IAP2-22-445

What controls post-seismic rock-slope instability?

Earthquakes trigger significant numbers of landslides in mountainous regions. The effects of earthquakes on rates and spatial patterns of landslides persist into the post-seismic phase. Regional rates of landslides remain elevated for many years after a large earthquake, posing ongoing risks to human populations living in these settings and holding significant implications for sediment routing and carbon transport in and from mountain belts.

Forecasting the evolution of the rate and spatial pattern of post-seismic landsliding is critical to risk management efforts, and in modelling the longer-term geomorphic evolution of active orogens. However, conventional regional-scale approaches to assessing rates of landsliding do not differentiate between the relative contributions of different types of landslides and, hence, we lack mechanistic understanding of the causes of ongoing and elevated rates of post-seismic landsliding; this limits our predictive capacity. Recent work (Kincey et al., 2021; 2022) has focussed on the significant contribution made by downslope runout of remobilised regolith as debris flows. Less attention has been given to the rate and style of first-time failures in bedrock that condition both the immediate and longer-term release of debris, and which may define the future evolutionary path of individual landslide footprints (Brain et al., 2022). These characteristics are important to understand because: (a) they control the magnitude and rate of sediment supply from landslides into the wider orogenic sediment cascade; (b) the way in which a rock-slopes fails is likely to affect secondary failure and sediment release; and finally (c) rock-slope failures are known to have significant long-term post-earthquake impacts on key infrastructure and livelihoods in mountainous regions. The aim of this collaborative project between Durham and Newcastle Universities is therefore to constrain the controls on, and rates and spatial patterns of, post-seismic rock-slope failure.

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Image Captions

Coseismic landslides in the Upper Bhote Kosi (2016). [Credit: Nick Rosser]

Methodology

Using Nepal as a case study and focussing on rock-slope behaviour following the 2015 Mw 7.8 Gorkha earthquake, the project will consider nested scales of analysis to address the aim. An indicative scheme of work may include (depending on the preferences and focus of the student):

Spatial analysis of recent multi-temporal landslide inventories mapped following the 2015 Gorkha earthquake (Kincey et al., 2021) will be used to quantify large-scale variability in rates and patterns of change to landslide in rock-slopes across the footprint of the seismically impacted region.

Targeted field surveys using repeat drone-based imagery, terrestrial laser scanning and GPS surveys will be used to validate the results of the remote sensing and to improve site-scale understanding of the processes of landslide evolution, focusing in particular on high-resolution assessment of landslide form, structure, failure mechanisms and rates and patterns of change.

Bespoke laboratory rock-testing methods will be used to simulate and characterise syn- and post-earthquake rock damage and how this evolves in rockslopes of variable configuration in response to differing environmental conditions that may drive incremental weakening of the rocks, and if/how this ultimately results in rockslope failure.

In addition to the scientific value of the project findings, the results will be of direct relevance to a range of partners in Nepal who are tasked with the management of landslide hazards. The student will have the opportunity to spend time on secondment with the National Society for Earthquake Technology (NSET), Kathmandu, Nepal, to understand and develop techniques and to determine how the results can be used effectively in landslide hazard and risk management.

Project Timeline

Year 1

Preparatory research and analysis of multi-temporal landslide inventories to establish broad-scale patterns of landslide evolution.
Selection of focus study sites and planning of initial surveys for first fieldwork season.

Year 2

First fieldwork season to survey key study sites and to collect rock samples for laboratory testing.
Laboratory testing of rock samples and analysis of rates and styles of cracking and failure, determined from strain and acoustic emissions monitoring.
Presentation of results at an international conference (e.g. EGU, AGU).

Year 3

Second fieldwork season in Nepal to re-survey focus study sites, followed by change detection relative to earlier 3D topographic datasets.
Ongoing laboratory testing and development of conceptual and numerical models to describe the processes that govern damage accumulation, healing and failure, the timescales over which they operate, and how they can be used to explain patterns of change observed in the field.
Begin write-up of thesis.

Year 3.5

Completion of thesis write-up and preparation of manuscripts for submission to scientific journals.

Training
& Skills

Training will be provided by supervisors and collaborators in the following:

• Remote sensing/GIS and field survey techniques (drone flying and laser scanning, SfM analysis and change detection); and
• Geotechnical laboratory testing (compressive and tensile strength testing, environmental simulation, acoustic emissions, creep testing).

The student will also have the opportunity to:
• undertake a placement with NSET-Nepal to work on the multi-temporal inventories and drone data.
• attend bespoke training courses, as appropriate. This may include the British Society for Geomorphology’s Windsor Workshop.

References & further reading

Brain, M.J., Moya, S., Kincey, M.E., Tunstall, N., Petley, D.N. and Sepúlveda, S.A., 2021. Controls on Post‐Seismic Landslide Behavior in Brittle Rocks. Journal of Geophysical Research: Earth Surface, 126(9), p.e2021JF006242.

Kincey, M.E., Rosser, N.J., Robinson, T.R., Densmore, A.L., Shrestha, R., Pujara, D.S., Oven, K.J., Williams, J.G. and Swirad, Z.M., 2021. Evolution of coseismic and post‐seismic landsliding after the 2015 Mw 7.8 Gorkha earthquake, Nepal. Journal of Geophysical Research: Earth Surface, 126(3), p.e2020JF005803.

Kincey, M.E., Rosser, N.J., Densmore, A.L., Robinson, T.R., Shrestha, R., Pujara, D.S., Horton, P., Swirad, Z.M., Oven, K.J. and Arrell, K., (In Press), Modelling post‐earthquake cascading hazards: Changing patterns of landslide runout following the 2015 Gorkha earthquake, Nepal. Earth Surface Processes and Landforms.

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