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.

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 (50)

  1. Passerby says:

    The lithospheric geodynamics of plate boundary transpression in New Zealand: Initiating and emplacing subduction along the Hikurangi margin, and the tectonic evolution of the Alpine Fault system . Kevin Furlong and Peter J. Kamp (September 2009). Tectonophysics 474(3-4):449-462.

    Reports indicated moderate-to-severe internal damage to many large buildings, but the extent will require engineering survey and inspection in the weeks ahead. Although the initial rupture lasted just 40-seconds, numerous large aftershocks gave the impression of near-constant shaking for more than a hour.

  2. Passerby says:

    Nearby forgot. Nice Job, Chris!

  3. bruce stout says:

    Yet again a fantastic post Chris. It is kind of ironic that the big risk touted to date for Christchurch was movement on the Alpine fault, which tends to slip in a series of M7 events every now and again (it is primed for another series apparently), and here we have what appears to be an extension of the Marlborough fault system hitting the city first.

    Christchurch is sometimes said to have a larger earthquake risk than Wellington (though personally I doubt it) because of it being situated on soft alluvial sediments near the ocean which will lead to liquification and severe shaking. The damage reports seem to confirm these fears. Darfield (which I have been to) is about three quarters of an hour’s drive from Christchurch but it appears damage was worse in ChCh than Darfield.

    So it looks like this quake was a blessing in disguise. Note that most of the collapsed buildings appear to be made of brick which has long gone out of favour in NZ (since the 1931 Hawkes Bay earthquake devastated Napier). So let’s hope this Darfield quake has effectively cleaned up the town of its residual killer buildings before any more big quakes hit the city when the Alpine Fault starts to move again.

  4. Lanthanide says:

    There are still lots of brick buildings left in the city, and many of the ones that have had damage, it has only been partial or superficial, so the whole building itself can still come down in the future.

    It appears Raikaia has been particularly affected, possibly due to recent wet weather rather than particular proximity to the epicentre.

    The quake was initially reported as being 7.4, and lated downgraded to 7.1, probably due to the “unconsolidated Quaternary sediments” mentioned in the post.

    Still getting aftershocks every 30-60 minutes, although just had about 3 in 15 minutes. I’ve got a glass with water on my desk as my earthquake detector.

  5. Lanthanide says:

    Also here’re links to the seismographs which I’m sure you guys will be interested in, if you haven’t seen them so far:
    http://geonet.org.nz/earthquake/drums/mqz-drum.html (last 24 hours, may vanish by midnight tonight)

  6. bruce stout says:

    How’s that glass of water Lanthanide? Looks like you just had another severe aftershock. Do you think they might start pulling down / reinforcing some of those buildings now? Nice as they are, they are real killers. I wouldn’t want to live in one.

    PS I must correct my own spelling above … it’s liquifaction not liquification (always get that wrong).

  7. Lynnie says:

    All I know is that my knees are still knocking!! Can you explain for us why the damage is so random? In one street all the chimneys are broken, yet in the next they’re fine; why half our ornaments fell down and were completely smashed, and the other half didn’t even seem to have moved – even when they were on the same shelf!!!

  8. bruce stout says:

    Actually, while I am at it, Chris, could you explain the other end of the Alpine fault (near Fiordland) me where the transition to a subduction setting appears to be really sudden. Is this the corrollary to the Marlborough Fault zone, i.e. where the southwards migration of the plate boundary leads to the MFZ, that same southwards migration encourages a sudden transition at 45°S? There’s been a lot of activity there too recently. Which all leaves me even more nervous about the bit in the middle.

    • Chris Rowan says:

      The change in deformation style at the southern end of the Alpine fault is due to another change in the nature of the plate boundary. On the South Island there is continental crust on both sides of the fault, but as it runs offshore in Fjordland there is oceanic crust, which can be subducted, on at least the Australian side of the boundary (it’s possibly still thinned continental crust on the Pacific side).

  9. bruce stout says:

    Lynnie, all I know is that the earth, particularly on soft alluvial soil, starts acting like water in a paddling pool. You will get lots of interference patterns as the earthquake waves bounce of the port hills and generally slosh around the Canterbury plains. So at one point you will get a lot of movement and relatively little at other points. The USGS have some amazing animations of quake simulations in California showing this.

