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Jamie Morton is the NZ Herald's science reporter.

What would happen if a big quake hit Wellington?

What would happen if big quakes hit Wellington or Auckland?

GNS Science experts have looked at both scenarios and crunched the numbers, finding potentially catastrophic scenarios for the capital and a much less likely - and less costly - quake probability for Auckland.

A 2014 report focusing on Wellington simulated the varying impacts of eight large earthquakes on the Wellington region, potentially affecting around 460,000 people and $78 billion worth of buildings.

One of the most deadly earthquake scenarios for the capital was a two-punch magnitude 8.9 earthquake and tsunami in the Hikurangi subduction zone, assuming a rupture extending across Cook Strait.

The report found this catastrophic event could cause 40 deaths due to earthquake shaking - but a further 3200 in a tsunami that followed.

Although scientists were uncertain whether a rupture could extend from the subduction zone to the Cook Strait, such an eventuality could send tsunami waves 5m to 10m high barrelling toward both sides of the strait.

It would bring "extremely damaging and deadly inundation" in low-lying southern suburbs of Wellington, along with damaging and moderately deadly inundation in busy areas bordering Lambton harbour.

If the rupture did not extend into Cook Strait, however, then inundation and damage in those areas would be relatively minimal.

In another severe Wellington scenario - involving a 7.5 rupture of the Wellington-Hutt Valley segment of the Wellington Fault - more people could be killed during the shaking.

Source / GNS Science
Source / GNS Science

The report estimated 2000 deaths due to earthquake shaking, and 300 killed in the following tsunami.

Both scenarios were modelled assuming the events took place during work days, with the equivalent night-time scenarios indicating much lower numbers of deaths.

The numbers of injured people were five times (workday) to 13 times (night-time) higher than the numbers of deaths, for earthquake shaking from a Wellington earthquake scenario.

For tsunami inundation in this scenario, the numbers of injured people approximately equalled the numbers of deaths for both workday and night-time scenarios.

The most damaging earthquake would be a Wellington Fault rupture, involving a $17 billion repair cost for buildings, many of which would need to be completely replaced.

By comparison, damage from tsunami was "mostly inconsequential", the report found.

Because there could be a 10 to 15 minute gap between the end of strong shaking and the arrival of a first tsunami wave at the south coast of Wellington, in a subduction zone scenario, there could be time for people to leave their buildings and move to safe ground.

Within the most at-risk zone, there were 180,000 people on a typical workday - 130,000 in Wellington and 29,000 in Lower Hutt - while at night there were 80,000 people in the area.

Damage caused by Monday's 7.8 earthquake at CentrePort in Wellington. Photo / File
Damage caused by Monday's 7.8 earthquake at CentrePort in Wellington. Photo / File

The report found it would be easier for people to evacuate to higher ground at night, with at least 60,000 being able to return to their homes once the all-clear had been given.

In the most damaging events, an estimated 50,000 to 70,000 people could be displaced from their homes for six to 12 months; and between 20,000 and 25,000 for more than a year.

Auckland quake probability remains low

New Zealand's largest city sits about 300km from the nearest zone of high seismic activity and historically has experienced very low seismicity.

The nearest known active faults are about 25 to 40km from the city's CBD and are thought to be capable of generating earthquakes of magnitude about 6.7 to 7.2, and a 2013 report found these were expected to rupture on average only every 10,000 to 20,000 years.

However, the understanding changed this year with new research that suggested the Kerepehi Fault could cause a large earthquake every 1000 years.

The rupture interval on a single segment of the fault was still likely to be several thousand years, but the region may experience a large earthquake every 1000 years.

Previous ruptures had involved up to 2m of vertical displacement of the ground surface per event, which suggests associated earthquakes were between magnitude 6.3 and 7.0 in size.

Auckland has rarely experienced even low-level earthquake shaking since Europeans first settled the area in the early 1800s, and there appeared to have been no historical earthquake casualties.

During its 170-year recorded history, Auckland had not experienced magnitude 7 shaking - and only occasionally had intensities between magnitude 4 and 5 been observed.

The highest intensity ever recorded was from a magnitude 6 quake that struck 50km away at the mouth of the Waikato River on June 23, 1891.

If that earthquake was to occur today, a loss of about $20 million could be expected in Auckland Region, but no casualties, the report found.

A 2013 GNS report into potential impacts on Auckland looked at a trio of scenario
earthquake sources - two were nearby active faults, the Wairoa North and the offshore segment of the Kerepehi fault, and the third was the more distant Kermadec Subduction Zone.

Auckland sits about 300km from the nearest zone of high activity and historically has experienced very low seismicity. Photo / File
Auckland sits about 300km from the nearest zone of high activity and historically has experienced very low seismicity. Photo / File

This was equated to the 526,000 buildings in the region, valued at $309 billion and housing 1.4 million people.In the first two scenarios, large quakes could cause between 45 and 53 and between 20 and 26 deaths in the region respectively, along with $1.9 billion and $1 billion in damage.

The third could mean $190 million in repair costs and between three and six deaths.

However, probabilities calculations suggested that such events were extremely unlikely.

The 2013 report found that, in a worst case scenario of a quake bringing down 79 buildings across the region and causing 67 deaths, the annual odds were just 0.0002 per cent - however the the Kerepehi findings may have altered this.

Auckland Civil Defence and Emergency Management states that, although the Auckland region lies in one of the lowest earthquake activity regions of the country, "earthquakes of varying magnitude are likely to occur at some stage in the future".

The extent of this damage would depend on the ground conditions of a particular site, building and infrastructure condition, response and recovery plans, and the community awareness of what to do.

Auckland was vulnerable to volcanic eruptions, as much of the city was built on the Auckland Volcanic Field (AVF), covering 360sq km and containing at least 50 volcanoes.

None of these existing volcanoes were expected to erupt again and the next one would likely be in a new, unknown location.

More information on natural hazards and how to prepare for them could be found on Wellington and Auckland's Civil Defence group sites.

- NZ Herald

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