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 (144)

  1. Warwick says:

    A question. Does Banks Peninsula as a plug of igneous rock into the crust move at the same rate with the plate as the surrounding geological strata? Could the Peninsula have a similar effect to say a tree trunk in a flooded river. The water flowing into the trunk (northern Banks Peninsula) rises up (as strata did on the 22 Feb quake centred on the Port Hills) and flows around the trunk with a compensating drop of water (strata) on the downstream side (south side of the Peninsula). Probably a fanciful idea, but as a resident of Akaroa, I have a vested interest as to whether this is a possibility or not.

  2. John Botha says:

    I compare banks peninsula to a nail in the ground that is holding the eastern side of the South Island from sliding along the Alpine fault. The stress has caused a break from the alpine fault to Rolleston (7.3). Followed by a break at the Port Hills (6.30) You will note that the shocks have not moved far out to sea past the Port Hills. If you take a piece of cloth and put you finger on it to hold it and then put your flat hand next to it and pull downwards you will see stress lines going to the top of your finger ( Port Hills) The majority of the shocks are ging to the north not the south of the hills. Sadly I agree there is more to come.

    • Warwick says:

      I find John Botha’s anology of a nail in the ground better than my analogy of Banks Peninsula being a tree trunk resisting the flow of water in a flooded river. It does seem to appear that the northeastern part of the Peninsula (near to the Port Hills, Lyttleton) could be acting as a pivot against which the southward movement of strata is oscillating. The pressure at this pivot could have been increased by the by the September 4 2010 quake centred near to Greendale leading to the Feb 22 2011 quake at the pivot. The thrust of the first quake has been mainly been shown as horizontal, whilst a feature of the Feb 22 quake seems to have been vertical movement. Thus the idea that the igneous core of the Peninsula may have acted as a barrier to the southward movement of strata compensated by upward movement of strata. I would be interested to see a 3D depiction of how the Peninsula relates to the Earth’s crust in relation to surrounding strata.

  3. Jeff Barnfield says:

    Hi Since the September earthquake, I have this theory based on the evidence shown on the Christchurch Earthquake Map website.That the faults are not singular in form but are spread out in forks near and under Christchurch much like a braided river i.e.( Like the Waimak river).
    That the faults venture out as far as a deep trench which quite some distance off of South New Brighton beach.Unfortunately the 6.3 February quake occurance was inevitable.I base this evidence on data from overseas Quakes that whenever (over 90% of the time) there is a large quake, say like a 7.4 then the next quakes or aftershocks should be in the 6 magnitude range, then slowly subsiding into the lower ranges.
    In the September quake we went straight from a 7+ to 5+ quakes which is a bit unusual to say the least.The February earthquake has performed more normally with 5+ aftershocks. Although I in no way want another large earthquake to occurr in Christchurch, I have this uneasy feeling that we have not seen the last of them as the South Island progresses on its travel very slowly northwards.

  4. dave nelson says:

    [quote] Although I in no way want another large earthquake to occurr in Christchurch, I have this uneasy feeling that we have not seen the last of them as the South Island progresses on its travel very slowly northwards.[/quote]
    except that most of the South Is is moving towards the SW not the north :)

    Dave

    • jeff barnfield says:

      hi Dave
      I have never heard of any evidence that the South Island is moving in a South West direction.Those scholars of some note are always claiming that a large part of the South Island is moving north at about 35 cm per year, that in several million plus years to come that the South Island will in fact break in two along the alpine fault????Perhaps one side is moving SW as you say, the other side heading NW?
      Jeff.

  5. jeff barnfield says:

    Looking at the maps above they show that perhaps the eastern side of the South Island is migrating south or to be more precise to the South East? All theory of course as none of us alive today will ever live long enough to see what really happens!
    Jeff.

  6. rust says:

    what up

  7. rust says:

    floppy

  8. dave nelson says:

    yes look at the map at the top of the page look at the arrows on ether side of the Alpine Fault, the majority of the Sth Is is moving in a roughly SW direction relative to the think slip of the west coast and Nelson Province.

    I was originally pointing out the inaccuracies by the OP concerning his overall Christurch region and northwards bound comments ;)
    CHCH and everything aroun d that region is principally (sp?) moving to the ~ SW relative to the West Coast.
    cheers
    Dave

  9. bruce stout says:

    oh, this discussion is still going.. nice to see.
    Yep, I’m with Dave, Banks Peninsula is not moving relative to the surrounding greywacke bedrock. In fact, doesn’t it more or less sit on top of the greywacke bedrock, drifting along for the ride so to speak?

  10. Warwick says:

    After yesterday’s 5.6 and 6.3 Richter shakes centered near to Lyttelton, an expert geoscientist made the comment that the events were related to the interface of the igneous rock of Banks Peninsula with strata moving southwards (related to plate movements associated with the Alpine fault – see original article). This relates to the question I asked in comment posted 30 March and comments that follows. More specific information about this possible interface interaction would be of interest particularly in relation to the original article by Chris Town et al.

