Alpine Fault quake: It will 'likely happen in the lifetime of many Kiwis'

Thursday, 30 August 2018

Dr Caroline Orchiston explains the Alpine Fault and the damage the earthquake will wreak. (Video first published in May 2018)

Most New Zealanders alive today will still be alive when the Alpine Fault rips in a magnitude-8 earthquake, leading scientists say.

Less certain is where the megaquake will hit, but the most likely is a south-to-north quake centred at Milford Sound, with intensity-7, 6 and 5 quakes rippling up the South Island.

The other options are a north-to-south tear starting in Greymouth and heading southwest, or an epicentre near Fox Glacier, heading up and down the fault.

The 7.8-magnitude Kaikōura earthquake ripped road and rail apart. Imagine what a magnitude-8 earthquake would do.

The scenarios were designed and developed as part of Project AF8, or Alpine Fault Magnitude 8, a partnership of all the Emergency Management Groups in the South Island.

**READ MORE:

A south-to-north earthquake is most likely along the South Island's Alpine Fault.

* Preparing for the big one on the Alpine Fault

* Team granted funding to plan response for Alpine Fault megaquake

Milford Sound is the most likely epicentre for a magnitude-8 earthquake on the Alpine Fault.

* Quake research a good reminder**

The massive Alpine Fault is due for another big earthquake and scientists have been drawing up a scenario of what the devastation would look like. (First published May 2018)

Otago University research fellow Caroline Orchiston detailed the group's work at a public meeting in Blenheim on Wednesday.

Regardless of the epicentre, Marlborough was looking at an intensity 6 or 7 earthquake on the Mercalli scale when the Alpine Fault finally gave.

'The Alpine Fault is a spectacular fault, it's more than 800 kilometres long at its full extent,' Orchiston said.

'The intensity of course depends on the type of ground you're standing on and how far away you are from the Alpine Fault.

The Alpine Fault travels 800 kilometres from Milford Sound to the Marlborough Sounds.

'For those of you who felt the earthquake in Kaikōura, those many minutes of shaking, this [the Alpine Fault earthquake] will feel similar for you in Marlborough.'

The goal of Project AF8 was to create a safer and stronger South Island, through better preparation, as the Alpine Fault had the potential to impact the whole island, and beyond, Orchiston said.

The team developed its co-ordinated response discussions around a south-to-north earthquake as it was most likely, she said.

'It was scientifically credible, but also it provided probably the most significant test for Civil Defence around the South Island,' Orchiston said.

Caroline Orchiston says the Alpine Fault is a 'spectacular' fault.

'As you can see, there's a potential for more damage up in the northern part of the South Island, where more of the population lives.'

The Ministry of Civil Defence approved a grant of $680,000 over three years for Project AF8, with work starting in 2016.

The Alpine Fault, running up the South Island, has an earthquake nearly every 300 years.

Camp Glenorchy project manager Steve Hewland talks about the resilience factors of the newly-built camping ground near Queenstown.

Studies at Alpine Fault excavation sites showed a large earthquake occurred on the fault every 300 years or so, with the last one occurring in 1717.

'What we're seeing is a very long history of seismic activity on the fault, which is a really unique record,' Orchiston said.

'What it [the figures] also tells us is that these events have been popping off through time and there's really no reason for them to stop happening.'

Kiwis needed to be concerned about an Alpine Fault quake, as it would likely 'happen in the lifetime of many New Zealanders', Orchiston said.

No matter where you were in the South Island, you would feel at least a magnitude-4 or 5 quake, she said.

But most dangers would not appear until after the Alpine Fault earthquake, Orchiston said.

'When we think of earthquakes, our mind goes straight to shaking, straight to the panic we feel when the ground moves,' Orchiston said

'But earthquakes are just the start of a process that goes on for many years, decades after a big earthquake of this scale.

'We call those long-term issues secondary hazards and these hazards can be more extreme than the earthquake itself.'

The 2004 Boxing Day tsunami and the 2011 liquefaction of buildings in Christchurch were two past examples of secondary hazards, she said.

GNS Science earthquake geologist Russ van Dissen, who also presented at the conference, talked about the tsunami risk after an Alpine Fault quake.

'I don't think the Alpine Fault will cause uplift in Marlborough; the land going up like it did in the Kaikōura earthquake,' van Dissen said.

'The tsunami, in the Alpine Fault case, might be [caused by] submarine landslides being generated off the West Coast, and it will impact the West Coast.

'Marlborough has, tsunami-wise, more to be concerned about from the Hikurangi Trench off the east coast of the North Island, which poses quite a threat.'

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