The Christchurch Earthquake: tectonic plates and fault lines

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 at www.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 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!

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About Keith Woodford

Keith Woodford is an independent consultant, based in New Zealand, who works internationally on agri-food systems and rural development projects. He holds honorary positions as Professor of Agri-Food Systems at Lincoln University, New Zealand, and as Senior Research Fellow at the Contemporary China Research Centre at Victoria University, Wellington.
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45 Responses to The Christchurch Earthquake: tectonic plates and fault lines

  1. Jamie says:

    Thanks for that summary Keith. I too, despite being young and having lost my home in the most recent shake, am quite happy to stick with Christchurch – as long as there is full disclosure from geologists and others regarding the risks. What I have seen recently, including following the Sept. quake, is a playing down of things in order not to spook people. I can understand that mentality to an extent, I would suggest that having full information is better than the complacency that was starting to set in until the most recent event (many seemed to think that nearly 6 months was enough, and there is a distinct lack of understanding around aftershock cycles and geological timeframes that need better explanation to the public, e.g. this seismic cycle could last years rather than weeks/months.).

    Have you seen this report from the BBC where the question of whether the Greendale & City faults might join up is raised?

    http://www.bbc.co.uk/news/science-environment-12668190

    Thanks again – you have done more for my understanding than most of the geologists!

    • Keith Woodford says:

      Thanks Jamie.
      No I had not seen this article. But I had seen another version of the interferogram. Both versions have suffered from either the absence of or a confusing interpretive key. The September 2011 earthquake involved horizontal slipping (very evident on Telegraph Road at Greendale) whereas the February earthquake has had vertical uplift (mostly on the Port Hills, according to the satellite data). I doubt whether there will now be a major quake between the two faults, given that over the last few months there have been plenty of aftershocks on an approximate line between the two quakes, and this should have released tension. I hope so because I seem to be living roughly on the intersection of the two (and about 9-10 km from the epicentre of the February quake) I suspect that there are many blind and jumbled faults across Canterbury, running at different angles (note that the faulting associated with the recent earthquakes is close to right angles to the Alpine Fault). My guess is that now that these two faults have released, then if there are more ‘within-plate’ earthquakes of major size, that they will be somewhere different within Canterbury. Following the September earthquake, most of the aftershocks seemed to be moving eastwards. Now the February earthquake seems to have moved further eastwards.
      KeithW

  2. anna says:

    Thank you for explaining this all. I have head the Alpine fault, tectonic plates and other such information bandied about but not to the extent that it made sense. We are lucky to have been largely unaffected by the quake physically, but mentally, it’s hard to trust anything when the ground you walk on isn’t safe!

    Cheers
    Anna

  3. Liz says:

    Thanks for some factual information regarding the tectonics within the South Island. There has been plenty of warped information going around so it is nice to read an article in easy to understand terms about what has happened and what is likely to happen (scary as it may be). Unlike the so called predictions many scared Christchurch residents are now petrified of, this is some what soothing for me to read. But I certainly won’t be moving to the West Coast any time soon!!

  4. Ryl Gormack says:

    Hi Keith thanks so much for the summary, very informative, very interesting. I am a Wellingtonian (previously a Christchurch resident) and lately have been quite nervous about whether or not the recent Christchurch earthquakes would have put presure on one of the 4 fault lines lying beneath Wellington. Do you have any theories on this at all? No one seems to have writen anything about whether or not the Christchurch earthquake will have a flow on effect to us here. Similar to the Napier earthquake that was precipitated by the Buller earthquake and then followed by two greater than 7 magnitude quakes in Paihiatua and the Wairarapa. I know we can’t predict them but do you think history might repeat itself? Any thoughts would be greatly appreciated, thanks so much, from a nervous Wellingtonian!

