A magnitude 5.5 earthquake that sent debris rolling down South Island mountains struck on a "dog's breakfast" of tectonic systems – but there's no indication it had any effect on the big-risk Alpine Fault.

Nearly 7000 people between Southland and Canterbury reported feeling the 10.35pm quake, which was recorded about 15km north of Milford Sound, at a depth of just 12km.

There had since been about 60 aftershocks – 16 of them registering at magnitude 3 or higher.

GNS Science seismologist John Ristau said the quake was a case of "reverse faulting".


This happened when the Earth's crust was being compressed, and one block of crust moved up and over the other.

Ristau said it was unclear whether the quake had struck on any pre-existing fault, as GeoNet's monitoring coverage wasn't as refined in Milford Sound's remote wilderness.

"The best we can give is an approximate location, within a few kilometres," he said.

"There hasn't been much in the way of detailed mapping in that area as it's hard to get to – and there are going to be many, many faults in that area."

The Milford area happened to be one of New Zealand's – and the world's – most seismically active regions.

More than 7000 people reported feeling the 10.35pm quake. Source / GNS Science
More than 7000 people reported feeling the 10.35pm quake. Source / GNS Science

"At the southern end of Fiordland, you have the Australian plate subducting beneath the Pacific plate, but, as you start to get further north, along the West Coast, we transition into the Alpine Fault, and the plates start to slide past each other," he explained.

"And so this earthquake was almost in that transition area – that's where things start to get really complicated. It's a like a little dog's breakfast of tectonics going on there, and we really can't calculate what the impact of stresses would be."

It followed another 5.5 quake at Fiordland's Big Bay in June, which might have been the most significant shake observed on the South Island's high-risk, 600km-long Alpine Fault in nearly two decades.


However, in both cases, the impact on the big quake-maker would have been negligible.

Last night's event also wasn't linked to other recent quakes that had struck south of New Zealand.

"It's always good to remember that Fiordland is the most seismically active area in New Zealand, and earthquakes of this size happen there on a regular basis, so there's nothing really unusual about this."