  11. dave nelson says:

    hi Warwick,

    M5.5 and M6.0 actually according to the GNS

    have you got a link to that article ?

    cheers
    Dave

  12. George says:

    lol @ Dave – Upgraded to M5.7 & M6.3 actually :) after reviewing all seismic stations. Sensors were going off blue (MMI=4) in Wellington and Southland on the Geonet website.

  13. Warwick says:

    Hi Dave, As said by George – 5.7 and 6.3

  14. Dave Nelson says:

    flaming GNS I get really annoyed at their inconsistancy !!!

    the main pages are still showing 5.5 and 6.0, its only when you delve deeper that you see 5.6 and 6.3 !!
    Its really annoying when they only update part of the site, cuz after you posted, Warwick,
    I even checked before I responded and still saw the 5.5 and 6.0 as is still on the main page. and only now after going deeper than that incorrect front page you see the 5.6 and 6.3 :(
    For a professional organisation its not a good look.
    The other thing they have still yet to explain to me is why there are 2 distinct locations published for the M7.1 last Sept. and quite some distance apart. And not only that but they are quite some distance from the fault rupture. The only way you can correlate the rupture with the epicentre is if the rupture was dipping very shallow to the north. I have not see (so far) any published evidence to support that.

    cheers
    Dave

    • Chris Rowan says:

      Actually, the confusion is because GNS is using the moment magnitude scale, whereas if you go to the Geonet pages I link to above, they’re reporting the Richter magnitude (which is calculated differently) AND the moment magnitude. There’s been some success in recent years in retiring the less useful Richter scale, but there are still some holdouts, it seems…

  15. Dave Nelson says:

    Did some site surveying of the rupture on my recent visit to CHCH area (early May)
    The largest fresh rupture I have so far seen in my travels. It was amazing to see the 3.5 – 4 metre offsets across several roads. And the still visible rupture going through farmland etc. Managed to trace it for a number of km’s. The farmers were very kind in letting me wander up and down access roads and paddocks.

    Dave

  16. Lava Flow says:

    Your podcast link is broke.

    Interesting data and analysis.

    Thanks!

  17. Chris Rowan says:

    For those following this comment thread: the most recent seismic activity is discussed in my latest post. I’d be interested in your thoughts.

  18. John says:

    There have been plenty of denials by experts that these earthquake vents have anything remotely to do with our Banks Peninsula volcanoes, but I have a theory that needs to be shot down in flames before it becomes dangerous…
    The Lyttelton volcano was created, as an example of intraplate volcanism, possibly by a thin spot in the Pacific tectonic plate moving over a fixed mantle plume, which then broke through to the surface. As the plate continued its NW movement, the weak spot continued past the plume and the Lyttelton volcano went extinct. The plume found another weak spot some millions of years later, at what is now Akaroa Harbour, SE of the Lyttelton event. It too went extinct after its weak spot continued past the plume. The plume may be finding another weak spot, some distance SE of Akaroa Harbour, extruding its magma up into the weak spot, beginning to form a volcanic hotspot and a possible consequent submarine volcano, such as occurs in the Hawaiian volcanic island chain.
    I’d guess that such an event in that location would result, in its earliest stages, in phenomena noticed in our earthquake events, including (1) the earliest earthquake events happening at the furthest distance from the hotspot (cracking of existing faults, leading to maybe the formation of new faults closer to the hotspot), (2) the raising of the surface closest to the hotspot (the Port Hills were reported as being 40 cm higher than pre-earthquakes, and the southern side of the Avon/Heathcote estuary fault is higher now that before). Also, since the 13 June quake events, there is a trending of subsequent quake locations in a line away from Christchurch, in a SE direction, towards the eastern-most tip of Banks Peninsula. A line continued out to sea in that direction, would reach some interesting undersea formations that show on Google Maps at the expected distance.
    Any and all attacks of this idea here would be much appreciated!

  19. Dave Nelson says:

    The volcanism of the eastern South Island, Primarily Banks Penin, Timaru, Oamaru areas and the “Dunedin Volcano” A quite complex multi stage system. There were all created when The South Island was undergoing extensional tectonics. Thisended ~ 8 million years ago, when the tectonic movement reversed and it became compressional, as it still is today, pushing up the Southern Alps, Kaikoura Ranges etc.

    It was the extensional tectonics that thinned the crust allowing the volcanism to occur, rather than “hot spot” volcanics. :)

    cheers
    Dave

    • John says:

      Thanks Dave – that will be a relief to many that all those coincidences I listed are simply that, and there will be no smoking object appearing out there. Though it would be a bonanza for our struggling tourist industry! :)

  20. Aparecida says:

    Hmmm .. interesting…. and like the tar baby, Chris aint sayin nothin…

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