    • Keith Woodford says:

      Hello Ryl
      I am not in the prediction game, but I have had something to say about Wellington risks in another post today at https://keithwoodford.wordpress.com
      I see some nice pictures on your site
      KeithW

      • Ryl Gormack says:

        Thanks Keith. I did read this post from you. Sort of reassuring but I guess living in Wellington has its risks :). We are well prepared in our household so all we can do is hope we never have to use our emergency kit. Today’s Memorial Service was sobering and I had a few tears, esp looking at the photos of the city I used to shop regularly in. Thanks so much for your comments re my photos :). I am not photographing much at the moment due to being a full time Mum but will get back into things soonish. I haven’t been able to receive your posts via my email despite confirming my address. Hmmmm. I find your posts very fascinating and they make for excellent and informative reading. Thank you for taking the time to research and write them. If you have any more theories re Wellington be sure to let me know!

  5. Brian Finlay says:

    Hi Keith,
    I have been trying to find some information on Tsunamis as related to Christchurch. I am old enough to recall the 1960 Tsunami that headed our way from Chile as my school was evacuated to the Port Hills where we proceeded to make huts in the lines of trees called the “Catapillars”. (Not much of them left today). That Tsunami petered out to a 12ft spring tide with some damage to Lyttelton Harbour and coastal areas but with little effect inland. At the time people said that this was because of our shallow coastal shelf as opposed to the deep waters of Japan which extend much closer to their coastline. I now understand that a shallow coastal shelf may exacerbate the effect of a tsunami in Pegasus Bay. I am now quite confused. Are you able to tell me why the 1960 tsunami was a “bit of a fizzer” or “flop”, and what would be the effect of a shallow 6.3 earthquake on Christchurch City if it occurred 30km due east of say, South Brighton This seems a more than likely scenario. Does the width of the coastal shelf also make a difference and if so, in what way. I realise that this may be outside your particular field but would be grateful for any guidance.
    Thanks, Brian

    • Keith Woodford says:

      Brian.
      My understanding is that the 1960 tsunami from the Valdivia earthquake arrived at approximately low tide, which was a help in minimising the effect. Given the enormity of that Valdivia earthquake (which was an offshore earthquake) I take some comfort that we are unlikely to get a tsunami coming from South America that is much bigger than the 1960 tsunami, although a natural high tide might make some difference.
      The biggest tsunami danger for us here in Christchurch relates to the Hikurangi Trench which runs north into the Pacific from about Kaikoura and is a major subduction zone. There could be an earthquake on this trench (which is a major fault line) or it could be combined with the unzipping of the Alpine Fault (to which it is linked). The risk from the Hikurangi Trench is enhanced by the possibility of an associated undersea landslide at the southern end (near Kaikoura) where there are huge undersea silt deposits at a menacing angle. [Intriguingly, it is these same deposits and the associated nutrient rich waters around them that attract the whales to Kaikoura.]
      You ask about the potential of an earthquake of say magnitude 6.3 about 30 km off Pegasus Bay to generate a tsunami. The conventional wisdom seems to be that it takes an earthquake of about 7.4 or greater to generate a major tsunami. Personally, I am more concerned about an earthquake on the Hikurangi Trench (which could be very large) from which the tsunami might get channeled south by the cliffs along the North Canterbury Coast to Pegasus Bay, and then come ashore through Christchurch’s eastern and north eastern bays, both north and south of the Waimakariri. The Kaikoura Coast itself is presumably at even greater risk. But all of this is not to say that there is no possibility of a tsunami generated from Pegasus Bay itself.
      Please note that my earthquake and tsunami interests are those of an amateur, but someone who tries to read the professional literature on these matters.
      KeithW

  6. Sam says:

    Thanks a bunch Keith. A great explaination with noble intentions; informing the lay people as to what the heck is going on!

  7. adam says:

    Keith, cheers for that info 😀 most of it i knew already. But i still learnt abit from it ty 😀

    But im still couriuos to know if canterbury quakes are actually fault line generated..or magma activity under us…we are right next to two “dead” volcanoes after all 😦

    • Keith Woodford says:

      Adam
      These earthquakes are definitely fault line and not magma related. The magma is a long way down and the ancient magma pathways are now solid rock. The chance of new volcanoes developing here in Canterbury is very unlikely. However, that is not necessarily the case in the North Island, and it is possible for new volcanoes to form in places such as Auckland where, on a geologic scale of thousands of years, new volcanoes do indeed occur. In contrast, the Banks Peninsula volcanoes are remnants from millions of years ago, when the plate movement was quite different to now.
      KeithW

