All quiet on the Alpine Fault?

A post by Chris RowanResearchBlogging.org The question of whether the Darfield earthquake was a ‘surprise’ depends greatly on how you look at it. The fact that Christchurch was shaken up by an earthquake was not a surprise at all, given that New Zealand is neatly bisected by a plate boundary.

The Pacific-Australian plate boundary on the South Island of New Zealand.

However, the precise location of the earthquake on 3rd September was perhaps a bit more surprising, given that the most obvious manifestation of the plate boundary on the South Island is not on the Canterbury Plains, but in the western foothills of the Southern Alps. Just like the Tetons, the steep western face of this range is effectively the scarp formed by several million years’ worth of motion on the Alpine Fault, and the 20 km or so of vertical motion pales in comparison to the several hundred kilometres of lateral motion between the Australian and Pacific plates that has been accommodated on this structure in the last 30 million years or so.

The southern end of the Alpine Fault. Source: Te Ara

Despite its obvious tectonic significance, the Alpine fault has not ruptured since European settlement in the 1840s. In fact, it’s quite the opposite: a map of all significant earthquakes in the past couple of centuries reveals a notable lack of seismic activity compared to the rest of the plate boundary region.

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

However, GPS measurements of present day deformation show that a large amount of elastic strain is constantly building up on the eastern side of the Alpine Fault, which will eventually have to be released by a rupture of the fault. The only question is, when?

Strain (deformation) rates in New Zealand, from ~10 years of GPS measurements. Source: GNS

The lack of a historical record for the Alpine Fault means that we must instead examine the geological record left by past ruptures. The most direct way to do this is by digging a trench across the fault zone itself, and carbon dating organic rich sediment layers that have been disrupted as the fault breaks through to the surface – a displaced layer provides a maximum age for the earthquake (it must have occurred after the sediment was deposited); an undisturbed layer draping across a scarp provides a minimum age. However, this is not the only effect of a large earthquake in a steep, mountainous region like the Southern Alps. It will also trigger landslides, which not only reshape the landscape in their own right, but dump huge amounts of sediment into the rivers running down from the mountains. As soon as the rivers reach level ground and slow, much of this sediment will be deposited, causing the river to build upwards – a process known as aggradation. But some of the excess sediment does make it all the way to the coast, causing the shoreline to prograde – grow outwards – around the river mouth.

How past earthquakes leave their mark on the landscape and vegetation.

The important thing is that all of these new landforms can be dated. Ages can not only be estimated from carbon dating of the sediments directly beneath aggradation and progradation surfaces, but also by dating the trees growing on top of them. An earthquake on the Alpine Fault sweeps away old forest and creates large tracts of clear ground for new vegetation to spring up upon. Tree ages in this entire region are therefore clustered around the times of major earthquakes. Even the trees that have survived an earthquake will bear the scars of the shaking, in the form of periods of poor growth recorded in their rings.

By combining all of these sources, New Zealand geologists have been able to piece together the timing of earthquakes on the Alpine Fault. A 2007 paper by Andrew Wells and James Goff further refined previous studies by looking in detail at the ages of coastal landforms, and provides the most up to date summary of the paleoseismic record. There appear to have been six distinct episodes of shaking in the last 1000 years, with estimated dates of 1230, 1410 and 1500 (older studies appear to have combined these two into one episode with an estimated date of 1460), 1615, 1717 and 1826. Four of these episodes – 1230, 1410, 1615 and 1717 – have been identified in trenching studies, so are definitely due to ruptures of the Alpine Fault. Furthermore, trench ruptures, landslides, and new fluvial and coastal landforms associated with these earthquakes are found all along the Alpine fault, suggesting that a substantial length – 3 or 4 hundred kilometres – of the whole active structure ruptured in one go, generating a magnitude 8+ earthquake. The other two shaking episodes in 1500 and 1826 only seem to have affected the southern part of the Southern Alps, and no evidence of a rupture has yet been found in trenching studies; this suggests that they may be due to earthquakes on the adjacent Puysegur subduction zone on the Fjordland coast. From these data, the average repeat time of the four confirmed ruptures of the Alpine Fault is around 160 years, and the maximum gap between ruptures is about 200 years. The last confirmed rupture, in 1717, was almost 290 years ago. The math is hardly reassuring, is it?

Past ruptures on the Alpine Fault (red lines) as estimated from paleoseismological studies. Both the average and maximum known recurrence interval were passed in the first half of the 19th century. The orange dashed lines denote events that probably occurred on the subduction zone to the south.

