Alpine Fault quake: Is the big one coming?

One of the country's potentially most destructive fault lines rumbled at the weekend - so does this mean a big one is on the way?

Queenstown residents were woken up early on Sunday morning by a relatively shallow magnitude 5.5 tremor, centred just northeast of Milford Sound on the Alpine Fault.

"This fault system has the potential for larger events and we would like to make sure that you are prepared for a large earthquake at all times, GeoNet said on its Twitter account.

How large? Magnitude 8 or larger, geologists said in a special edition of the New Zealand Journal of Geology and Geophysics last year. And it's overdue, having last ruptured in 1717.

A magnitude 8 quake would release twice as much energy as the 2016 Kaikoura 7.8 quake, according to the US Geological Survey's 'How Much Bigger?' calculator.

GNS Science duty seismologist Sam Taylor-Offord says the weekend's quake hasn't changed the outlook for the Alpine Fault.

"The chance of a larger earthquake happening is still about the same as it was before this quake," he told Newshub on Monday.

"There is always the potential for a large earthquake on this fault, as there is for any other fault in New Zealand. We can't say that a big one is not on the way, but we don't believe that this quake has significantly increased the chance of a big one."

Where the quake was felt.
Where the quake was felt. Photo credit: GeoNet

Geologists say you generally can't predict whether a big quake is about to happen based on smaller foreshocks -  while many large quakes are preceded by smaller ones, just as many aren't.

Nor do small shocks reduce the chance of a big one happening - they're just not strong enough to release the tension.

"It would take 32 magnitude 5s, 1000 magnitude 4s, or 32,000 magnitude 3s to equal the energy of one magnitude 6 event," the USGS says on its site.

That's because the magnitude scale is logarithmic - a magnitude 6 quake isn't just a bit stronger than a magnitude 5 - the seismic waves are actually 10 times stronger, and the energy release a whopping 32 times. That's why you wouldn't even feel most magnitude 3 quakes, but a magnitude 6.3, like the one that hit Christchurch in 2011, can knock down buildings - it's literally 89,000 times more powerful.

Taylor-Offord says the Alpine Fault is feared because "there is a clear geological record showing that many earthquakes above magnitude 7 have occurred on this fault".

"We know that the fault will likely rupture again in such an earthquake, and that the impact of such an earthquake would be significant. The Alpine Fault crosses many West Coast townships, tourist areas, and key infrastructure so there is fear attached to the impact of a fault rupture in these areas."

The Alpine Fault.
The Alpine Fault. Photo credit: NASA/Newshub.

The Kaikoura quake was complex, with several faults rupturing at once - including a number that were previously unknown.

"In contrast, unknown faults are just that - unknown - and so there is not so much anxiety attached to them," says Taylor-Offord.

"Our well-known faults always get more attention because people have an understanding of their potential."