Failure mechanisms of Ana Slide from geotechnical evidence, Eivissa Channel, Western Mediterranean Sea

Lafuerza, S., Sultan, N., Canals, M., Lastras, G., Cattaneo, A., Frigola, J., Costa, S. and Berndt, C. (2012) Failure mechanisms of Ana Slide from geotechnical evidence, Eivissa Channel, Western Mediterranean Sea. Marine Geology, 307-310, 1-21. (doi:10.1016/j.margeo.2012.02.010).

Abstract

This work deals with the failure mechanisms of Ana Slide in the Eivissa Channel, in between the Iberian Peninsula and the Balearic Islands, under the effects of gas charging and seismic loading. In situ geotechnical tests and sediment cores obtained at the eastern Balearic slope of the Eivissa Channel suggest that the basal failure surface (BFS) developed as a result of subtle contrasting hydro-mechanical properties at the boundary between a fine-grained unit (U6) overlying a methane-charged relatively coarser unit (U7). Past methane seepage is inferred from seismic reflection profiles and high magnetic susceptibility values in sediments from the slide headwall area. Past methane charging is also supported by further seismic reflection data and isotopic analyses of benthic foraminifera published separately. The possibility of failure for different critical failure surfaces has been investigated by using the SAMU-3D slope stability model software taking into account the role of free methane in the development of the landslide. Failure would occur after SAMU-3D if the undrained shear strength of units U6 and U7 is strongly degraded (i.e. 95%). Wheeler's theory suggests that a 9% free gas saturation would be required to reduce the undrained shear strength by 95%. However, the theory of the undrained equilibrium behaviour of gassy sediments for this methane concentration shows that the excess fluid pressure generated by gas exsolution, estimated at 12% of the effective stress, is not high enough to bring the slope to fail. This led us to consider seismic loading as an additional potential failure mechanism despite the lack of historical data (including instrumental records) on seismicity in the Balearic Islands, therefore assuming that the historical period is not necessarily representative of seismic activity further back in time (i.e. when Ana Slide occurred ~ 61.5 ka ago). Considering current slope conditions, the most critical failure surface obtained by SAMU-3D relates to peak ground accelerations (PGA) of 0.24 g, which relates to magnitude moment Mw = 5 at epicentral distances of 1 km, and 7 ? Mw ? 5 at epicentral distances ? 15 km to Ana Slide. However, no active faults have been identified at so short distance from Ana Slide. Only when shear strength is degraded due to the presence of free methane in units U6 and U7 is considered, the most critical failure surface obtained by SAMU-3D fits with lower magnitude and larger epicentral distances. Consequently, the most plausible hypothesis to explain the occurrence of Ana Slide is the combination of free gas and seismic loading.

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