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Unlocking the secrets of slow-slip events - 06/10/2011

A three-year science project to decipher the low frequency ‘chatter’ that occurs several kilometres underground during ‘slow earthquakes’ under Poverty Bay has won funding from the prestigious Marsden Fund.

Scientists Stephen Bannister (left) and Ted Bertrand, of GNS Science, are leading a three-year investigation into the mechanisms involved in slow-slip earthquake events that occur periodically under Poverty Bay. The Marsden funded project also involves scientists from Japan.   Photo: Margaret Low, GNS Science

Scientists Stephen Bannister (left) and Ted Bertrand, of GNS Science, are leading a three-year investigation into the mechanisms involved in slow-slip earthquake events that occur periodically under Poverty Bay. The Marsden funded project also involves scientists from Japan. Photo: Margaret Low, GNS Science

The GNS Science-led project, also involving scientists from Japan, has been awarded $765,000 spread over three years to investigate the mechanisms involved in slow-slip events. The funding is administered by the Royal Society of New Zealand

The project stems from the discovery by researchers at GNS Science a decade ago, that large patches of the subduction interface between the Pacific and Australian tectonic plates under the North Island’s east coast periodically slide past each other in what is called a slow-slip event – effectively a very slow earthquake.

The discovery of this phenomenon in New Zealand, Japan and California has led to a dramatic shift in the understanding of the mechanisms at play at subduction zones, where two tectonic plates meet. However, it has also raised a whole range of questions, which have yet to be answered.

The project is called ‘Capturing the gurgling and chatter from slow-slip deformation: Unlocking the role of fluids with magnetotellurics and seismology’.

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The slow-slip events that occur under Poverty Bay are notable for being some of the shallowest anywhere in the world.

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Project leader Stephen Bannister, of GNS Science, said a growing number of scientists believed that fluids at depth played a key role in slow-slip events. But the exact mechanisms that caused slow-slip events to start, continue, and stop were not known.

“The slow-slip events that occur under Poverty Bay are notable for being some of the shallowest anywhere in the world,” Dr Bannister said.

“They offer an unprecedented opportunity for close-up observation of the physical processes involved in this phenomenon.”

In early 2012 the scientists will set up a temporary network of geophysical instruments between Mahia and Gisborne to capture data on the slow-slip events that occur there at intervals of between one and two years.

These events are small episodes of plate movement lasting a couple of weeks and occurring at depths of 5km to 15km beneath Poverty Bay. Typically large parts of Poverty Bay move eastwards by up to 30cm during each episode. Movement is measured in millimetres per day and it can only be detected by special GPS instruments.

If slow-slip events were compressed into a few seconds, rather than spread over a couple of weeks, they would be equivalent to a magnitude 6.3 to 6.8 earthquake.

Slow-slip events in Japan and in western Canada are often accompanied by low-frequency tremor, or ‘chatter’, and a range of other seismic phenomena that can only be detected by sophisticated instruments.

“Scientists believe the tremor, or chatter, is the seismic signature of the slipping between the two plates.

“Our feeling is that movement of fluids at the interface between the two tectonic plates acts as a kind of trigger for slow-slip events under Poverty Bay.”

The scientists plan to use a technique known as ‘magnetotellurics’ to detect the movement of fluids at depth and will then pair this with very accurate earthquake recordings from a network of 10 seismometers to be deployed in the Gisborne region.

“We believe the findings from this project will greatly benefit the understanding of the physical processes that occur at subduction zones. It will also put New Zealand at the forefront of this type of research internationally.”