Alpine Fault spreads across South Island, researchers say
The Alpine Fault flattens across part of the South Island, painting a different picture of the areas at risk when the fault ruptures.
The fault, which runs for about 600km up the South Island, has produced four earthquakes of around magnitude 8 in the past 900 years, GNS Science said.
Victoria University geologist Dr Simon Lamb said it had been presumed to be a near vertical crack, where the Australian and Pacific plates met.
But after new research, Lamb said for about 350 kilometres of the South Island around the southern lakes, "the land mass of the Pacific Plate is actually sitting and sliding right on top of the Australian Plate".
Along that section the "stacked" area could be up to 100km wide in some places.
"So, rather than thinking of the fault line as a vertical crack, we should be thinking of it as a nearly horizontal one that curves up to the surface where the fault line is exposed."
That would change how the risk of an earthquake on the Alpine Fault was viewed.
"Someone in the centre of the South Island, for instance, might think they are miles away from the fault line, when, in actual fact, the fault could be right underneath them, making these regions more vulnerable than first thought," Lamb said.
The area of the fault with the "flat shape" was toward the south, from about Lake Pukaki to Lake Wakatipu.
Looking at the fault from space, if we were able to remove the overlying features, "it would look sort of like an elongated triangle with its tip north of Tekapo," Lamb said.
There had been a number of clues that something different happened to the fault in that region, including the widening of the Southern Alps and a higher frequency of small earthquakes in the southern zone even though the area was "well away" from the fault.
It also helped to explain why some of the highest mountains, like Aoraki/Mt Cook, were found further down the South Island.
Lamb said further work would be necessary to determine how the new model would change predictions of shaking from an Alpine Fault earthquake.
"It does change the way we think about the response during a big earthquake, what might actually happen...I think that's something that needs to be thought about."
"The better we understand these things, the better prepared we are."
Lamb and his colleagues' conclusions were drawn from research into the thickness of the South Island crust and the speed of seismic waves.
"The crust is very thick beneath the South Island, which is not what you would expect if the two tectonic plates were just sliding past each other on a near vertical fault," Lamb said. Also, seismic waves tend to travel faster the deeper they go, but seismic waves get slower under the Southern Alps.
The findings did not change the Alpine Fault's status as a slip-strike fault, Lamb said, but instead of the two plates running straight past each other one partially straddled the other.
Lamb's research was published in G³, an American Geophysical Union journal.