  8. john.mason@doubleglaze.co.nz says:

    Hi Keith have you considered the possibility that the new fault lines from the Alpine fault are likely to connect with the extension of the Hikurangi Trench much in the same way that it flows through at Kaikoura. Further that eastern side of the Alpine fault is in fact being pulled or stretched south and eastwards. The ruptures and fault lines seem to indicate that as seen in ther Kaikoura area

    • Keith Woodford says:

      John,
      The faults that are causing so much trouble in Christchurch,(i.e. the Greendale Fault and the Port Hills Fault and branches thereof) are not part of the Alpine Fault. In fact they run roughly at right angles to the Alpine Fault and relate to boundaries between crustal subplates on the Pacific Plate.
      My own interpretaion of this is that we are in a transtion zone from where the Pacific Plate is being pushed under the Indo-Australian Plate (from about Kaikoura northwards, as indicated by the Hikurangi Trench) and the zone of the Alpine Fault where the two major plates are butting up against each other, and slipping (with the Pacific Plate moving southwards) but where neither plate is riding up over the other. At this boundary there is lots of buckling and twisting of the crustal subplates.
      The Alpine fault is connected to the Hikurangi Trench by the Hope fault, which has several branches, and there is a good chance of the Hope Fault going at any time with a quake of about M7.5, (The last big one was in 1888, but only on a portion of the fault.) As for the Alpine Fault itself, when that goes it is likely to be of about magnitude 8. This fault is believed to have last moved in about 1717 and a new quake is arguably overdue. When it does go there will be huge landslips along the Southern Alps. Possibly the worst place of all to be is Franz Josef, where I am told that a huge section of unstable rock has the potential to bury the town. But there will be many places where it will not be nice to be.
      Coming back to the Greendale and Port Hill Faults, my theory there is that north of the faults the land is moving west, and south of the fault it is not really going anywhere, But it gets a bit more complex than that, because its a bit like the front row of a rugby scrum, where there is plenty of buckling and twisting, and this can have somewhat unpredictable effects. For example, On Telegraph Road (running northwest off the Main South Road between Burnham and Dunsandel) the land on the north side of the fault appears to have actually moved east rather than west.,
      I note that an American geologist who is here in Christchurch studying our earthquakes made the point today that we don’t actually know much about crustal earthquakes occurring on plate fragments. We know a lot more about major faults between the big plates. Personally, I am working on the assumption that there could be more local eathquakes to come, and in rebuilding Christchurch we have to work on that assumption. The irony is that these local faults are really quite small. It is just unfortunate that they are shallow and right beneath us!
      KeithW

  9. John B says:

    Hi Keith, what is your take on the June 13 event?

  10. John B says:

    Keith, I think you have just answered my question with your post of 15 June.
    I have spoken to a number of geologists and others, and there appears to be a growing concern generally of a much larger event, which is why only remedial work is being done on buildings as opposed to new construction. Realistically I am told the rebuild of Christchurch and whatever shape the CBD will take is likely to take up to 20 years.

    • Keith Woodford says:

      John
      One of the sobering facts of the latest earthquake of June 13, is that the epicentre is so close to the February 22 earthquake. I find it difficult to think of this as an aftershock given that both it and the Feb22 earthquake were M6.3. Rather, I see them as both being outcomes of a much bigger force. Looking at the intensity maps of the September earthquake, the accelerations were actually quite modest in the eastern suburbs, but there was still extenive damage. So that reinforces the notion that these eastern suburbs are indeed very vulnerable to any future earthquakes.
      KeithW

  11. Karen Smithream says:

    Hi Keith
    Just wondered what your comment would be to a piece of information going around that says there is a high possibility of the greendale fault and port hills fault joining up. The result could cause a very large earthquake that would probably be centred near Halswell. What do you make of this rumour?