Note that I have refrained from using words like ‘overdue’. Whilst it’s certainly true that we are currently at the wrong end of the longest known period between ruptures on the Alpine Fault, we need to bear in mind that from a geological perspective, our record is still woefully short – there is no way of knowing if the last millennium accurately reflects the long-term seismic behaviour of the fault. There could be periods of activity separated by longer periods of relative quiescence, for example. That said, from what we do know, there will be a large earthquake on the Alpine fault in the not-too-distant geological future, and it would be no surprise if it happened tomorrow. It would also be no particular surprise if it didn’t happen for another 50 years; unlike we impatient humans, a fault would hardly know the difference.

As for the impact of such an event on New Zealand: well, Christchurch would certainly get shaken up, but perhaps not much more than it was a couple of weeks ago, due to the greater distance from the Alpine Fault. Settlements on the west coast like Greymouth and Hokitika would be far more severely affected; roads and railways across and along the Southern Alps could be destroyed; and there must be some risk of damage to some of New Zealand’s many hydroelectric dams, which are a significant contributor to the country’s overall generation capacity. Fortunately, as the past couple of weeks have shown us, the Kiwis are neither ignorant of these risks, nor unprepared for them.

Wells, A., & Goff, J. (2007). Coastal dunes in Westland, New Zealand, provide a record of paleoseismic activity on the Alpine fault Geology, 35 (8) DOI: 10.1130/G23554A.1

Categories: earthquakes, geohazards, geology, geomorphology, tectonics
Tags: , , ,

Comments (21)

  1. bruce stout says:

    “The other side of uh-oh”, (to plagiarize your post) has just been entered into my permanent vocabulary. ;-)

    Great graphics and brilliantly informative yet again. Thanks very much! So glad I stumbled upon this blog.

  2. Kim says:

    Nice.

    The Christchurch EQ made me wonder if the plate boundary is gradually shifting to faults to the east? (The San Andreas is doing something similar – the existing fault isn’t in quite the right place to accommodate the plate motions.) Have you read any of Kevin Furlong’s recent papers? (I haven’t, but I know from short media blurbs that he’s working on the transition from subduction to transform motion on the north end of the South Island.)

    • Chris Rowan says:

      That’s certainly how I’d interpret it. I tried to explain the earthquake in these terms in my initial post the Friday before last, but obviously I wasn’t quite clear enough…

      • Kim says:

        Maybe that was where I heard about it. (Sorry – I’m skimming blogs occasionally right now, and reading very closely, even when posts are very interesting.)

  3. Dan McShane says:

    Great write up. Thanks

  4. bruce stout says:

    Another thing that intrigues me about this region is that if you look at the geological map of NZ
    http://maps.gns.cri.nz/website/geoatlas/viewer.htm
    you can see at least two distinct steps in the original AF up towards the NW (in the Nelson region, too lazy to look for the name of these faults at the moment, sorry!). Moreover, the rocks of the seaward Kaikouras match those of the eastern part of Wairarapa but with a similar sort of offset, i.e. as though the northern part of the South Island has moved NW by about 50 km relative to the rest of the country. Does this have something to do with the southwards migration of the plate boundary?

  5. Brian Sandle says:

    Hi,
    My Christchurch earthquake latitude, longitude and depth 3D representation movie

    Canterbury Public Issues Forum now contains my following file

    Name: Canterbury_quakes_3D.mov
    Tags: “attachment”
    Type: video/quicktime
    Size: 3229KB

    The red is the west 172 degrees, and the green the more east to 173
    degrees range

    At the beginning of the movie the north is towards you, range -43.3 to
    -43.7

    Sorry the depth is upside down. 0 to 30km.

    The co-ords come from Geonet.org.nz. The depths they give also have an
    estimated accuracy attached. So far I have only plotted ones where
    they say the depth error is up to about 1.8 km.

    The program I have used to plot is Graphing Calculator 3D free version
    which requires entering co-ords one by one. Maybe someone else has a
    program which is quicker to use. to see what it looks like with more
    co-ords though lower accuracy. It may be harder for Geonet to get good
    depth accuracy of quakes nearer the surface.

    From the data plotted it seems that the quakes are roughly on several
    lines going down into the earth on various angles, though with some
    clusters about 5 to 7km deep on those lines.

    I think the greywacke bedrock of Canterbury Plains is some 0.5km deep,
    with shigle and 3 to 5 aquifer layers above.