  10. Glenys says:

    I wonder if the reason why there is less damage in Darfield itself, despite being the epicentre, is that Darfield is built on the remains of a riverbed and Christchurch is on a swamp so Darfield is effectively cushioned whilst Christchurch just slips around?

  11. Lanthanide says:

    There’ve been a few aftershocks since my post above, included the biggest so far at 5.4M. Now getting them about every 60 minutes, although we had a little swarm around 7pm. We were at the supermarket when one of the smaller 7pm ones struck, and they had large (presumably) plexiglass windows blocking off the back off the checkouts from the common foyer. The windows started flexing in and out, like big wobble boards, was pretty interesting.

    Most of the aftershocks have been longish (7-12s) rumblers, rather than sudden jolts. The jolts have generally been the smaller ones, and make me start out of my seat for the doorway, while the rumblers are mostly over by the time I decide whether I really need to move or not.

    Also the town that got it the worst is Kaipoi, not Raikia (which I spelt wrong above).

  12. Lanthanide says:

    Oh, I forgot to mention, there was quite a lot of rain dumped in Canterbury yesterday, which would’ve encouraged liquefaction, especially after a generally wet week. Today was bright and sunny and pretty warm though, so we were lucky. Tomorrow is going to be a very warm gale with some possible rain in the evening, so that may do a bit more damage.

  13. Chris Rowan says:

    Some great comments and eyewitness accounts here – thanks. Keep them coming!

  14. Passerby says:

    Christchurch is built on estuarine sediments deposited at the confluence of two large rivers. River sand, silt and gravel deposits are spatially uneven (depth, type, extent) distribution in the area due to the heavily braided river channel topography.

    No surprise that Kaiapoi, situated to the NW of Christchurch, on the largest river’s flood plain (see photo, Waimakariri River), was reported to be the hardest hit.

    The earthquake hazard In Christchurch: a detailed evaluation

    A New Probabalistic Seismic Hazard Model for New Zealand. M. Stirling.

    >The New Zealand earthquake hazard model has been updated to incorporate new geological data, an improved historical earthquake catalogue, newly-developed methods for the treatment of historical catalogue data, and new attenuation relationships for peak ground acceleration and response spectral accelerations. Maps produced for the Canterbury region show higher estimates of PSH than those produced in the earlier PSHAs (1998).

    The larger picture of deformation forces in North-central South Island:

    A Letter-article, published recently in Nature, on the Marborough fault system. See Fig 1.

    Fluid and deformation regime of an advancing subduction system at Marlborough, New Zealand. Nature 460, 733-736 (6 August 2009).

    >Our data imply three distinct processes connecting fluid generation along the upper mantle plate interface to rock deformation in the crust as the subduction zone develops. *Massive fluid release just inland of the trench induces fault-fracture meshes through the crust above that undoubtedly weaken it as regional shear initiates.
    * Narrow strike–slip faults in the shallow brittle regime of interior Marlborough diffuse in width upon entering the deeper ductile domain aided by fluids and do not project as narrow deformation zones.
    * Deep subduction-generated fluids rise from 100 km or more and invade upper crustal seismogenic zones that have exhibited historic great earthquakes on high-angle thrusts that are poorly oriented for failure under dry conditions.

  15. Passerby says:

    Canterbury earthquake really three quakes?

    Ken Furlong (Penn State Univ, on sabbatical at Univ. Canterbury and cited above by me), would be a good contact to query and invite for a guest post here.

  16. Lanthanide says:

    It certainly seemed to me like the shaking got more intense moments after I got under the doorway. When the shaking started I was in bed and woke up thinking ‘wtf’, then after 2-3 seconds realised it was an earthquake, got up and ran for the door. Then it seemed to get more intense, and stay at peak intensity for about 7-8 seconds, before tapering down. Shaking continued for about 40-50 seconds in total.

    Managed to sleep through many aftershocks this morning without waking at all, two of which were rated at 4.6 and 5.1, so I think for me to be awoken up initially like that the shaking must have been on the order of 5.5-6.0 or so? Still using my glass of water, and placed it in front of the LEDs in my alarm clock so I could easily see it at night, if required, but luckily no more big quakes.

    So far we’ve had 7 aftershocks between 5.0 and 5.4:

    Note that the 7.4 and 7.1 entries refer to the same first big quake, with the 7.1 being an update to the initial 7.4 report, rather than a second earthquake.

  17. Passerby says:


    Okey dokey. Although the US Military has offered help and been turned down, there is something they can do that will help NZ cope with recurrent earthquakes from these active fault zones in the future.

    Coordinate with NZ military to capture satellite-level images at a variety of focal depths, to document spatial variation in property damage density and extent. It should be gridded and shared with Google, who can map the data to various on-line visualization apps like Google Earth and Google Maps, which can make them available to engineer, planners and emergency management personnel.

    This should be completed ASAP, before major cleanup removes evidence visible from space.

    What we want is to overlay geological and soil/sediment maps with the damage survey images, using estimated damage index as vulnerability proxies. Goal is to map the shallow subsurface at fine-scale, to refine knowledge of risk variability over large urban areas. These would be used alongside probabilistic models, to refine building codes for high-risk structures in vulnerable areas of Metro ChCh.

    Feedforward of knowledge, for reducing earthquake severe damage risk in the Wellington Metro and other major metro areas with significant seismic liquifaction risk would be A Good Thing.

  18. Wild Land says:

    A 5th magnitude foreshock four seconds before the main event has now been identified by USGS and GeoNet.

    The identification of a multiple mainshock event will be interesting, but I guess that will take some time. GeoNet have already hinted at it in their initial report –

    The liquefaction headaches for Kaiapoi are staggering but seem to be explained by the high rainfall and soft soils.

    I now wonder if they will find some parallels with the “terrible twins” of 1942 when resonance in Wellington caused the toppling of thousands of chimneys by the mag 7 Masterton quakes. Could similar resonance effects be causing the random damage effect described by Lynnie?

    • Chris Rowan says:

      I think that’s very likely. Resonance effects in both the subsurface and within particular structures could both contribute to very large differences in the amount of shaking.

  19. bruce stout says:

    sounds good to me Passerby, in fact obvious. I imagine the authorities will be on to it shortly as many sources have pointed to the immediate surface strata as now being a prime factor in damage. Looks like the new way to go in terms of mitigating earthquake damage and NZ has been pretty on the ball in this field in the past so I imagine this will be the way they go in future.

    And thanks for all the links everybody! How I wish this was my paid job to actually read this.

    As for the bigger picture, a few pennies have dropped for me. This quake now makes it clear to me why the Mt. Cook region has the biggest mountains in the South Island. It is not so much slippage along the 200 to 300 km of alpine fault that is the issue but the fact that the Mt. Cook region represents the fulcrum around which the Australian and Pacific plates are stuck. No doubt there will be movement as in the past along the AF in the future but the MFZ is now spreading a long way south and it is not far away from the Haast region (in lateral terms) where the Australian plate starts subducting. I dunno, but common sense tells me that the Mt. Cook region is going to get pretty screwed up in all this. Does anyone know of any other comparable plate collisions in the world? What can we expect in future? Somehow the past scenario of steady slippage along the AF looks kind of simplistic. (Yes, I realize Nelson once sat by Otago but how is this meant to work in future when the MFZ has migrated so far south that it is almost opposite the Haast subduction zone?)
    The other thing is this Darfield quake is still effectively a side show to the main AF scenario, i.e. it is a reaction to stresses along the main plate margin rather than being an expression of movement along the margin itself… or have I got this wrong? (and yes, I’d be grateful for any correction on anybody’s part!!) Whatever the case, it looks to me like this quake has made movement on the AF far more likely than less.

    The other thing I’d like to visualize is what is happening under the plate/mantle boundary in the Haast region. The Australian plate is being subducted and moving northward relative to the Pacific plate but here it must be ploughing into the aesthenosphere as it goes which in itself must generate some interesting stresses… or, converserly, the aesthenosphere is in fact surprisingly accomodating and offers little resistance until the D II region is reached, as Passerby pointed out recently at the Eruptions thread, implying that the main point of collision is indeed fixed on the crust around the Mt. Cook region.

    The other question is the absence of volcanism in Fiordland.. why so? Has the Australian plate not reached the necessary depth to induce melt?

