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New Zealand, the Shaky Isles of volcanoes and earthquakes

By David Barber     Jul 11, 2002 in Technology
WELLINGTON (dpa) - New Zealand is not called the "Shaky Isles" for nothing. It has about 14,000 earthquakes every year - though people only actually feel up to 150 of them.
The country straddles the boundary of two of the earth's giant tectonic plates which are constantly grinding together below the surface. It also contains five active volcanic areas. To geologists it is one of the most unstable parts of the globe.
Most New Zealanders get on with their everyday lives and do not dwell on the issue. The last major fatal quake (which took 256 lives in Hawke's Bay, on the east coast of the North Island) was 71 years ago, and another "Big One" is statistically well overdue.
Advice from civil defence authorities centres on precautions against loss of life and damage from earthquakes, but recent reports have highlighted the risk of an equally devastating volcanic eruption.
Auckland, the biggest city with a regional population of 1.1 million, sits on an active field of about 50 volcanoes and, although none are simmering, the local council has drawn up a contingency plan.
Local scientist Ian Smith said the city could be hit by a volcanic hazard at any time and although seismic monitoring to detect rising molten rock beneath the earth's crust would give warning of an eruption, that could be no more than a week.
Although Smith said volcanic fall-out would be restricted to about a three kilometre radius, the impact would be enormous because of disruption to the country's biggest port and airport.
But Auckland is not just threatened by volcanoes close to home. Mount Taranaki, 400 kilometres to the southwest, showered ash on Auckland when it last erupted about 200 years ago.
The 2,518-metre mountain blew its top eight times in the previous 300 years and the Institute of Geological and Nuclear Sciences says there is no evidence to suggest it has finally ceased erupting and become extinct.
"Rather it must be regarded as an active volcano in a state of quiescence and is one of a number of volcanoes in New Zealand where future eruptions are to be expected," the institute says.
White Island, 50 kilometres off the east coast of the North Island and at the northern end of the Taupo volcanic zone, is almost constantly active, pouring a plume of steam into the air.
In the centre of the island, about 350 km south of Auckland, lies Lake Taupo, formed about 1,800 years ago with an eruption that blasted volcanic rock over 20,000 square kilometres and affected the atmosphere over China.
New Zealand's largest lake at 619 square kilometres, it covers a series of calderas (or collapsed volcanoes, distinct from the more widely recognised volcanic cones) with many underwater vents.
An institute study just released shows that parts of the region have risen and fallen by up to seven centimetres over 10 years.
Although it was known that the Pacific plate beneath New Zealand moves 50 millimetres a year - about the rate human fingernails grow - scientist Brad Scott said monitoring over 20 years showed movements at Taupo were "probably unrivalled in their rates and complexity".
But he said there was no cause for alarm. "Taupo is an active volcano. It has erupted 28 times in the last 26,000 years. It will erupt again, but there's nothing to suggest it's going to happen in the very near future."
The institute has warned: "Even a small volume explosive event at Taupo will be very destructive and disruptive to human life and activity throughout the North Island."
Apart from White Island, there are three on-shore active volcanoes within the Taupo volcanic zone - Tongariro, its subsidiary cone Ngauruhoe and Ruapehu, at 2,797-metres the North Island's highest mountain.
Ruapehu last erupted in 1995-96, showering volcanic ash as far as Wellington, 233 kilometres away, and closing the mountain's three ski-fields.
It caused a disaster on Christmas Eve 1953 when its crater lake of steaming sulphurous water broke its banks, sending a lahar down the mountainside which swept away a bridge, derailing the Wellington- Auckland express and killing 151 passengers.
Summing up the volcanic situation, the institute says: "Although the probability of an eruption affecting a large area of the North Island is relatively low in any one year, the probability of an eruption occurring in the future is high."
While it cannot predict when the next eruption will occur, the institute says its permanent surveillance at active and potentially active volcanoes can detect early signs of increasing seismic and volcanic activity.
Although all New Zealand's volcanic activity is in the North Island, the lesser populated South Island has its own threat.
The Alpine Fault, which runs for about 600 kilometres up the spine of the island, is the boundary of the Pacific and Australian Plates and is responsible for creating the Southern Alps, some of the country's most spectacular scenery.
It has ruptured four times in the past 900 years, the last about 1717, each time producing an earthquake of about magnitude 8.
The institute says the fault has a "high probability" of rupturing again in the next 40 years, adding: "The rupture will produce one of the biggest earthquakes since European settlement of New Zealand, and it will have a major impact on the lives of many people."
More about New Zealand, Earthquakes, Islands, Volcanoes