On The Role of Volcanic Material in Submarine Landslide Initiation Processes

Along active continental margins earthquake shaking was singled out as one of the most important triggering mechanisms for submarine landslides, which may trigger a tsunami wave and have tremendous impact on coastal communities and infrastructure. It is known that seismic loading may lead to sudden strength loss in loosely deposited granular soils such as sands and silts due to liquefaction. Volcanic arcs provide a source of granular sediment along active margins through ejection of volcaniclastica. Volcanic ash is of sand to silt size, such that it is presumed to act preferentially as gliding plane in submarine landslides. However, volcanic ash differs significantly in its geotechnical properties compared to commonly studied sands and silts. This dissertation tackles the question to what extent physical properties of volcanic sands and silts affect their response to gravitational and seismic loading in comparison to common sands and silts. Advanced laboratory shear experiments (direct shear and triaxial shear) were conducted with the purpose to investigate the shear behavior of volcanic material. In generic laboratory studies it is shown that properties like angularity, roughness and particle strength (i.e. crushability) play a key role in sediment stability and majorly determine the shear behavior of volcanic ash at effective stress < 0.5 MPa. Furthermore, a case study was conducted on a subaqueous landslide located in the earthquake prone area of South-Central Chile. Recovered sediment cores show volcanic fall-out ash a few mm from the basal shear plane of a landslide that occurred ~ 6 k years ago. In a highly detailed analysis that gathers seismic profile data with CPT data and advanced geotechnical laboratory shear experiments, it is shown that the sole presence of volcanic ash was not sufficient to result in liquefaction failure due to earthquake shaking.