NZ’s next large Alpine Fault quake is likely coming sooner than we thought, study shows

NASA/JPL/NGA, CC BY-ND

Jamie Howarth, Te Herenga Waka — Victoria University of Wellington and Rupert Sutherland, Te Herenga Waka — Victoria University of Wellington

The Alpine Fault marks the boundary between the Pacific and Australian plates in the South Island of New Zealand.
Author provided

The chances of New Zealand’s Alpine Fault rupturing in a damaging earthquake in the next 50 years are much higher than previously thought, according to our research, published today.

The 850km Alpine Fault runs along the mountainous spine of the South Island, marking the boundary where the Australian and Pacific tectonic plates meet and grind against each other, forcing up the Southern Alps. Over the past 4,000 years, it has ruptured more than 20 times, on average around every 250 years.

Alpine Fault earthquakes are recorded in lake sediment deposits.

The last major earthquake on the Alpine Fault was in 1717. It shunted land horizontally by eight metres and uplifted the mountains a couple of metres. Large earthquakes on the fault tend to propagate uninhibited for hundreds of kilometres.

Until now, scientists thought the risk of a major earthquake in the next 50 years was about 30%. But our analysis of data from 20 previous earthquakes along 350 kilometres of the fault shows the probability of that earthquake occurring before 2068 is about 75%. We also calculated an 82% chance the earthquake will be of magnitude 8 or higher.

Alpine Fault earthquakes in space and time

From space, the fault appears like a straight line on the western side of the Southern Alps. But there are variations in the fault’s geometry (its orientation and the angle it dips into Earth’s crust) and the rate at which the two plates slip past each other.

These differences separate the fault into different segments. We thought the boundaries between these segments might be important for stopping earthquake ruptures, but we didn’t appreciate how important until now.

Differences in geometry and the rate of slip between the tectonic plates create sections along the Alpine Fault.
Author provided

We examined evidence from 20 previous Alpine Fault ruptures recorded in sediments in four lakes and two swamps on the west coast of the South Island over the past 4,000 years. From this evidence, we built one of the most complete earthquake records of its kind.

Once we analysed and dated the sediments from lakes near the Alpine Fault, we were able to see new patterns in the distribution of earthquakes along the fault. One of our findings is a curious “earthquake gate” at the boundary between the fault’s south western and central segments. It appears to determine how large an Alpine Fault earthquake gets.

Some ruptures stop at the gate and produce major earthquakes in the magnitude 7 range. Ruptures that pass through the gate grow into great earthquakes of magnitude 8 or more. This pattern of stopping or letting ruptures pass through tends to occur in sequences, producing phases of major or great earthquakes through time.

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Forecasting the next Alpine Fault earthquake

From the record of past earthquakes it is possible to forecast the likelihood of a future earthquake (i.e. a 75% chance the fault will rupture in the next 50 years). But from these data alone it is not possible to estimate the magnitude of the next event.

For this we used a physics-based model of how earthquakes behave and applied it to the Alpine Fault, testing it against data from earlier earthquake sequences. This is the first time we have been able to use past earthquake data that span multiple large earthquakes and are of sufficient quality to allow us to evaluate how such models could be used in forecasting.

The physics-based model simulated Alpine Fault earthquake behaviour when we included the variations in fault geometry that define the different fault segments. When the simulation is combined with our record of past behaviour it is possible to estimate the magnitude of the next earthquake.

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The Alpine Fault earthquake record shows the past three earthquakes ruptured through the earthquake gate and produced great (magnitude 8 or higher) earthquakes. Our simulations show that if three earthquakes passed through the gate, the next one is also likely to go through.

This means we’d expect the next earthquake to be similar to the last one in 1717, which ruptured along about 380km of the fault and had an estimated magnitude 8.1.

Our findings do not change the fact the Alpine Fault has always been and will continue to be hazardous. But now we can say the next earthquake will likely happen in the next 50 years.

We need to move beyond planning the immediate response to the next event, which has been done well through the Alpine Fault Magnitude 8 (AF8) programme, to thinking about how we make decisions about future investment to improve infrastructure and community preparedness.

Jamie Howarth, Senior lecturer, Te Herenga Waka — Victoria University of Wellington and Rupert Sutherland, Professor of tectonics and geophysics, Te Herenga Waka — Victoria University of Wellington

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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Rainforest giants with rare autumn displays: there’s a lot more to Australia’s red cedar than timber

Peter Woodard/Wikimedia

Gregory Moore, The University of MelbourneNative deciduous trees are rare in Australia, which means many of the red, yellow and brown leaves we associate with autumn come from introduced species, such as maples, oaks and elms.

One native tree, however, stands out for its leaves with soft autumnal hues that drop in March and April: Australia’s red cedar. Don’t be fooled by its common name — red cedar is not a cedar at all, but naturally grows in rainforests throughout Southeast Asia and Australia.

