Feature

Wild West Coast

The spectacular scenery of the South Island's West Coast hides clues to past and future dangers.

Veronika Meduna

Everything about the West Coast has an element of drama, but few people are aware of how close they are to potential devastation as they travel along the road that lies at the abrupt foot of the South Island's main mountain range. Driving along State Highway 6 and past the new petrol station in Franz Josef Glacier, the slight rise in the road goes mostly unnoticed. Yet it identifies the exact location of New Zealand's largest active fault, which stretches about 480km from Milford Sound to Blenheim.

Many communities along the West Coast lie close to the Alpine Fault, the boundary of two major tectonic plates. The fault has long been silent but has produced devastating earthquakes in the past and has another big one in store for us soon.

Andrew Wells, a PhD student in Lincoln University's ecology and entomology group, and Dr Tim Davies, a reader in natural resources engineering, have focused their scientific interest on the Alpine Fault and the outcomes of its past activity. Their work provides insights into the Coast's seismic potential and power.

While one researcher looks back at the history of major ruptures and the other looks forward to methods of hazard risk assessment and prevention, they both paint a picture of major damage and destruction when they talk about the next earthquake.

Wells has spent the last three years piecing together the history of major natural disturbances in Westland, using trees as indicators of environmental stress. He worked closely together with geologist Mark Yetton, who spent many hours in trenches along the fault line trying to work out the timing of past eruptions. He identified three time periods in the past 600 years during which major earthquakes have happened.

"But you can't really tell from this," says Wells, "whether the earthquakes were synchronous across Westland or what impact they had on the landscape. So the idea with trees was to look at the impact of the earthquakes on the landscape and to try and figure out when exactly they occurred."

Revealing Tree Rings

In the case of the most recent natural disaster, the analysis of tree rings helped to narrow down the estimated time period to one year -- 1717 AD. The previous events occurred sometime around 1620 and 1450. Each caused devastation of the landscape severe enough to justify describing them as natural catastrophes, most likely caused by an earthquake of a magnitude greater than eight on the Richter scale.

The research project involved taking tree-core samples from defined areas of the Karangarua valley, south of Fox Glacier, to determine the ages of cohorts of trees that had colonised after disturbance events.

"In Westland," Wells says, "most of the conifer trees will live up to 800 years quite happily, unless they get killed by a landslide or other disturbance. If there is a landslide, it creates a devastated site, and new trees will colonise that really quickly, especially rimu, cedar, totara and kahikatea. They are often some of the first species to come back after a landslide, or after an event such as tree windfall and earthquake shaking which will topple a lot of forest.

"These trees will come back within 30 to 40 years and will establish new forests. So when you get a forest disturbance you can easily date it by ageing a sample of the trees growing on the disturbed area, and the ages will tell you the date of the event to within a few decades."

Often, he found a few gnarly survivors surrounded by a cohort of younger trees. The distinct age groups alone are evidence that something devastating has happened in the area, he says, but the rings in the older trees provide further proof.

"The way the tree rings show that is, generally, as really marked sudden suppressions in growth which can last for 10, sometimes even 100, years. When a tree has been affected by a landslide, it often suffers great damage to its roots, which affects its whole ability to grow and maintain itself; sometimes you get almost a total shut-off from growth."

Wells says he originally set out to study the history of major disturbances unaware of what was being done in geology, purely from a perspective of wanting to understand forest development. It was only when he met Mark Yetton that he realised that his own results confirmed what Yetton had found and even provided better refinement of the timing.

"What the forest record tells us is that there were three periods in the last 600 years of massive natural disturbance throughout Westland. Cohorts of trees of these ages are on many different new surfaces, such as landslide surfaces, young terraces, debris avalanches, but also on old, stable surfaces which weren't affected by erosion or sedimentation; there had obviously been some major disturbance to the forest by treefall, be that earthquake shaking or wind."

While the tree study alone could not exclude massive storms as culprits, the geology data from the fault line itself confirmed that the damage was caused by earthquakes.

"All indirect evidence for landscape disturbance, such as forest ages and even dating of landslides and debris avalanches, does not actually directly link it to an earthquake. To do that you need information from the fault line itself, and it was the geology work that pinned down these landscape impacts in Westland to Alpine Fault ruptures."

