Alpine Fault moves more than any other known land fault in the world

MICHAEL DALY

March 8 2016

GNS SCIENCE

The Alpine Fault, which runs up the spine of the South Island, has ruptured five times in the past 1100 years - producing an earthquake of between magnitude 7 and 8 each time.

The Alpine Fault in the South Island has moved more in the past 25 million years than any other known fault on land in the world.

In that time, the two sides of the South Island have shifted more than 700 kilometres relative to each other along the Alpine Fault. That is 250km more than previously thought.

The full extent of the movement was masked because the rocks first moved 250km in one direction, then went back the other way – retracing the first 250km and adding a further 450km.

GNS Science earthquake geologist Robert Langridge has been studying why the Alpine Fault is so susceptible to earthquakes - it's since been discovered that it may be the world's fastest-moving known fault line.

The extent of the movement was worked out by researchers from Victoria University and GNS Science, with the findings published in the American Geophysical Union journal G-Cubed.

JOANNE CARROLL/FAIRFAX NZ

GNS Science earthquake geologist Robert Langridge studying layers in the trench across the Alpine Fault at Springs Junction. New research has found that the fault line may be the world's fastest-moving, having shifted around 700km in 25 million years.

"I don't think anybody in their wildest dreams would have thought that displacements on the fault could be so large, and also change direction so dramatically through time," Associate Professor Dr Simon Lamb, from Victoria's School of Geography, Environment and Earth Sciences, said.

The researchers made the discovery by looking at geological maps together with studies of the direction of magnetisation in the rocks.

The finding could have implications for why the Alpine Fault was the main hazard for earthquakes in the South Island, Lamb said.