Plates, Faultlines and Tectonics Explained In English

Posted 30 Jun 2011 by user1 Popular
Posted in Earthquake Facts

 

This piece was written by Keith Woodford and originally featured on his personal blog on Wednesday June 29th, 2011.

In an attempt to understand the Christchurch earthquakes I have been forced to do a lot of reading. There is a lot of good material ‘out there’, but much of it assumes a prior knowledge of geology. Some of the best material for non-geologists is in the Learning Section atwww.gns.cri.nz. Another site that I found to be very informative was the Mt Aspiring College website. The starting point (hard to find from the home page) is http://mtaspiring.school.nz/Tephra/Thehowwhatandwhereofanearthquake.htm (Alternatively, search Google using the terms ’Mt Aspiring’ and ‘earthquake’.) From there, go through the various pages within the earthquake section. The nice thing about the Mt Aspiring College material is that it is put together by professional educators, who know how to write for a non-expert audience, and also know how to put together a professional website.

In this post I report in summary terms what I learned from these plus many other reports and websites. Hopefully, other lay people will find this useful in understanding something of the ‘big picture’, and it is on this basis that I share my learnings. What I did learn along the way is that there is general agreement about that big picture, but the details can be debated, even by the so-called experts. Despite all the modern tools of science, it is not easy to see everything that is going on deep beneath the earth.

Tectonics

Here in New Zealand, we lie at the interface of the Pacific Plate and the Indo-Australian tectonic plates. These plates have been grinding against each other for millions of years. However, the outcome of the collisions has three different outcomes which vary according to the location within New Zealand.

In the North Island, and the top of the South Island including Marlborough, the relatively light Indo-Australian Plate is riding-up over the denser Pacific Plate at a rate of between 10 and 40mm per year. Multiple fault lines extend north across Marlborough, then cross the Cook Strait, then up through Wellington and the Wairarapa, eventually exiting through Hawkes Bay, Gisborne and the Bay of Plenty. Most of these faults fracture at intervals between 500 and 2500 years. The Wellington Fault, which extends up through the Hutt Valley, is widely considered the most likely one of these to fracture in the next hundred or so years. But it could be another fault either adjacent to Wellington (such as the Ohariu Fault) that fractures first, and it could either be in the near future or not for several hundreds of years. Or it could be one of the faults in the northern part of the South Island that is the next fracture.

Although Wellington is actually on fragments of the Indo-Australian Plate, the Pacific Plate lies only about 30km below. So when earthquakes occur some of these are likely to be at this depth, which is sufficient to cushion some of the shaking. Wellington’s recent Magnitude 4.7 earthquake on 4 March 2011 was actually on the Pacific Plate lying underneath the Indo-Australian Plate at about 30 km depth. A slightly smaller earthquake three days previous was at about 40km. All major North island earthquakes – such as the Napier earthquake of 1931 and the Wairarapa earthquake of 1855 – lead to the land being uplifted by what is known as ‘subduction’ of the Pacific Plate, which gets squeezed beneath the Indo-Australian Plate. Eventually, the leading edge of the Pacific Plate gets forced down to depths of about 600 km where it melts into magma.

To the west of the main North Island faults, fissures develop in the Indo-Australian Plate. In effect, the plate cracks as it is forced up over the Pacific Plate. These cracks provide a means for magma to escape, and hence we have the North Island volcanoes. In contrast, on the eastern side of the major fault we have the Hikurangi Trench and the Kermadec Trench. These are caused by the Pacific Plate being forced downward, with some of that having occurred nany millions of years ago.

In Fiordland, at the southern end of the country, the reverse is occurring. Here, the Indo-Australian Plate is being subducted under the Pacific Plate. In contrast to the subduction in the North Island which involves a relatively shallow dive, in Fiordland the Indo-Australian Plate is taking a steep dive. In Fiordland it is common to get big earthquakes, but many of them tend to be at considerable depths (because of the steep dive), and hence their effects on the surface are less.

The Alpine Fault

In between these two zones lies the Alpine fault, which extends south from about Greymouth, and disappears into the Ocean about 12km north of Milford. In this zone, and for a distance of between 450 and 600km, there is a stalemate between the Pacific and Indo-Australian Plates, with neither being able to ride up over the other. Along this Alpine Fault – which actually lies some distance from the high mountain peaks and well down on the West Coast side of the Southern Alps – the stalemate leads to a relative absence of small earthquakes. Instead, the two plates lock together, building up pressure causing the rocks at depth to melt, until there is a cataclysmic release. The West Coast side slips north, and east of the fault the land slips south. Typically, each major earthquake on the Alpine Fault is of Magnitude 8 or greater. Horizontal slippage is 8-10 metres. There is also uplift of about 4 metres along the edge of both plates . Over many millions of years the uplift has totalled about 20,000 metres, but erosion has limited the mountain tops to less than 4000 metres.

