Tree stumps old fault theories

A West Coast tree stump has revealed that the Alpine Fault is moving more quickly than thought.

Researchers working at Inchbonnie, a dairy-farming settlement about 60 kilometres from Greymouth that lies on the fault, dated wood from the dead podocarp, believed to be a matai, sitting in water at the southern end of Lake Poerua.

The dating was part of a series of investigations in the area in February 2008 to determine movement on the fault and how strain is transferred from it to the nearby Hope, Clarence and Kakapo faults at the western edge of the Marlborough Fault System.

Until now, the average annual horizontal slip rate of the Alpine Fault at Inchbonnie was believed to be about 10 millimetres a year.

The latest calculations, based on when the stump died as a result of an earthquake, show it is moving at an average 13.6mm a year, plus or minus 1.8mm. That equates to about an extra metre of movement each time the fault ruptures – about every 300 years.

The Alpine Fault is the boundary between the Australian and Pacific plates, which are moving past and pushing against each other, forcing the Southern Alps higher.

The fault is visible from space and runs for about 650km from the Wairau River valley through the central upper South Island, down the western side of the Southern Alps and out to sea near the Milford Sound entrance.

The last time the fault moved was about 1717, generating an earthquake of at least magnitude 8.0.

GNS Science earthquake scientists Rob Langridge and Pilar Villamor, Lincoln University soil scientist Peter Almond and two Spanish scientists have published a paper in Lithosphere outlining their Inchbonnie findings.

A slab of wood was cut from the stump and age analysis showed when the tree began growing and when it died as a result of a major Alpine Fault earthquake upsetting the water table.

"Podocarps are intolerant of saturated soil... When the local groundwater table rose, this tree effectively drowned and was preserved in place."

Dating indicated the tree was about 570 years old when it died between 947 and 1064 years ago.