Planning starts to cope with megaquake

A megaquake along the Alpine Fault with the potential to ‘‘unzip’’ the South Island happens roughly every 300 years – the last one was in 1717.

The Government has put together a team of scientists and civil defence experts to investigate the possible 8.0 magnitude quake, which could tear chasms in the ground, cause landslides, damage state highways and topple cellphone towers.

The Ministry of Civil Defence and Emergency Management has granted them $490,000 to create a high-level response plan within two years.

GNS Science earthquake geologist Robert Langridge said the most likely outcome of the megaquake was the South Island ‘‘unzipping’’ from Milford Sound to Hokitika.

Although the last Alpine Fault rupture happened 299 years ago, a similar event could still be 50 years away. But the longer it took, the worse it would be as pressure built beneath the Earth’s surface, he said.

‘‘We just need to start to be prepared for an event that will eventually happen.’’

Response efforts would likely be coordinated from Marlborough as the main access point between the North and South Island, with agencies from across the country working from the emergency operations centre in Blenheim.

The team of scientists and civil defence experts had split into three panels: one to ‘‘draw a picture’’ of poss- ible scenarios, one to show how the landscape would be affected, and one to coordinate the civil defence response, or the ‘‘human outcome’’, Langridge said.

They would liaise with agencies, ‘lifeline’ groups and regional civil defence departments to create the plan.

‘‘An earthquake of that magnitude would rupture the seismogenic crust.

‘‘It’s a bit like a zipper in that the energy and force of the earthquake moves along the fault line, and the energy is great enough to shift the surface of the earth.’’

The effect would be similar to the chasms found after the 2010 earthquake in Canterbury where the ground was pulled apart, Langridge said.

‘‘That earthquake ruptured the surface, with roads, fences, tree-lines offset by four or five metres. So when the Alpine Fault goes, the whole South Island won’t split apart, but it will displace roads and fences.’’

Main highways and roads could be damaged or blocked by landslides, electricity networks could be cut, phone lines and cellphone towers could be damaged and many homes could be too dangerous to return to, Langridge said.

‘‘The whole purpose of the project is to think carefully about what this event could do in terms of damaging state infrastructure.’’

Agencies such as Land Search and Rescue, St John and Civil Defence would come to the aid of people affected in the West Coast, Canterbury, Southland and Otago. ‘‘Lifeline’’ utilities such as electricity and cellphone providers would also contribute to the plan, Langridge said.

Marlborough was identified as the most likely response base, co-ordinated by civil defence officers.

Marlborough District Council emergency manager Brian Paton said the region needed to prepare for an influx of people displaced from their homes.

After the Canterbury earthquakes, about 10,000 people left the region, many settling in Marlborough.

‘‘We really weren’t prepared for that,’’ Paton said.

Refugees from Canterbury needed housing, jobs and medical support, and the demand would be greater when the Alpine Fault ruptured, he said.

‘‘Recovering from these events, as Christchurch has discovered, takes years and years and years. And that’s where it can become unstuck,’’ Paton said.

The panels would come together again in August to compare notes, Langridge said.

‘‘Simply by geologically mapping the South Island, you know the land has been there for hundreds of thousands or millions of years. It’s not about to drop into the ocean,’’ he said.

‘‘On the other hand you can’t underestimate the effects of the earthquake, so that’s why we need to give reasonable advice, and prepare for it.’’