During the last 1000 years, the Alpine Fault has fractured four times, at intervals of about 150-350 years. The last fracture is widely believed to have been in 1717. So the maths is not comforting. According to the GNS website, there is a strong likelihood of a Magnitude 8 earthquake on this fault within the next 40 years. The greatest destruction would likely be along the aWest Coast. Towns such as Greymouth, Hokitika, Ross, Harihari and Whataroa, plus the Glacier Towns of Franz Josef and Fox, would probably take the greatest hits. The effect in Christchurch from a Magnitude 8 earthquake on the Alpine Fault would likely be similar to the Canterbury 7.1 earthquake of September 2010, but hopefully much less than the 6.3 eathquake of 22 February 2011, which was almost underneath the city and at very shallow depth. An 8.5 or even greater earthquake, which is defintely possible, hardly bears thinking about. But even then, our recent experience seems to say that genuinely modern buildings built on firm ground, and to the latest building code, are incredibly robust.

Christchurch

So how does the Alpine Fault explain the 6.3 magnitude Christchurch earthquake of February 2011? In a direct sense, it does not, in that neither the September 2010 or the February 2011 earthquake involved fracture of the Alpine Fault. But these earthquakes were related to the pressure exerted between the two great plates, of which the Alpine Fault is the most explicit sign. Instead of fracturing at the boundary, these recent earthquakes involved fractures within fragments of the Pacific Plate on which Canterbury lies. In other words, they were ‘within-plate ‘ earthquakes but caused by tension between the plates. There is also speculation that the North island subduction zone is slowly (over many millenia) shifting southwards and this could be part of the story.

If we go back some millions of years, there is evidence that at one time the Pacific Plate was actually climbing over the Indo-Australian Plate along much of the length of the South Island. This is the most likely explanation for the Banks Peninsula volcanoes. This would also explain the extinct volcano of the Dunedin Peninsula in Otago, and the volcanic origins of Mt Somers in the Canterbury foothills. But those events were all many millions of years ago.

Looking back over the last 150 years – a mere instant in geological time – there have been several earthquakes in Canterbury of about Magnitude 7. Given the particular geology of Canterbury, and the associated risk of liquefaction, the damage would seem to have been higher than would normally be expected with this magnitude of earthquake. It would seem reasonable to expect more of these earthquakes over time, although where in Canterbury they will be is probably impossible to predict. There may be a greater likelihood for these to occur in North Canterbury and along the Kaikoura Coast than in South Canterbury.

Christchurch versus Wellington

When I was young, and indeed until recently, the conventional wisdom was that Wellington was the region of New Zealand with the highest earthquake risk. This assessment was based on the known effects of the 1855 Wellington /Wairarapa earthquake, plus the high visibility of major fault lines. In Canterbury, where the fault lines are buried beneath the shingle, and where there has never been a Magnitude 8 earthquake within recorded history, the risk was assessed much lower. But that has now changed. An increasing knowledge of the tectonics of the Alpine Plate has created an understanding that the likelihood of a Magnitude 8 or greater earthquake on the Alpine Fault is very high. And the two recent earthquakes on fragments of the Pacific Plate that lie beneath the Canterbury Plains have provided new understandings of how the tectonic forces are occurring. Over time, there is considerable potential for further ‘within-plate’ earthquakes of Magnitude 6 or 7, and for these to occur at very shallow depths.

Personally, I am happy to stick with Canterbury as my home. However I am one of the lucky ones who has a house that is largely undamaged. I might see things differently if I were a young person with a destroyed house. What I do now know is that in Canterbury we are all truly part of the Shakey Isles. Whether the risk going forward is actually more in Christchurch than in Wellington I do not know. At least here in Christchurch we are 100km from the Alpine Fault, whereas Wellington sits right on top of major faults. Of course one could go and live in Auckland. There the risk of earthquakes is much less, although they too are still part of the Shakey Isles. However, Auckland has the additional risk of volcanoes, as do most parts of the North Island lying west of the plate boundary. Those Auckland volcanoes are only sleeping, as is Mt Taranaki and many others. At least there are no Aussie snakes anywhere in the Shakey Isles!

Discussion

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