26 February 2011
A clearer picture is beginning to emerge of landslide losses caused by the Christchurch earthquake this week. Unfortunately, it is now apparent that some of the deaths were caused by rockfalls and that ongoing slope problems are likely to be a legacy of the earthquake in some areas. In terms of loss of life, two people are reported to have been killed by rockfalls triggered by aftershocks as they tried to hike back to their homes in Lyttleton, and at least one person was killed by a rockfall in Sumner.
A particularly badly-affected area appears to be the town of Sumner, where houses are located close to steep slopes in volcanic materials. Several houses were damaged by rockfalls in this area, and Star Canterbury reports that 12 streets in the Redcliffs area have been evacuated as a result of concerns about rockfalls. 3News reports that the concern is that further rockfalls are occurring during aftershocks (this is very possible), but also that there may be victims located under the debris. Probably the best depiction of this problem is this aerial image from MSNBC:
Meanwhile, the threat posed by rockfalls is probably best illustrated by this remarkable story from Lyttleton of a house that was struck by a boulder dislodged from the slope above:
The boulder in question is now located at the bottom of the slope:
Newer buildings in New Zealand are generally timber-framed and quite lightweight, but the level of damage as the boulder has passed through is interesting. A few things to note – first the big gouge in the ground in the first image suggests that the rock was both bouncing and rotating. It is this rotation that allows boulders to build huge amounts of momentum. Second, the boulder was clearly airborne when it passed through the house – note where the gouge stops and then the lack of damage to the floor of the house. And finally, the boulder is heavily weathered (i.e. it is not a fresh detachment from a rockface) and for a large part the weathered surface appears undamaged. Still, bearing in mind that the house was subjected to very intense shaking and the impact of this rock it has not done too badly.
Finally, not really a landslide story of course, but on a related issue, as I mentioned in my earlier post liquefaction has been a very major problem in this earthquake. This image from MSNBC does illustrate remarkably the magnitude of the impacts of this process: