Plans made for ‘catastrophic’ quake

Tremor threat: The South Island's alps PICTURE: NZ HERALD

Civil Defence groups are developing a plan to deal with the catastrophic threat from the South Island’s Alpine Fault.

GNS Science maintains the rupture would produce one of the biggest earthquakes since European settlement of New Zealand and have a “major impact” on the lives of people across the country.

Delegates at a Civil Defence conference in Queenstown last Friday were updated on what’s called Project AF8 – a joint effort to develop a co-ordinated response in the event of a potentially devastating rupture on the fault, running up the spine of the South Island.

Once complete, the research would inform a series of workshops with the South Island’s six Civil Defence Emergency Management (CDEM) groups to help develop a plan, due to be finalised within the next two years.

It’s the first time a collective South Island-wide response plan had been attempted.

At last week’s South Island Civil Defence Emergency Management Conference, Dr Caroline Orchiston of Otago University’s Centre for Sustainability says: “We are still talking about a South Island-wide impact that will be felt across all of those different CDEM regions – that’s why we need to do this work so we can get all of the plans in place.

A focus of the conference was a potential Alpine Fault quake striking Queenstown, which would require CDEM planners to evacuate those trapped in the town if road closures left it isolated.

“It was just a way to get people thinking about the broader outcomes of an Alpine Fault earthquake; it’s not just going to be an Otago response, or a Southland response – it’s going to be one that affects all of those groups.

“Lifelines, power, communications, all of those things would be significantly disrupted for quite a long period of time.”

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Research over the past four decades has added considerably to the understanding of the fault’s rupture history and the likely impacts from its next big earthquake, which could change the face of the country and lives within it forever.

On average, earthquakes of magnitude 7.5 or larger strike along the Alpine Fault every 300 years – a relatively frequent rate in geological terms – and scientists say it’s highly likely the next big quake will happen in the lifetime of today’s population.

It last ruptured 299 years ago – producing a massive earthquake of about magnitude 8.0 – and has an average 30 per cent probability of rupturing in the next 50 years.

Orchiston expects most people in the South Island were aware of the Alpine Fault, “and certainly, there is a level of awareness about what it could mean for the South Island”.

“But I think we’ve still got a long way to go in terms of building more awareness, and certainly preparedness, from individuals about what could happen after an event … but the great thing is that all of the evidence and these CDEM groups are really coming together.”

Satellite view: A shaded and coloured image of the Alpine Fault. IMAGE: NASA
Satellite view: A shaded and coloured image of the Alpine Fault. IMAGE: NASA

Meanwhile, GNS Science is undertaking a range of research to understand the fault’s behaviour, but it is no easy task establishing how often it moves, how big its earthquakes are, and how widely the impacts will be felt.

One of them is exploring natural “environmental archives” preserved in sediments at the bottom of alpine lakes Ellery, Paringa, Christabel, Kaniere, and Mapourika, which are adjacent to the fault.

As part of the new study, Dr Jamie Howarth and colleagues have established a new sub-discipline in New Zealand earthquake science, called “lacustrine paleoseismology”.

In this case, earthquakes shake the landscape, steep hillsides are mobilised, and there are tell-tale influxes and disruptions of sediments in lakes.

To investigate it, researchers need to collect drill cores of the sediments, analyse them, determine the timing of the sediment influx events using radiocarbon dating, and then interpret the results.

While the rupture history of the southern segment of the Alpine Fault between Haast and Milford Sound is reasonably well understood from research efforts in the past two decades, the history of the middle segment between Haast and Fox Glacier remained less clear.

But early results from the new approach have proven encouraging, and could offer an exciting new way to address uncertainties.

Scientists now have a record of 12 ruptures on the middle segment of the fault spanning the past 3500 years, providing a more complete picture of the rupture history, and boosting risk modelling and preparedness.

NZ Herald