Tectonics of the M7 earthquake near Christchurch, New Zealand

A post by Chris RowanThis post was written in response to the Darfield earthquake in September 2010. The most recent seismic activity is discussed here.

[Updated 8th September 1200 GMT - see bottom of post. And check out the PodClast discussion of this earthquake, featuring Kiwi Chris Town, Ron Schott, and myself.]

The South Island of New Zealand has just been shaken by a large earthquake, reported as a magnitude 7.0 by the USGS. It appears to be quite a shallow rupture, on the Canterbury Plains close to Christchurch, and the focal mechanism indicates largely strike-slip motion.

Focal mechanism of September 3rd earthquake, and it's location with respect to the plate boundary in New Zealand

As the figure above illustrates, New Zealand is not just located on top of the boundary between the Pacific and Australian plates: it is located at a point where the nature of that plate boundary changes in some rather fundamental ways. The subduction zone running down the East Coast of the North Island terminates off the Northeast coast of the South Island, about 100 kilometres north of Christchurch, and gives way to a transform boundary cutting through the continental crust of the South Island, where the plate motions are accommodated by largely dextral strike-slip on the faults of the Marlborough Fault Zone (MFZ in the figure above) and the Alpine Fault (AP). Whilst this latest rupture clearly occurred some way south of both of these fault systems, the focal mechanism can be interpreted as showing as dextral strike-slip on an east-west oriented fault, suggesting that it is still linked to deformation at the plate boundary.

New Zealand is a region of distributed deformation: the relative motions between the Australian and Pacific plates are not accommodated on one or two faults in a narrow zone, but on many faults across a much wider zone. It is therefore perhaps not surprising to observe large earthquakes accommodating plate motions some distance from where the two plates actually meet. However, the occurrence of such earthquakes in this particular region of the South Island is probably also linked to ongoing changes in the nature of the plate boundary at the junction between the subduction zone and the continental transform. If you look at the displacement history of the individual faults in the Marlborough Fault zone, the northern faults are older, were more active in the geological past, and have quite small recent (in the geological sense of ‘the last few 100,000 years’) displacements; the southern faults are younger, and have much larger recent displacements. The most obvious explanation for these changes is that the most northern of the Marlborough faults was originally directly linked with the end of the subduction zone, but that these two structures moved out of alignment as the subduction zone moved south, causing new strands of the Marlborough Fault system to grow in order to more efficiently accommodate plate motions.

Growth of new plate boundary faults on the South Island of New Zealand in response to southward propagation of the subduction zone

This tectonic evolution is ongoing, and since the end of the subduction zone is now actually to the south of the southernmost and youngest of the Marlborough faults. Some of the plate boundary deformation is probably therefore being shunted into the region around Christchurch, where it needs to be accommodated by dextral strike-slip faulting. Eventually, over geological time, this deformation will lead to the formation of a new, more southerly strand of the Marlborough Fault system. It also means that earthquakes of this type of size are unlikely to be a one-off event in this area. Unsurprisingly, then, seismic surveys have identified a number of active faults beneath the recent sedimentary cover on the Canterbury plains (although they were identified in the linked study as reverse faults accommodating compression, strike-slip deformation is very difficult to identify if you only have a 2 dimensional cross section to work with).

Whilst this map of large historic earthquakes in New Zealand shows that earthquakes of this magnitude can occur pretty much anywhere in New Zealand, seismicity in this particular area has some particular hazards; it is close to a heavily populated region (Christchurch) built mainly on unconsolidated Quaternary sediments, which will intensify the potential shaking and damage to unreinforced buildings. Fortunately, whilst this earthquake appears to have caused a fair amount of damage, from the early reports casualties seem to be light.

Update: 4/9/10 Here’s a couple more plots to that help to further put this earthquake in context. Via GeoNet, here’s a plot of all the earthquakes in New Zealand over a ten year period, including magnitude 3 and 4 tremors that only really disturb seismometers. This gives us a much more complete picture of how the crust in New Zealand is deforming, and shows us that yesterdays earthquake occurs towards the edge of, but still within, a zone of distributed plate boundary deformation on the northeast South Island.

>M3 shallow earthquakes in the New Zealand region over a 10 year period. Source: GeoNet

The historical seismicity map from the USGS shows that in the last couple of decades there were a couple of earthquakes with very similar strike-slip focal mechanisms a bit further to the north-west, closer to the actual plate boundary, with magnitudes of around 6-6.5.

