Horizontal fault line find changes New Zealand earthquake risk

One of New Zealand's major earthquake fault lines extends beneath more of the country than previously thought, according to academic study out Wednesday.

The Alpine Fault, which runs almost the entire length of the South Island, had been assumed to be a near vertical crack, but was in fact nearly horizontal and only curved up to the surface where the fault line was exposed, said Victoria University scientists.

"What we've found is that for approximately 350 kilometers of the length of the South Island, the land mass of the Pacific Plate is actually sitting and sliding right on top of the Australian Plate," Associate Professor Simon Lamb said in a statement.

The region where the Pacific Plate was stacked on top of the Australian Plate was believed to be up to 100 kilometers wide in some places.

"As well as vastly increasing the area where the two plates are in contact with each other, the research tells us that the effects of earthquakes may be quite different, and in some big earthquakes, the rupture zone may never break the surface," said Lamb.

This posed a very different geological problem when assessing earthquake risk.

"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," he said.

The conclusions were drawn from research into both the thickness of the South Island's crust and the speed of seismic waves.

Scientists have evidence that the fault has ruptured 24 times in the past 8,000 years at an average interval of 330 years, although individual intervals ranged from 140 to 500 years.

The fault last ruptured in 1717 and scientists estimated it has a 28 percent probability of rupturing in the next 50 years, which is high by global standards. Endi