Quake mystery: Did 5.5 jolt really strike on big-risk Alpine Fault?

Scientists are questioning whether a magnitude 5.5 weekend quake did indeed strike on New Zealand's most dangerous mainland fault.

If the event - recorded near Big Bay in the far north of Fiordland at 3:24am on Sunday - did actually rupture on the Alpine Fault, it would have been the most significant shake observed on the South Island's high-risk, 600km-long quake-maker in nearly two decades.

Only, seismologists still weren't so sure.

"The reason we are putting a bit of ambiguity around whether the earthquake happened on the Alpine Fault is just because of the quality of our earthquake location data in that part of the country is not particularly good," GNS Science duty seismologist Sam Taylor-Offord explained.

"So, when we are looking at the map of Big Bay and the epicentre of the main shock in that earthquake, the amount of error in that location could mean the earthquake was anywhere between the Alpine Fault trace … or 5km to 10km inland towards Southland."

Reported by more than 600 people around Queenstown and Wanaka, the quake was part of a swarm of tremors recorded around Milford Sound that morning.

What made the picture so complicated was that the region was notoriously seismologically and tectonically active – and the quake could have happened on faults nearby.

"So, while the epicentre is sitting conspicuously close to the Alpine Fault trace, when we think about that uncertainty and the location, we have to kind of back step a little bit and say, well, that could be on the Alpine Fault, but with the data that we have, we can't actually confirm that."

It was something that might never had been able to be confirmed had a GNS team not co-incidentally deployed a temporary seismic network in the area.

"When they go out and get the data from that, they should be able to actually locate the earthquake very quickly and very reliably, and tell us with certainty whether it occurred on the Alpine Fault."

In any case, scientists were expecting further aftershocks over coming days, and then a return to background levels of seismicity.

"We're not expecting this to be a precursor to a large earthquake - although that's not to say that one couldn't happen."

Taylor-Offord also expected that some stress transfer had resulted from the quake – and some of it had likely been loaded on to the Alpine Fault.

"We would expect to see a reduction in stress around where the actual rupture has actually happened in this earthquake," he said.

"Because there has been movement on that part of a fault, it will actually shift some of that stress on to adjacent areas.

"So I would expect there has been some increased stress on the Alpine Fault, but because it's been a relatively small earthquake, it wouldn't be a very large increase – an infinitesimal increase, I would say."

OUR MONSTER FAULT

The fault, which runs up the western side of the South Island between Milford Sound and Marlborough, posed one of the biggest natural threats to New Zealand.

Over the last 12 million years, the Alpine Fault had driven an incredible 20km of uplift, making the Southern Alps its most recognisable marker.

Only the much faster pace of erosion has kept the alps' highest point below the 4000m mark.

This rapid, dramatic process has also hauled up faulted rock from deep below the surface, giving scientists critical insights into the constant geological scrum taking place under New Zealand.

The fault had a clear geographic record of rupturing around every three centuries - and scientists recently marked the 300th anniversary since an 8.1 quake that shunted the fault's southern side eight metres further south in a matter of seconds.

Until now, scientists knew of four large quakes along the Alpine Fault over 1000 years; today, they could point to 27 over 8000 years.

Further, they've also learned that the southern section of the fault has ruptured repeatedly with similar characteristics in size and in time intervals between events.

These intervals have ranged between 100 and 500 years – a trend that's enabled scientists to roughly work out the probability of a quake over any given future time period.

Given that the Alpine Fault last rumbled to life in about 1717AD, the median estimate for the next one happened to be back in 2010.

But since time periods always varied, we couldn't truly say the fault was "overdue" a big show – nor could we say whether the Canterbury quakes, or the 7.8 Kaikoura Earthquake, had eased or added pressure.

What we could say, however, was the effects of a major Alpine Fault quake would be catastrophic.

Recent research suggested a large event could strand some 10,000 people living in affected areas, along with several thousand tourists.

The inter-agency AF8 project has developed scenarios so South Island communities can plan how they might respond to impacts such as road closures, power losses, fatalities, injuries, and social dislocation.

Based on the first seven days of emergency response, AF8's planning covered such things as the shelter and care of displaced people including the potential for tens of thousands of tourists, an immediate medical response and the restoration of priority telecommunications.

A Tier 4 national Civil Defence exercise was planned to be held next year to build awareness and resilience and to test the planning and national response capability.

• Read an in-depth Herald feature on the latest research on the Alpine Fault here .