    • Keith Woodford says:

      Karen,
      I believe that this suggestion has come from reputable geologists.
      Personally, I think the next big earthquake is no more likely to be at Halswell than anywhere else.
      However, you may have noticed that last nights 5.4 earthquake (upgraded from 5.3) has also been relocated to Halswell (rather then between Little River and Akaroa). That earthquake, plus all the rumbing aftershocks that went on all night, were indeed rather unpleasant for those of us living at Halswell, but of course very minor compared to the major trauma that others have faced from the larger quakes.
      Events of the last 10 days have emphasised that in Canterbury we are un unknown territory, because these are shallow crustal earthquakes, and also because of the apparently unique features of the boundary between the Alpine Fault and the subduction zone north thereof. These last 10 days tell us that we cannot be confident as to how long we are going to have to wait until the earthquakes stop. And that greatly complicates the rebuilding phase.
      KeithW

      • John B says:

        I spoke to an American geologist who is here at Canterbury for a semester. He specialises in volcanoes. His take was quite interesting.

        Firstly below CHCH there is a complex structure of volcanic formations. The problem is that when a fissure or fault occurs in volcanic rock it inevitably triggers another one, which then triggers another one…see the domino effect here? As a result we [unfortunately] have experienced over several thousand, whereas in Japan they are in the hundreds.

        The bottom line is that it is impossible to predict when these shakes may start and/or stop. Realistically it could be a few years or even a few lifetimes. NZ unfortunately sits on the Pacific Plate, which is the most active plate o the planet. The movement of this plate annually is three times that of any othe plate on the planet. So no doubt there is an influence there as well.

        Given this information I can now see why the powers to be have a conundrum of mammoth proportions in deciding which way to move forward for the best. Essentially the experts can only hypothosise on likely outcomes with complete uncertainy!

  12. Andy says:

    Thanks for a brilliant article. I’m finding it hard to understand how we have warm springs in Lyttelton Harbour, yet geologists are saying in the press this week there’s no need to be concerned about volcanic activity associated with the earthquakes as the nearest pool of magma is 500km away in the North Island. Surely heated water has to come from relatively close by and not too deep.

    • Keith Woodford says:

      Andy,
      The hot springs are caused by presure along mini fault lines. The pressure between the plates causes the rock to heat up. When the pressure gets sufficient then the rock gets hot enough to melt and we have an earhtquake. But in some cases the heat is also transferred to undergound water which is also under pressure, and this bubbles up through fissures as hot springs. So hot springs are often found near fault lines (such as at Hanmer Springs, or at Welcome Flats in the Copland valley south of Fox Glacier, and in places on Banls Pensinsula) . Earthquakes can create new fissures and hence new sptrings. So this is indicative of plate movement and faults, but not of volcanoes, which require magma to well up from much greater depths where all the rocks are in liquid form.
      Where one plate is riding up over another plate then cracks occur and this can allow magma to find its way thought the cracks to form volcanoes. This is what is happening in the North island where the Indo Australian plate is climbing up over the Pacific Plate. Hence the possibility of new volcanoes appearing in places like Auckland. But there is no evidence for this occurring in Canterbury for about 8 million years. So we are fairly safe from volcanoes.
      KeithW

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  14. milale says:

    Hi Keith,
    The information you provided on this web page is brilliant, exactly what I was looking for. It wasn’t easy to find though. Maybe you could try to make it easier for people to find.
    Really interesting stuff.
    Thanx

  15. Lynne says:

    Extremely interesting and informative. I live on the West Coast at Hokitika so we know we are going to get “The Big One” one day but living in a wooden house on piles makes me feel much safer than if i was living in brick or block as these material are unforgiving where as wood has a degree of flexability.

  16. Karen Wright says:

    Fantastic blog Keith, easy reading and very informative.

  17. Frank Fontein says:

    Great Blog!

    A few days into the new year – many new shakes later and having studied the patterns on Quake Map since 4 Sept 2010 – I’m looking at the bigger picture and despite what Geologists say about what’s ‘likely to happen’, I think there is going to be a connection between the current fault extension and the Kaikoura basin. It all began in Darfield, moved to the CBD, then to the coast near New Brighton (June) and now off the coast. I cannot help but liken it to a crack in a windscreen. A ‘small’ crack will remain there for some time, but slowly extend. If there’s another (separate) crack not far away, eventually the two will meet (seeking the weakest connection). Isn’t that what we’re seeing now between ChCh and Kaikoura? I’m sure it will crack and connect, the question is only when.
    Do Gelogists really know what’s 10-20km deep between here and Kaikoura? You can easily measure on land, but has the sea floor off our coast been accurately explored and measured?

