4 August 2011

Images of landslides from the Christchurch earthquakes part 3: topographic amplification at the cliff tops

Posted by dr-dave

This is part 3 of this series.  The other posts in this series are as follows:

Part one: boulder damage to a house

Part two: evil dancing rocks

Part three: topographic amplification at the cliff tops

Part four: large-scale cliff collapses

Part five: landslides that involve sliding

One of the key aspects of landslide and rockfall triggering in earthquakes is the process that we term topographic amplification.  The idea is simple, which is that the topography causes enhanced shaking at key locations.  We believe that this is particularly significant at ridge tops and breaks of slope (locations in which the slope gradient changes).  In effect it is a resonance effect that causes the ground to shake much more violently in those locations.  We think that this is a key process in the triggering of landslides and rockfalls in earthquakes.

Of course it is also potentially bad news for any structures located at the top of slopes too, so I was interested to see how the houses that had occupied those beautiful cliff top locations in the hills to the east of Christchurch had performed.  as the pictures below show, the answer is that they did not do well.  It is important to stress that these images show shaking damage, not landslide problems (though there is some of that on the ridge tops too – see the post tomorrow…).

The best illustration of the magnitude of the shaking on the ridge tops was found on this balcony:

If you look carefully you can see that the plant plot has been smashed.  It has clearly been lifted vertically and then dropped, which suggests vertical accelerations considerably in excess of 1 g:

There was huge disruption to even sturdy structures in this area, but similar houses set back from the cliff remained intact.  The tile roof of this houses suggests high levels of acceleration:

Note the cliff collapses.  One or two houses collapsed completely:

Whilst others suffered very signficant levels of damage:

The next post will look at what this meant for the cliffs below.  The other posts in this series are as follows:

Part one: boulder damage to a house

Part two: evil dancing rocks

Part three: topographic amplification at the cliff tops

Part four: large-scale cliff collapses

Part five: landslides that involve sliding