18 September 2010
Yesterday I was exceptionally fortunate to be able to spend the morning looking at the surface expression of the fault responsible for the Darfield earthquake, which hit the Canterbury plains area of New Zealand a fortnight ago. I was kindly guided around by Russ Van Dissen of GNS Science, and the visit was organised by Chris Massey, also of GNS – so many thanks to them. This GNS map shows the surface expression of the fault across the Canterbury Plain:
These pictures will work from east to west.
The east-most expression of the fault occurs close to the town of Rolleston, where a railway track crossed the fault. The rails have been repaired, but the kink in the formerly straight railway track is fairly obvious.
Just around the corner a road crosses the fault as well. Here the damage, again to the formerly straight road, is unrepaired as the fault is expressed as a broad deformation zone accommodating about 60 cm of movement:
If you are struggling to see the feature, remember that the road stretching into the distance (not the bit at the very bottom of the picture) was straight before the earthquake. You should be able to see that from the middle of the image the road has been shifted to the right. In this area there is little evident vertical change, bearing in mind that the surface was not perfectly flat before the earthquake.
A few kilometres to the west there is another formerly-straight road crossing the fault. Here we see a little more lateral movement – actually about a metre. Again, there is no vertical movement. The movement is evident in this image – look at the alignment of the edge of the road:
Lets now jump a few more kilometres to the west, where things start to get really interesting! Another formerly straight road is our indicator of strain – but now it is becoming difficult to believe that this happened in a single 30 second event (I can assure you it did):
Note here the highway people have resurfaced the road, so the original cracks are no longer visible. There is a ditch running down the side of the road that also shows the deformation rather clearly:
The movement here is about 3 metres or so. Again there is little evidence of vertical deformation.
The farmer at this site very kindly allowed us to enter his field (please do not do this without permission), where the array of cracks, and associated deformation, is astonishing. GNS have an aerial view of the field here; below is the ground view:
The movement of the fault is evident in the power lines that cross the fault here. The movement of the fault has put the cables under tension, meaning that the insulators (the pieces that join the cable to the pylon) are no longer hanging vertically:
A few more kilometres to the west and we are into the maximum displacement area. Here we see over 3 metres of horizontal deformation and about 1.5 metre vertically. This is the view from the hanging wall side – the drop down onto the footwall, and the lateral motion should be evident:
The picture below was taken on the footwall side looking back towards the fault – note the horizontal motion (the road has been patched up) and the vertical change. Remember that this was a straight road across a flat plain before the earthquake:
The maximum deformation is recorded a little to the west again, where a road is offset by almost its entire width, indicating movement of about 4 metres. There is some vertical deformation to
o, but this appears to be more of a ground roll than a true vertical movement of the hanging wall block:
Our last site is at the western end of the fault trace. Here the surface expression of the fault is reducing, leaving a small step in the road that is quite hard to see. The best evidence is from the power cables, some of which are now very tight (those on the upper right), whilst others are very slack (those linking to the pole on the far right). Note how the tension cable supporting the pole on the left has also gone loose – this would have been taut before the earthquake:
It is at this point, almost perfectly on the fault, that the highest accelerations were measured.