Le vendredi 29 janvier 2021, Anne-Sophie MREYEN  présentera l'examen en vue de l’obtention du grade académique de Docteur en Sciences (Collège de doctorat en Géologie) sous la direction de Hans-Balder HAVENITH et Frédéric NGUYEN.

Cette épreuve consistera en la défense publique d’une dissertation intitulée :

« Integrated Structural-Geophysical-Geomechanical Models of Landslides in Seismic Regions».

Abstract

Slopes in hilly or mountainous regions can present complex morphologies due to erosional, gravitational and seismotectonic processes. In seismically active regions, mountain ranges are usually characterised by numerous slope failures in the form of shallow to deep-seated landslides; yet, their origin is often not clearly established and must not necessarily be seismic.

This thesis investigates deep-seated landslides of unknown origin (and age) that hint at a probable co-seismic development in regard to their regional seismotectonic context, site-related structural and geological setting, and distinct geomorphological shape. Such investigations include the reconnaissance of surface as well as subsurface structures with a variety of methods; here, we analyse landslide surfaces with satellite imagery, through geomorphological observations and structural measurements, while multiple geophysical techniques are used to reveal subsurface structures and to infer geotechnical properties of the ground.

The work is focused on two study areas presenting rather ancient slope failures: the seismogenic Hockai Fault Zone in East-Belgium, with the Bévercé and the Pays de Herve landslides, and the seismic Vrancea-Buzau region in the Carpathian Mountains of Romania, with the Eagle's Lake and Balta landslides. A third study area, the Longmenshan Fault Zone in Central China with the Daguangbao and the Qinglincun landslides, is used as reference site to discuss landslides actually triggered by a recent earthquake, i.e. the 2008 Wenchuan event, and highlights mechanisms leading to co-seismic slope failure.

A decisive approach of this work is the integration of the collected data into 3D geomodels of the respective sites; these allow to link the geophysical subsurface information to observed surface structures, as well as to assess landslide dimensions and to estimate the volume of landslide debris still in place. For two study areas, the Bévercé and Balta landslides, we present a numerical back-analysis with distinct element modelling in the 2D and 3D domain, respectively, to further comprehend mechanisms influencing the genesis and development of slope collapses possibly induced by earthquakes. These computations are based on geomodelled pre-failure slope shapes, structural settings derived from field measurements and geotechnical properties inferred from geophysical measurements. For both sites, numerical computations indicate the necessity of ground acceleration to achieve similar failure patterns as observed in their actual post-failure state.

At last, we introduce conclusive key elements that link the observed markers deduced from the field measurements and numerical analyses to distinctive processes that are crucial components to comprehend the development of slope failures in seismic regions.

The larger interest beyond the direct scope of this study is the deeper comprehension of regional seismotectonic processes that were active in the past to allow for better constrained seismic hazard evaluations in the future.

Le Jury sera composé de :

M. A. DEMOULIN (Président), Mmes et MM. C. BOURDEAU (Université Gustave Eiffel), X. FAN (Chengdu University of Technology), T. FERNANDEZ-STEEGER (Technische Universität Berlin), H.B. HAVENITH (Promoteur) (Secrétaire), M. MICU (Institute Geography Bucharest), F. NGUYEN (Co-promoteur).

Conformément aux consignes institutionnelles relatives à l’épidémie du coronavirus, cette défense de thèse se fera en visioconférence via Lifesize.

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