What Lies Beneath the Canterbury Plains? A Fault Revealed

By Jesse Dykstra 07/09/2010 14


This is the second post of a 3-4 part series on the Canterbury Earthquake.

Canterbury Earthquake, Pt II

Source: GNS
Source: GNS

Was this Canterbury’s ‘Big One’?

When I was shaken out of a deep slumber at 4:36 am last Saturday, I couldn’t help but think that ‘this is the big one’.   The intensity of the shaking was certainly more than anything that I have experienced before. As I initially struggled to come to my senses, to even know if I was in the middle of a particularly vivid dream, it seemed that everything that wasn’t nailed down was jumping around. Over the next 20-30 seconds, the violent shaking and crashing escalated until it was nearly deafening.  But not deafening enough to drown out the roar of the wave that was bearing down on us from the west at tremendous speed.  A wave travelling through solid earth. When it arrived, the wave bore the entire house up upon its crest, and dropped us down the other side, as if we were afloat on some tempestuous solid sea. The 107 year old house flexed and groaned as the wave passed, protesting the immense strains that must have tested its solid wood-frame construction. The wave and its accompanying roar sped off towards central Christchurch, and the violent shaking and crashing resumed for a few more seconds.  I don’t know exactly how long the main event lasted for, but at a guess, I would say that it was something like one minute. By then, as the shaking diminished, I knew that it wasn’t an Alpine Fault earthquake, which when it ruptures in the not-too-distant future, will generate strong shaking for several minutes.  I will examine the likely impacts of the future Alpine Fault earthquake, and the implications of Canterbury’s earthquake for planning and preparing for the real ‘Big Event’ in another post in the near future.

A Previously Unknown Hazard Lurks Beneath the Canterbury Plains

Saturday’s magnitude 7.1 earthquake was centered near Kirwee, approximately 44 km west of Christchurch, at a depth of only 10 km. So why didn’t we know that this potentially damaging fault existed beneath the Canterbury plains, quietly building up enormous energy, so close to Christchurch?

The media has been portraying this as a ‘new fault’, which has not ruptured in the past. This is partly true in that this particular section of fault has not ruptured historically (i.e. within the last 200 years), and had not been previously identified.  However, that does not necessarily mean that this fault hasn’t been active in the past.  As noted by Dr. Mark Quigley, professor in Geological Sciences at Canterbury University, and one of the lead investigators on the ongoing assessment of the fault, it is possible that the recurrence interval on this fault is less than a few thousand years.  Indeed, the modern day surface of the Canterbury plains is relatively young, having been steadily built out over the last 16,000 years, as the great braided rivers of Canterbury transport vast quantities of material from the Southern Alps.  The Canterbury plains and the braided rivers that nourish them are, by nature, a very active landscape.  So while this may indeed be a “new fault”, it is also very possible that it has ruptured in the past, but the surficial evidence of that rupture is no longer evident.

Seismically active faults are often initially identified by visual cues.  The presence of a fault may be given away by its surface expression, such as fault traces (visible line of disturbance), or offset/displaced topography (such as river valleys).  However, where these faults occur beneath or within very young and active surfaces, such as the Canterbury plains, their surface expressions may be periodically erased by active surface processes, such as deposition of gravels by braided rivers. Whenever the fault ruptures, its surface expression may be rewritten.  Here is a typical view of the 20+ km surface expression of the fault following Saturday’s earthquake in Canterbury:

 

Source: GNS (text & arrows added)
Source: GNS (text & arrows added)

Geophysical Investigation Methods

Surface expression is not the only way to identify an earthquake fault. Geophysical investigation methods, including seismic refraction surveying, and ground penetrating radar (GPR) can show the subsurface structure of the earth.  The seismic reflection profile shown below suggests a series of faults, many of which are blind (i.e. do not show any surface expression) in part of the northwest Canterbury Plains:

Seismic Reflection Profile, Northwest Canterbury Plains
Seismic Reflection Profile, Northwest Canterbury Plains

 While geophysical survey methods allow scientists to look into the earth, these techniques are time consuming, labour intensive, and require specialized, often expensive equipment. For these reasons, areas of interest (e.g. surface fault traces) are often identified prior to any major geophysical investigation. It would take a tremendous concerted effort and a great deal of resources to undertake a detailed geophysical investigation of a region the size of Canterbury.  Consequently, it is very possible (perhaps even probable), that faults buried beneath the ephemeral surface of the Canterbury Plains may not have been discovered yet.