You may be more familiar with its timber, which I’ve been acquainted with all of my life. My grandmothers had cedar chests of drawers they had inherited from their mothers or grandmothers, and I had assumed they were made from one of the Northern hemisphere cedar species. The wood still smelled of cedar after all this time in family homes – a scent I associate with grandparents and country homes.

By the time I was given one of these chests to restore, I knew much more about the tree and valued the chest of drawers all the more. So, with autumn putting a spotlight on Australian red cedars, let’s look at this species in more detail.

Majestic giants of the rainforest

I first encountered red cedar trees in the sub-tropical rainforests of Queensland and New South Wales in the 1980s. Then, its scientific name was called Cedrela toona and later Toona australis. Now, it’s recognised as Toona ciliata.

The various names reflect a taxonomic history in which the Australian species was once regarded as being separate from its Asian relatives, but all are now considered one.

Native red cedar trees can grow up to 60m tall.
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The trees are awe-inspiring. Under the right conditions, it can grow to 60 metres tall (occasionally more) with a trunk diameter of up to 7m.

After losing its foliage in autumn, the new foliage in spring often has an attractive reddish tinge. In late spring it has small (5 milimetres) white or pale pink flowers, but they usually go unnoticed in the rainforest because of their height or the density of other tree canopies growing beneath.

Older red cedars have wonderful buttresses at the base of their trunk, a characteristic shared by many tall tropical trees. These buttresses have long been considered an advantage for species that can emerge above the canopy of a rainforest where winds are much stronger, with the buttresses and expanded root systems providing greater strength and resistance to the wind.

These buttresses also greatly increase the surface area of the base of the trees exposed to air, which facilitates the uptake of extra oxygen as the activity of micro-organisms in the soil can leave it oxygen-depleted.

Tiny white flowers are hard to see from the ground in a rainforest.
Forest and Kim Starr/Wikimedia, CC BY-SA

Logged to near extinction

With a wide distribution throughout Asia and Australia, its uses in ancient times were many and varied. In traditional medicine, bark was used or digestive remedies as well as wound dressing and its resin was used for treating skin conditions.

Dyes, oils and tannins used for preparing leather could also be extracted by boiling various plant parts. Today the wood is used for culturing shiitake mushrooms, which are much in demand in restaurants.

But the recent history of red cedar is a typically sad colonial tale. The species belongs to the same family as mahogany (Meliaceae) and, not surprisingly, was exploited for its timber from the early days of colonisation.

You can find red cedar timber in many public buildings across Australia.
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The timber is durable, lightweight and suitable for naval use and so was very heavily logged, right along the east coast of Australia from the early 1800s until the early 20th century.

The rich deep red colour of its timber and the fact it was soft and easily worked meant it was used for furniture, ornate carvings in public buildings, town halls and parliaments, such as the State Library in Melbourne. It was also used for implements and handles, and for sailing and racing boats.

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You’ve probably had a close encounter with the lovely red banisters on some of these old buildings that were made of red cedar, often darkened under the patina of so many hands.

The once common and widespread species was logged almost to extinction along the east coast by the mid-1900s, and to the point of practical commercial extinction with little timber available to industry by the 1960s.

So valued was the timber that in the late 1970s, a plan was hatched to remove red cedar from Queensland National Park rainforests using helicopters. Luckily, the idea did not fly and so some great trees persist. The species has a conservation status of concern, but is not considered to be endangered at present.

The leaves of red cedar begin to fall in late March.
Peter Woodard/Wikimedia

A terrible pest

The fact they are deciduous makes them potentially very interesting and useful for horticultural use, but that potential remains largely unrealised. And given the value and quality of its timber, you may be wondering why it’s not being grown in plantations across the continent.

The reason is a native moth called the cedar tip moth (Hypsipyla robusta), which lays its eggs on the main growing shoot of the tree. When the eggs hatch the larvae bore down the shoot, which not only results in shoot dieback but also causes the trees to develop multiple stems and branches which reduce its timber value.

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White cedar is a rare bird: a winter deciduous Australian tree

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Despite this, they are still planted as a quick-growing ornamental tree for their shade in other parts of the world, such Hawaii and Zimbabwe.

The moths are attracted to the scent of the tree, so they’re very difficult to control. The moth does not attack the tree in South America, for instance, because the moth has not established there, so there are large plantations of red cedar in Brazil.

It’s an interesting reminder: often it’s the little things in ecology that can affect success, or failure. When we humans meddle without knowledge, things don’t necessarily go to plan, usually to our cost.

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Gregory Moore, Doctor of Botany, The University of Melbourne

This article is republished from The Conversation under a Creative Commons license. Read the original article.