31 August 2017
The Maca landslide: a large, slow-moving slide in Peru
In an excellent article in Nature this week (Palmer 2017) discussing the importance of understanding slow-moving landslides, Jane Palmer features the Maca landslide in the upper Colca Valley in southern Peru. This landslide was also described in an article (Zerathe et al. 2016) published a year ago. This is a very complex landslide system, with components on both sides of the river. The image below shows one element – a large, active slide around about 1000 metres long on the northern side of the river:
Whilst on the other side of the river is a second broad component of the Maca landslide complex:
These images show the scale of the problem at Maca – Zerathe et al. (2016) calculated that the landslide complex has a total width of about 2.7 km, a length of about 1 km and affects an area of 1.7 km². The total volume is about 60 million m³. The morphology of the landslide is very complex, with multiple blocks bounded by active scarps (in effect faults), leading to patterns of movement that are similarly complex. This mirrors the findings of our studies of the Utiku (Massey et al. 2013) and Taihape (Massey et al. 2016) landslides in New Zealand. This complex topography is rather nicely shown in the following Panoramio photo by Daniel Horns:
Zerathe et al. (2016) looked at the movement of the landslide on both long (decadal) and short (interannual) scales. They found that:
This study reveals three main driving factors acting at different timescales: (i) over several decades, the river course has significantly changed, causing the Maca landslide reactivation in the 1980s due to the erosion of its toe; (ii) at the year scale, a minimum amount of rainfall is required to trigger the motion and this amount controls the landslide velocity; (iii) transient changes in slide velocity may occur anytime due to earthquakes.
This is a pattern that we often see. Over longer timescales the landslide responds to changes to the overall geomorphic system, going through periods of comparatively rapid motion and periods of quiescence. In those movement periods the landslide responds to rainfall inputs, but in a highly non-linear manner because the rate of motion is so sensitive to groundwater level in which, beyond the threshold at which movement starts, small increases in groundwater level (pore water pressure) cause large increases in landslide velocity. And then of course in a tectonically-active area such as this, there is the unpredictable impact of earthquakes, although even in this case the amount of movement is probably also controlled by the groundwater level at the time of shaking.
The impact of this slow-moving landslide is clearly visible: in recent years, it has destroyed a section of the region’s main road and torn apart farmland, threatening the community’s key source of income. What is not clear is the landslide’s future: whether it will continue to lurch along as it always has or speed up dramatically, potentially endangering lives. “It’s like a sword of Damocles hanging over the town,” says Pascal Lacroix, a geoscientist at the Institute for Earth Science in Grenoble, France.
Massey, C.I., Petley, D.N., McSaveney, M.J. and Archibald, G. 2016. Basal sliding and plastic deformation of a slow, reactivated landslide in New Zealand. Engineering Geology, 208, 11-28.
Massey, C., Petley, D.N. and McSaveney, M. 2013. Patterns of movement in reactivated landslides. Engineering Geology 159, 1-19.
Palmer, J. 2017. Creeping earth could hold secret to deadly landslides. Nature, 548, 384–386.
Zerathe, S., Lacroix, P., Jongmans, D., Marino, J., Taipe, E., Wathelet, M., Pari, W., Smoll, L., Norabuena, E., Guillier, B., Tatard, L. 2016. Morphology, structure and kinematics of a rainfall controlled slow-moving Andean landslide, Peru. Earth Surface Processes and Landforms 41 (11), 1477-93.