Big Bay quake on Sunday largest along Alpine Fault since 2001

Monday, 10 June 2019

Dr Caroline Orchiston explains the Alpine Fault and the damage the earthquake will wreak. (Video first published in May 2018)

Aftershocks are already dying down from what is believed the Alpine Fault's largest earthquake since 2001, a quake scientists say has many interesting aspects about it.

Sunday morning's magnitude 5.5 shake close to Big Bay in the far north of Fiordland, about 10 kilometres northeast of Lake McKerrow, has generated about 70 aftershocks so far.

The 3.24am quake was very shallow, with a focus about 5km deep. Its largest aftershock, a magnitude 4.3 at 2.45pm on Sunday, was centred a few kilometres away near Lake Wilmot.

Big Bay in northern Southland/Fiordland. The Alpine Fault runs from about the middle of the photo on the left obliquely along the valley in the centre of the image, where Sunday's magnitude 5.5 quake was centred, to Lake McKerrow on the right.

GNS Science duty seismologist Sam Taylor-Offord said the Big Bay quake happened in an area where the characteristics of the Alpine Fault were changing as it approached the offshore Puysegur subduction zone.

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The location of more than 70 aftershocks near Big Bay since Sunday morning's magnitude 5.5 quake on the Alpine Fault, centred in the middle of the cluster.

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The magnitude 5.5 earthquake on Sunday morning was centred on the Alpine Fault close to Big Bay in the far north of Southland and Fiordland.

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Mapping has shown the Alpine Fault runs directly under some Franz Josef businesses, including the petrol station, the Helicopter Line offices and part of the supermarket.

'This is the most significant earthquake to occur near the southern end of the Alpine Fault in a few years.

'It is, as many earthquakes are, a reminder of the place we live in and that large earthquakes can come at any time.'

Victoria University of Wellington PhD student Calum Chamberlain at one of the seismic recording sites in the Southern Alps where very small earthquakes on the Alpine Fault have been measured. (File photo)

Following Sunday morning's quake, GeoNet tweeted that the Alpine Fault had 'the potential for larger events and we would like to make sure that you are prepared for a large earthquake at all times'.

The last sizeable quake on or very close the Alpine Fault was also 5km deep but of magnitude 5.8 on December 8, 2001, about 35km further northeast near Lake Ellery, closer to Haast.

The Alpine Fault crossing Calf Paddock at Marble Hill, near Springs Junction. The wall was built in 1964 to measure movement on the fault, but it hasn't moved - yet. The Pacific Plate is upthrust on the left, with the Australian Plate on the right of the scarp.

The Alpine Fault is one of the world's most intensively studied faultlines. Visible from space, it marks the boundary between the Pacific and Australian crustal plates and runs along the western spine of the Southern Alps for about 650km from Marlborough, heading out to sea close to the northern side of Milford Sound.

Long-term geological records show it ruptures roughly every 300 to 350 years, generating magnitude 8 earthquakes. The last great quake was in 1717.

Taylor-Offord said the Big Bay quake appeared to have ruptured the fault vertically on the Pacific Plate – eastern – side, as well as horizontally.

There were many other faults in the area and it was possible the aftershocks, and even the main quake, could have actually happened on these instead.

'This is one of the more seismically active areas through which the Alpine Fault passes.'

The quake had not been large enough for GNS Science to deploy extra instruments, he said.

'We have observed the aftershock sequence taper off significantly already.'

Pennsylvania State University seismologist Professor Kevin Furlong said the quake 'seems to be very clearly on the Alpine Fault'.

'It is in the region where the plate boundary starts doing some interesting things with the transition from the Alpine Fault into the Puysegur subduction section of the plate boundary.

'What I think will be interesting is if any aftershocks (or continuing activity) occur further north along the Alpine Fault, or the activity occurs more to the south, associated with the more seismically active northern Puysegur system.'