>M 5 earthquakes on the South Island since 1990. Source: USGS

Some other useful links:

Update: 5/9/10 More useful links:

Update: 6/9/10

  • New Zealand geologists have been doing a sterling job of getting good information out to the public. In this press release from New Zealand’s Science Media Centre, Professor Euan Smith of Victoria University in Wellington has done a great job of describing the quake (he thinks that we’re looking at sequential ruptures of the same fault) and the seismic hazard going forward.
  • The British Geological survey have put out a bulletin (pdf) that includes a plot of every >M 6 earthquake since 1843 – which must be pretty much the entire historical record.

    Magnitude 6+ earthquakes in New Zealand since 1843. Source: BGS

    The most striking feature of this map is the section of the Alpine fault in the central South Island that has not ruptured in the last couple of centuries – which suggests there might be a fair amount of strain belt up waiting to be released.

  • There is a Christchurch earthquake group on flickr, with a good collection of earthquake damage photos.
  • For those interested in learning more about liquefaction, this (pre-quake) poster from Environment Canterbury (pdf) gives both general background and a hazard map for the whole region – it would be interesting to see how it matches up to the reality.


Update: 8/9/10

  • New Zealand’s GNS have posted a video of their survey of the fault surface rupture on their YouTube channel.

  • GNS have also been calculating the focal mechanisms for all of the past weeks’ aftershocks. The pattern of strike-slip to the east of the main shock and compression to the west is quite interesting. Thanks to commenter Lanthanide for the link.
  • Many people, myself included, have contrasted the Canterbury earthquake with the Haiti earthquake as an illustration of how poverty, and the consequent lack of building standards or preparedness, contributed to large differences in the damage and casualties. This is certainly an important point, but via Andy Revkin on Dot Earth, a comparison of the shaking intensity and population exposure for these two earthquakes should caution us about pushing the comparison too far. [Update: As is discussed in the comments, this USGS chart seems to underestimated the intensities in New Zealand: Christchurch, for example, is more like a VI-VII than a V. Nonetheless, the point still stands.]

    Comparison of shaking intensity and population exposure for Canterbury and Haiti earthquakes. Source: USGS, via Dot Earth

Categories: earthquakes, focal mechanisms, geohazards, tectonics
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Comments (144)

  1. Pamela Morrison says:

    If the AF is likely to be M8 or so, but further distant from CHCH than the Darfield quake, what is our experience of it in CHCH likely to be relative to the Darfield quake? Worse, the same or ‘better’?

    • Lanthanide says:

      From what I understand, due to the distance it should be about the same, although the shaking could last longer as there are likely to be multiple quakes occurring within minutes of each other. Aftershocks will probably be worse too, as the fault line is much longer.

      West coast, unfortunately, will be screwed, and all of the road links from Canterbury will be impassable. They’ll have to get support crews in by air and sea.

    • Dave Nelson says:

      Hi Pamela,
      the extra 100km odd to the Apline Fault will attenuate the seismic waves a bit … but due to the much larger sized event its still going to produce severe shaking in CHCH and the whole region.
      one thing that will vary the period of shaking will be determined by whether the AF bi-laterally (from a semi central point in 2 directions) or if it uni-laterally ruptures.
      A bilateral rupture will produce a shorter period of shaking as the length of the rupture is accomplished quicker.
      A unilateral rupture… then the fult unzips (ruptures) say from sth to nth or visa versa will in effect double the period of shaking.
      This has a major effect on the occurrence of liquifaction and how much. Also buildings that may have withstood 1 minute of severe shaking without collapsing may not withstand 2 minutes of shaking.

      Please remember everyone all the comments on here by myself and others in the know, are NOT to scaremonger and frighten people. But just to try and give as best and educated info as possible :)
      Although I have pretty reasonable knowledge of the region … I am by no means an expert !!
      BE PREPARED :)

      • Pamela Morrison says:

        Thanks guys, really informative, as is this whole discussion. the best site on the whole event that I have come across, and should be compulsory studying for all 2ndary schools in the middle South Island. As you say Dave, it’s not about scaremongering but about being informed and prepared. We’ve long known that the big AF is primed to go… if the (fortunately) casualty-free Darfield quake wakens people to the reality that we’re living in a geologically active area that can pop these surprises on us at any moment, it might actually prove to be a blessing in disguise.

  2. ChCh Resident says:

    Hmm… given that the MMI scale deals with the impact of the earthquake on the built environment, it seems inaccurate to attribute differences in MMI ratings between Haiti and NZ to differences in “shaking intensity”. Particularly if the point is to provide context to discussions of building standards between the two countries.

    In addition, a rating of “V” for ChCh proper seems off the mark to me. my neighborhood got off lightly and it was definitely a VI and possibly a VII.

