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Unprecededented land movement under Gisborne

Published: Thu 19 Dec 2002 03:27 PM
Scientists Record Unprecededented Movement Of Land Near Gisborne
Scientists have recorded land near Gisborne moving about 20mm to the east during an eight-day period in October.
The motion is thought to have been caused by movement on the boundary between two tectonic plates under the seafloor in Poverty Bay.
It is the first time that land deformation of this type has been recorded in New Zealand, although it has been observed above plate boundaries in other parts of the world in recent years.
The movement was recorded on two continuously-recording Global Positioning System (GPS) instruments, one of which was installed only five months ago, said Laura Wallace and John Beavan, geophysicists at the Institute of Geological and Nuclear Sciences Ltd (GNS).
One of the instruments is near the centre of Gisborne and the other is about 12km west of the city.
The instrument west of Gisborne captures data from GPS satellites every 30 seconds and sends "packages" of data every hour to GNS data processing centres in Wellington and Wairakei, near Taupo. The instrument in Gisborne is run by Ian Bell, a local GPS operator.
In the months leading up to the event, the instruments showed that tectonic forces were pushing land at Gisborne slowly to the west by about 5mm-a-year.
" It's almost as if land around Gisborne sprung back slowly to the east to relieve some of the stress it was under," Dr Beavan said.
" We routinely see horizontal movement of land in numerous parts of New Zealand of 20mm or more a year. But to see that amount of movement in eight days is extraordinary."
The sloping boundary between the Pacific and Australian tectonic plates is located about 15km below Gisborne and gets shallower offshore in Poverty Bay.
Earthquakes occur occasionally at plate boundaries throughout the world as the two plates stick together for many years and then suddenly slip past each other.
Scientists believe that the movement at Gisborne may have been caused by a "slow earthquake" where the two sides of the plate interface slipped past each other by about 20cm. However, the slip took place much more slowly than in a normal earthquake, so did not cause the shaking and damage of a normal quake.
Scientists at GNS were not expecting to see such dramatic motion so soon after the GPS instruments were installed, and checked the data thoroughly to make sure it was not a technical anomaly. The fact that the movement was observed on two instruments separated by 12 kilometres gave them confidence in the regional nature of the event.
" This type of event has major implications for understanding earthquakes and the level of hazard that parts of New Zealand are exposed to.
" If tectonic stress is being relieved periodically by movement such as this in parts of New Zealand, we may need to reassess earthquake hazard models."
Scientists are unable to say how much land was affected by the event.
" At this stage, all we can say for certain is that an unknown area of land on the North Island's east coast moved quite rapidly to the east by about 20 mm during mid-October. With more GPS stations in the North Island, we'd be able to say with more certainty how much of the North Island was affected," Dr Beavan said.
GNS and Land Information New Zealand have recently joined forces to set up 13 new permanent GPS stations between Wellington and Whangarei to detect land movement associated with the Australia-Pacific plate boundary through New Zealand.
Additional GPS stations will be deployed through the GeoNet project with funding from the Earthquake Commission.

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