6 November 2014
Earthquakes and landslides in Peru
Peru is a country that has an abundance of landslides, resulting from a combination of high rates of tectonic uplift (which creates mountains that can be eroded), frequent earthquakes and regular intense rainfall events (often linked to El Nino episodes). It has been known for a while that the rate of landsliding in Peru has changed dramatically in the past, and it has been widely hypothesised that this might have been linked to periods of more intense precipitation associated with phases in which El Nino events were larger and/or more frequent. Clearly this is interesting not just from a scientific perspective but also because it might provide insights into the role if future climate change in generating landslide hazards.
In a paper just published in Nature Geoscience, McPhilips et al. (2014) have explored this hypothesis by examining cobbles located in the Quebrada Veladera river channel and an associated fill terrace. The have used 10Be concentrations in individual cobbles to determine erosion rates for the catchment. The modern cobbles in the river channel give an indication of the current erosion rate, which is occurring when the climate is very arid (less than 200 mm rainfall per year on average, whereas the terrace is dated from 16,000 years BP, when the climate was sufficiently wet to maintain a 60,000 square kilometre lake on the altiplano.
The results are quite surprising. In the words of the abstract:
The distribution of 10Be concentrations in terrace cobbles produced during the relatively wet climate before about 16,000 years ago is indistinguishable from the distribution in river channel cobbles produced during the drier climate of the past few thousand years. This suggests that the amount of erosion from landslides has not changed in response to climatic changes.
In other words, it does not appear to be the case that during this phase of higher rainfall in Peru the occurrence of landslides increased. This is counter-intuitive in many ways, and it is not at all clear to me as to why this should be the case. The authors hypothesise that the area might be so arid that even wet phases do not generate sufficiently high pore pressures to trigger landslides, but this does not seem to agree with what our modern day obserations . If the paper is right, then an alternative trigger must be responsible for the landslides that are recorded in the sediments in the river channels. McPhilips et al. (2014) suggest that these might have been triggered by large earthquakes. It has been hypothesised previously that in very dry environments with high uplift rates earthquakes must do most of the work to trigger landslides, whereas in equivalent wet climates rainfall may be the primary factor. This is one of the first studies to provide field data in support of this idea.
The implication of course is that a large earthquake in this area would be devastating in terms of landslides, and the 2010 earthquake event may support this idea. Given that the return period for great earthquakes in Peru is about 100 years, this is a sobering thought.