Auckland eruption scenario: 435,000 displaced

A previous artist's impression of a volcanic eruption in Auckland's Waitemata Harbour. A new scenario imagines an event at Mangere Bridge. Photo / File

Scientists have imagined a two month-long volcanic eruption near Mangere Bridge, in which 435,000 residents would be forced from the area.

The paper, focusing on what would happen to Auckland's transport network, found most physical damage to road and rail transportation would happen from the volcanic eruption itself, destroying anything within a 0.5 to 2.5km radius, but with little impact from earthquakes leading up to the eruption.

In this scenario, Auckland Airport would not be directly affected by physical damage, but would have restricted access from evacuation zoning and airspace restrictions, forcing limited domestic and cargo flights to redirect to Whenuapai and Ardmore Aerodrome.

After the eruption itself, the researchers say that the most important factors for keeping the transportation network open are cleaning bottleneck areas like bridges of tephra, ensuring electricity supply for the trains, airport and traffic signals, as well as ensuring continued fuel supply.

The University of Canterbury, University of Auckland and GNS Science researchers behind the new study, published in the Journal of Volcanology and Geothermal Research, said the most extensive service reduction across all transport networks would occur around six days before the eruption onset.

This was largely because of the evacuation zones that would be implemented, disrupting crucial north-south ground links through Auckland.

In the main scenario, ash was deposited on parts of the road and rail network in Auckland, which would likely cause some disruption for over a month.

"The volcanic activity in the scenario progresses from seismic unrest, through phreatomagmatic explosions generating pyroclastic surges to a magmatic phase generating a scoria cone and lava flows," the researchers reported.

"We find that most physical damage to transportation networks occurs from pyroclastic surges during the initial stages of the eruption."

The scenario also meant visibility would likely be reduced, and lower speed restrictions might have to be implemented to reduce the amount of ash being re-suspended into the air.

The amount of disruption to transportation and locations of impact would also strongly depend on wind directions and strengths at the time of the ash-producing eruptions.

In modified scenario explored by the scientists, there were "extended uncertainties" on the potential location of the volcanic vent.

In this case, large evacuation zones could remain in effect for longer than if an eruption did occur, and residents would therefore be displaced for longer periods of time.

The researchers said that working with transportation infrastructure providers and emergency management officials was "crucial" to help examine what might occur on Auckland's transport networks during an eruption and the subsequent recovery.

Much of Auckland is at risk - more than 50 volcanoes lie beneath a field stretching across 360sq km - and more than one million people live on areas where an eruption could occur.

An eruption from one of the Auckland Volcanic Field's many volcanoes could blast out an explosion crater 1 to 2km across, destroying everything in it, but planners believe people in its path would be evacuated well before an event.

Over recent years, researchers have published several major papers attempting to assess what a major eruption would mean for the city.

One study suggested a worst-case eruption near the CBD could cause a 47 per cent reduction in gross domestic product, while another 2015 paper found a catastrophic volcanic eruption in Auckland's industrial heart could have an economic impact of up to $10 billion in the first year - and knock out a large chunk of the city's GDP.

In that study, researchers said it was in the core industrial hotspots of Penrose, Onehunga, Otahuhu and East Tamaki where such a disaster would be most damaging to Auckland's economy, costing on average $8.3b (in 2007 dollar terms) in lost GDP from businesses, factories and supply chains being taken out.