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Supercomputer helping to solve slow-slip riddle

Combining supercomputers with seismic CT scans is the latest approach scientists are using to better understand the causes of slow-slip earthquakes on Hikurangi subduction zone off the East Coast.

Slow-slip events can release the equivalent of a magnitude-7 earthquake, but they happen so slowly they escaped detection until about 20 years ago.

The mechanisms that govern slow-slip events still remain unclear so researchers at GNS Science and the University of Texas at Austin set out to test several theories by pairing very powerful computers with high quality seismic data.

Their research has been published in Nature Geoscience as part of a special edition focused on subduction zones.

“Subduction zones are the biggest earthquake and tsunami factories on the planet,” said co-author Laura Wallace, of GNS Science and the University of Texas Institute for Geophysics (UTIG).

“With more research like this, we can really begin to understand the origin of different types of earthquake behaviour at subduction zones.”

Understanding the timing and likely location of a large subduction zone earthquake can only happen by first solving the mystery of slow-slip events, Dr Wallace said.

“One of the theories we tested, called rate-state friction, failed to hold up as well as expected. That means we can probably assume there are other processes involved in modulating slow-slip events, such as cycles of fluid pressurisation and release,” Dr Wallace said.

The study's lead author, Adrien Arnulf of the University of Texas, said the seismic imaging data of the Earth's sub-surface used in the research was gathered off the southern Hawke's Bay coast in 2005 as part of a New Zealand government-funded survey. Applying next-generation techniques to this data had provided a whole new view of parts of the Hikurangi subduction zone.

“It was turned into detailed images by using similar techniques to those used in medical imaging, so geoscientists could visualise the interface between the Pacific and Australian tectonic plates to figure out what's going on underground,” Dr Arnulf said.

Scientists then used a supercomputer at the Texas Advanced Computing Centre to look for patterns in the data. The results showed how weak the fault had become and where pressure was being felt within the joints in the Earth's crust.

“The study has enabled scientists to narrow their focus on the physical conditions in the fault where slow-slip events occur, providing an important test of a range of theories for why they occur,” Dr Wallace said.

Dr Arnulf said if scientists ignored slow-slip events, there was a risk they would miscalculate how much energy was being stored and released as tectonic plates moved around the planet.

“They are an important part of the earthquake cycle because they occur in the same places as high magnitude earthquakes, and release tectonic energy slowly over weeks to months.”

He added that the Hikurangi subduction zone was ideal for studying slow-slip quakes because they occurred shallow enough to be imaged at high resolution using seismic techniques. This aspect had therefore attracted numerous international science agencies to New Zealand, bringing sophisticated technology and know-how to focus on our highly active plate boundary.

The study's seismic data was provided by GNS Science and the Ministry of Economic Development. The research was funded by UTIG and a Ministry of Business, Innovation and Employment Endeavour fund grant to GNS Science.

SLIPPING AWAY: The Hikurangi subduction zone which lies 40km from the East Coast. Image supplied
COMPUTING: Dr Laura Wallace from GNS Science. Picture supplied