The South Island of New Zealand has just been shaken by a large earthquake, reported as a magnitude 7.0 by the USGS. It was 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:
New Zealand geologists have already identified a 13km fault trace with 3-4 m of right lateral, strike-slip offset, and variable vertical movement of up to 1 m. Follow the link for more photos.
There are indications that there may have been multiple ruptures during Friday’s event, which could either be due to sequential failure of adjacent strands of the same fault, or possibly instantaneous triggering of nearby faults. The linked article is a bit confused, but seems to be suggesting there was a magnitude 5.8 foreshock, followed within seconds by a main shock with 2 distinct pulses. It will probably take a while to definitively untangle the sequence of events.
GeoNet’s shakemap also has access to instrumental readings of the shaking (via the pull-down in the top right-hand corner).
Over on sciblogs.co.nz, Chris McDowell has produced a cool Google Maps visualisation of the last few days’ seismic activity around Christchurch, which has also been turned into an animated gif (source)
Mainshock and aftershock sequence
A summary of the earthquake by another NZ science blogger, David Winter, included a nice explanation of how shaking and liquefaction of unconsolidated sediments can produce sand and mud volcanoes, which seem to have popped up on streets and in gardens all over Canterbury.
Photo by Jon Sullivan, on Flickr
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.
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.
Magnitude 6+ earthquakes in New Zealand since 1843. Source: BGS
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.
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
Christchurch, the largest city on the South Island of New Zealand, was once again shaken by a large earthquake. The USGS page reports it as a magnitude 6.3, with the rupture occurring just 5 km beneath the surface near the port of Lytellton, only a few kilometres south of Christchurch itself. This is significantly closer that September’s magnitude 7.0 earthquake, which was 45 km to the west; because the energy of seismic waves spreads out and dissipates the further away you are from the rupture point, the shaking experienced in Christchurch today was probably just as, if not more severe, than that experienced in September, even though the quake was smaller in magnitude. The proximity of the rupture, combined with the fact that many buildings in Christchurch had unrepaired damage from September’s earthquake, the timing (in the middle of the day rather than the middle of the night) and the ever-looming spectre of liquefaction, which severely magnifies the effects of shaking, have sadly resulted in collapsed buildings, and at least some casualties. When it comes to the impact on people and infrastructure, earthquake magnitude is only part of the story.
Location and focal mechanism of the Feb 21 2011 M6.3 earthquake near Christchurch. The location and focal mechanism of the September 2010 M7 earthquake are also plotted for reference. Orange dots are >M5.5 aftershocks of today's quake.
The focal mechanism for this earthquake plotted in the figure above, courtesy of the USGS, shows that it is transpressional – a combination of mostly east-west compression, with some right-lateral strike slip motion mixed in – and on a north-south trending fault [update: what I really mean here is more N-S trending than the Darfield fault; as Kim points out in the comments, if my interpretation above is right the actual fault plane is NE-SW oriented]. Superficially, this seems very different from September’s earthquake, which consisted of mainly right lateral motion on an east-west trending fault. However, strike slip on an east-west trending fault and compression on a north-south trending fault are in fact fairly equivalent in tectonic terms – they can be produced by pretty much the same regional tectonic forces. The transpressional deformation in today’s earthquake is fairly consistent with the overall sense of motion across the plate boundary that bisects New Zealand.
Location of Christchurch earthquakes in relation to the plate boundary running through New Zealand.
The other thing worth noting is that today’s rupture occurred in a region of crust that, according to modelling, saw a significant stress change as a result of last September’s earthquake. This seems unlikely to be a coincidence. We’re looking at a grey area between an ‘aftershock’ and a ‘triggered earthquake’, in that the Darfield earthquake probably helped to push the fault that ruptured today over the threshold, but that most of the stress released in this earthquake has been building up since long before six months ago.
Aftershocks and changes in crustal stress due to the Darfield Earthquake in September 2010. Source: Stuff.co.nz
What does this mean for the seismic risks for the residents of Christchurch in the days and months ahead? Well, there are going to be more aftershocks, more than there would have been otherwise. Beyond that, I’m afraid to speculate: I can only hope that there aren’t any more nasty seismic surprises lying in wait beneath the Canterbury Plains, and that Christchurch and New Zealand continue to show their characteristic resilience in the face of this latest disaster. I’ll update this post as necessary, as more concrete information comes in: please feel free to add any relevant links and information in the comments. Update: 22 Feb 2011 Here’s the shaking recorded by a seismogram close to Wellington, on the Southern North Island, via Shaking Earth:
Click for a larger version. Source: Shaking Earth
From Geonet, you can view a map of reported shaking intensity, coded according to the Modified Mercalli Scale:
Reported shaking from the 21 February Earthquake, according the Mercalii Intensity Scale: 8 - orange; 7 - light orange; 6 yellow; 5 - green; 4 - blue. Source: Geonet
Note how the maximum values are clustered in Christchurch, close to the rupture, and fall away fairly quickly outside it. This contrasts with the shakemap for September, where intense shaking was felt across a much wider region. This shows that yesterday’s magnitude 6.3 quake released much less energy in total than September’s magnitude 7, but due to its location the energy it did release was focussed on a built-up area.
