HERE’S YOUR WARNING
About 1300 deaths, thousands of injuries and extensive destruction along the North Island’s east coast – this will be the result of a tsunami triggered by an earthquake in the Hikurangi subduction zone under a scenario presented by GNS Science.
The scenario, in a report prepared for the purpose of developing a response plan, is considered serious and credible but comes with the obvious caveat that there is no way of telling what any earthquake or tsunami would look like.
This scenario is based on a magnitude-8.9 earthquake in the southern part of the Hikurangi subduction zone, creating a strong slip on the ocean floor in the southern Wairarapa/Cook Strait area.
It hits just after 9am on a winter school day. It sees intense shaking along the whole east coast of the North Island, and initiates a tsunami that rapidly reaches the coast, with wave heights of seven to 10 metres, and 20m in some localised places.
A preliminary impact assessment by expert panel members shows severe impacts in Wellington, Napier and Gisborne by both the earthquake and tsunami, leading to widespread destruction, about 1000 deaths and some 10,000 injuries.
In Wellington, the shaking is intense, with destruction in the city’s CBD similar to that seen in Christchurch in the 2011 quake.
‘‘A large landslide near Karori has engulfed 20 homes, and one near Days Bay has destroyed 15
houses. Ten cars are trapped on State Highway 2 within a landslide between Wellington and Petone’’ in the scenario.
Landslides isolate the city, liquefaction is widespread and fires break out.
‘‘The first tsunami waves reach the south coast of Wellington after 10 minutes and the inner-harbour areas like Petone after about 30 minutes,’’ it said.
The largest waves strike 40 to 60 minutes after the quake. Waves reach a height of seven to eight metres at the coast. Inundation in Wellington is mitigated by the ground being uplifted 1.5 to 2 metres but still the waves inundate 2km inland of Petone.
Worst hit are Lyall Bay and Eastbourne but there is significant inundation in Seatoun, Evans Bay, the CBD, Petone and Seaview. The southern end of the airport’s runway is covered by centimetres of sand and debris.
People rush to higher ground but some are inevitably caught in the waves, especially the less mobile. The elderly, the disabled and those living in areas where the tsunami arrived most quickly are disproportionately represented among the estimated 500 casualties and 5000 injured.
In the Napier CBD, 10 buildings collapse, liquefaction is widespread and there is widespread tilting of buildings on their foundations. Roads and bridges are also damaged.
The first major wave arrives about 20 minutes after the quake, peaking at 30 to 0 minutes. A second, more severe, wave hits 140 to 160 minutes after the quake.
The waves cause widespread damage to houses in Westshore, Ahuriri, Clive, Haumoana and Te Awanga. There is severe damage to the port, with logs and containers exacerbating damage to Westshore and Ahuriri.
Napier is cut off by road, the airport is unusable due to damage and liquefaction. A large-scale evacuation of the city CBD sees people rush up Bluff Hill but those in other areas – particularly Westshore and Clive – have trouble evacuating. About 200 people die and 700 are injured.
In Gisborne, 10 to 20 buildings collapse before the first large wave rises about 10 minutes after the quake. The wave peaks 20 to 40 minutes after the quake, and is larger again 80 minutes after.
The area is cut off but the airport remains usable. About 90 people are killed and 500 injured.
In Tauranga, there is intense shaking but minor damage to buildings and minor liquefaction. A small series of tsunami waves follow but there are no deaths and about 50 injuries.
The rest of the country is also hit, with total casualties at about 1000, and 10,000 injuries.
The report considers four other scenarios, with variations to the type of slip and the location. These result in more or less damage in each city depending on the scenario.
The report writers stressed that the scenario was created for planning exercises and ‘‘must not be used as the basis for assessing the risk to any particular property or person and must not be used as the basis for any sitespecific engineering work’’.
‘‘A striking feature of the scenario is the wide geographical spread of damage that can occur in such a large earthquake, which will present serious challenges for how to restore normal living conditions,’’ the writers said.
Jeremy Holmes, of Wellington Region Emergency Management Office (WREMO), said the likelihood of a large earthquake and tsunami was low but the potential impact could be significant.
‘‘A major rupture of the Hikurangi subduction zone is likely to be a national level emergency, similar to the Christchurch earthquake in 2011.’’
‘‘The first tsunami waves reach the south coast of Wellington after 10 minutes.’’