It is New Zealand’s biggest seismic hazard, short of a Hikurangi Trench subduction zone rupture. The Alpine Fault earthquake that is expected to occur in the next 50-100 years has been well publicized. But how much are communities close to the fault doing to prepare for a magnitude 8.0+ earthquake?
The Alpine Fault, New Zealand’s answer to the well known San Andreas Fault in California, is the tectonic plate boundary between the Pacific Plate and the Australian Plate where they intersect in the South Island. Every 300 years or so this fault line ruptures in a magnitude 8 earthquake. A sequence of 24 events over the last 8,000 years points to earthquakes in 1100AD, 1450AD, 1620AD and finally around 1717AD,
No part of the South Island will be spared prolonged shaking. Many people, especially i in the lower North Island, will also notice the earthquake. Shaking intensities along the rupturing segment of fault are likely to be up to MMX, which is strong enough to heavily damage all structures, with many failing and large objects such as televisions and microwaves being moved about. Liquefaction, lateral spreading, landslides and seiching of lake bodies will occur as well.
Alpine Fault Magnitude 8 is a collaborative and ongoing project to improve the readiness of councils across the South Island in terms of their ability to respond to such an event. It has buy in from emergency services, Civil Defence, social groups, the N.Z.D.F., agencies working with lifeline infrastructure and others. The aim is to improve modelling of the potential hazard, engage emergency management and planning experts and use the knowledge gleaned to fill gaps about how to respond.
I anticipate that much of the work that has been done will have been brought into sharp focus by the Kaikoura earthquake in 2016. This was the largest onshore earthquake to hit New Zealand since Murchison in 1929. It caused widespread damage across the northern South Island and lower North Island. The quake exposed weaknesses in transport arrangements with both the railway line and State Highway 1 closed – traffic had to be rerouted through the Lewis Pass in order to reach Picton.
Despite the Kaikoura earthquake and lingering shadow of the Christchurch earthquake, not all councils appear keen to progress their disaster planning. Westland District Council found itself in hot water in 2016 for rejecting Plan Change 7, which sought to address the planning issues that Franz Josef township finds itself confronting. The township straddles the Alpine Fault, which is clearly visible from the air as a crude gash in the landscape. Critics pointed out that the council has a duty of care to all in the District and that by failing to address the risks posed, it leaves itself open to court action by anyone in the District at the time of such an earthquake.
Yet the risk remains. Other councils are pressing ahead with their own plans individually, to be fed into the overall A.F.8. planning framework. It is a proactive council that stands the best chance of success, for no one knows when 300 years of seismic stress on the Alpine Fault will give up the ghost. The only certainty is that with the same confidence that darkness will come into a room when the light goes out, one can conclude it is inevitable.