    I would be interested to hear of other 3D studies of aftershocks.

    Brian Sandle

  6. Jason says:

    Informative blog and I’m glad you have remained neutral (apart from the uh-oh comment ;) in presenting these facts, rather than the Armageddon doom and gloom reports about this next big event currently circulating the media, people are already on tender hooks after the 7.1 in Darfield, sure be prepared, but lets not spread further fear and panic than is necessary, if anything the Chch quake will at least open all of our eyes to the very real danger of earthquakes in Aotearoa, BTW the quake hit September 4 not 3.

  7. Andrew Wells says:

    Thank you. I enjoyed reading this article. Great summary!

  8. James Goff says:

    Like Andrew says, thanks you – I really enjoyed reading this article. I stumbled across your blog while searching for something else. Nice summary.

    There is an earlier paper by the way that people might be interested in reading that is a precursor study:

    Goff, J. and McFadgen, B.G. (2002) Seismic driving of nationwide changes in geomorphology and prehistoric settlement – a 15th Century New Zealand example. Quaternary Science Reviews, 21, 2313-2320.
    DOI: 10.1016/S0277-3791(02)00033-1

    There is also another that in its own way is relevant to Christchurch – showing what WILL happen when the Alpine fault goes next time.

    McFadgen, B.G. and Goff, J. (2005) An earth systems approach to understanding the tectonic and cultural landscapes of linked marine embayments: Avon-Heathcote Estuary (Ihutai) and Lake Ellesmere (Waihora), New Zealand. Journal of Quaternary Science, 20, 227-237. DOI: 10.1002/jqs.907

  9. emma says:

    I cant locate that article on the net. Can you email a copy or suggest a way to find it. Many thanks

  10. Barbara says:

    Great article. Thanks so much for sharing it with us. Keep up the good work. It would be great to have a Q and A blog or section.

  11. harley says:

    between the 16 20 march how big will the earthquake be i been told it will be a 9 more do you think its the alpine

  12. Krish Sammy says:

    Great Article!!

  13. Grant says:

    Excellent article. I have to admit the ‘gap’ in earthquakes over the main portion of the Alps has always be curious to me – good to have an account of it.

    BTW, the strain deformation map isn’t showing up for me. (I’m guessing the image has moved at GNS’s end.)

  14. Joanne says:

    My question is, what is being done to prepare the people on the Coast and Buller for this terrible scenario? Are there any plans in place and meetings to inform people of what they must do when the Alpine Fault goes?

  15. Kane says:

    Hey it’s all great but we ( i live in christchurch ) didnt have and earthquakeon the 3rd of september ? not sure where you heard that. it was on the 4th at 4,36 am

  16. Brian Sandle says:

    I know that the destructive effect of a quake of a certain magnitude depends not only depth and distance away. It also depends on ground acceleration and I am not sure how that is related to the magnitude.

    Quake waves take some time to arrive and ones from a long fault will keep arriving for a while owing to the differing distance to the various points on the fault.

    Thew magnitude seems to be the total energy released and on a long fault it would be interesting to know the energy released per second as teh rupture moves along the fault, which might give some indication of acceleration.

    A long duration of shaking may of course be doing little bits of more damage for a longer duration, like several smaller quakes adding.

    The last writer mentioned some towns, and it would be interesting to know an estimate of ground acceleration at them from a long fault rupture.

    Christchurch suffered extreme acceleration from a 6.3 quake quite close and shallow. Magnitude 9 wouild be something like 500 times the energy and though Japan had that recently they may not have suffered the same acceleration in cities as Christchurch?

    Brian

    • Doug says:

      I think the energy released from a 9 as apposed to a 6.3 would be nearer 18000 times as much.
      Cheers Doug

    Links (7)
  1. Pingback: Open Laboratory 2010 – submissions so far | A Blog Around The Clock

  2. Pingback: Open Laboratory 2010 – three weeks to go! | A Blog Around The Clock

  3. Pingback: Open Laboratory 2010 – two weeks to go! | A Blog Around The Clock

  4. Pingback: Open Laboratory 2010 – only eight days till the deadline! | A Blog Around The Clock

  5. Pingback: Open Laboratory 2010 – the final stretch! | A Blog Around The Clock

  6. Pingback: Open Laboratory 2010 – submissions now closed – see all the entries | A Blog Around The Clock

  7. Pingback: After the (blog)storm: following up on the big geological stories of 2010 | Highly Allochthonous