Great Alpine Fault earthquake talks

This is a summary of the talk given by Associate Professor Tim Davies at Cromwell, Wanaka and Queenstown, 8-10 October 2007.

Natural event and human consequences in Queenstown Lakes and Central Otago.

Tim Davies, Canterbury University
Mauri McSaveney, GNS Science

Introduction

Earth has a mobile surface divided into "tectonic plates". The New Zealand continent is on one of the plate boundaries.

The Alpine fault connects two "subduction" margins where the ocean floor descends into the Earth's mantle. At this point the surfaces of the two plates making up the South Islands alpine fault meet.

The Pacific Plate on the easterly area of New Zealand is moving westwards and the Australian Plate, on the western side of New Zealand moves eastwards. They move at a relative rate of about 45mm per year.

As these two plates move against each other enormous pressure builds up which must eventually be released through earth movement. The result is a major earthquake along the Alpine fault. The pressure has been continually building for about 280 years since it was last released by a large earthquake in 1717 AD.

This earthquake is a normal part of New Zealand's evolution. We are a young country and learning to adapt to it is a necessity if you and your community are to get through.

The alpine fault earthquake will be but one event of many different types of natural hazard events that will occur in New Zealand's future.

Alpine fault 

Where will the next great alpine fault earthquake begin

The next earthquake centred on the Alpine Fault should begin in South Westland and will probably have a magnitude of 8+.

It will be felt throughout the South Island and as far away as Sydney.  The map below indicates the range of the earthquake and its shaking intensity (shown in Roman numerals).

The next great earthquake

When will the next great alpine fault earthquake occur?

The historical patterns of earthquakes and current research on the alpine fault indicate that it is likely to rupture very soon. It is 280 years since the last earthquake. The current pressures in the tectonic plates make it probable that the next earthquake will occur in the next 1-20 years. The further we look into the future, the less likely it becomes because it is most likely to occur now! The probability is increasing every year; presently it is about 1% p.a, 15-20% in the next 20 years, about 50% in the next 100 years.

But probabilities don't tell us when, they just tell us how unsurprised we should be when it happens. About half of the time it may go for more than 400 years between ruptures. But the longer it goes without rupture, the bigger it will be, and the worse are its consequences. Expect and be prepared now for a major shake in Queenstown, Wanaka and parts of Central Otago.

How will the next great alpine fault earthquake happen?

This earthquake will occur with no recognisable warning. The rupture of another faultline nearby may trigger it, but we will only know this afterwards. The alpine fault earthquake will alter tectonic stress distribution, and other faultlines may rupture in the days or years following it.

The length of the rupture will be up to 400 km, eg. on Haast and spreading north to Ahaura. The earthquake will last for about two minutes.

The alpine fault may rupture along part of its length, with lower magnitude, and be followed shortly by rupture of the rest. Two very large earthquakes or a series of large earthquakes are also realistic scenarios.

The magnitude of the earthquake

With an expected magnitude of 8+ this will be considered a "great earthquake" not simply a strong one. The force will result in a horizontal earth shift of up to 8 metres, and a vertical displacement of 4 metres. The effects will be worst in West Otago, diminishing eastward.

Significant building damage can be expected in Queenstown. Damage in other parts of Otago will be irregularly distributed depending on the aspect and topography. Less damage should occur in Wanaka than Queenstown, much less in Cromwell. Some minor damage will occur in Dunedin.

What will happen to the earth during the earthquake?

The effects will be amplified in South Island mountainous regions and high country where enormous damage can occur to peaks and ridges. Countless landslides can be expected of all sizes. In areas where the magnitude is plus or minus 9, many tens of millions of cubic metres of rock and scree may collapse from slopes.

Damaging aftershocks are likely to continue for several weeks afterwards and the event will have disastrous consequences across many regions. Less intense shaking will continue for months. Liquefaction and widespread ground damage will occur.

What will happen to the rivers and lakes?

Landslide dams with breakout flash flooding are very likely. Aggradation will deposit high sediment volumes in riverbeds lasting for many months.  Huge sediment and gravel deposits will have downstream effects for years.  Areas such as the Shotover River may be radically transformed.

