9 December 2016
The landslide distribution from the M=7.8 Gorkha Earthquake in Nepal
In the aftermath of the M=7.8 Gorkha Earthquake in Nepal last May, there has been a sudden and very welcome interest in landslides in that country. A number of groups undertook mapping of the landslide distribution, and the publications are now starting to appear. In a paper just published in Landslides (Martha et al. 2016), a group from the National Remote Sensing Centre and India have analysed an inventory of coseismic landslides triggered by the earthquake. The results are interesting. They have used a range of high resolution satellite instruments to generate a high quality landslide map. India has some wonderful satellites that represent ideal tools for this purpose, although the team have also used a range of other instruments as well. The upshot is probably the best landslide inventory published to date.
The raw statistics are important. Martha et al. (2016) have mapped 15,551 landslides triggered by the M=7.8 Gorkha Earthquake in total, including 213 landslides triggered by the large aftershock in Dolakha. The total volume of the landslides is about 620 million cubic metres. Whilst this number of landslides sounds high, it is probably much lower than we had anticipated for an earthquake of this magnitude in the highly unstable mountains of Nepal. The reasons for this remain unclear.
The most interesting aspect of the paper though is that the spatial correlation between the landslides triggered by the earthquake and the peak ground acceleration is weak. There is a strong correlation with slope angle (this is always the case), but the landslides occurred in a zone to the north of the earthquake affected area that does not correspond with the high peak ground accelerations. This diagram, from the paper, shows this rather nicely:-
Whilst it might be tempting to explain this by the distribution of steep slopes, I do not think that this alone explains the result. Thus, in the case of the M=7.8 Gorkha Earthquake, the landslide distribution is more complex than one might have expected. Martha et al. (2016) make the point that the landslide distribution seems to have been strongly controlled by the behaviour of the fault rupture, and that the spatial termination of the landslides towards the east seems to be controlled by the termination of the rupture event. I think this is spot on. Below is an INSAR map of the tectonic ground deformation caused by the earthquake. You can get an idea of the location in relation to the map above using the location of the epicentre of the mainshock to the west and the M=7.3 Dolakha aftershock to the east (with the cluster of aftershocks that followed that event:
And this is a map of the aftershocks, published in an open access paper by Ichiyanagi et al. (2015). Note that this is from an array in Kathmandu, so the distribution towards the west may not be as complete:
In general the high density of aftershocks lies in the northern area of the fault rupture, as does the landslides. These aftershocks did not trigger the landslides, but the similarity in extent suggests that the dynamics of the behaviour of the fault plays a strong role in controlling the landslide distribution.
The authors of the paper in landslides have made the inventory data available via the Bhuvan tool.
Ichiyanagi, M., Takai, N., Shigefuji, M., Bijukchhen, S., Sasatani, T., Rajaure, S., Dhital, M.R. and Takahashi, H. 2015. Aftershock activity of the 2015 Gorkha, Nepal, earthquake determined using the Kathmandu strong motion seismographic array. Earth, Planets and Space, 68, 25. DOI: 10.1186/s40623-016-0402-8.
Martha, T.R., Roy, P., Mazumdar, R. et al. (2016). Spatial characteristics of landslides triggered by the 2015 Mw 7.8 (Gorkha) and Mw 7.3 (Dolakha) earthquakes in Nepal. Landslides. doi:10.1007/s10346-016-0763-x