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Central North Island A Mobile Area Scientists Find



Parts of the Taupo region have risen and fallen by up to 7 centimetres over 10 years, scientists have found in a long-term study of the area.

The vertical movements, or deformation, are a direct result of Taupo's location which spans an active caldera volcano and a volcanic rift system where tectonic forces are pulling the earth's surface apart at a rate of several millimetres a year.

The measurements, taken between 1979 and 1999, are part of the volcano surveillance programme run by the Institute of Geological and Nuclear Sciences Limited (GNS).

Published in the New Zealand Journal of Geology & Geophysics, the report is a summary of data from 22 measuring stations around the edge of the 600sqkm lake, and along a belt of active faults northeast of Taupo

Although vertical movement data from other parts of New Zealand is sparse, it is thought that movements at Taupo are probably unrivalled in their rates and complexity.

GNS volcano surveillance co-ordinator, and joint author of the report, Brad Scott, said the data showed parts rising steadily for several years then stopping abruptly, and sometimes sinking just as fast.

Mr Scott said the observations provided a valuable insight into the underlying dynamics in a region undergoing "continuous extension, frequent earthquake swarms, and occasional volcanism."

He added that the lake levelling technique, combined with Taupo's high deformation rates, allowed changes in deformation trends to be detected over short intervals.

During the study, each measuring station was visited up to four times a year, although more frequent visits were made when scientists suspected more rapid rates of deformation.

Scientists observed four distinct periods of deformation during the 20-year study. The first was a four-year phase when they recorded steady subsidence in the Taupo Fault Belt, northeast of Taupo.

The second episode started in 1983 and consisted of rapid uplift in the Taupo Fault Belt. During the third phase, between 1984 and 1996, the Taupo Fault Belt subsided again at an average rate of 11mm a year.

The final episode, between 1996 and 1999, featured uplift at the southeast shores of Lake Taupo.

Mr Scott said the behaviour of caldera volcanoes such as Taupo was generally more difficult to understand than cone volcanoes such as Ruapehu, Egmont/Taranaki, and White Island.

Overseas evidence showed it was not uncommon for the land around caldera volcanoes to deform by up to 2m vertically without any serious volcanic activity.

" A difficult feature of caldera volcanoes is that even deformation of two metres does not guarantee there will be volcanic activity. But you would certainly be watching it closely."

In this context, deformation of 7cm over 10 years was nothing to be alarmed about. It was consistent with deformation at many other caldera volcanoes throughout the world.

" It's simply a fact of life in an area that is geologically very active."

Studies by GNS scientists show that Taupo has erupted 28 times during the past 26,500 years, and it last erupted 1800 years ago.

" It's important not to get too hung up on the average eruption interval at Taupo, because there have been periods of up to 3000 years with no eruption and then two in 500 years."


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