Alpine Fault quake will melt rocks

MICHAEL WRIGHT
Last updated 12:54 15/03/2013
ALPINE FAULT: An 800 kilometre-long fault which runs along the western edge of the Southern Alps from Marlborough to Milford Sound.
GNS Science

ALPINE FAULT: An 800 kilometre-long fault which runs along the western edge of the Southern Alps from Marlborough to Milford Sound.

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The next time the Alpine Fault "goes", it is likely to be a big one, but the earthquake on the surface will be a tiny fraction of the power unleashed beneath.

University of Canterbury researcher Carolyn Boulton is finishing her research on the plate boundary that splinters the South Island and what happens underground when it ruptures.

The energy that went into surface shaking was less than 5 per cent of the energy released by the fault rupturing, she said.

"A lot of it is just used up on the fault itself,'' she said.

"Most of [it] is used up as heat on the fault and it's also used up to crush and pulverise [rocks]. The remainder, whatever is left, that's what's sent out as seismic waves."

Dry parts of the fault line can get so hot - about 1000 degrees Celsius - that the rock melts.

The "main divide" of the Alpine Fault is that dry only at great depths, but branches at its northern end - the Hope, Clarence, Wairau and Awatere faults - also have little water.

Boulton recreated Alpine Fault conditions at three university laboratories in the United States and China for her research and built an instrument to simulate quake shaking here.

"We have a machine [in Christchurch] that we put the rocks into and we simulate earthquake conditions,'' she said.

"We have learnt that the fault gets stronger with increasing temperature and pressure with increasing depth [and] that when an earthquake occurs, the Alpine Fault's strength dramatically decreases."

The Alpine Fault's last major quake was in 1717 and had a magnitude between 7.9 and 8.1.

GNS Science estimates there is a 30 per cent probability of the next quake on the fault occurring in the next 50 years.

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