'Surprising amount' of aftershocks following Fiordland earthquake, scientist says

Tuesday, 13 August 2019

GNS seismologists say there is no real cause for concern after a 5.5 magnitude quake struck near Milford Sound on Monday night.

A strong Fiordland earthquake that shook several thousand South Islanders was just a run-of-the-mill event, GNS Science says.

The magnitude 5.5 quake at 10.35pm on Monday was centred about 15 kilometres north of the Milford Sound settlement and was 12km deep.

According to GeoNet, its epicentre was near wetlands along the John O'Groats River, a couple of kilometres from the onshore end of the Alpine Fault.

The Alpine Fault comes ashore near the mouth of the John O'Groats River - which runs into the sea near to the entrance to Milford Sound in the centre left of this photo, close to the epicentre of Monday night's magnitude 5.5 quake.

However, the Crown research institute and University of Otago professor of earthquake science Mark Stirling believed it was not generated by that fault.

**READ MORE:

Fiordland is the most seismically active part of New Zealand and a magnitude 5.5 quake is nothing really out of the ordinary. (File photo of Milford Sound)

* Alpine Fault is better understood – but that won't stop the Big One

* Big Bay quake on Sunday largest along Alpine Fault since 2001

The earthquake was centred around wetlands on the John O'Groats River (white blob), not far from where the Alpine Fault heads offshore between Yates Point and Stripe Point close to the entrance to Milford Sound.

* Scientists examine Alpine Fault for signs of stress following earthquake

* 'Soft rocks' prevented earthquake damage

The epicentre of Monday night's magnitude 5,5 quake (white blob) was on the last couple of kilometres of the onshore section of the Alpine Fault.

* Our Australian cousins just got closer**

More than 6800 felt reports of the quake have been sent to GeoNet, most from people across Otago and Southland and up the West Coast, but a few from Canterbury and Marlborough and parts of the North Island too.

There have also been reports of several small rockfalls around Milford Sound and three avalanches being triggered above State Highway 94 near the Homer Tunnel.

On June 10, another magnitude 5.5 quake occurred a little further north close to Big Bay, in the far north of Fiordland.

Stirling said it appeared the shake was initiated in the zone where the Australian crustal plate was being subducted beneath the Pacific Plate.

'It looks like a typical thrust earthquake on the subduction zone, caused by compression.

'It's an area of transition from the subduction zone moving into the Alpine Fault area, with lots of complexity there.

'In the scheme of things it's a pretty moderate-sized earthquake and in terms of stress changes it has probably not done much. There have been quite a few aftershocks – a surprising amount,' Stirling said.

More than 25 felt aftershocks of between magnitude 2.3 and 3.7 had been recorded by late Tuesday morning.

Pennsylvania State University seismologist Professor Kevin Furlong, who was at the University of Canterbury during the 2010-11 quakes, said the location of the earthquake was interesting.

'It is the area where the plate boundary switches from the Alpine Fault to the Puysegur subduction zone. That transition is fairly complicated, so these earthquakes, although relatively small, do provide some insight into how that plate boundary transition actually works.

'The events … probably reflect effects of compression between the Australia and Pacific plates. Essentially the crust is being stressed by movements below it and these earthquakes are the result,' Furlong said.

GNS Science duty seismologist Elizabeth Abbott said in a statement it was not known which fault the quake occurred on.

The aftershocks also appeared to be on the same fault.

It was 'an almost textbook example of a reverse fault, where the earthquake is caused by compression of the Earth's crust on either side of the fault'.

'It is not unexpected for this region.'

It was unlikely to have had an effect on the Alpine Fault, was not linked with recent magnitude 5.6 and 6.3 quakes near the Snares Islands southwest of the South Island, and was also unlikely to be related to last week's very shallow, 1km deep, magnitude 3.6 quake near Haast.

There was nothing unusual or concerning about the quake, she said.