  20. Mac says:

    Very good to have some solid scientific analysis of this earthquake; thank you, Chris. It fascinated me as a child growing up in ChCh to learn that the top of the cathedral spire was remade in copper sheathing, following 2 or 3 topplings of the original stone, due to (presumably smallish) quakes back in the 1800s. Now living in Dunedin, where many of the larger public/commercial buildings are 19th or early 20th century, I suppose a similar vulnerability would apply in the event of a major quake. But I see on your map that East Otago (and upper Northland) look to be historically almost earthquakeless (although I did feel the ChCh quake ever so slightly — I don’t think I awoke, but had had a dreamlike qualitiy of a very slight mm or 2 of sway, which I could interpret later in the day, when the news came through). Chris, if you would comment, does this history allow a degree of reassurance, and perhaps a less pressing need for quake-proofing, in this city (and maybe also Kaitaia and other Far North)?

  21. massimo says:

    Congratulations for the escaped treath.
    Someone can tell something about the local amplifications measured (g) ?

  22. Dave Nelson says:

    @ Bruce Stout…
    Quote ..The other thing I’d like to visualize is what is happening under the plate/mantle boundary in the Haast region. The Australian plate is being subducted and moving northward relative to the Pacific plate but here it must be ploughing into the as it goes which in itself must generate some interesting stresses… End Quote

    the Australian Plate isnt really being subducted … rather the Pacific Plate is just riding up over the Australian Plate in an oblique slip motion, for every ~3 metres of horizontal movement there is ~1 metre of vertical movement along the central portion of the Alpine Fault. So no “ploughing of it into the aesthenosphere” is occurring. In my university studies of the Apline Fault at Otago Uni during the ’90’s we did a serious field study of the central portion of the fault from Orita in the nth to Haast in the south.

    your next comment
    Quote… The other question is the absence of volcanism in Fiordland.. why so? Has the Australian plate not reached the necessary depth to induce melt?End Quote

    see above comments re Australian Plate, also things change quite dramatically in Fiordland from Haast southwards. The main plate boundary becomes quite steep
    as has been shown by spacial locations of quakes in the region and it is still the east side of the boundary ( the Pacific Plate) the has the signif. upwards motion compared to a lack of signif. vertical motion (up or down) on the west side (Australian Plate) see http://www.sydneystormcity.com/FiordlandW-E_crosssection.jpg

    ohhh I love this science 🙂
    Dave N

  23. Dave Nelson says:

    After posting, I was trying to think of a way of relating the motion in a better way along the Alpine Fault Zone. In relation to the Australian Plate not subducting and the Pacific Plate riding up and over the Australian Plate…. Look at it this way…. the narrow strip of Westland, from the fault trace to the ocean (lets look at roughly Kumara to Haast) is the Australian Plate doesnt have any significant vertical motion relative to sealevel. Where as the Pacific Plate is thrusting upwards at an avg amount of ~ 1m / 1000 yrs.
    see quick drawing http://www.sydneystormcity.com/STH_IS_X.BMP
    This relatively rapid ascent of deep hot rock accounts for numerous hot springs along the fault go have a look along the access roads to the Fox and Franz Joseph Glaciers 🙂

    Another interesting thing to look at is the Plate motion models and the “Stage Poles” of rotation over the last 30 odd million years and how this has affected the types of motion along the boundary region…… http://www.sydneystormcity.com/NZ_ROTAT.BMP

    Significant subduction doesnt really start to occur again to you leave the SW corner of the South Island and head down towards Macquarie Island and along that section subduction tectonics looks like the normal style that we see off the east coast of the North Is. and up to the Fiji area. ( the Hikurangi – Kermadec Trench region)


  24. Dave Nelson says:

    Edit to my first post … typo ……fault from Orita in the nth to Haast in the south….
    Orita should be Otira 🙂

  25. Passerby says:

    Very nice update of your post, Chris!

    From the BBC this morning:
    >…. 430 houses and another 70 buildings had already been earmarked for demolition by assessment teams (so far). About 100,000 of the 160,000 homes in the Christchurch, Selwyn and Waimakariri areas had sustained some damage…

    “It is still possible that we’ll have a magnitude 6 in the next week, and people ought to be aware of that, particularly if they are around structures which are already damaged,” Ken Gledhill of the Institute of Geological and Nuclear Sciences told the NZPA news agency.