  3. Dave Nelson says:

    @ Pamela Morrison
    Yup exactly :) and more than one blessing in disguise….
    it has weeded out a few more of those unsafe buildings that “historical societies/ Heritage Trust” love to keep around. They probably dont have to live and work in any of them ;)

    Dont get me wrong… I love seeing the old style architecture (sp) but seriously they are deathtraps waiting to happen

  4. Dave Nelson says:

    bruce stout says:
    September 8, 2010 at 3:49 pm
    Dave, the cross-section of the middle of the South Island is great schematic representation. I’d love to see similar cross sections going all the way up (and down) the island.

    hey Bruce….. A few comments sprimkled amongst yours :)

    I will see what I can dig up… I no longer have access to the transect work done by the uni of Otago and others during the ’90′s
    One book you may find interesting… if you havent already seen it…
    Landforms of New Zeland 2nd Edition, by JM Soons and MJ Selby pub 1992 Longnman Paul Ltd, NZ

    1.Particularly, how rapid is the transition from strike slip movement such as we just saw south of Darfield to the thrusting that forms the Kaikouras? Does the Pacific plate thin out here and the Australian plate thicken?

    The oblique slip motion is still very prevalent on the faults of the MFZ as was witnessed by the large horizontal offsets in the 1848 Awatere Valley event with up to 8metre movements recorded
    Without immediate access to a transect, I’m not really familiar with how different things look deep down in the kaikoura’s reg compared to the ~ cntrl AF reg.

    2. And why do the Kaikouras end abruptly to the north (apart from the obvious reason to grace the world with fantastic Sauvignon Blanc?), and

    hahahaha … isnt that good reason enough ;) between the Nth and Sth Islands the tectonics gets really messy and the discussions on what is going on there is still being pondered on. But over a relatively short distance there is an abrupt change from the classical subduction we see under the Nth Is. to the oblique slip thrusting of the Sth Is.
    I would imagine that must have a considerable effect on the type of landforms created

    3. while I’m at it, looking at that stage pole diagram, why is there thrusting and mountain building going on at all in the Kaikouras when the relative plate motions look much more conducive to strike-slip movement?

    For the few events I have focal mech’s on in the NE reg, thay primarily still show the same style as furthe south see this pic I have put together back in the ’90′s and have added to as new info comes to hand http://www.sydneystormcity.com/quakes.htm
    note the 2 events I have never been able to obtain CMT’s for…. Edgecumbe and Napier

    4. Am I correct to infer that this latest quake should relieve tension on the northern part of the AF but increase it on the southern part?

    Yes possibly, tho, I recall the stressfield work done on the Northridge and other Californian events and it plainly showed that the stress was just moved along to the next section and/or area of faulting within that region
    will see if I can dig up that reference :)
    We still have to consider that with no significant event in the MFZ reg in the last 100 or so yrs … i.e. M8+ … a M7 event doesnt really do a lot to reduce the overall long builtup stresses in the region. Think of the number of M7′s needed to release the similar amount of stress released in a M8. (personally, and those more in the know than me please correct me :) … I would feel that the stresses released in this latest event would be relatively nearfield and not extending much into the farfield)
    cheers
    Dave

  5. Chris Rowan says:

    With regards to the USGS MMI estimates – as I understand it, they’re based on an automated algorithm that will not take into account the local effects that probably increased the damage in Canterbury – namely, the fact that it sits on a huge pile of unconsolidated sediments, and liquefaction. Interestingly, though, the chart above conflicts with the USGS shakemap, which seems to have Christchurch pegged at VI-VII. Closer the the GNS version, but still not quite as severe: either the GNS have a better local model, or their magnitude estimate of 7.1 makes a difference.

    I’d just like to thank everyone for contributing to this great discussion. It’s been great to read.

    • bruce stout says:

      I’ll ditto that. Thanks very much to you Chris and Dave and everyone else. It’s a fascinating subject.

  6. In regard to the southward progression of faults towards the Christchurch area, the sinmplest structural model for this is strike-slip duplexing (reference: N.H. Woodcock and M. Fischer, Strike-slip duplexes. Journal of Structural Geology 8 (1986), pp. 725–735).

    I have long held the view that the major transcurrent Alpine fault system shows properties of such duplexing, with each new strand located south of preceding strands.

    The locus of the recent quakes is within a block oriented northwest-southeast, between the Hororata Fault to the northwest and a new fault to the southeast. This block is spatially related to the Banks Peninsula, which might have played a role in ‘fixing’ this block. I would be most interested to see if the focal plane solutions to these quakes in this area include soume normal faulting, indicative of an incipient pull-apart basin.

