More than 430,000 Kiwis live in tsunami evacuation zones

Wednesday, 19 August 2020

More than 430,000 Kiwis – 9 per cent of the New Zealand population – live in tsunami evacuation zones, Niwa researchers have calculated.

There are more than 490,000 buildings in the zones, of which nearly 400,000 are residential. The zones also include 6370km of roads, 411km of railway lines, and Auckland and Wellington airports, which are in yellow tsunami zones.

Most residential buildings in New Zealand were one or two storeys high with timber construction frames, “a building typology highly susceptible to complete damage when tsunami depths exceed two metres,” a report into the study said.

It also noted that airports were “complex facilities with people often distributed over a broad area, posing considerable logistical issues for organising a safe evacuation, especially in response to short wave arrival times from a near-field tsunami event”.

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In the Wellington region, yellow zones were potentially at risk of tsunami inundation from near-field sources that could generate tsunami arriving within 30 minutes.

Published in the journal Geosciences, the report contains a welter of statistics highlighting New Zealand’s vulnerability to tsunami from seafloor earthquakes both nearby and far away.

The first two decades of the 21st century have highlighted the risks posed around the world by tsunami. The most deadly was the Indian Ocean tsunami on Boxing Day 2004, caused by a massive earthquake near Indonesia. That is estimated to have killed around 228,000 people in 14 countries.

Another massive earthquake, this one off the Pacific coast of Japan, caused the 2011 Tōhoku tsunami. Japanese police put the death toll around 16,000, with about 2600 more people missing and presumed dead.

Closer to home, the Samoa Islands tsunami in September 2009 killed at least 192 people. It was caused by a magnitude 8.1 earthquake between Samoa and American Samoa, and according to the US National Oceanic and Atmospheric Administration generated tsunami waves up to 22 metres high.

The magnitude-7.8 Kaikōura Earthquake in November 2016 created New Zealand’s largest tsunami generated close to this country’s shoreline since 1947.

The tsunami wave height was probably 3-4m above normal sea level when it reached Goose Bay, near Kaikōura, and had a 6.9m run up on the steep beach, GeoNet said.

New Zealand has tsunami evacuation zones along much of its 14,000km of coastline. The zones are classed as red, orange and yellow.

The red zone covers low-lying land up to 2m elevation above mean high water springs. The orange zone is the area that is expected to be inundated by a tsunami that could happen on average every 500 years and that would take at least an hour to arrive. The yellow zone is the area expected to be inundated in an event that could happen on average every 2500 years, or in a larger earthquake close to shore.

The study noted about 72,000 people and more than 49,000 buildings could in any case be exposed to coastal flooding from present-day extreme sea levels expected to happen on average once a century. A mean sea level-rise of 1m would more than double that to 177,000 people and 125,000 buildings.

Those low-lying populations and buildings were connected by an extensive transport network with more than 1400km of roads, 86km of railway and 13 international and domestic airports on land exposed to coastal flooding from once-in-a-century sea levels, the study said.

The red zone covers 17 square km of built-up land and 100sq km of farmland. It has 11.4km of high traffic volume roads and 25.7km of medium volume roads. About 3900 people live in the red zone, where there are nearly 7800 residential buildings.

The orange zone covers 158sq km of built-up land and 1126sq km of production land. About 168,000 people live in the zone and there are about 160,600 residential buildings and 139 retirement homes. It has 261km of high traffic volume road and 756km of medium volume roads. The zone also has 186 buildings used for hospital and medical purposes, 228 used for emergency management, and 453 used for infrastructure utility.

“The complex arrangement of built environments highlights a need for disaster risk managers to proactively identify and prepare populations for evacuation based on their vulnerability to harm from tsunami and ability to access resources for recovery after the event,” the researchers said.

New Zealand was exposed to numerous tsunami sources.

The Hikurangi subduction margin, off the coast of the east of the North Island was capable of producing tsunami waves from “great” earthquakes of greater than magnitude-8.0 strength, the report said.

The Hikurangi margin posed the greatest concern to New Zealand due to its potential to create large waves with onshore flow depths of more than 5m and short arrival times of less than 30 minutes.

“The Hikurangi margin has not ruptured in the last 200 years. However, 10 possible subduction earthquakes have occurred over the past 7000 years,” the study said. The most recent event was 540-590 years ago in the southern Hikurangi margin and there was evidence for a full margin rupture 885-940 years ago.

The Tonga-Kermadec trench northeast of New Zealand, posed the greatest regional tsunami hazard threat, with waves heights up to 10m generated from earthquakes greater than magnitude-8.0 expected to reach coastlines of New Zealand’s northern regions at least every 500 years. The waves could arrive within four hours.

In the last 150 years, three tsunami generated by earthquakes off the Peru–Chile trench had caused inundation along New Zealand’s eastern coastlines. That area posed the most frequent far-field tsunami hazard threat, with waves typically reaching coastlines 12 to 15 hours after generation.