The Alpine Fault ruptures in a massive earthquake every 300 years, on average, and the last big one was in 1717.
The fault runs hundreds of kilometres up the spine of the South Island, from Fiordland along the western edge of the Southern Alps.
Research has extended the record of earthquakes on the fault back 8000 years, revealing it produced 27 quakes of magnitude 8 or greater over that time. Scientists say there is no reason why the pattern should change now.
More and more evidence is being found showing these quakes happened with remarkable regularity – the latest from sediment in two sites in Fiordland.
Based on this evidence, scientists say there’s a 29 percent chance of another significant earthquake on the Alpine Fault in the next 50 years.
No matter where along its length the quake starts, the ground will shake throughout the South Island, causing the worst damage within 100km of the fault itself. For the lower North Island, it will be at least as strong as in the 2016 Kaikōura quake.
“The ground shaking will last three to four minutes across the South Island,” University of Otago scientist Caroline Orchiston says.
Close to the fault, the ground could lift up to three metres and move sideways by eight metres.
Because a big quake is a South Island-wide event, officials are now planning how all the six civil defence regions will work together in an emergency response.
“It’s really focused energy in terms of the need for civil defence emergency management to work collectively across the South Island,” Orchiston says.
The project, AF8, uses a scenario of a 400km rupture of the fault in a quake of about magnitude 8, starting in south Westland and rippling north.
Most damage to buildings and other structures would be in western Southland and Fiordland, the Queenstown Lakes, the West Coast, inland Otago and Canterbury, and southern parts of Tasman and Marlborough districts.
Power would be out in much of the South Island as hydro stations shut down, telecommunications cut, airports will be unusable or closed to check for damage, local health services will struggle to deal with injured and sick people, and towns and rural communities will be cut off.
Shaking will bring down huge rockfalls and landslides, blocking roads, taking out rail lines and plunging into rivers to create dams. These would be vulnerable to breaking in an aftershock leading to flooding. In a big quake, tsunami waves are likely in lakes and Sounds close to the fault line.
“The glacier towns of Fox and Franz Josef lie right on the Alpine Fault itself so chances are those towns will be cut off for a significant period of time,” says Orchiston, who is lead scientist for the AF8 project.
Regions will gear up to receive evacuees and send support into other areas worse hit - Otago civil defence would if needed help out the southern West Coast and the glacier townships for example. “[That] has been less the case in the past where the requirement has been to plan within each region,” says Jon Mitchell, programme manager for the AF8 project.
Helicopters will be key in evacuating seriously injured people and dropping supplies in the first few days – and co-ordinating fuel supplies will be part of that.
Rather than civil defence and emergency authorities responding alone, the plan is for DOC, tourism operators and iwi to help identify the worst hit areas.
In a quake of this scale, people need to prepare to look after themselves for at least seven days, Mitchell says.
Close to the fault line communities could be cut off by road for days or months. Getting supplies of water, food, shelter and fuel to communities suffering damaged homes and food sources will need national, if not international, support.
As with Kaikōura in the 2016 earthquake, thousands of visitors will be at tourist spots. Visitors and seasonal workers swell the population of Queenstown Lakes, Otago and the West Coast, taking day trips to Milford Sound in the summer or travelling to ski areas in the winter.
“The actual population during peak season can be easily four times the resident population,” Mitchell says. At peak summer season, Glenorchy, near Queenstown, permanent home to 400 people, could have another 800 visitors on walking tracks in the area.
Watch - Steve Hewland, Camp Genorchy project manager:
Watch - Tom Thomson of Christchurch firm EPL on rebuilding a company after the 22 February 2011 quake:
The plans will be tested late 2020 in a national exercise for the Alpine Fault, including NZDF, major tourism operators, DOC and transport providers.