected for closer analysis.
The study found the fault was being compressed, broadly in an east-south-east to west-north-west direction, along most of its 600km length.
"For the fault to slip and produce a large earthquake under these conditions likely requires either high fluid pressures or low-friction materials within the fault core," said Professor John Townend, who, with Professor Martha Savage, supervised study leader Konstantinos Michailos.
The team also looked at whether recent large quakes – including the main Canterbury earthquakes and 2016's 7.8 Kaikoura Earthquake – had increased the pressure.
"We did not observe any systematic effects on the stress field acting on the Alpine Fault or throughout the Southern Alps," Townend said.
"Regional earthquakes do induce small stress perturbations, which can be calculated using models of the Earth's crust, [but] these perturbations are too small to be detected in earthquake catalogues such as the one we have analysed."
Among the biggest questions scientists want to answer were why the fault tended to rupture with such regularity, whether quakes typically began in the same position, which direction they travelled in, what caused the ruptures to stop, and what role fluid pressure played at different points in the fault's apparent cycle.
Although scientists may never be in a position to warn of an impending quake, Townend said, new research meant we could better understand how ruptures developed.
"It's important to focus on being prepared and developing the resilience of people, communities and infrastructure."
The study, which also involved GNS Science researcher Dr Emily Warren-Smith, has been published in the journal Tectonophysics.
It comes as scientists this month warned against any future development at Franz Josef Glacier in a 200m-wide area defined as a "fault avoidance zone".
However, Westland District Council scrapped the plan because of legal threats.
That was despite scientists warning that the ground some buildings sit on could be displaced 8m horizontally – and 1.5m vertically – in the next Alpine Fault quake.
One previous proposal even explored shifting the entire township to Lake Mapourika, about 10km away.
The Alpine Fault
• Running about 600km up the spine of the South Island, the Alpine Fault is the boundary of the Pacific and Australian tectonic plates.
• The fault has ruptured four times in the past 900 years - 1717AD, 1620 AD, 1450 AD, and 1100 AD - each time producing an earthquake of about magnitude 8.
• The fault has an estimated 30 per cent probability of rupturing in the next 50 years. The rupture will produce one of the biggest earthquakes since European settlement of New Zealand.
• Researchers have estimated a major rupture could strand at least 10,000 people and block roads and highways in 120 places.