  7. Matt - Christchurch resident says:

    This Geonet map shows the recorded peak ground acceleration for the region. http://www.geonet.org.nz/images/news/2010/darfield_pga.png

    It appears that Christchurch was very lucky and didn’t bare the full brunt of the earthquake. A lot of credit has been given to the building codes. Is a PGV of 30 strong enough to put a lot of strain on the structure of modern buildings?

    It’s interesting that the Selwyn district escaped with mostly minor damage considering it felt the largest ground movement. Presumably the ground is more consolidated and liquefaction wasn’t an issue.

  8. Tony Norriss says:

    Hi,

    Someone much more knowledgeable than myself might be able to answer my question. I understand the amount of movement in the AF is actually quite high. It would seem intuitively that where movement is easy that stresses aren’t so acute, and that stresses are more likely to build up where plates become “stuck” and then release the pressure.

    Am I correct in this understanding? If so, are the suspect sections of the AF ones that are currently “stuck” and building up pressure to release?

  9. bruce stout says:

    Matt, that’s an excellent map, it shows how the southern side of the fault felt the full brunt of the movement, which ties in, I guess, with the talk of it being a unilateral rupture (testing new vocabulary here, but I guess that is what is meant by unilateral rupture with the southern side suddenly shunting 4 m towards Australia). My cousin in Darfield (on the northern side of the rupture) said it was an absolute brute so lord only knows what people 20 km further south thought where the PGV was more than double what they had in Darfield.

    Tony, I’m certainly not more knowledgeable than you but I’d be happy to share the little I do know, which is something a US geologist once said to me and that is the ground behaves like that silly putty stuff kids play with. It can accommodate a certain amount of strain but every now and again if the stress gets too high it will snap. Consequently, the degree to which movement occurs along a fault ranges from slow earthquakes like they measured near Gisborne a couple of years back (only noticed from GPS measurements) to sudden shockers like this Darfield quake. As far as I know there is still a lot of research going into figuring out under what conditions a fault will slide easily and often or, conversely, stick, store up stress and finally snap. Quite a complex business, the whole thing, depending on the type of fault, the stresses involved, the molecular bonding of the rocks involved, water etc. etc.
    So, yes, the parts of the AF that haven’t moved are cause for concern, particularly given the amount of activity both north and south of the “quiet” portion of the AF. Dave said above that he thought the stress transfer from this Darfield quake would
    be “near-field” which I interprete to mean it has little significance for the AF but I could be wrong on that! (If we are lucky one of the professionals here will chip in and put us right! ).

    All in all I find the whole plate juncture peculiar and I’m having trouble getting my head around all the vectors involved. On the one hand you’ve got this twist in the Pacfic plate which goes from being subducted under the Australian plate to colliding with it, and then this transition area is itself migrating south as the Australian plate slips NE relative to the Pacific plate. The resulting twisting is expressed in this Darfield quake, but if you extrapolate the direction of movement of the area south of the fault it points directly towards the Southern Alps. I dunno, but it feels to me that the collisional stress must be building there, which should result in vertical thrusting. At the same time I have no idea what impact this Darfield quake has on the horizontal strike/slip component of the AF, which is the dominant factor there. It will be interesting to see what vertical component and what horizontal component there will be in the next major series of quakes on the AF.

  10. Chris Rowan says:

    The plate motion vector stays pretty much the same, actually: the Pacific plate is moving ~WNW with respect to the Australian plate at 30-40 mm/yr. However, because as you move south down the North Island, oceanic crust transitions into thicker and more buoyant continental crust, which cannot be subducted, so the subduction zone is effectively jammed up and the motion is transferred west onto the S Island, where it is accommodated by (mainly) strike/slip faulting. The reason the boundary changes orientation is because while compressional structures need to be oriented at right angles to the plate motion vector to accommodate the deformation, strike-slip structures need to be parallel.

    However, the plate motion vector has rotated clockwise a bit since the Alpine fault first initiated (from SW to WSW), so now there is a component of compression that needs to be accommodated, too, which is why the Southern Alps are there. So a rupture will probably combine vertical and horizontal movements: if you’re standing on the west coast, the mountains will move south and up.

    It seems unlikely that this earthquake will have affected the stress built up on the Alpine Fault too much, especially since from the aftershock pattern most of the stress transfer was to the east rather than the west. As for the general hazard, I’m just finishing off a post that addresses that very issue. Watch this space.

