Scientists mark 300 years since last great Alpine Fault quake - preparing for the 'inevitable'

Tuesday, 11 September 2018

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

Scientists say the last major Alpine Fault earthquake, in 1717, brought long-term disruption across the South Island and 'there will be a similar earthquake soon'.

The 1717 quake is estimated to have had a magnitude of around 8.1, according to a special anniversary edition of the New Zealand Journal of Geology and Geophysics, published on Tuesday. That estimate comes from evidence for the ground breaking along 380km of the South Island.

'Land on the south-eastern side of the fault moved up relative to the north-western side in most places and 7–8 metres south in a matter of seconds,' an editorial said. 'Intense shaking caused numerous large landslides, forest damage, and long-term changes to drainage and sedimentation patterns in the Southern Alps, across the coastal plain, and offshore.'

Another great earthquake on the Alpine Fault is considered to be inevitable.

The fault remains of global importance for researchers, the journal noted. 'It provides an opportunity to study a major fault late in its typical cycle of stress accumulation ahead of a future earthquake, to search for precursors to large earthquakes, and to understand the long-term evolution of faults and the formation of fault rocks.'

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The editorial was co-written by chief guest editor Dr Phaedra Upton of GNS Science, and several other prominent New Zealand earthquake researchers.

GNS Science earthquake geologists studying layers in a trench across the Alpine Fault at Springs Junction.

They said the plan was to publish the edition exactly 300 years after the earthquake, but the November 2016 7.8 magnitude Kaikōura Earthquake intervened. It was 'a timely reminder of how little we know', the editorial said.

'The Kaikōura Earthquake kept many of the editors and authors of this volume busy during the rapid response required to understand what had happened. However, the surprises and insights we gained from witnessing a major surface-rupturing earthquake in modern times have made us even more motivated to compile and present current research on the Alpine Fault.'

Research has included drilling into the Alpine Fault.

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.

The evidence for the 1717 date for the last big quake on the fault came from trees, sediments and the West Coast landscape.

'For example, the specific date for this great earthquake comes from very old trees. Tree ring patterns from various species (cedar, silver beech and mountain beech), and from localities spread between Hokitika and Milford Sound, show a marked decline in growth between 1717 and 1719,' the editorial said.

'The timing of the disturbance that caused this decline can be narrowed down to taking place after the 1716 AD growing season and before the end of the 1717 AD growing season - i.e., the earthquake occurred early in the year 1717 AD.'

Insights weren't just about the size of the quake, 'but also the immediate and long-term cascading nature of the damage wrought to mountains, waterways, and forests'.

That 'havoc' was just the most recent in a regularly repeating pattern of movements of the Earth's crust.

'The Alpine Fault is the boundary between the Pacific and Australian tectonic plates that are moving relentlessly past and towards each other. There will be a similar earthquake soon. In fact, the last great earthquake occurred over 300 years ago and the average recurrence for such events is less than 300 years.'

In one of the papers in the journal a team led by Dr Jamie Howarth of Victoria University reviewed research into past large earthquakes on the Alpine Faults.

It was known that large to great earthquakes had occurred on the Alpine Fault quasi-periodically every 250-300 years for at least the past 8000 years, the paper said.

There was evidence for seven earthquakes replicated at multiple sites, and a record of 27 earthquakes for one part of the fault.

'It is in part the large number of past earthquakes we now have evidence for - demonstrating that the next event is inevitable - that has motivated additional effort to prepare for the next Alpine Fault earthquake, for example through initiatives such as the AF8 project,' the paper said.

'We see that surface-rupturing earthquakes occur remarkably regularly on the South Westland section of the fault.' For the South Westland section of the fault, the likelihood of a surface-rupturing earthquake in the next 50 years was 29 per cent.

'We are starting to understand that each section behaves differently and hence that differently sized earthquakes occur on this fault.'

There was evidence of large landscape impacts from Alpine Fault earthquakes, with periods of increased sediment flux lasting up to 50 years.