Big Alpine Fault quake would cut off 10,000 residents

A big quake on the Alpine Fault could block South Island highways in more than 120 places and leave 10,000 people cut off, new research has estimated.

The fault, which runs about 500km up the western side of the South Island between Milford Sound and Marlborough, poses one of the biggest natural threats to New Zealand.

It has a clear geographic record of rupturing around every three centuries - and last year marked the 300th anniversary since an 8.1 quake that shunted the fault's southern side eight metres further south in a matter of seconds.

One of a series of scientific papers published today suggested a large event could strand some 10,000 people living in affected areas, along with several thousand tourists.

Roads would likely be blocked at State Highway 6 just south of Franz Josef, 20km northeast of Haast and 10km east of Haast, and at State Highway 94 just south of Milford Sound and State Highway 73 just north of Arthurs Pass.

If these sections were cut off, reaching the West Coast would only be possible via State Highway 7 - and with access only to 50km south of Hokitika.

The study pointed out that 1.3 million travellers visited the West Coast each year, with enough capacity for 4000 visitors each night in the popular Franz Josef area.

The Milford Sound area alone attracted 650,000 visitors each year - equating to 1700 each day - and in the event of a quake, hundreds could be spread out in the region and on foot.

The study, led by Tom Robinson of from the UK's Durham University, suggested the number of tourists requiring evacuation by sea and air would be five times that of 2016's 7.8 Kaikoura Earthquake, which was used as a guide in the modelling.

Specifically, the scenario assumed a 7.9 earthquake on the fault, with 350km of surface rupture between Charles Sound in Fiordland and Hokitika.

The study said there should be contingency plans for the evacuation of large numbers of people and the provision of emergency supplies to local populations who could be isolated for extended periods.

"Examining the response to the Kaikōura earthquake is a useful analogue to highlight valuable lessons that can help with planning for the next Alpine Fault earthquake," said Robinson, who worked at Canterbury University for five years before taking up his present position.

The study was one of 12 research papers featured in a special issue of the New Zealand Journal of Geology and Geophysics.

Chief guest editor of the special issue Dr Phaedra Upton, of GNS Science, said the fault had a well documented history of producing large earthquakes at fairly regular intervals and there would be another one at some time in the future.

"Whatever we can learn about this fault and how it moves will help us understand and prepare for the next great earthquake," Upton said.

The lengthy Alpine Fault, which runs along the spine of the Southern Alps, has garnered much attention as it has a clear geographic record of rupturing every 300 years or so. Image / GNS Science

There had been intensive research on the fault over the past two decades and the special volume provided a sample of the latest thinking across a number of disciplines.

It contained papers on tectonics, seismology, paleoseismology, landscape evolution, physical impacts and social science.

"The collection provides an insight into the range of research currently being undertaken, but it is by no means a complete picture."

One of the papers introduced Project AF8, a collaborative effort to save lives by preparing for a magnitude 8 earthquake on the Alpine Fault.

Lead author, Dr Caroline Orchiston of the Centre for Sustainability and the University of Otago, said the potential impacts of the next rupture on the fault continue to increase as New Zealand's population grew beyond 4.5 million people.

"Project AF8 represents the first effort to co-ordinate disaster response planning for a major Alpine Fault event in the South Island, as well as other large sources," Orchiston said.

The project has developed scenarios so South Island communities can plan how they might respond to impacts such as road closures, power losses, fatalities, injuries, and social dislocation.

Based on the first seven days of emergency response, AF8's planning covered such things as the shelter and care of displaced people including the potential for tens of thousands of tourists, an immediate medical response and the restoration of priority telecommunications.

A Tier 4 national Civil Defence exercise was planned to be held in 2020 to build awareness and resilience and to test the planning and national response capability.