  26. Dave Nelson says:

    that news report is a bit old 😉
    over nite there has been a 5.2 and 2 x 5.4 ashks … my son and daughter are not getting much sleep. The nerves are very frayed 🙁


  27. Roger Jordan says:

    What part does the Lyttleton laval cone play in all of this? i.e. is it moving with the surface or is it a fulcrum providing the differential in movement resulting in compression and stress fractures?

  28. Lanthanide says:

    Yeah, we’ve now had 3 aftershocks at 5.4, and two of them were last night. Slightly worrying is that I managed to sleep through one of them without waking up at all.

  29. Lizzie Cook says:

    This is a quirky post about my mudfish (kowaro) in my garden in the centre of Christchurch. I have a knee deep pond in the ground approx. 3m long and 1m wide lined with polythene over a thick paper underpad and over tin sides with bricks in concrete as an edging (now cracked). After the quake it was quarter empty of water with azola (sp?) (nitrogen fixing fern which floats on water) and mudfish thrown out as far as half a meter. When I put the mostly still alive (some were killed by frost) mudfish back in the water they wriggled off to hopeful continuation of life.

  30. Lanthanide says:

    You might be interested to know that there’ve been 3 quakes in the North Island now. GNS Science are saying they’re unrelated to the Canterbury quake except that they’re happening to the same plates.

  31. Lanthanide says:

    New graphic on geonet with beach-balls for the mainshock and the more significant aftershocks:

    • Chris Rowan says:

      Nice! It’s interesting how all of the aftershocks to the east of the mainshock have E-W strike-slip focal mechanisms, while the ones west towards the southern alps have SE-NW compressional focal mechanisms. A definite change in tectonic style there: same stress, different way of accommodating it.

  32. michael says:

    is it possible that, the “claimed” undiscovered fault near at darfield, is due to past fault line between the two volcanoes erupted in akaroa and lyttelton hundered years ago???

  33. i hope n pray that everyone safe and also all pple around the world are worried what is happening in christ church(NZ).leave each n every minute in GODS hand becozs he is the one who going to solve the problem.takecare pple of christ church(NZ).all the best.

  34. bruce stout says:

    To Dave Nelson,

    Thanks Dave! That’s excellent feedback. (and thanks for reading my midnight rambling!!)
    With the “ploughing into the aesthenosphere” bit, I didn’t mean along the middle section of the Alpine Fault but south of Haast (well somewhere between Hasst and Milford Sound) where the fault moves offshore. I always thought there was some sudden transition in this region, with dextral faulting appearing in the Australian plate (striking WNW offshore) and the appearance of deeper (up to 100 km) quakes to the east of the boundary under Fiordland. But, if I understand you correctly, the transition is much more gradual which makes the contrast between the abrupt absence of slippage on AF and the huge amount of activity in Fiordland recently all the more intriguing. In your opinion, does this Darfield quake make movement on the AF more or less likely? Or to put it another way, will this southerly extension of the MFZ change the nature of movement on the AF in the long term?

    • Dave Nelson says:

      its been an enjoyable discussion with great input and questions by various participants (thanks Chris for starting it all )

      Well not too gradual, the transition is occurring over ~ a 300km stretch of the boundary from ~ the Milford Snd area to a bit south of the SW tip of Fiordland.
      That pic in my previous post shows how and where the transition is occurring. Changing from the oblique slip in the cntrl and nthrn Sth Is to the traditional subduction south of the Sth Is.
      I feel that this latest event is probably not doing too much strain relief on the main central section of the AP.( yes a little but not signif.) its probably done more to release strain in the Marlborough FZ. In fact as has been hinted at by others, I am “excited” to think that this could be the next significant splay off the AP., as with the Hope, Awatere and Clarence Faults.
      I feel the same way about all the events from ~ the Arthur’s Pass area and nthwrd.
      Its still that section from sth of Arthur’s Pass to Haast that the stress is building up and no significant continual releases. Off the top of my head there was a ~ M6 at the head of the Godly River (25June1984) and other than that, nothing else to speak of.
      The general consus of opinion is that the avg return time of maj events on the AP is ~ 360 yrs +-40 yrs The last event has been timed at ~ 340 yrs. Its also thought that it rupture in a series of maj hi M7 events over a minute or so. This is a common occurrence in very large ruptures.