From the analysis of Westland's 600-year history of natural disasters, it seems obvious that the next major earthquake is overdue.

"The intervals between the most recent earthquakes were about 100 and 170 years, but since the last one 280 years have elapsed, so you could quite reasonably say that we must be overdue."

Tim Davies agrees. He says statistically there is a 16% chance of a big earthquake in the next 10 years. "The most likely date for it to happen is today. Geologists are expecting it any time."

The West Coast's spectacular beauty is mostly due to the fact that it is an active landscape, changing at a fast pace.

"It's a major plate tectonic boundary in the Roaring Forties, and that's probably one of the worst combinations in the whole world for generating an active landscape -- or the best combination, depending on which way you look at it.

"The tectonic plates are moving very rapidly with respect to one another, so it's an area in which there are frequent severe earthquakes, it's an area of very high uplift of the mountains, it's an area of very high rainfall."

Davies says while all landscapes evolve, most of the landscapes in which civilisations have been established for a long time, like Europe, are evolving fairly slowly.

"The West Coast of New Zealand is evolving very rapidly -- the mountains are going up about one centimetre a year on average. A consequence of this is that every 200 to 300 years, the Alpine Fault moves and there is a very serious earthquake."

When it does move, there will be massive erosion, he says. Major landslides and rock avalanches will deliver large volumes of debris to the rivers, which will spread it out on the plains at the foot of the mountains.

"These are the plains on which people have established what they think are permanent facilities like roads, bridges, houses, hotels, airfields. The infrastructure of society over there was designed to be permanent, but in fact it is very temporary."

Davies has studied the way in which communities inhabit natural landscapes and cope with the hazards Nature occasionally throws at them. He says people have two options: either they work out strategies to manage the risk of natural hazards or they look for a safer place to live. But nature on the West Coast is too powerful to manage, he says. However, few people there realise that they live on borrowed time.

It's Happened Before

Even in the short time since people began recording natural events, the Alpine Fault has demonstrated its power regularly and Davies has plenty of material to illustrate the frequency of natural calamities.

For example, a major shake near Murchison in 1929 caused a landslide which blocked the Matakitaki River and created a huge lake. In the same event, a river broke another landslide dam, transporting mud and silt down with it and killing people in a mining settlement.

In the course of his work, Davies has completed a hazard risk assessment for the Franz Josef area and says minimization of major damage and loss of human life is essential, particularly in the light of the importance of tourism to the West Coast.

He says there is tension between what he can see the landscape is likely to do and what people want to do with the landscape, and some of his strategies may seem radical. For example, he believes the traditional strategies for controlling West Coast rivers are unreliable and may in some cases exacerbate the hazard. He has pictures of newly built dwellings sited on active debris flow fans, where the next rainstorm might well destroy the dwelling without warning.

However, safer areas are often within a few metres of where people intend to or have built tourism and other facilities, and relocating somewhere safe only means putting a respectful distance between yourself and the natural spectacle -- "and that's got to be worth it. You can't really manage hazards -- you can only manage people, and in the long run the most economic solution to all serious hazard situations is to get out of the way. In New Zealand we usually have space available to do this, and we should make better use of it".

Such remarks apply particularly urgently to the West Coast, where the probability of a serious hazard situation is extraordinarily high, he says.

The West Coast can be clearly seen as a place under strain in this diagram from Caught in the Crunch. The strain is caused by movements of the plates in either side of the country. When the strain at any one point becomes too much, a fault breaks causing an earthquake. All of New Zealand's large earthquakes, such as those at Murchison and Inangahua, occur within this zone.
Caught in the Crunch by Rebecca Ansell and John Tabor (Harper Collins, 1996; 188 pages; $34.95) provides an excellent geological, historical and personal look at earthquake and volcanic activity in the "Shakey Isles", as well as practical advice of what to do if you get "caught in the crunch". Well illustrated and well written, it's a fitting tribute to Rebecca's late father, Jim Ansell, Professor of Geophysics at Victoria University.

Veronika Meduna is a freelance journalist with an interest in science stories.