    • Keith Woodford says:

      Hello Frank
      I doubt whether there is a direct connection between the Christchurch faults and the faults off the Kaikoura Coast, although of course they are both part of the tectonic battle between the Pacific and the Indo-Australian Plates. But I do see considerable potential for furher quakes in various parts of the South Island, including the Hikurangi Trench which runs north from about Kaikoura. The reality is that there are major faults in most parts of the South Island. Although the Alpine Fault is the largest, there are plenty of other faults with potential for quakes of more than magnitude 7. Over the last 100 years or so we allowed ourselves to be lulled into a false sense of safety. But the human record from the 19th century, and the longer term geologic records visible in the Aps and foothills, tells us to be wary almost everywhere in the South Island. The particular risk with the Hikurangi trench is a tsunami that could sweep south into Pegasus Bay. This could be triggered by the Alpine Fault (which is due to go) or perhaps linked to the Hope fault, or perhaps it could go all be itself. With hindsight, building Christchurch in a swamp was not a very bright thing to do.
      KeithW

  18. Keith, once more – thank you. You confirm my understanding, and Frank Fontein’s addition is appealing to me, too. Noting the easterly trend and the general timing, it’s easy to suggest ‘the windscreen’ is being torn or sheared (or whatever) gradually, it taking some months of small movement for the stress to grow to the point where the material gives way and the crack advances a further few kilometres along with its epicentre, progressively out to sea. The crack would advance with major shocks which are hopefully less destructive of the city as their locations ‘move’ further away. Meanwhile the earlier broken edges of the plate (under the city as elsewhere) could grind together with much smaller, localised tremors. I’d ‘predict’ many years of major quakes every few months, less severely felt in the city with the passage of time, followed in each case by many weeks of minor local shaking all along the previous path.
    But what would I know?
    P

    • Keith Woodford says:

      The reality is that none of us, including the supposed experts, can predict what will happen next. But I do see in the media in the last few days some reference to the Kaiapoi Fault, which, based on its apparent length of about 30km, is believed to have potential to release at about magnitude 7. I have not mentioned the Kaiapoi Fault before, but it does seem very much ‘in reach’ of the recent sequence of quakes.

  19. stephen says:

    Hi Keith,
    Thank you for your post. I’ve just left Chch and moved to Dunedin due to work commitments and I was just wondering if you knew of any faults in, or around Dunedin, and what type of impact the alpine fault would have here, if it were to go? Thanks again

    • Keith Woodford says:

      Hello Stephen. I am an amateur on these matters. So all I can use is “first principles” There is no evidence that I have seen of big earthquakes near Dunedin, at least since European colonisation. But that cannot necessarily be taken as comfort. It may simply mean that pressure is building up. Dunedin is deifnitely somewhat further from the Alpine Fault than is Christchurch, so that is somewaht comforting. But in the same way that the Christchurch earthquakes have been occurring on subsidiary faults, I would expect there would be similar faults in Otago. That is certainly the case for the Waitaki catchment and I presume also further south. I liken it somewhat to the twisting of a rugby scrum in which the two loosehead props each twist and buckle under (one on each side). And in the centre the two hookers pop up. I think we have to assume that almost everywhere in the South Island is at some risk of a big one. But it really was bad luck that the spate of Christchurch earthquakes were almost right below the city and at very shallow depths. If I had to guess on the safest city in the South Island it would be Invercargill. That may seem surprising, as Invercargill gets more earthquakes than Dunedin. But these earthquakes tend to be at considrable depth (where the Indo plate is slipping under the Pacific Plate) and this dampens the effect for Invercargill. I also think that south of Christchurch (say Timaru) is somewhat safer than to the north of Christchurch. But it is all about probabilities, and Nature is never predictable. As for the Alpine Fault, the more I learn about it the more I fear its capacity to trash considerable swathes of the South Island. It is now 295 years since it last went (we know that by the tree rings; essentially none of the trees in the mountains grew during 1717, 1718, and 1719 as they re-established their damaged roots), and that means a lot of built up pressure.

