Scientists examine Alpine Fault for signs of stress following earthquake

Tuesday, 6 January 2015

The Alpine Fault scarp crosses fields near Haast before heading southwest into isolated country and finally going out to sea at the mouth of Milford Sound.

It wasn't the Alpine Fault, for which we can all be glad.

But yesterday's magnitude 6.0 earthquake, close to the head of the Waimakariri River and within 30km of Arthurs Pass, was near enough to it to get the scientists calculating what the shaking may have done to the South Island's most dangerous feature.

The Alpine Fault scarp becomes a prominent ridge as it approaches the St Arnaud township.

GNS Science researchers hope today to find out whether the Alpine Fault has become more, or less, stressed by yesterday's Wilberforce quake.

More stress on the Alpine Fault could hasten a major earthquake.

When the initial difficulties in determining the quake's location, magnitude and depth were resolved, it was found to have been generated about 5km down in the greywacke basement rocks, just on the Canterbury side of the main divide in extremely remote country.

As the kea flies, it was about 20km east of the Alpine Fault, the boundary between the Australian and Pacific crustal plates, which are moving past and pushing against each other, forcing the Southern Alps higher.

It was also about 15km southwest of the June 18, 1994 magnitude 6.7 quake, which caused widespread alarm and some damage in Christchurch. Worse damage occurred in Arthurs Pass that day and State Highway 73 through the Otira Gorge was blocked by a large rockslide.

The Alpine Fault is one of the world's major structures. Visible from space, it runs about 650km from the Wairau River valley through the central upper South Island, down the western side of the Alps and out to sea near the Milford Sound entrance.

The last major rupture on the fault was in 1717, which generated an earthquake of at least magnitude 8.0. There is also evidence of a similar-sized quake about 1620, and other fault movements around 1450 and 1100.

Some studies have shown an Alpine Fault earthquake occurs every 250 to 300 years on average and that the South Island could be about due for another big one.

Geonet public information specialist Caroline Little said yesterday's 6.48am quake, felt by thousands across the island, was 'strike-slip', in which the movement on either side of the fault was largely horizontal.

It was unlikely the fault responsible was a splinter of the Alpine Fault, she said, but it had much the same southwest-northeast orientation. Chances were the fault might not have ruptured to the surface, in which case it would be very difficult to identify.

'In the region around the Alpine Fault there are lots of faults. This was a result of the stresses of the two plates colliding [and] building up.

'[This quake] has the potential to change stress on different parts of the Alpine Fault, depending on the orientation of it. Some could be increased and others decreased.

'We're running models to see if we can see what those parts are, and by how much,' Little said.