Insights into the edifice stability of oceanic volcanoes from direct shear experiments and numerical models

Volcanoes are mainly related to plate boundaries but can also be found at intraplate settings related to mantle hot spots. Moreover, volcanoes can be located on land or in the ocean, i.e., coastal and ocean island volcanoes (oceanic volcanoes). Beyond the primary hazards commonly associated with volcanic activity, oceanic volcanoes pose the additional risk of secondary hazards such as tsunamis, which are frequently triggered by flank or sector collapses. This thesis aims to contribute to the understanding of the processes and conditions related to flank and sector collapses at oceanic volcanoes and thus, to help to better assess the associated risks in the future. A key parameter controlling the stability of the volcanic edifice is the frictional properties (cohesion, peak friction, residual friction, rate-and-state friction) of the rocks that built the volcanic edifice. Direct shear experiments were carried out on samples from two case-study volcanoes Anak Krakatau (Sunda Strait, Indonesia) and Kilauea volcano (Hawai’i, USA). The experimental results were used to inform finite-element models representing the case-study volcanoes to get a better understanding of edifice stability, evaluate the influence of internal structures on edifice stability and evaluate the stress field within the edifice regarding edifice deformation. It is shown that volcanic rocks exhibit high friction (µ ≥ 0.6). The composition, minerology and rate-and-state friction however are highly variable between samples from the same and different volcanic edifices. Thus, whether a rock and subsequently a volcanic edifice might fail catastrophically highly depends on the individual rocks in which the failure surface forms. Using finite-element models it can be shown that the observed deformation pattern from geodetic measurements can give a first insight into the presence of major internal structures. Hence, it is evident that the internal structures greatly influence the deformation of the volcanic edifice and knowledge of the lithologies and internal structures is important when studying edifice stability. However, if knowledge on lithologies and internal structures is limited, numerical models can be used to get a first insight into the expected deformation behaviors in different parts of the edifice.