  20. Erica Bertram says:

    Hi Keith I have a question for you that I cant seem to find the answer to what type of plate movement caused the christchurch earthquake? was is subduction ? slppage or neither? please help 🙂

    • Keith Woodford says:

      Hello Erica
      Definitely not subduction as that is where one plate slips under the other. North Island earthquakes are typically subduction where the Pacific Plate is slipping unde rthe Indo-Australian Plate. The recent Canterbury and Christchurch earthquakes are probably best described as slippage, However, even then it is not the classic slippage as we would expect to occur when the Alpine Fault slips.
      A distinguishing feature of the Christhcurch earthquakes has been that they are on the shallow crust. Christchurch is only about 100km south of where the northern subduction zone ends and the two plates are in a ‘standoff’ with neither subducting to the other. Down in Fiordland the subduction is the other way round, with the Indo-Austalian plate diving donw under the Pacific Plate. In the zone between the subduuction zones there is lots of twisting and buckling and the surface platelets get moved around in different directions. So there are elements of both horizontal and vertical movement. I liken it to a rugby scrum where on each side one prop goes down and the other comes up, and the poor hookers in the middle both pop up in the air when the pressure on them becomes too much.
      I think the reason that geologists have apparently struggled to put names on the type of earthquakes we have been having is that the conditions here in Canterbury with the standoff between the plates combined with subduction zones to both the north and south are very unusual.
      KeithW.

  21. Tony Harcourt says:

    Hi Keith
    Is there any evidence that the Port Hills uplift has been slightly greater in the eastern (Sumner) area than that in the western area? That is, has the rock mass of the port hills tilted ever so slightly in an east west direction? Our house situated close to bedrock on the south side of Hackthorne Road (No 74a) appears to have been tilted slightly in this direction, even though there is no apparent foundation subsidence. A recently installed cavity slider door that was installed dead level now closes (east to west) at the slightest touch!

    • Keith Woodford says:

      Tony
      I cannot give an authoritative reply, but I think it is possible. The effect may well be local, with different bits of the crust tilting in different ways. In the Souterh Alps we can see sedimentary strata that have tipped, over the millenia, in all sorts of directions.
      Keith W

  22. elleeeennn says:

    hey keith. I’m just a student looking for some info on the christchurch earthquakes back in 2011. do you maybe have any notes on the reasons to why this earthquake was about? ( tectonic plates + more ). keeping in mind science is not my subject therefore that is why I’m looking for some pointers. I know this was along time ago but i was hoping you would still reply. thank you 🙂

    • Keith Woodford says:

      All I have is the various posts here in the earthquake category

      • elleeeennn says:

        Thank you for your time to look at comment I really appreciate it. Are you sure you don’t have any short explanation. It would really help. thank you in return.

      • Simply put, I think the surface of the globe is unstable because we are all supported on one or other of huge tectonic plates floating on a ‘sea’ of magma (not unlike ice floes, in a sense) and what we call earthquakes are what we experience as the edges of the floes grind very slowly together. Why not try google or wikipedia?

  23. Pingback: Earthquake Plates – Earthquake

  24. Shelly says:

    Hi. Firstly, this is really helpful information. Just one thing: when you say that the plate fractured within itself in the Christchurch Earthquake, is this called a ‘fault’. If not, what’s the difference between a fracture and a fault? Thanks, and once again great article. Very informative!

    • Keith Woodford says:

      Shelly,
      The fracture is what happens, the fault determines the location. Just like when a dinner plate breaks (I have experience of that!) it sometimes occurs at a pre-existing crack ( the fault or weakness).

      It is sometimes claimed that all earthquakes occur on pre-existing faults, But I don’t think that has to be the case. It would imply that all faults were present when the earth was initially formed. But all other things being equal, an earthquake will occur at a pre-existing weakness.
      Keith

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