Alpine rupture a scary scenario

Mid Canterbury will shake with a rolling motion for two to three minutes when the Alpine Fault ruptures in coming years.

Shaking could be of the level to make it difficult to stand, as well as causing substantial damage to fragile or unsecured objects, and damaging a few weak buildings.

The Alpine Fault is the focus of attention this week as the New Zealand Journal of Geology and Geophysics has published a special issue with contributions from a range of researchers.

It is 301 years since the last time the fault ruptured, making it overdue for an earthquake, which scientists predict will be in the magnitude of 7.8 to 8.2.

It could result in reshaping the South Island, with the last quake measuring approximately 8.1 and displacing the south eastern side about eight metres along the north-western side in seconds.

“Intense shaking caused numerous landslides, forest damage, and long-term changes to drainage and sedimentation patterns in the Southern Alps, across the coastal plain, and offshore,” researchers said.

They have forecast varying shaking intensities in the South Island according to a model for a magnitude 8 quake starting in Milford Sound,  and ranked them on the Modified Mercalli Intensity Scale. They range up to a level of 10, with pockets at this “very destructive” level occurring along the West Coast.

At the level of 10, many buildings are damaged and most weak buildings are destroyed.

In the model, Mid Canterbury has effects ranging from six (slightly damaging) to seven (damaging) on the scale.

Level six means the quake would be felt by all, people and animals would be alarmed, objects would move and fall from walls and shelves, and slight non-structural damage to buildings could occur. Level seven means there would be general alarm, with people experiencing difficulty standing, substantial damage to unsecured objects and a few weak buildings would be damaged.

Science lead on one of the papers in the journal issue, Caroline Orchiston, said with Mid Canterbury some way from the fault, which is along the western side of the Southern Alps, shaking would not be severe like it had been in Christchurch on February 22, 2011. And the district would be likely to experience a rolling motion, due to the sediments the district was located on, and this would reverberate for two to three minutes.

“Canterbury is sitting on a basin of sediments and there is not a lot of hard rock, so seismic waves bounce around in those sediments, which means shaking would go on for longer,” Orchiston said.

Orchiston said if the rupture in the fault occurred at a different location to the modelled location of Milford Sound, such as near Hokitika, the intensity for Mid Canterbury would still be likely to go no higher than the level of seven.

– Susan Sandys