7.8 quake raised risk of another biggie, say experts

International scientists have claimed the 7.8 Kaikoura earthquake has raised the potential threat of another large event facing New Zealand in the future.

But a top GNS Science natural hazards expert says there’s no indication that the November 14 quake would prove a trigger for “the big one”.

In an article this month in Science Bulletin, scientists from the US, China and Singapore wrote the rupture created by the Kaikoura quake — which set off at least 10 faults after first striking at an epicentre near Culverden — was “far too complex to have [ been] predicted, even if we knew all of the previously unrecognised faults in the system”.

They said scientists may now have to reconsider the way earthquakes were modelled in complex systems.

Further, they said stress changes caused by the quake warned that faults in the southern part of the North Island, near Wellington, may be “closer to failure than the past”.

The fact that the Kaikoura earth- quake occurred along the comparably smaller Marlborough Fault System, instead of the much larger Alpine Fault on the west of the South Island, could also signal “the coming of a great earthquake along this plate boundary fault”, they wrote.

On average, earthquakes of magnitude 7.5 or larger strike along the Alpine Fault every 300 years — a relatively frequent rate in geological terms — and scientists say it’s highly likely the next big quake will happen in the lifetime of today’s population.

It last ruptured 299 years ago — producing a massive earthquake of about magnitude 8.0 — and has an average 30 per cent probability of rupturing in the next 50 years.

But GNS Science principal scientist Kelvin Berryman said investigations had so far indicated the Kaikoura quake was unlikely to have loaded pressure on the Alpine Fault, and instead had applied pressure northeastward toward the Cook Strait.

The article’s authors also suggested the increase in the number of slow-slip events along the Hikurangi plate boundary underneath the North Island, triggered by the Kaikoura earthquake, had “encouraged re-evaluation of the potential of a great megathrust earthquake in this region in the near future”.

Berryman said while there had been some loading on the subduction interface, there had been many major quakes the size of the Kaikoura event in the 600 years since the last big subduction quake — and none had proven enough to be a trigger.

“Sooner or later, there might be a trigger — but there’s no reason to think this was the one.”