Scientists probe alpine fault

High-powered stethoscope-like devices are to be used to investigate the mystery of why New Zealand's alpine fault in the Southern Alps appears to have few earthquakes.

The equipment will be funded as part of a $810,000 grant over three years from the Marsden Fund.

Researcher Tim Stern, of Victoria University's institute of geophysics and earth sciences, said very little was known about how quakes occurred on faults like the one that runs through New Zealand's major tectonic plate boundary.

Only a few others like it exist. Turkey and the Philippines have such faults and California's San Andreas Fault is perhaps the best known.

Stern said where the mountains on the central section of the alpine fault in Westland between Whataroa and Fox Glacier were highest, there had been little seismic activity in the past 200 years.

"This could be because the fault is very strong and locking up and storing what could be called elastic strain and that could be released in an earthquake of up to magnitude 8.

"Or the other possibility is that the fault is very weak and the strain could be released in smaller earthquakes and at irregular intervals."

A magnitude 8 earthquake has not been felt in New Zealand since 1855.

Stern said one in the alps would provoke severe shaking in the Christchurch region, but he was reluctant to make any further predictions.

There was evidence to suggest high-pressure water existed in the fault zone, and the rock coming up one side of the fault was hot and wet, he said.

"This has given us the idea that maybe the fault may move in a different manner from regular faults."

To test the idea researchers from Victoria, with Professor Peter Malin from Auckland University, plan to drill eight bore holes of up to 250 metres then lower the seismograph, which he compared to a stethoscope, into the holes, which are 10cm in diameter.

"What we want to do is try and listen and hear the chatter, if you like, of what happens deep down of the tectonic plates – and we suspect there are many small earthquakes."

At present only earthquakes greater than 2.5 on the Richter scale can be detected by GeoNet, New Zealand's quake monitoring network. "If we can get into bore holes we think we can get down to magnitude 0.5. We're talking about earthquakes a thousand times less energetic than magnitude 2.5."

A Japanese survey that used bore hole seismometers was able to determine when fault movements occurred without any earthquakes – known as slow or silent earthquakes. "This is very important now for trying to learn about how these major faults of the world are working."

Some slips on the fault are occurring without generating what he called "a high-frequency ping" through rock.

The sliding action happened over days, weeks or even months, manifesting itself as a slow-moving rumble.

Slow earthquakes had already been detected beneath the North Island's east coast and Manawatu regions, but it was unknown whether the central section of the alpine fault also experienced them.

"What makes the alpine fault a compelling research focus is that it dips at about 40 to 60 degrees into the crust and it has had nearly 20km of uplift and erosion on its eastern side," he said. –Dominion Post

- © Fairfax NZ News