Failure mechanisms of Ana Slide from geotechnical evidence, Eivissa Channel, Western Mediterranean Sea S. Lafuerza a, b, ⁎, N. Sultan a , M. Canals b , G. Lastras b , A. Cattaneo a , J. Frigola b , S. Costa b , C. Berndt c, d a IFREMER, Géosciences Marines, BP 70, 29280 Plouzané, France b GRC Geociències Marines, Departament d'Estratigrafia, Paleontologia i Geociències Marines, Universitat de Barcelona, Martí i Franquès s/n, 08028 Barcelona, Spain c Leibniz-Institute for Marine Sciences (IFM-GEOMAR), Wischhofstr. 1-3, 24148 Kiel, Germany d National Oceanography Centre, European Way, Southampton, SO14 3ZH, UK abstract article info Article history: Received 20 August 2010 Received in revised form 13 February 2012 Accepted 21 February 2012 Available online 3 March 2012 Communicated by D.J.W. Piper Keywords: Eivissa Channel Ana Slide methane seepage seismic loading undrained shear strength in situ geotechnical measurements 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 crit- ical 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 epi- central distances. Consequently, the most plausible hypothesis to explain the occurrence of Ana Slide is the combination of free gas and seismic loading. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Ana Slide covers 6 km 2 and is located on the eastern Balearic flank of Eivissa Channel (Fig. 1a). This small landslide is aligned with three other landslides (Joan, Nuna and Jersi, Fig. 1b) along the 0°48′E meridian at water depths ranging from 600 m depth at the rim of the shallowest headscarp (Joan Slide) down to 900 m at the lower edge of the deepest landslide toe (Jersi Slide) (Lastras et al., 2004, 2007). The current seafloor expression of Ana Slide ranges from 635 m to 790 m of water depth, with an average slope of 1.6° (Lastras et al., 2004)(Fig. 1c). Previous work based on multibeam ba- thymetry, backscatter and high-resolution seismic reflection profiles indicated that modest mass transfer accompanied the downslope propagation of the deformation front of Ana Slide, which extensively remoulded the underlying slope sediments without necessarily displacing them too far downslope. Only a slight displacement is ob- served for most of the sediment in the central and lower sections of the landslide (Lastras et al., 2004). Extensional ridges in the headwall area of Ana Slide correspond to detached subvertical slabs of partially Marine Geology 307–310 (2012) 1–21 ⁎ Corresponding author at: IPGP, Géologie de Systèmes Volcaniques, 1 Jussieu, 75005 Paris, France. E-mail addresses: sara.lafuerz@gmail.com (S. Lafuerza), Nabil.Sultan@ifremer.fr (N. Sultan), miquelcanals@ub.edu (M. Canals), glastras@ub.edu (G. Lastras), Antonio.Cattaneo@ifremer.fr (A. Cattaneo), jfrigola@ub.edu (J. Frigola), sergiocosta@ub.edu (S. Costa), cberndt@ifm-geomar.de (C. Berndt). 0025-3227/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2012.02.010 Contents lists available at SciVerse ScienceDirect Marine Geology journal homepage: www.elsevier.com/locate/margeo