Scientists discover Alpine Fault's last big earthquake more recent than first thought

Wednesday, 19 June 2019

GNS Science earthquake geologist Robert Langridge has been studying why the Alpine Fault is so susceptible to earthquakes - it's since been discovered that it may be the world's fastest-moving known fault line.

Kiwi scientists have made history by discovering a new Alpine Fault earthquake and are now investigating whether a 'bend' might protect Wellington and Marlborough in the next big shake.

Painstaking work by Victoria University of Wellington palaeoseismologist Dr Jamie Howarth and GNS Science senior earthquake scientist Dr Rob Langridge casts doubt on 1717 as the last time the fault caused a major earthquake.

Instead, they have evidence from the back country behind Hokitika that the northern part of the fault ruptured more recently, sometime between 1740 and 1840, possibly generating a quake with a magnitude of 7.0 to 7.5.

The Alpine Fault runs northeast from the northern side of the entrance to Milford Sound, along the western side of the Southern Alps for about 800 kilometres before morphing into the Marlborough fault system beyond St Arnaud.

They say it is too early to tell how their discovery may change the probabilities of when the next big, possibly magnitude 8-plus, Alpine Fault quake is due.

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GNS senior earthquake scientist Dr Rob Langridge inspects the fault zone in the Inchbonnie trench across the Alpine Fault.

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The southern part of the fault has been found to rupture on average every 291 years, plus or minus 23 years, and there is a 27 per cent chance of that happening again in the next 50 years.

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The 'big bend' in the Alpine Fault between about Springs Junction and Lake Rotoroa, near St Arnaud.

The southern part of the fault has been found to rupture on average every 291 years, plus or minus 23 years, and there is a 27 per cent chance of that happening again in the next 50 years.

Howarth said they were intrigued about the 'big bend' in the Alpine Fault between about Springs Junction and Lake Rotoroa, near St Arnaud, which might help stop a rupture and protect people in Wellington and parts of Marlborough from severe shaking.

GNS Science earthquake geologists studying layers in the trench across the Alpine Fault at Calf Paddock near Springs Junction. (File photo)

The bend – where the fault turns from its northeast-southwest orientation to run in a more north-south direction – was a 'big unknown' in terms of how it interfered with ruptures.

'It might be that it acts as a barrier. It's about whether that provides some protection for those population centres in Marlborough and Wellington further down the fault. It's a puzzle.

Cows in front of the Alpine Fault scarp at Inchbonnie (file photo).

'Bends like that usually stop earthquakes.'

Investigations of the Alpine Fault at Inchbonnie near the Taramakau River have been carried out by studying trenches dug across the fault and by using ground-penetrating radar. (File photo)

The Alpine Fault from Mount Robert above St Arnaud. The fault crosses Lake Rotoiti and the Brunner Peninsula before striking northeast up the Wairau River Valley as the Wairau Fault. (File photo)

The Alpine Fault is the boundary between the Pacific crustal plate and the Australian plate. It runs northeast from the northern side of the entrance to Milford Sound, along the western side of the Southern Alps for about 800 kilometres before morphing into the Marlborough fault system beyond St Arnaud.

Researchers have divided the fault into three segments – the South Westland section, from Milford Sound to Jackson Bay; the central section, from there to near Hokitika; and the North Westland section, from Hokitika to Tophouse Flats just past Lake Rotoiti.

For their date discovery, Howarth and Langridge used new techniques of radiocarbon-dating to reanalyse organic material found in trenches on the section of the fault between the Toaroha River, about 20km southeast of Hokitika, and Springs Junction.

Howarth said each section had its own characteristics. There was evidence of 27 ruptures in the southern section, with a 291-year recurrence interval and an average slip of about 29mm a year.

The central section had evidence of eight events with a return period of 263 years plus or minus 68 years and slip of 28mm a year. But there was only evidence of two ruptures in the 'under-studied' northern section, which meant a robust recurrence internal could not be calculated.

The slip rate here was much lower, only about 14mm a year, probably because movement was transferred in the northern section on to the Hope, Awatere and Wairau faults.

The two quakes they had found evidence for were the big quake in 1717 and the more recent quake, sometime between 1740 and 1840, he said.

'But we don't really know any more than that. We don't have enough data to say. It could be a lower-magnitude local rupture in the [magnitude] 6s, or it could have ruptured a larger part of the North Westland section and be in the low [magnitude] 7s.'

Howarth said 1740 was the earliest likely year for the recently discovered quake and it was unlikely to be much later than 1840, otherwise it would have been felt by South Islanders and talked about.

More work was being done to refine the date of that last quake.