Natural hazards

Natural hazards

The Grand Canyon is 446km long, up to 29km wide, 1500m deep. The Hokitika and Cook canyon system is at least 650km long, up to 20km wide and up to 1000m below the surrounding seafloor, reaching depths below sea surface of about 3750m.

A NIWA study reveals vast underwater canyons around New Zealand, December 2005

The natural forces that create New Zealand’s stunning scenery also present many natural hazards – and earthquakes, volcanic eruptions, floods and landslides are part of life. New Zealand lies in a geologically unstable zone, straddling two moving sections of the Earth’s crust – the Pacific and Australian tectonic plates. The vast majority of New Zealanders (95 percent) live within 200km of the boundary where these plates meet.

Rain, rain, rain

Floods caused by intense or prolonged rain are by far the most frequent and costly disaster in New Zealand, as the Roaring Forties weather systems and mountainous terrain increase the rainfall produced by frontal storms and tropical cyclones. Insurance industry statistics show that between 1968 and 2004, natural hazards were responsible for NZ$1.5 billion in insurance claims. While the event leading to the largest total claim was the Bay of Plenty earthquake in 1987 ($392m), weather-related hazards were responsible for 74 percent of all claims.

Most flood forecasting in New Zealand involves projecting river flows from rain gauges. Depending on the catchment, there can be as little as six hours between the time the rain falls and the time it turns up in the river – which barely gives authorities enough time to evacuate affected residents, let alone organise sandbagging or other efforts to reduce flood damage. An alternative is to derive river flows from quantitative rainfall forecasts produced by weather models, which can produce up to 36 hours’ warning. The National Institute of Water and Atmospheric Research (NIWA) is currently trialling such flood forecasting systems for four catchments around the country.

Shaky isles

Under relentless pressure, New Zealand is shaken daily by earthquakes. GNS Science records about 14,000 earthquakes in and around New Zealand each year. Most are small, but between 100 and 150 are big enough to be felt. The reason for the large number of earthquakes is New Zealand’s position on a plate boundary. To the east of the North Island the Pacific plate is being forced under the Australian plate. Under the South Island the two plates push past each other sideways, and to the south of New Zealand the Australian plate is being forced under the Pacific plate.

The heaviest toll was in 1931, when 256 people died in the magnitude 7.8 Hawke’s Bay earthquake. Parts of New Zealand moved by as much as three metres since this last major lethal jolt, and an inevitable consequence of such movement and ground deformation is the periodic release of huge amounts of stored energy in the form of an earthquake and volcanic eruptions in parts of the North Island.

GNS Science staff also record a number of slow, silent earthquakes, occurring deep under New Zealand and pushing parts of the lower North Island out of shape. It is likely these events have always been occurring, but scientists have only been able to detect them recently with the advent of global positioning satellite equipment that can detect land movements smaller than a centimetre.

Volcanoes

New Zealand has many active and dormant volcanoes, and the largest city, Auckland, is built on a volcanic field. The most recent eruption took place about 600 years ago, when Rangitoto Island was formed. More than 26,000 years ago, the Taupo eruption produced a massive caldera, which is now filled by Lake Taupo. The Mt Tarawera eruption in 1886 was the most destructive, killing at least 120 people. The volcanoes of Tongariro National Park have erupted many times in the past two centuries, spreading volcanic ash and disrupting travel, but also increasing the fertility of the region’s soils. The same magma (molten rock) that feeds the volcanoes powers the geysers, hot springs, and mud pools of New Zealand’s geothermal areas.

Measuring the forces building up under New Zealand is the focus of the GeoNet project – a 10-year programme to upgrade and sustain the equipment that monitors earthquakes and volcanoes in New Zealand. With the first batch of equipment already installed, higher quality seismic and ground movement data are being collected from many parts of New Zealand.

Tsunamis

Tsunamis are generated by movements of the sea floor during earthquakes, by volcanic eruptions and by landslides. Offshore earthquakes in New Zealand have produced tsunamis up to 10m high. Tsunamis from other parts of the Pacific have also hit our coasts. Only one death from a tsunami has been recorded since European settlement, but Māori tradition suggests earlier deaths.

Landslides

New Zealand’s uplifted land is rapidly eroding, creating deep valleys with hillsides prone to collapse. Many landslides are triggered by heavy rain or earthquakes. In 1979, part of the Dunedin hillside suburb of Abbotsford gave way due to unstable rock layers under the area. This wrecked 69 homes.

By Veronika Meduna

Further reading and Websites 

Natural hazards and disasters - Te Ara Encyclopedia of New Zealand website

GNS Science website 

National Institute of Water and Atmospheric Research (NIWA) website

Geonet project website

Image courtesy GNS Science.

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