    • Dave Nelson says:

      Thanks Chris ….
      that pretty much confirms all I said in an earlier post concerning stress relief including the comments about the roatation of the motion vector as shown on the Rotation Pole diagram
      I posted :)

    • bruce stout says:

      that’ll teach me for using terms I don’t have mastery of. I don’t have problems with the plate vectors as such (and the shift in the pole of rotation over time).

      where my mind turns into boggle mode is trying to visualize how the change in the nature of the crust and the plate boundary results in various land forms and fault systems. For instance, are the Kaikouras on the Pacific plate like the Southern Alps or are they blocks of the Australian plate getting thrown up by the various fault systems of the MVZ, or are parts of the Pacific plate (i.e. the bit between Nelson and the Kaikoura coast suddenly finding themselves on the other side of the plate boundary, i.e. the Australian side? (I’ll ignore the political ramifications for the moment).

      • bruce stout says:

        sorry that was a bit garbled.. what I meant was this:
        For instance, are the Kaikouras
        1. on the Pacific plate (like the Southern Alps) that are getting thrown up by plate collision
        2. or are they blocks of the Australian plate getting thrown up by the various fault systems of the MVZ arising from the plate collision (which I doubt, but the plate now seems to be EAST of them), or
        3. are they originally parts of the Pacific plate that now suddenly find themselves on the other side of the plate boundary?

        or even 4. is this talk of plates a bit misplaced as actually the entire area was once part of one plate (the doomed and submerged Zealandia) that is in a process of getting rent asunder? with the plate boundary still exploring its way through the middle of it?

  11. Barbara Hepburn says:

    Thanks for such a brilliant resource Dave and if you don’t mind I’ve just added your page to our high school library database. However I can’t open the link to the “map of large historic earthquakes in New Zealand” – the one that shows that earthquakes of this magnitude can occur pretty much anywhere in New Zealand?

  12. Sam says:

    Hi Chris,

    It would seem that based on the map of relative plate motion (first figure at the top of the page) the pacific plate is moving WSW relative to the Australasian plate. Do you mean overall the plate vector is WNW but closer to New Zealand it is rotated due to the subduction zone?

    • Chris Rowan says:

      That was actually a typo on my part, sorry. WNW should be WSW (and NW should be SW). I’ve amended my comment – thanks for the spot!

  13. Dave Nelson says:

    wonder if anyone else has noticed the rise in seismicity in the Kaikoura region over the last 3-4 days. There is an ongoing large number (15+ a day) of small events and close to the khzo seismograph
    http://www.geonet.org.nz/earthquake/drums/khz-drum.html

    cheers
    Dave

  14. Tony Norriss says:

    Hi, thanks for the answer to my question above.

    Something else I was wondering.

    The projections for the next rupture of the AF are based on 4 ruptures over the last 1000 years. Firstly, this seems like a relatively small sample of events on a geological time scale. Secondly, there was several ruptures in relatively quick succession (1600-1700 time frame). Could it be that those two ruptures settled the fault down so we are now enjoying a period of hiatus? Also, could it be that the activity over the last 1000 years represented a cluster of activity for the fault and that there may be a reasonably extended time period before the next rupture? Probably wishful thinking on my part :smile:

    • Dave Nelson says:

      the projections for the next rupture of the AF are based on 4 ruptures over the last 1000 years. Firstly, this seems like a relatively small sample of events on a geological time scale.

      not really considering that those 4 ruptures were quite consistant in their return period Work done by William Bull (Journal of Geophysical Research Vol 101 No.B3, pgs 6037 – 6050, Mar 10, 1996) ……
      his calculated event occurrences/return periods
      calendric age Time between events
      in yrs, AD (years)
      1996 248 +- 10 Elapsed Time

      1748 +- 10
      259 +- 14
      1489 +- 10 Event
      263 +- 14 Recurrence
      1226 +- 10 Intervals
      259 +- 14
      967 +- 10

      Secondly, there was several ruptures in relatively quick succession (1600-1700 time frame).
      there has been no ruptures between 1600 and 1700 … see above comments for return times

      • Dave Nelson says:

        ….. no major ruptures identified…..

      • Dave Nelson says:

        grrrr that layout of dates and return periods was screwed up by the blog formatting :( lets try this……

        date at time of report 1996 248 +- 10 Elapsed Time

        previous event ………..1748 +- 10
        recurrence interval …………………… 259 +- 14
        previous event ………..1489 +- 10
        recurrence interval …………………… 263 +- 14
        previous event ………..1226 +- 10
        recurrence interval …………………… 259 +- 14
        previous event ………… 967 +- 10

        Secondly, there was several ruptures in relatively quick succession (1600-1700 time frame).
        there has been no major ruptures between 1600 and 1700 … see above comments for return times

  15. Chris Rowan says:

    In answer to Bruce’s question above:

    are the Kaikouras
    1. on the Pacific plate (like the Southern Alps) that are getting thrown up by plate collision
    2. or are they blocks of the Australian plate getting thrown up by the various fault systems of the MVZ arising from the plate collision (which I doubt, but the plate now seems to be EAST of them), or
    3. are they originally parts of the Pacific plate that now suddenly find themselves on the other side of the plate boundary?

    or even 4. is this talk of plates a bit misplaced as actually the entire area was once part of one plate (the doomed and submerged Zealandia) that is in a process of getting rent asunder? with the plate boundary still exploring its way through the middle of it?

