John Roughan: Interesting twist to the quake tale

The Southern Alps are the buckled top of the Pacific plate, which continues to push up the mountains as fast as wind and ice are eroding them. Photo / Dean Purcell

My brother in Christchurch said it was like a train coming through the house. I remember earthquakes in the south but they were never that loud. They were silent, eerie moments when the ground shifted, the house shuddered and everything rattled.

A big one lasted maybe five seconds. None lasted long enough to send me to a doorway or under a table. None brought the aftershocks that continued to shake Canterbury this week.

My father sounded more nervous after two more jolts on Wednesday morning than he was last Saturday. They have had a trauma the rest of us can only imagine.

I wonder whether it was better or worse for those who have read a little geology. All Black Keiran Read said his wife, a geography major, beat him to the doorpost in their house. But I bet she was fascinated too.

Somebody said the sensation was one of being jolted sideways, then lifted and twisted. That more or less is what you read has been happening to the tectonic plates under the Southern Alps for millions of years.

The world is a cracked egg shell and we live on one of the cracks. Geologists have found the crack takes an interesting twist under the South Island.

Where shell plates meet, the edge of one usually descends under the other to melt back into the Earth's upper mantle.

That is what the Pacific plate edge is doing when it runs into the Australian plate just before it reaches the North Island.

But down at Fiordland and further south the reverse is happening.

The Australian plate is going under the Pacific plate. The reason for this is that plates contain two varieties of crust: granite that is lumpy and forms land, and basalt that forms ocean floor.

Basalt is heavier and more fluid and it is the one that goes under when they meet.

Between the North Island and Fiordland the plate boundary crosses North Canterbury and runs down the western edge of the Southern Alps.

Along the Great Alpine Fault granite is meeting granite and neither plate is "subducting". Instead they are grinding past each other, the Australian plate going northeast and the Pacific plate southwest.

Canterbury, Otago and Southland are being carried in the opposite direction to the rest of the country.

They are moving away at a rate of 36mm a year and it adds up to 460km so far. Rock at Milford Sound used to be at Nelson.

It is thanks to the absence of subduction that we have the Southern Alps. They are the buckled top of the Pacific plate which continues to push up the mountains as fast as wind and ice are eroding them.

The uplift is about 10mm a year in the central Alps. It might be some consolation in Canterbury that the seismic activity that has shaken them up is the reason they still have land to live on.

In fact considering the pressure that must build up along the Great Alpine Fault, geologists are surprised the region does not suffer more upheaval.

In an article for a book published just two years ago by the Geological Society and GNS Sciences, Alan Cooper and Richard Norris cited excavations at Haast that indicated three major ground ruptures in the past 800 years, each creating an 8m movement along the fault.

"The amount of strain needed to cause this much movement would accumulate in about 350 years," they wrote.

"The timing of forest regrowth on slopes disturbed by earthquake induced landslides, and periods of anomalous tree ring growth, indicate that the most recent large Alpine Fault earthquake occurred in 1717.

"Based on this date, and the estimated return period for large ruptures of the fault," they said, "there is a high probability that a large earthquake will occur on the Alpine Fault in the near future."

Have we just seen it? Geologists say no. This was probably the result of compression of crust well back from the plate boundary. They hadn't known there was a faultline in the crust near Darfield on the Canterbury Plains.

Another article in the Geological Society-GNS Book, A Continent on the Move, (New Zealand is the top tenth of a submarine continent) notes that the fault lines where plates meet on continents might not be as narrow and clear-cut as the visible trenches on the seabed.

Tim Stern and Fred Davey say the South Island Alpine Fault zone widens as it goes deeper and it behaves differently at depth. The effects have been observed 100-200km from each side of the fault line.

The convulsion that hit Christchurch last Saturday leaves geologists with more to learn.

And once the shaking stops and the city is repaired, Cantabrians will be left with a minute of their lives they will forever share.