Scientists have detected what they call a "slow earthquake" underneath Poverty Bay, offshore from 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 a day.
It is still moving east, although scientists expect the rate of movement will start to within days.
Geological and Nuclear Sciences geophysicist Laura Wallace said yesterday the motion was almost certainly caused by movement between two tectonic plates about 20km under the seafloor in Poverty Bay.
Two millimetres a day is 10 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 a 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.
GeoNet operates four continuous GPS instruments in the Raukumara Peninsula region, but only one has recorded the latest motion, suggesting the movement is quite localised.
The latest recording appears quite similar to the 2002 event, in which an area of Poverty Bay moved eastward for just over a week and then stopped.
Dr Wallace likened the easterly movement to 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."
The ability to detect slow earthquakes has come about only in the past six years with the installation of continuously recording GPS instruments.
- NZPA
Herald Feature: Conservation and Environment
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