Faults Beneath The Canterbury Plains Predicted by Scientists?

This is not to say that the presence of such faults hasn’t been predicted by scientists.  An offshore fault system has been identified in Pegasus Bay, and may be related to the relatively well-established Marlborough fault system. The Pegasus Bay faults trend NE-SW, and likely extend under the Canterbury plains.

Source: Walters et al., 2006. NZ Journal of Marine & Freshwater Research, Volume 40, Issue 1
Source: Walters et al., 2006. NZ Journal of Marine & Freshwater Research, Volume 40, Issue 1

As mentioned by professor Quigley on his website, the presence of such hidden, or ‘blind’ faults were predicted by University of Canterbury scientist Dr. Jarg Pettinga and colleagues from GNS and Geotech Consulting Ltd in a report produced for the Canterbury Regional Council in 1998. However, predicting the existence of such faults is only the first step towards identifying their actual locations, and the likely recurrence intervals of associated earthquakes. Perhaps, Saturday’s powerful

The Marlborough Fault System & Pegasus Bay Fault (19)
The Marlborough Fault System & Pegasus Bay Fault (19)

earthquake in Canterbury will provide the impetus to get this important task underway, so that Cantabrians have a better idea of what to expect next time.


14 Responses to “What Lies Beneath the Canterbury Plains? A Fault Revealed”

  • Excellent article and photos Jesse.

    Just a heads up that your link to Dr Mark Quigley’s website doesn’t work as it’s duplicated (needs to be reduced to http://www.drquigs.com).

    Cheers, that should be working now! Jesse

  • Don’t mean to be disrespectful but this piece is hilarious. So, it takes expensive equipment and expensive people to do a full geophysical survey and work-up of Canterbury. But, you have to concede, that rebuilding a City and treating the injured isn’t exactly cheap either. Betcha I could get the time of geologists a lot cheaper than, for example, an orthopaedic surgeon & theatre team rebuilding a crushed leg. People like that use very expensive equipment too….

  • Great piece Jesse 🙂
    Just a response to Miles – absolutely agree. But a key issue behind trying to understand how, and where, and the magnitude of impacts from another earthquake is so that we can understand the risk and try and reduce the vulnerability. Hopefully with that done, we wont need to use the orthopaedic surgeon etc etc.

  • Thanks for your comments Miles. It certainly wasn’t my intention to write a “hilarious” post! Rather, the main point that I had hoped to get across was that while the science community does have the technology and know-how to properly survey the siesmic geology of the Canterbury Plains, the political will and economic resources to do this are not necessarily available. Your point about the “cost” of the disaster is well taken, but I would remind everyone that all the scientific investigation in the world isn’t going to prevent such disasters from happening in the future. Hazardous natural processes will continue to work as they always have. At best, we will be better prepared for the next one.

  • Alan, I would be very surprised if there were any truth to that, but then again, I’m not a city planner. I imagine that it would be more practical to improve the building standards in the CBD, than to try to relocate the whole works.
    Also, if you look at the ECan liquefaction hazard mapping at http://ecan.govt.nz/publications/General/solid-facts-christchurch-liquefaction.pdf
    you will see that the CBD is not actually any more prone to liquefaction than most of the surrounding suburbs. I’m sure that this map will be updated after the latest earthquake, but I wonder, where would you move the CBD to? Certainly not to the East, North, or South! Perhaps it’s actually in about the best place where it is now. In my opinion, the extensive damage in the CBD is more related to the generally older buildings there, and the density of structures, than to location. But I would love to hear from some engineers, planners, etc. on that! Cheers,
    Jesse