    You’re pretty close with 3. The Marlborough/Kaikoura region was initially on the Pacific plate: when the plate boundary first propagated through the South Island (probably around 30 million years ago), there was just one, single, proto-Alpine fault accommodating all of the motion (see the top of the second figure on my post). However, as the plate boundary region has evolved, the effective plate boundary in this region has moved to the south east as new faults have initiated; what you’re in effect getting is bits of the Pacific plate being cut off by faulting and amalgamated to the Australian plate.

    It’s not quite as clear-cut as this, because there is basically distributed deformation across the whole region, but if you were forced to to draw a linear plate boundary, It would currently run along the Alpine fault up to Hokitika, then along the Hope Fault to Kaikoura, before linking up with the Hikurangi subduction zone offshore. 10 million years ago, it would have run all the way along the Alpine-Wairau fault to Cook Strait; a few million years from now, it will probably branch away from the Alpine fault further south and cut across to somewhere near Christchurch.

    • Dave Nelson says:

      Primarily … when you look at the subduction going on under the Marlborough and Nelson provences region you would be inclined to think that ALL of the Marlborough area would be Australian Plate material.
      But a lot has to be said for accretion of Pacific seafloor sediments onto the leading edge of the Australian Plate
      So personally I like your suggestions of 2 and 3 :)

  16. Tony Norriss says:

    Dave: “here has been no ruptures between 1600 and 1700 … see above comments for return times”

    Thanks for that Dave. Is this an area where there is academic disagreement. That is because the Wiki reference refers to ruptures at 1100, 1450, 1620 and 1717 AD which is different to the timings you have given above (http://en.wikipedia.org/wiki/Alpine_Fault)

    • Dave Nelson says:

      well the inaccuracy of the first statment in the wiki doesnt give me much confidence in the rest of its reporting…..
      “The Alpine Fault is a geological fault, known as a right-lateral strike-slip fault, that runs almost the entire length of New Zealand’s South Island.”

      they could have at least got their opener correct !! :)

      • Dave Nelson says:

        seems that they are the dates that the GNS are pushing :)
        will have to see if I can contact my 2 professors at Otago Uni and see what light they can shed as they have meen the main 2 doing the research Prof’s Norris and Cooper

        I am always willing to be swayed by newer info thanks for the references :)

  17. Dave Nelson says:

    I have emailed the GNS to see if they can cite references as to where this info comes from :) will be interesting to see if they respond

  18. Bill Christchurch Resident says:

    Hi Everyone, I dont know much about the technical side of geology and I think I understand what you are all talking about a tiny bit. Please excuse me for changing the subject a little…. I was at work when the earthquake struck, infact I was in the control room at the Lyttelton Road tunnel. What was interesting and some thing that we dont quite understand, was that just before the quake hit, there appeared to be what looked like a shock wave travel through the Tunnel from East to west. ( From Lyttelton through the 2km long tunnel and exit at Heathcote, the Christchurch end). The shock wave looked similiar to a very fine dust cloud, however it dissiapated way to fast to be dust.
    Anyway, given that the quake was beleived to have originated south east of darfield I would have expected a shock wave, if any, to have travelled west to east.
    After the quake we went back and viewed the cctv footage and noticed that the shaking started in Lyttelton (East) before it started in the Heathcote end (West).. thus confirming the visable shockwave.
    The other thing we observed on the cctv footage was what appeared to be a type of lightning (Earthquake lightning)?? Only it appeared to be comming out of the ground. Now that sounds weird I know, but members of the police on duty that night also reported it..
    Maybe you can help me out with an explaination to these occurrences.

  19. Chris Rowan says:

    There has been – understandably – a fair amount of work done on the Alpine Fault in the past decade, which has considerably refined the dates of the last few earthquakes. I’ve just put up a post on this very subject – follow the link above if you’re interested…

    • Dave Nelson says:

      I got a response from the GNS and a link to many references, its “forced” me to spend copious amounts of $$ to join the Geological Soc. of America so that I can download some of the papers and catch up on the research :)
      hopefully it will be worth the spend ;)

  20. Dave Nelson says:

    hey gang
    following my comments a couple of days ago concerning activity in the Kaikoura region…. things are continuing to get more active. with a couple of M4+ events this evening after another day of many smaller events…
    The $6million questions….
    Is this just going to be a swarm with no defined large event ?, or….
    Is it the prelude to something bigger ?

    strap yourselves into your armchairs and await the outcome with baited breath ;)

    • bruce stout says:

      Ha, that’s a good way of stopping any activity, Dave. It’s kind of like Schrödingers cat… you watch it and all activity dies… (well I’ve got my fingers crossed anyway).