      Finally… Something I didnt add to volcanism comments the other nite….
      one of the other features that halts volcanism in and east of Fiordland is the cuz the whole region (include the whole Sth Is) in under compression rather than extension as it was 5 – 10 Ma when the Dunedin Volcano and the Banks Peninsula volcanics. That is, there is no extensional basin area with assoc volcanics, as in the North Island where the subduction tectonics is more conventional.
      ok that will do for this waffle 😉

  35. Jane says:

    Do you agree that there could likely be another earthquake of this magnitude or higher? Due to the aftershocks getting a lot stronger?

    • Dave Nelson says:

      what makes you say they are getting a lot stronger ?
      there ‘s still just lots of 2’s 3’s and 4’s sprinkled with the intermittant 5 and a bit
      the overall frequency/avg of M5.0 – 5.5 hasnt changed significantly

  36. Dave Nelson says:

    quote..Chris Rowan says:
    Nice! It’s interesting how all of the aftershocks to the east of the mainshock have E-W strike-slip focal mechanisms, while the ones west towards the southern alps have SE-NW compressional focal mechanisms. A definite change in tectonic style there: same stress, different way of accommodating it.end quote

    One thing it gives good indication of is that the rupture was unilateral rather than bi-lateral
    ie.in this case its started in the west and movd eastwards. And going by the locations of some of the ongoing a.shks (this morning’s M5.1 under Cashmere) that rupture process is still moving eastwards.
    Those western events are indicitive of the reverse thrust faulting that is found along virtually the whole sthrn 2/3’s of the Sth Is. in the foothills and mountains east of the AP.

  37. Ryan says:

    Guys we should talk more about the next quakes to come around here they have now said
    this is a real problem and we should be ready for another 7+

    but id like to know abit more on when the next will go off weeks, months, years?

    • Dave Nelson says:

      quote”but i’d like to know abit more on when the next will go off weeks, months, years?

      any or all of the above…
      Unfortunately earthquake prediction is far from being an accurate or perfected science and you question just cannot be answered. The 2 things that can be said are….
      1) … the region, from Arthur’s Pass and to the NE tip of the South Is is struck by regular >M6.5 events on the various faults, and the chance of a M7 event within the Marlborough FZ within the next 10-15 yrs is prob, a good guestimate, based on the last 100 yrs of activity ~ 40 – 50%.
      2) … Seeing that prediction is almost impossible, the best thing you can do is be prepared … have a structurally sound home, tie things down, keep supplies of medical, food and other emergemcy items etc and be ready to ride out “the storm”

  38. Ryan says:

    + the after shocks are getting closer to the surface. and thats a bad sign =S

  39. rochelle says:

    hi there has been some worry today about sulphur smell being present what causes this??
    as i no a few ppl who are thinking along the lines of weather the volcano port hills could become active ..

    • Dave Nelson says:

      I guess the posibility and couldnt be totally dismissed, but its highly unlikely considering the tectonic setting of the region.
      the sulphur smells .. interesting … wonder if its gasses being expelled in the “sand volcanoes” etc (the liquifaction process) coming from the very long time buried organic materials in that whole estuary area
      mmmm jst a thought 🙂

    • Passerby says:

      Estuarine sediments covered and entrapped by river silt and sand deposits are old organic-rich, submerged microbial ‘mat’ communities. Within these thick mats below the photic zone, anoxic black ‘ooze’ layers have sulfur-specialist colonies that convert sulfur-containing organic matter into inorganic gases and sulfur precipitates. These old deposits may have been disturbed by fissuring or mud expulsion during the eruption, exposing the sulfur-rich sediments, which then off gas as hydrogen sulfide – it can be detected at pretty low levels by human olfactory receptors.

  40. Chris Rowan says:

    The Lyttelton and Akaroa cones are both extinct, and have not erupted for at least 8 million years or so. So there is no danger there.

    I agree with Dave that it’s far more likely that the sulphur has been released from sediments disturbed by the earthquake – hydrogen sulphide is a common by-product when bacteria consume organic matter in low/no oxygen conditions, and if it was brought to the surface it would be oxidised to sulphur dioxide.

  41. Sam says:

    I heard yesterday that new hot springs have been created on the beach or somewhere at Raupaki near Lyttleton as a result of the earthquakes.

  42. bruce stout says:

    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.
    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?
    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
    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?
    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?

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