Quantifying the role of sandy–silty sediments in generating slope failures during earthquakes: example from the Algerian margin

Abstract

Abstract:The Algerian margin is a seismically active region, where during the last century, several large magnitude earthquakes took place. This study combines geotechnical and sedimentological data with numerical modelling to quantitatively assess the present-day slope stability of the Algerian margin. Geotechnical laboratory tests, such as cyclic triaxial tests, oedometric tests and vane shear tests were carried out on sediment cores collected on the study area. The liquefaction potential of a sediment column located about 30 km from the Boumerdès earthquake epicentre of 21st May 2003 was evaluated theoretically for an earthquake of M w = 6.8. We show that thin sand and silt beds such as those described on recovered sediment cores are the main cause of sediment deformation and liquefaction during earthquakes. Numerical calculations showed that the slope failure may occur during an earthquake characterised by a PGA in excess of 0.1g, and also that, under a PGA of 0.2g liquefaction could be triggered in shallow silty-sandy deposits. Moreover, comparison of the predicted slope failure with failure geometries inferred from seafloor morphology showed that earthquakes and subsequent mass movements could explain the present-day morphology of the study area.

Searching for the seafloor signature of the 21 May 2003 Boumerdès earthquake offshore central Algeria

“…The aim of this paper is to provide a full overview of the offshore impact of the 2003 Boumerdès earthquake, using detailed swath bathymetry, backscattering imagery, highresolution side-scan sonar data gathered in the area during the MARADJA 2003 and MARADJA2-SAMRA 2005 cruises (Déverchère et al, 2005;Domzig et al, 2006;DanUnterseh et al, 2011) and the PRISME 2007 cruise (Dan et al, 2009;Cattaneo et al, 2010;Nouguès et al, 2010). The fragmented tectonic setting of the Algerian margin is an impellent reason to study the record of past earthquakes in great detail in order to evaluate the size of faults and the related earthquake magnitudes.…”

“…Because of the tectonic over-steepening of their slope gradients, the scarps are preferential sites for submarine landslides Nouguès et al, 2010). Facilitated by the great slope gradient, instability is most likely triggered by soil acceleration leading to the liquefaction of thin silty layers interbedded in the dominant hemipelagic sediments (Dan et al, 2009;Nouguès et al, 2010).…”

“…However, a precise age of each failure has proven elusive until today to ascertain with the available data, in particular in absence of pre-and post-event bathymetric survey (see, for example, the 2002 Stromboli event case history in Chiocci et al, 2008). Although no direct evidence of a submarine landslide triggered by the 2003 earthquake is available, the soil acceleration of such an event is compatible with the sediment failure in areas where submarine landslides are present (Dan et al, 2009;Nouguès et al, 2010). Furthermore, it is possible that a submarine landslide, which probably originated along one of the seafloor scarps described above, is at the origin of cable breaks either directly or indirectly by transforming into a turbidity current, as proposed in the interpretation of the causal mechanisms for cable breaks in Fig.…”

Mass-Transport Deposits on the Algerian Margin (Algiers Area): Morphology, Lithology and Sedimentary Processes

“…Recent studies focused on a few selected cases (S1 escarpment- Dan et al 2008;S2 escarpment-Nouguès et al this volume) allowed the identification of triggering mechanism of different slope failures. Slope stability evaluation under gravity loading, where only the static physical properties of the sediments (unit weight, internal friction angle and undrained shear strength) are considered, showed that failure does not occur for a slope value of 5° (Dan et al 2008).…”

“…Following the Boumerdès event, numerous studies focussed on the area offshore of the epicenter in order to identify related tectonic deformation and earthquake-triggered deposits (Déverchère et al 2005;Domzig et al 2006;Dan 2007;Dan et al 2008;Domzig et al 2008;Yelles et al 2009;Cattaneo et al this volume). The central Algerian margin is characterized by a continental shelf of variable width and a steep continental slope (Fig.…”