  21. Chris Rowan says:

    I’d advise some caution here. Firstly, is this kind of activity unusual for Kaikoura, or is it just that people are (understandably) paying more attention to the seismometers at the moment? A major active fault (the Hope Fault) runs through this region, so you’d expect some background level of seismic activity.

    Secondly – and perhaps more importantly – there is no reason to suppose that a burst of moderate seismic activity is the prelude to a bigger earthquake. If only it were that simple!

    • Dave Nelson says:

      I’d advise some caution here. Firstly, is this kind of activity unusual for Kaikoura, or is it just that people are (understandably) paying more attention to the seismometers at the moment? A major active fault (the Hope Fault) runs through this region, so you’d expect some background level of seismic activity.

      ohhhh this is substantially way more than background activity Chris
      this has been a sudden burst of high activity over ~ 4-5 days
      Has nothing to do with me paying more attention…. I monitor the seismometers around the country daily and have done so for many many years

      Secondly – and perhaps more importantly – there is no reason to suppose that a burst of moderate seismic activity is the prelude to a bigger earthquake. If only it were that simple!
      Thats true, as stated in my earlier comments, it may just be a swarm of events.
      Also, as a side interest, this swarm may or may not have been induced by the activity further south in Darfield. Its well known that large quakes can induce other regional activity, but far outside the aftershock zone.

  22. ratio152 says:

    Great to have found the information. Thank you

  23. Crile says:

    What a great post – thanks. Before shifting to Lyttelton some 13 years ago, I was aware of subduction under the NI and strike-slip in the SI and wondered what was happening where the two processes met. I came across an “Ask a Geologist” sort of site from GNS but didn’t get a very satisfying answer. I must say that I learned more from this post than I have from a fair bit of reading since arriving on this still shaky isle. Many thanks!

    Fascinating stuff about the tunnel, Bill.

    Now if I could just get my head around focal mechanism diagrams…

  24. John says:

    Well, I am not a geologist, just a network engineer, but I beleive I have seen reports of electrical discharges associated with earthquakes before. Most likely sounding mechanism to me is static discharges caused by gas exiting from cracks in the ground under pressure. It is quite well known that a release of steam (eg from a safety valve) can cause static discharges, and there is no reason why the same should not occur if gas is forced out of the ground under pressure. The gas could be natural gas from pockets of decaying matter underground, or CO2 for that matter. Of course, if a natural gas pipeline under high pressure is broken, the same thing can happen, and the gas can self ignite from its own discharge. So I am not saying that is the definitive explanation, but it is a possible one. I will leave the possible reverse direction of the shock wave to the experts…

    The news of small tremors along the Hope fault does not surprise me too much…you have a system of large blocks of terrain all pushing on each other, and one joint pins, builds up stresses, then releases. This has to change the stresses on the other blocks around it that were pushing on it or being pushed on, so to see some accomodation would be reasonable. You wouldn’t necessarily expect it to be a harbinger of another big event, since the general stress levels should have been reduced by the movement that has taken place. If one movement did change things in such a way as to permit another to take place, I would expect the second event to happen fairly quickly, and I wonder if in fact something like this may have happened with the main event here, where there seems to have been a moderate foreshock followed quickly by the big one. But most of my knowledge of structures and their failure modes is for much smaller scale and better defined structures and stresses, so I had better shut up before I mire myself to deeply.

    John

  25. Carolyn@UC Canterbury says:

    Hi Bill Christchurch Resident. I am really interested in your footage showing lights emitted from the ground. Is it clear that the emissions are from the ground and not electrical infrastructure? I am a student at UC and one of my research topics in electromagnetic radiation emitted from fractured rocks. Quartz (which is piezoelectric) stores electrical charges which are emitted at many different frequencies when rocks break. This may result in lights, but it is hard to get evidence of this in cities which have light pollution. I am really interested in viewing the footage if that’s possible. Thanks for posting.

  26. Warren Fowlie says:

    To Bill, the Christchurch Resident. Any chance of getting NZTA to release the surveillance footage of the event that was seen in the Tunnel. I will try to contact you through the system, but in the meantime you would be doing a great service to a wider group of experts if this footage could be made available. If it is already out there, could you direct us to the site. Thanks.