Shakemap for the September 2010 M7 Earthquake. Colours as above. Source: Geonet
There are lots of photos coming out of the damage in Christchurch, but this video shot from a helicopter provides a good overview. Obviously some buildings have collapsed completely, but it should be noted that many more structures have remained standing (although many of those will probably be in need of extensive repairs). It is cold comfort to those who have been trapped or injured, or the friends and families of the several hundred casualties, but New Zealand’s stringent building codes have probably once more saved many lives. At the end of the video I linked to above, there are also some shots of extensive liquefaction caused by the shaking, which probably had a strong influence in the distribution and magnitude of the damage.
Water forced to the surface by liquefaction. Source: TVNZ
Update: 23 Feb 2011 New Zealand’s geologists have once again been doing an excellent job of explaining this earthquake, and the risks going forward, to the media, and through them, the Kiwi public.
There have also been some compelling, often harrowing eyewitness accounts of the earthquake and it’s aftermath:
The racing editor of the NZ Herald took ‘a walk through sorrow’ in the centre of Christchurch the evening after the earthquake hit:
Everywhere I look buildings I have dined in with friends, bars I have visited, banks and shops I have been to are ruined. Not damaged, ruined.
A resident of Lyttelton, which was even closer to the epicentre of the quake than Christchurch, in an interview with the Australian Broadcasting Corporation yesterday:
really 80 per cent of the township, if you like, the heart of Lyttelton, I would say is lying in little pieces. Now you’re not talking everything levelled to the ground, but it’s parts of buildings fallen off into the streets. And it’s not just one, it’s every second or third building, you look at it and go, “Well that’s a write off. No business can operate there.”
A journalist for the Christchurch paper the Pres, whose headquarters close to the Cathedral was heavily damaged in the earthquake:
Outside the inner CBD looked like a war zone. Outside on the street strangers were holding each other and crying and gazing bewildered at the gutted ghetto surrounding us.
(she also describes how one of the many areas overwhelmed by liquefaction “looks like Rotorua“)
Some more photos and videos:
Also worth reading is Dave Petley’s analysis of the reasons why the damage to Christchurch was much more severe than that caused by last September’s larger earthquake. Fortunately, it seems that New Zealand’s Earthquake Commission can cover the costs of further rebuilding. Update: 24 Feb 2011 GNS have posted another nice video explaining the different types of seismic waves generated by the earthquake. Part way through, there is a plot of the aftershocks that have continued to rattle Christchurch in the past few days (red dots in the screenshot below – green dots are aftershocks of September’s quake). Most of them are found along a northeast-southwest trending line that probably represents the trace of the fault.
Aftershocks of the magnitude 6.3 earthquake (red) and suggested trace of the rupture (dashed yellow line). Source: GNS
This article in the New Zealand Herald raises the interesting possibility that geological structures in the region may have acted as a ‘seismic lens’, focussing the seismic energy released in the earthquake towards Christchurch. My latest post explains this concept in a bit more detail. For those involved in the assessment and the communication of seismic hazards, one of the hard lessons coming out of both of the Christchurch earthquakes is that you can’t just focus entirely on the ‘Big Ones’ at large plate boundary faults. Smaller earthquakes on lesser-known or totally unknown faults near a plate boundary zone can be just as dangerous if they run near to, or even underneath a city. The northwest USA is one place where these risks need to be taken seriously: the Cascadia subduction zone poses a major regional seismic (and tsunami) threat, but cities like Portland may also face more local earthquake hazards, as this excellent article points out. Update: 26 Feb 2011 ABC News in Australia has posted some striking before and after satellite photos of some of the more heavily damaged areas of Christchurch. I’m especially struck by the amount of debris that has been thrown into the streets even from buildings that are not obviously damaged from above (which doesn’t mean that they’re not) – this is probably mainly from collapsed brick facades. Early estimates suggest that up to a third of buildings in central Christchurch may need to be demolished and rebuilt. Here’s a map of Christchurch showing the areas that suffered from liquefaction: the northeast of the city seems to have been particularly badly affected. Thanks to all-geo.org for this awesome information