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Slow earthquakes pushing NZ out of shape - 05/08/2005

Slow, silent earthquakes occurring deep under New Zealand are pushing parts of the lower North Island out of shape, scientists say.

Slow Earthquake diagram

Up to seven "slow earthquakes" have been recorded by scientists at Geological and Nuclear Sciences Ltd (GNS) since 2002. It is likely these events have always been occurring, but scientists have only been able to detect them recently with the advent of global positioning satellite equipment which can detect sub-centimetre land movements.

GNS geophysicist Laura Wallace says continuously operating GPS equipment is adding to the understanding of earthquake hazards in New Zealand .

The Pacific plate descends westward beneath the eastern North Island, but for most of the time the descending Pacific plate and the over-riding North Island are "stuck" together at their interface, which causes large parts of the eastern North Island to be pushed to the west.

Dr Wallace says that slow-slip quakes are a sign that this tectonic stress is being relieved by 'silent' land movements that are the result of slow slip occurring on parts of the plate interface at 15 - 30km depth beneath the North Island

They manifest themselves as large areas of land moving eastward by up to 30mm over days, weeks, or months. Some scientists believe that these movements can shift stress within the Earth's crust and trigger earthquakes. So they are not necessarily benign events.

GNS has detected these events at GPS sites at Gisborne, Hastings, Wanganui, Ashhurst, Dannevirke, and Paekakariki. Each event has a characteristic signal. Some happen within days and others are more leisurely, taking some months to settle.

Some of the events may occur quite regularly, with an event near Gisborne repeating after about 2 years. Some cause ground movement of just 5 mm, while others have caused 30mm of ground displacement.

One of the best-documented slow slip earthquakes in New Zealand has occurred from January to June of this year beneath the Manawatu region. This event caused 10 to 30 mm of eastward movement of GPS sites near Ashhurst, Wanganui, and Dannevirke.

The GPS site near Ashhurst has uplifted by up to 30mm over the last six months. GNS scientists suggest that this ground displacement has been caused by approximately 150mm of slip on the plate boundary beneath the Manawatu region. It is possible that this event triggered some of the many small to medium sized earthquakes in the lower North Island early in 2005.

Repeated GPS measurements have also enabled GNS scientists to deduce that the area where the plate interface is most 'stuck' is beneath the southern North Island , but that there are also small areas where the plate interface is stuck beneath the northeastern North Island.

As more continuous GPS instruments are installed as part of the GeoNet project, it will enable scientists to track these events with more precision. This will lead to greater understanding of the seismic hazard posed by the plate interface beneath the North Island .

Contact: Laura Wallace