Detailed Analysis of a Submarine Landslide
(SAR-27) in the Deep Basin Offshore Algiers
(Western Mediterranean)
A. Nouguès, N. Sultan, A. Cattaneo, G. Dan, K. Yelles, and PRISME team
Abstract On May 21, 2003 an earthquake with a magnitude of 6.8 (Mw) struck
the city of Boumerdes, located on the coast near Algiers, and generated significant
gravity flows recognized by numerous submarine cable breaks. In order to investigate a possible offshore imprint of past earthquakes in terms of sediment instabilities, we analyzed one of the numerous escarpments of tectonic origin existing on
the Algerian margin. This escarpment, located about 50 km NE offshore Algiers
is within water depths between 2,600–2,900 m and has an average slope of 4°.
A variety of geotechnical (piezocone), geophysical (swath bathymetry, Chirp echosounder and high-resolution deep-towed side scan sonar) and sedimentological
(sediment cores) techniques were used to realize this work. Our study focuses on a
well-defined submarine landslide scar (called SAR27) revealed by side-scan sonar
images. The SAR 27 slide is small in size (0.5 × 1.7 km), but exhibits a complex
morphology including a 10-m high headwall scar, tilted blocks, an area of reworked
sediment and a distal deposit. Correlations between in-situ measurements, sampling
and CHIRP profile provided a comprehensive understanding of the slide geometry
and emphasized the role of thin silty sand beds in the slide initiation. These silty
sand beds are probably acting as liquefied slip surfaces, implying a translational
displacement mode which correlates the slide morphology.
A. Nouguès, N. Sultan, A Cattaneo, and G. Dan
IFREMER, Géosciences Marines, Laboratoire Environnements Sédimentaires, Plouzané, France
A. Nouguès ()
Institut Polytechnique LaSalle Beauvais, Département Géosciences, Beauvais, France
FUGRO FRANCE S.A.S., Nanterre, France
e-mail: a.nougues@fugro.com
K. Yelles
CRAAG, Centre de Recherche en Astronomie, Astrophysique et Géophysique, Bouzaréah,
Algiers, Algeria
D.C. Mosher et al. (eds.), Submarine Mass Movements and Their Consequences,
Advances in Natural and Technological Hazards Research, Vol 28,
© Springer Science + Business Media B.V. 2010
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A. Nouguès et al.
Keywords Submarine landslide • Algeria • CPT • triggering mechanism •
liquefaction
1
Introduction
Although evidences of submarine landslides are regularly highlighted on very gentle
slopes, the triggering mechanisms of such slides remain quite imprecise. Obviously,
the static driving stresses implied by the low slope angles are insignificant, and
additional external mechanisms (earthquake, fluid flow, free gas …) are needed to
trigger a landslide (Lee et al. 2007). Cyclic loadings generated by earthquakes
contribute to this external driving stresses but may especially induce the liquefaction
of granular sediment (silt or sand) and degradation and softening of soft clays properties (Pestana et al. 2000). In case of interlayered sand and clay levels, the sand
layers reaching a liquefied state may become preferential slip surfaces leading to
slope destabilisation (see L’Heureux et al. 2007; Dan et al. 2008 for examples of
liquefaction-induced submarine slides and Bardet et al. 1999 for general considerations about liquefaction-induced ground displacements).
The Algerian margin, embodying the boundary between the European and the
African plates, is a tectonically active margin (Déverchère et al. 2005). Among
the several strong to moderate earthquakes which occurred in the last century on
the coastal zone, the last violent seismic event (Mw = 6.9) struck the city of
Boumerdes on May 21st 2003. The MARADJA 2 survey (2005) aimed at recognizing possible slope failures upslope from submarine cable ruptures which were
recorded following the seismic shaking. A well-defined slide showing a freshlooking submarine slide scar was revealed on a deep-towed side-scan sonar
profile (the SAR27 profile). The so-called “SAR27 slide” occurs at 2,700 m of
water depth on a seafloor escarpment probably of tectonic origin (S2 escarpment, Déverchère et al. 2005). This morphological feature, located approximately 50 km offshore the coastline and characterized by an average slope of 4°,
displays numerous slide scars (Fig. 1). Previous studies (Dan 2007; Domzig
et al. 2009; Cattaneo et al. this volume) showed the widespread presence of such
small-size slides (usually about 0.2 km2) across the Algerian margin. In addition,
thin sand beds interlayered in mostly clayey deposits have been described in
sediment cores and determined as the main predisposing factor. The recurrent
seismicity and subsequent danger of liquefaction have been pointed out as the
main triggering factors explaining these numerous small-sized slides in the area
(Dan et al. 2008).
The present study deals with a comprehensive analysis of the SAR27 slide.
A multidisciplinary approach including geophysical, sedimentological and geotechnical data was used to constrain the geotechnical character of the slide together with
its detailed architecture. In addition, we aimed to assess the presumed triggering
mechanisms of the SAR27 slide and to reconstruct the sliding scenario.