  27. mark quigley says:

    you guys might be interested in a recent lecture i gave a u canterbury on this event

    ftp://ftp.geol.canterbury.ac.nz/Furlong-Quigley%20Talks%2016%20&%2027%20Sep/20100927.mov

    cheers

    mark

    • bruce stout says:

      Thanks for sharing this Mark, that was a really excellent talk. When things have quietened down it would be great to hear some more from you about this event, for instance, I am intrigued about this reactivation of ancient faults that originated from the Chatham rise and the implications this has in the wider setting. In the blog entry above Chris suggested this event is related to the southwards migration of the MFZ. Can we expect more of this kind of EW strikeslip faulting to happen in the coming millenia and how do you think the fault systems will adjust to the fact that they do not strike in quite the right direction to accommodate the plate motion. Will we get lots of parallell EW striking faults between here and the Ashley fault or will eventually a new fault form with a more traditional NE/SW strike?

  28. william slater says:

    Excellent presentation and lively discussions.Do you think that after a month of continuous “aftershocks” (1600 up to M4.5 ) that the Pacific Plate will continue to “slide” under the Australian Plate for a long time in the same pattern?Are there other examples of this behaviour elsewhere in the world?Are there chances of new volcanoes appearing on the Canterbury plains ,say in 50,000 years?After all Lyttleton Harbour is the collapsed cauldron of 2 ancient volcanoes?Look forward to the learned answers.Bill Slater (PEng. -ex Canterbury College 1953)

  29. william slater says:

    Mark,I found your talk quite exciting and educational.The fallen trees and extent of the liquefaction and extensive damage without fatalities,like in Haiti,is remarkable.Great research record.Bill Slater. Toronto,Canada.

  30. litiane says:

    god bless my family in nzed hope thre or right

  31. George says:

    So do you guys think the Alpine Fault may rupture soon? Lots of deepish earthquake across the fault line the last few weeks, or this not really an indication? Cheers.
    P.S. GREAT & VERY INFORMATIVE ARTICLE A++++

  32. VEERAPERUMAL says:

    IN THIS MAJOR EARTHQUAKE IS CONNTUNIUSELY PROCESSS IN THIS AREA. SO PROPER SERVE THIS AREA WE ARE STUDIED. FIRST ONE IS PEOPLE AWARENESSSSSSSSSSSSSSSS. ALL MAN STUDENTS WILL STUDY TO MUST IN THIS STORY

  33. bill blyth says:

    Is there any likely hood of this quake being generated by the HAARP earth quake weapon that was recently used in China and Haiti.

    • Chris Rowan says:

      No. Mainly because there is no such thing as a ‘HAARP earthquake weapon’.

      • Dave Nelson says:

        although there is such a thing called HAARP :) a wonderful American hi power transmitter located in Alaska. Used for ionising patches of the Earth’s Ionosphere for auroral research.
        the paranoia around the world blame it for everything from crop circles -> weird circles on Australian weather radar (in reality just radar reflection artifacts) -> weather controlling -> and now to earthquake generation
        Just google HAARP to see the garbage ;)

        cheers
        Dave

  34. Chris (UK) says:

    Just a note of appreciation for such an informative piece. I too have trouble getting my head around some of the geometry of the plate movements. I hope that more will be added after the recent and disastrous quake on Feb 22.

  35. Chris says:

    Did the recent solar flare that was a X2.2 that hit on Feb 14, 2011 cause the earthquake?

  36. Chris says:

    The earth core is magnetic right, so with the solar storm that hit China could this not cause the earthquake activity? I seen an article from NASA saying they found a hole in the magnetic field from the equator to the South pole.
    http://science.nasa.gov/science-news/science-at-nasa/2008/16dec_giantbreach/

  37. Chris Rowan says:

    There is no conceivable way that these two events are connected. Whilst there is some evidence that faults generate some sort of electromagnetic signal as they start to rupture (although not in a way that makes such signals predictive, as far as we can tell), that’s the exact opposite of EM signals triggering an earthquake.

  38. jess says:

    this article is really interesting and informative!

    however i’d be really keen to hear what everyone thinks of the nelson regions fault lines? i have always heard there are about 3 – 5 quite active fault lines (in fact i hear the nelson hospital is built on one??) but in the quake graphs above, the nelson region is very VERY quiet on the earthquake front.

  39. janine says:

    Oh joy … ive been seeking the truth for days.. i stumbled on this page.. thanks heaps.. maybe my family will stop laughing at me now.. please continue with your wonderful research.. i now have an informed opinion

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