  • Very interesting, as was your all-too-short radio interview. I agree that all the exploration of this fault will not stop another such event occurring (which is hard for us to adjust to), but it can only make us more aware, and hopefully prepared to withstand the effects. We will never find anywhere on our planet that is entirely safe, and in NZ? Well I always thought Wellington was the earthquake prone city, Auckland maybe to volcanic action, and other centres would all have some potential hazard. We live on a moving planet and are only now becoming aware of it. Keep up the good work and keep us informed. Thanks.

  • Jesse, what now? You were obviously referring to the previous quake but on Tuesday 22 February 2011 Christchurch has been hit with the ‘big one’ I would imagine. Surely rebuilding on the old site, specially if the ‘heart’ of the city i.e. the cathedral is condemmed is pouring good money after bad?? I agree the CBD shouldnt be relocated to the north, south and cant go east, but what about west? Buy up some farmland. The origins of where the CBD is now was marshland when the first settlers walked over the hill wasnt it? Would love to hear your thoughts on these queries. Cheers Pam

  • I gather that the whole of New Zealand has an earthquake risk with the City of Wellington supposedly at very high risk as it is supposed to located on a major fault line. Surrounding countries and Islands are supposedly also at risk from Tsumanis possibly generated by these quakes.
    The Christchurch Cathedral should be rebuilt as it is part of Christchurch’s hertiage. It could be strenghtened against future earthtquakes.

  • What is your opinion on artificial earthquakes? Do you believe it is possible? Can magnetic waves, directed into the earths ionosphere and reflected back to earth, have an effect on a fault line ready to slip? im not saying it was created, but there are alot of speculations leading to this theory, such as the beaching of 107 pilot whales a few days before the earthquake. Maybe the magnetic waves could be heard by them, and thats what lead them to be stranded. Nowadays, there are such technologies that are being used for military purposes, scientific research, weather manipulation. But is it possible?

  • “I will examine the likely impacts of the future Alpine Fault earthquake, and the implications of Canterbury’s earthquake for planning and preparing for the real “Big Event” in another post in the near future.”

    Did you ever do this? Am interested in your comment “will generate strong shaking for several minutes.”
    Information and discussion about the Alpine Fault seems to have been dropped in the light of the 2010 and 2011 events.

    • Cheers for the nudge Jane. I certainly haven’t forgotten about addressing the future Alpine Fault EQ in this blog, but have left it aside for the time being, for a couple of reasons:
      1) The impacts of the event of 22 February in Canterbury have been all-consuming for many of us. While I very much want to utilize the heightened awareness resulting from the Canterbury events, to increase our collective awareness of Alpine Fault hazards, I am also wary that people just want to get back to “normal”, without earthquakes ruling every moment of our lives. However, things do seem to be quieting down, and the Chch rebuild will (hopefully) start in earnest soon, so I think that it will be a good time to discuss the Alpine Fault in the near future.
      2) I’m writing up my PhD thesis at the moment, and there just isn’t much spare time!
      Your point is a good one, I’ll get onto it soon (probably early in the new year).
      Jesse

    • Just one more quick thing, to clarify “strong shaking for several minutes”.
      The duration of shaking from any earthquake is largely determined by the length of fault rupture. So for the September EQ, which was caused by a fault rupture length in the range of 30-40km, the duration of shaking was less than one minute. The Alpine fault is a very long fault (~400km), which could potentially rupture all at once, or partially. This means that the duration of ground shaking will likely be in the range of 2-4 minutes. The shaking will be most severe near the fault, which is 140km away from Christchurch at it’s closest. The shaking intensity felt in Christchurch will probably be similar to that felt in the September earthquake, but for a significantly longer period of time. Dunedin is further away from the Alpine fault, but other towns such as Queenstown, Wanaka, Franz and Fox, Hokitika, Greymouth and Westport are closer, and will experience higher shaking intensity