4 November 2015
Hillslope preconditioning is the term we use to describe the possibility that the behaviour of a slope during an event might also be influenced by the legacy of previous trigger events. This idea arises from a damage mechanics approach to landslides, in which the slope evolves to failure through the accumulation of defects – damage – with time. Hence, each successive trigger event causes the slope to degrade until it is ready – preconditioned – to collapse. There are different ways to conceptualise the damage process – for example, it could be that damage occurs through the progressive development of a sliding surface, such that failure occurs in the trigger event that causes it to become complete. Alternatively, damage might accumulate as a degradation and general weakening of the entire rock mass.
The damage approach to landslide failure is appealing in many ways, not least because it fits with observations of slopes prior to failure, which often show large amounts of deformation prior to collapse. This is one of my favourite examples, from Malaysia. Note the extensive damage to the main (rock) slope on the left side of the image:-
A closer look shows the massive displacement on the shear surface and the damage to the rock mass that forms the landslide:
These images were taken in 2006. The slope has yet to collapse.
Whilst hillslope preconditioning is a really interesting idea, there is little quantitative analysis to ascertain whether it is a real factor in slope behaviour. In a paper just published in Earth Surface Dynamics (Parker et al. 2015) – this is an open access paper, so you can download it for free – we have tried to explore this effect by looking at the effects of two earthquakes in New Zealand. These earthquakes occurred in 1929 – the so-called Buller Earthquake – and in 1968 – the Inangahua earthquake. Both triggered large numbers of landslides, which were subsequently mapped, primarily by my good friend Graham Hancox at GNS Science in New Zealand. The beautiful thing about these two events is that they occurred reasonably closely together (the epicentres were 21 km apart), such that there was an area that was affected by both earthquake events, as this image from the map shows:-
This work was led by my then PhD student Robert Parker, who hypothesised that if the hillslope preconditioning idea is correct then in this overlap zone it might be expected that the probability of landslides being triggered by the second – 1968 – earthquake would be higher than in the rest of the area affected by this seismic event. In other words, we think that the 1929 earthquake might have preconditioned the slopes in the overlap zone, making them more susceptible to failure. This should be characterised by an unexpectedly high occurrence of landslides in this zone compared with the remainder of the area affected by the 1968 earthquake.
The idea is simple, but investigating this hillslope preconditioning effect is actually very challenging. We had to use a complex statistical approach – I won’t describe it in detail here, but you can read about it in the paper. Essentially we had to model the effects of time independent factors – slope angle, lithology, etc – and then correct for these factors in the overlap zone. The analysis suggests that once these factors have been controlled the probability of failure in the overlap zone is higher than would be expected, suggesting that another factor – hillslope preconditioning – is having an influence.
In the paper we emphasise that these results are tentative, and that more work is needed. In particular, the dynamics of the interactions between slopes and seismic waves are so complex that it could be that there is another factor, for which we have not controlled, having an influence. Indeed it could be that the results are just random effects. However, the result does seem to be robust and fits with our understanding of slope behaviour.
If this result is correct then it potentially makes the analysis of the potential for slope failure in a future earthquake even more difficult. On the other hand, it might also open up new possibilities for analysing slope response to seismic events, as well as changing the way that we conceptualise the development of failure.
Parker, R.N,., Hancox, G.T., Petley, D.N., Massey, C.I., Densmore, A.L., Rosser, N.J. 2015. Spatial distributions of earthquake-induced landslides and hillslope preconditioning in the northwest South Island, New Zealand. Earth Surface Dynamics, 3 (4), 501-525.