Rock avalanches falling into the water bodies may cause tsunami in lakes, rivers, and fiords. Areas such as Lakes Wakatipu, Wanaka, Hawea, Te Anau, Manapouri, Tekapo, Milford Sound Doubtful Sound are at risk from tsunami induced by massive landslips into the water.

Geomorphic consequences

Immediate societal consequences from the earthquake

A sudden onset national civil defence emergency of long duration will be called. It is to be expected that medical services and other civil defence emergency services will be overwhelmed and severely inhibited in their rescue efforts by the scale of damage to roads and buildings. Overseas rescue and medical assistance will be required.

Roading

Expect many bridges to fail in West Otago areas as there is no bridge design that performs well during a fault rupture. Many rivers and streams will become impassable. Roads will suffer serious damage, some areas will be become isolated immediately. Transalpine routes and roads in mountainous areas will be impassable for weeks, therefore, tourists and other travellers are likely to be stranded. Any ski-fields that were operating will pose severe rescue difficulties.

Kawarau Gorge, Kingston, Haast Pass and the Glenorchy routes may be cut in many places, mostly by landslides and dropouts. Large sections of SH8 (Haast Highway) may be out for up to a year.

Even with concentrated repair resources, susceptible major highways can still be out for over a month with multiple blockages.

Electricity supply

Damage to hydro electrical generation plants and transmission lines will result in an immediate shutdown of South Island power generation and widespread disruption of reticulation. Electricity supply is likely to be unavailable for many weeks or even months in some remote areas.

The Clyde Dam has been built to very high specifications and it is unlikely it would suffer catastrophic damage.

Damage to buildings and infrastructure will cause uncontrollable fires.

The nature and location of the earthquake indicate that a relatively small number of people will be killed. However, a large number of people will suffer disabling injuries.

Communications and public services

All communication systems including land and cell-phones may be down in many parts of West Otago. Satellite based telephone systems will initially be the main means of communication. Queenstown could become completely isolated if the airport is damaged.

Water, sewerage, energy, transport, health, and social services may be disrupted for weeks.

Cessation of most commercial activity may occur in many parts of the South Island, however, many local economies will be maintained solely by recovery activities. As in any disaster some people may suffer terribly, and others will profit.

People trapped on roads and tracks, or in accommodation will need to be looked after where they are for days due to road blockages, airport damage, and limited means of transportation.

Agricultural production will  be disrupted, and dairy herds may be unable to be milked in some areas due to electricity outage.

A major problem constraining repair and rebuilding will be the shortage of trades people and materials.

Provision of emergency medical facilities for many major trauma victims and the rescue of trapped people on roads will be severely disrupted for 3-4 weeks.

Earthquake damage

What you can do now to mitigate this catastrophe

Look at your home, property or business and develop scenarios about the risk factors. Develop a plan about for what you need in order to be prepared.  Encourage others to do likewise.

Contact the Civil Defence emergency manager at your district council and find out what to expect from them, and what you should do. Request information on what to expect in the event.

Share your scenarios with friends and other people, and share theirs.  Talking about surviving and coping with an earthquake will help you prepare.  Find out what your friends and neighbours expect of you and tell them what not to expect from you.

There will be many other families to check on, the emergency services will not be able tocheck them all as may be needed.  Plan to be self sufficient for 2-3 weeks.

This event is going to affect all of us, so the better prepared you are the better you will get through.

Put your plans into action.  Do things to help you and your family get through.  E.g. Carry locators, food, drink, blankets, medical supplies in your vehicle when travelling.

Store enough essential supplies (fuel, food, spares, radios, generators, machinery, medical supplies etc) in safe secure locations.

Keep a store of construction and weather proofing materials in a secure site on your property.

Inventory your possessions and machinery and maintain both electronically and as hard copy available to civil defence services.

Identify high-risk parts of your property and gradually strengthen, or redevelop to safer areas - indoors and out.

You will never forget this experience. It may be the most frightening thing ever for you, so prepare to live to tell the story to others. It will be the worse for the next generation if it doesn't happen in the next ten years.

Prepare to adapt to the affects of this earthquake now and give yourself the best chance to survive it.

For more information contact the Emergency Management Officer at the District Council or visit:

www.getthru.govt.nz


Tim Davies
University of Canterbury
Private Bag 4800
Christchurch
www.canterbury.ac.nz

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Dunedin 9054

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