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Store enough food, water and medication to last seven days in case the Alpine Fault ruptures, Cantabrians told

Friday, 5 May 2023

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

Cantabrians need to store enough food, water and medication to last seven days in case the Alpine Fault ruptures – more than twice the three days of supplies recommended a decade ago.

Under the worst case scenario, Canterbury residents can expect one to three minutes of “strong and sustained shaking” if the Alpine Fault ruptures in the worst possible way.

The September 2010 and February 2011 quakes both lasted less than a minute.

An earthquake caused by the Alpine Fault could see hydroelectricity dams in the Southern Lakes shut off and they will probably have to be checked before coming back online, Dr Matthew Hughes​ told a public meeting in Christchurch this week.

Getting power back could take days, weeks or even months, depending on how much damage is discovered, he said.

Residents clear the streets following the 2011 Christchurch earthquake.
Residents clear the streets following the 2011 Christchurch earthquake.

Hughes, a senior lecturer in civil and natural resources engineering​ at the University of Canterbury, was speaking on behalf of AF8,​ a programme of scientific modelling, response planning and community engagement in anticipation of a magnitude 8 on the Alpine Fault.

If the power is out, so is the internet and the eftpos network. Cellphone coverage will also be down, at least until generators can be fired up, he said.

Liquefaction is probable where it occurred in 2010-2013, although that depends on how high the water table is at the time the Alpine Fault moves and other factors, said Hughes.

Liquefaction covered roads and properties in Christchurch. Image taken in 2011.
Liquefaction covered roads and properties in Christchurch. Image taken in 2011.

AF8 speakers like Hughes have been presenting to public meetings all around the South Island since 2019. Toka Tū Ake EQC​ partly funds them. They have been particularly active on the West Coast, because that’s where Alpine Fault desolation will be the worst.

Everywhere they’ve presented, the core message has been the same.

The Alpine Fault is a 800km-long seismic feature on the west side of the Southern Alps. It runs roughly from Milford Sound in the South to Lewis Pass in the north. Over the last 7000 years, it has significantly ruptured 27 times, roughly every 300 years.

The last big movement was in 1717, 306 years ago.

There’s a 75% chance it will rupture in the next 50 years, within the lifetime of many South Islanders, Hughes said. There’s an 82% chance it will be magnitude 8 or slightly higher.

Worst case scenario

Dr Matthew Hughes, a senior lecturer at the University of Canterbury, speaks at a Christchurch public meeting on Alpine Fault Magnitude 8 (AF8).
Dr Matthew Hughes, a senior lecturer at the University of Canterbury, speaks at a Christchurch public meeting on Alpine Fault Magnitude 8 (AF8).

Under AF8, experts and modellers from New Zealand universities, Crown Research Institutes, national and regional civil defence organisations and others considered scenarios along the fault.

The worst case, they decided, was a rupture that started in the south, somewhere near Milford and ripped northward for hundreds of kilometres. Quakes that started elsewhere, or didn’t release more parts of the Alpine Fault, would be awful for the West Coast, but not necessarily national events, Hughes said.

Franz Josef on the West Coast sits on top of the faultline.
Franz Josef on the West Coast sits on top of the faultline.

One animation created by AF8 shows the energy from a Milford quake passing under the southern hydro lakes less than a minute later. The energy hits Timaru and the loose sediments of the Canterbury Plains about the 1.45 minute mark.

The shaking arrives in Christchurch about 2.20mins after the initial rupture and lasts more than a minute. The shaking keeps travelling north and slams Marlborough-Nelson and Wellington.

The animation suggests shaking under Christchurch won’t be magnitude 8 – that’s for the West Coast – but nonetheless we’ll get a “good old shake”, Hughes said.

It’s entirely possible every alpine highway over the Southern Alps will be shut by landslides, some possibly for years. The only immediate access to the West Coast could be on helicopters and perhaps planes, if runways survive.

Cantabrians should store enough food, water and medication in case the Alpine Fault ruptures.
Cantabrians should store enough food, water and medication in case the Alpine Fault ruptures.

On the East Coast, there’s some good news.

There’s evidence few of those 27 historical large quakes on the Alpine Fault caused significant rockfall or cliff collapse in the Port Hills or Banks Peninsula, Hughes said.

The worst liquefaction zones were red zoned. Many pipes were replaced.

Most of Christchurch’s “dungers” (older, mostly commercial buildings) have been demolished. But people living in unrepaired or poorly repaired houses can probably expect more damage than those living in well-repaired or newly built homes.

Alpine Fault facts: