Thousands to be evacuated, highways blocked for months when Alpine Fault ruptures

Saturday, 26 May 2018

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

Up to 5000 tourists spread out over more than 10,000sq km of the West Coast could need evacuating after an Alpine Fault rupture, and major roads will be blocked, a new study estimates.

Using the November 2016 7.8-magnitude Kaikōura Earthquake as a guide, study author Tom R Robinson calculated landslides could block up to 120 sections of road in an Alpine Fault rupture.

'This suggests that blockages can be expected along State Highway 6 between Hokitika and Haast, on SH94 before Milford Sound and on SH73 around Arthur's Pass,' Robinson, from the Geography Department of Durham University in the UK, wrote in the study published in the New Zealand Journal of Geology and Geophysics.

When the Alpine Fault ruptures, road access could be cut for months to much of the West Coast.

Previous studies had suggested several sections of those roads could take more than six months to restore. 'Contingency planning for the evacuation of large numbers of spatially distributed tourists and the provision of sufficient emergency supplies to local populations is urgently required.'

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Robinson, who was based at Canterbury University in New Zealand for five years, used an Alpine Fault earthquake scenario from Project AF8, which is preparing a coordinated response for a major quake on the fault. The scenario developed involves a 7.9-magnitude earthquake with about 350km rupture between Charles Sound and Hokitika.

Several sections of road were consistently at risk of landslide blockage: SH6 immediately south of Franz Josef; SH6 about 20km northeast of Haast, SH6 about 10km east of Haast, SH94 immediately south of Milford Sound, and SH73 immediately north of Arthur's Pass.

If those sections were blocked access to the West Coast region would only be possible on SH7 - which joins SH6 at Greymouth - and then only to about 50km south of Hokitika. There were no alternative road routes to Milford Sound, Franz Josef, Haast or Arthur's Pass.

That would mean about 10,000 local people would be cut off by road. Depending on the time of year, several thousand tourists could also be affected.

Following the Kaikōura Earthquake, the immediate priority was to provide essential supplies to isolated communities and evacuate stranded tourists. That was only possible by air and sea, but poor weather meant flying was not possible on several days, and the small size and damage to the port at Kaikōura restricted the number and size of ships that could dock.

Similar issues were expected following an Alpine Fault earthquake, Robinson said. Weather conditions west of the Southern Alps could be difficult and changeable, few locations in the West Coast region had access to airstrips, and ports in the region and Milford Sound were small and likely to be damaged. There were no viable berthing spots for ships between Hokitika and Milford Sound.

Considerably more tourists could be in the area than the 1000 evacuated from Kaikōura. The Franz Josef area had the capacity for up to 4000 visitors a night, and there could be about 1700 a day in Milford Sound.

'Although at night visitors cluster in major townships, during the day most will be spread out across the region, with many visiting remote areas on foot,' the study said. 'Consequently, it is possible that following an Alpine Fault earthquake, nearly five times as many tourists will require evacuation than following the Kaikōura earthquake, with these people distributed across a region of more than 10,000sq km.'

Another issue was getting emergency supplies to isolated local populations. Many West Coast communities would be isolated for far longer than the three days for which people were advised to be self-sufficient.

Continuous resupply by sea and air was unlikely to be sustainable, Robinson said. 'One possibility is the temporary evacuation of isolated local populations until road access can be restored. However, this is likely to prove controversial and poses questions as to how to respond to anyone refusing to leave as well as the management of safe return.'

In the quake scenario used in the study, seven sections of state highway were within 50 metres of a known surface trace of the Alpine Fault and so at risk of being blocked. Most were between Franz Josef and Haast, with another between Franz Josef and Hokitika.

Surface fault ruptures from previous Alpine Fault earthquakes were thought to have averaged about 8m horizontally and 1-2m vertically, suggesting those sections of road would be completely impassable, the study said.

Robinson calculated the number of sections of road that would be blocked by landslides at between 17 and 120. He looked at three shaking variables, with the calculations for two of them in reasonably close agreement while the third was much lower. The ranges for the number of landslide blockages were 17-24, 54-91 and 69-120.

The Alpine Fault runs along the western edge of the Southern Alps for about 411km and forms the onshore plate boundary between the Australian and Pacific plates, the study said. It has generated large (8.1 + or - 0.2) earthquakes regularly over the past 8000 years, with a relatively invariable average recurrence of about 341 years or less.

The last known quake on the fault was in 1717, giving a conditional probability of rupture of about 26 per cent in the next 50 years.