Big bang opens tectonic plate to Vic researchers

HUNDREDS of kilograms of dynamite have let Victoria University researchers catch a glimpse of what lies deep beneath Wellington.

Their imaging of the bottom of the Pacific tectonic plate has been hailed as a breakthrough internationally, with the result published in prestigious journal Nature today.

They discovered a 10km-thick area of soft rock that may act as a lubricant, allowing the plate to slip along, unconnected to the layers of the earth below it.

Victoria University professor Tim Stern said very little was known about the underside of tectonic plates before his team’s findings. The only information geologists had from the bottom of the 73km-thick Pacific plate came from earthquake shockwaves – not exactly a high-resolution or easy-to-organise testing method.

But 500kg dynamite explosions 50m below the surface sent sound waves down into the ground, some of which echoed back, allowing scientists to make an image of the plate, Stern said.

‘‘It’s just like CAT scanning in medical science, but while there you’re working on the scale of 10cm effectively, here we’re working on a 100km scale.’’

In 2011, Stern, GNS Science’s Stuart Henrys and their team took a crosssection of the lower North Island, from Kapiti to the opposite Wairarapa coast. With 60 volunteers, they planted 12 dynamite charges and buried almost 900 seismographs to record waves along this route.

Over two nights, these explosives were set off one by one, in blasts heard 10km away – detonated in the early hours of the morning while people were asleep. ‘‘These instruments record very, very tiny ground motion. A car or truck going by would wipe out a seismograph record nearly a kilometre away.’’

The resulting 900 seismograph readouts were displayed side by side and the images told researchers a lot about where the Australian plate met the Pacific plate under the North Island. But they also spotted two distinct lines about 100km and 110km down – which they have interpreted to be a soft rock layer at the bottom of the Pacific plate.

‘‘Very small volumes of magma are thought to pond at this boundary because the upper plate is rigid.’’

Stern said this build-up of magma could act as a lubricant, the liquid allowing the plate to slip along in the same way a tiny bit of melted water below a ski on snow gave it its smooth glide.

It suggested the plate’s movement was due to being both dragged downwards by gravity on one side and pushed at the high oceanic ridge where it was newly made.

Stern said while the discovery would not directly lead to improved quake prediction, the better geologists knew what was deep down, the more they would understand the forces and stresses at plate boundaries where life-threatening seismic shakes occur.

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