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Scientists Record A Second 'Slow Quake'

17 November 2004

Scientists Record A Second 'Slow Quake' Near Gisborne

Scientists have detected what they call a 'slow earthquake' near Gisborne the second time this unusual phenomenon has been observed at Gisborne in the past two years.

From about October 31, an area of land near Gisborne started moving eastward at nearly 2mm per day. It is still moving east, although scientists expect the rate of movement will start to slow any day now.

The motion has almost certainly been caused by movement on the boundary between two tectonic plates about 20km under the seafloor in Poverty Bay.

Two millimetres a day is ten times faster than normal motion of land on the North Island's East Coast due to tectonic forces. Normally, tectonic forces push Gisborne slowly to the west by about 5mm per year.

As in 2002, the movement was detected on a continuously recording GPS instrument run by the GeoNet project, which is funded mainly by the Earthquake Commission and operated by Geological and Nuclear Sciences Ltd (GNS).

The instrument that picked up the motion is also part of Land Information New Zealand's network of instruments.

GeoNet operates four continuous GPS instruments in the Raukumara Peninsula region, but only one has recorded the latest motion. This suggests the area of movement is quite localised.

The latest recording appears very similar to the 2002 event, in which an area of Poverty Bay moved eastward for just over a week and then stopped.

The GPS instruments operated by GeoNet capture data from GPS satellites every 30 seconds and send "packages" of data every hour to GeoNet data processing centres in Lower Hutt and Wairakei, near Taupo.

GNS geophysicist Laura Wallace said the easterly movement was like Gisborne springing back slowly to relieve some of the stress it had been under from years of constant westward motion.

" We routinely see horizontal land movement around New Zealand of 30mm per year. But to see that amount of movement in two weeks is extraordinary," Dr Wallace said.

The ability to detect slow earthquakes has come about only in the past six years with the installation of an expanding network of continuously recording GPS instruments in New Zealand.

" Until now we really had no idea how frequently slow earthquakes occur in central New Zealand. On the western coast of Canada they occur almost every year. It could be that Poverty Bay has a two-year cycle.

" Slow earthquakes are becoming increasingly important in reassessing the seismic hazard in particular parts of New Zealand, as they may relieve some of the stress that would normally be released in damaging earthquakes."

This is the third recorded occurrence of this phenomena in New Zealand - two have been detected at Gisborne and one on the Kapiti Coast, north of Wellington. Scientists first detected slow earthquakes overseas about eight years ago. Since then they have been recorded in Canada, Mexico, Japan, Costa Rica, and New Zealand.


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