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Quake Swarm And Unusual Land Movements Link

Wed, 26 May 2004

Scientists LinkQuake Swarm And Unusual Land Movements

Scientists believe they are a step closer to understanding the way tectonic stresses translate into earthquakes in New Zealand.

In what scientists consider a significant advance, they have linked unusual land movements north of Wellington to a recent swarm of earthquakes under Upper Hutt.

In recent years, Paekakariki, a coastal township 40km northwest of Wellington, has been moving gently west at 25mm-a-year.

Scientists at Geological and Nuclear Sciences Ltd (GNS) have found that movement at Paekakariki has suddenly slowed to 15mm-a-year. And 12 months ago, a hill above Paekakariki started rising at 10mm-a-year.

At about the same time, Upper Hutt began experiencing a swarm of more than 40 earthquakes - a rare event in the Wellington region.

GNS scientists say their modelling shows the quake swarm and the unusual surface movements near Paekakariki appear to be linked.

GNS geophysicist John Beavan said the two tectonic plates colliding about 35km under the Kapiti Coast appear to have slipped past each other by 50cm in the past year.

This would account for the change in surface movement measured by a GPS instrument near Paekakariki in recent months.

Traditionally scientists have regarded the Pacific and Australian Plates as beling locked together under Wellington. However, this latest finding suggests slipping or "slow earthquakes" are occurring.

Slow earthquakes are a relatively new phenomenon that have been observed in only a handful of places worldwide, and only in the past eight years thanks to the advent of continuous GPS recording instruments.

A slow earthquake happens over weeks or months and its only surface manifestations are small ground movements picked up by highly accurate GPS instruments.

The 50cm of slippage at depth - possibly triggered by highly pressurised water moving up the plate interface - would have been enough to produce the 10mm-a-year change observed near Paekakariki, Dr Beavan said.

The plate movement, or slow earthquake, would have increased the tectonic stress under Upper Hutt by the right amount to trigger a swarm of small to moderate sized earthquakes.

" We're now able to observe phenomena such as slow earthquakes because we have new technology. It's likely they have been occurring for many years in New Zealand, but no-one has been able to detect them," Dr Beavan said.

" The measurements we've recorded in recent months are changing the traditional view that the two tectonic plates are tightly locked under the lower North Island.

" It seems that tectonic stress is being relieved occasionally at the deeper part of the interface.

" Periodic changes in the stress regime under the lower North Island may advance the timing of earthquakes on some faults, while reducing the likelihood on others."

The earth deformation near Paekakariki was recorded by a global positioning system instrument capable of detecting horizontal movement down to 2mm.

It transmits data continuously to the GeoNet operations centre run by Geological and Nuclear Sciences. More information on what is happening under the lower North Island will become available as more GPS instruments are installed during the next few years.

The Earthquake Commission and Land Information New Zealand are funding the installation of the instruments.


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