Detailed Analysis of a Submarine Landslide
543
Figure 2
Slide scars
SAR27 slide
37°N
A lgiers fan
Dellys
A lgiers
Bourmerdes
M w = 6 .9
(2 0 0 3 )
4°E
3°E
Fig. 1 Morpho-bathymetric map of the S2 escarpment, in the deep Algiers basin. Numerous slide
scars (underlined in red) are observed in the vicinity of the SAR27 slide. The red star stands for
the 2003 Boumerdes epicentre after Semmane et al. (2005)
2
Data Set and Methods
Deep-towed side-scan sonar imagery and 3.5 kHz profiles using the S.A.R. system
(Système Acoustique Remorqué) as well as very high-resolution Chirp profiles
were collected during the MARADJA 2 survey (Savoye 2005). Further data were
acquired along the SAR27 profile during PRISME survey (Sultan 2008): five
CPTu (Cone Penetration Test with pore pressure measurement, Meunier et al. 2005),
three Küllenberg cores and interface-gravity cores. The cores were performed (1)
upslope the headwall in an undisturbed area, (2) in the disturbed deposits and (3)
at the base of the slope (Fig. 2). For general information about CPT tests and its
interpretation readers are referred to Lunne et al. (1997). Evaluation of the liquefaction potential was performed according to the procedure depicted by Youd
et al. (2001). Sediment core logging was performed using a Geotek Multi Sensor
Core Logger (http://www.geotek.co.uk) in order to characterize the sediment
density or unit weight (g). The volume of the slide was evaluated by reconstructing the initial slope from the present bathymetric profile and by calculating the
missing sediment thickness. Thus, the volume of sediment involved in the slide
was extrapolated from the scar width and the deposit extent, based on sonar
image and backscatter data.
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Fig. 2 SAR imagery (a) and associated 3.5 kHz profile (b) acquired along SAR27 slide
3
3.1
Results
Morphology of the Slide Area
The morphology of the SAR27 slide is clearly expressed on the side-scan sonar
image (Fig. 2a). The scar consists of a 560 m wide amphitheater-shaped area
including numerous arched extensional cracks. These cracks delineate terraces up
to 500–700 m long, tens of meters wide and a few meters high. The arched terraces
are perpendicular to the longitudinal axis of the slide (N-S) and are visible from the
headscarp until 1 km downslope. The eastern flank of the slide scar reveals multiple
Detailed Analysis of a Submarine Landslide
545
Table 1 Characteristics of SAR27 slide from SAR data, CHIRP profile and reconstructed slope
Slide scar
Deposit
Volumes
Width:
560 m
Length:
1.7 km
Headwall height:
~10 m
Extension:
1.2 × 0.8 km
Thickness of the deposit:
< 8m
Run-out distance of blocks:
0.65–1.85 km
Volume of the empty scar:
6.3 × 106 m3
Volume of Base-of-Slope deposit:
3.2 × 106 m3
Volume of evacuated sediments:
2.9 × 106 m3
walls that probably correspond to successive, intersecting scars which seem to
propagate upslope. The echo-sounder profile associated with the SAR image
(Fig. 2b) confirms the presence of (1) a well-defined headscarp cutting undisturbed
parallel reflectors corresponding to undisturbed slope deposits and (2) of tilted
blocks in the upper part of the slide. Compressive ridges are observed at the base
of the slope, followed by a transparent lenticular seismic facies suggesting the presence of a mass-deposit. Additionally, the SAR image shows that blocks of 3–5 m in
height have been rafted, up to 1 km away of the slope break. Table 1 summarizes
the main quantitative characteristics of the SAR27 slide.
3.2
Sediment Core Data
Core KS28 (Fig. 3a) is 11-m long and was collected upslope the slide headwall in
order to provide the reference lithology of the undisturbed sediments. The top 1.3 m
thick sediment consists of soft silty clay deposits characterized by low unit weight
value (g = 16 kN/m3). From 1.3 m down to 10 m below seafloor (mbsf) the sedimentary column is characterized by alternating silty-clay and thin cm-thick sandy layers
corresponding to repeated turbidite beds (Fig. 3a, p1). From 10 mbsf to the bottom
of the core, the sediment is essentially clayey. The clayey intervals from 1.3 mbsf
to the bottom have a nearly constant unit weight (g = 18.5 kN/m3).
Core KS29 (Fig. 3b) is 5-m long and was collected within the slide scar. The
upper 3.5 m, consists of alternating clayey intervals and silt laminae that exhibit
numerous small-scale discordances (Fig. 3b, p1) and a overall inclined bedding
surfaces with 25° to 60° dip (Fig. 3b, p2). A clear discordance observed at 3.5 mbsf
likely corresponds to a slip surface. The lower 1.5 m (from 3.5 to 5 mbsf) consists
of horizontal, probably undisturbed clayey layers (Fig. 3b, p2). The interface gravity core recovered nearby the core confirmed that the tilting of the layers is real, and
exists beneath the upper 15–20 cm corresponding to the youngest hemipelagic clays
(Fig. 3b, pi).
Core KS30 (Fig. 3c) is 9-m long and was collected at the base of the slope. The
upper 4.3 m of sediment is essentially clayey with fine silts laminae and show discrete internal shear surfaces (Fig 3c, p1). From 3.6 mbsf and down to 4.3 mbsf, the
layers appear distorted and the layering is no more visible beneath 4 mbsf, possibly
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Fig. 3 Synthetic logs, unit weight (g), water contents (w), and selected sample photos (p1–3, pi)
for cores KS28 (a), KS 29 (b) and KS 30 (c). D: discordance, black dashed lines: internal shear
surfaces. pi: photo of the associated interface core
indicating a greater degree of reworking (Fig 3c, p2). At 4.3 mbsf, the reworked unit
overlies undisturbed turbidite beds (Fig 3c, p2 and p3). The upper unit (from 0 to
4.3 mbsf) is characterized by low and highly varying unit weight and high water
contents compared to the underlying unit and to the reference core (KS28).
3.3
Stratigraphy Inferred from Seismic Data and CPTu
Several well-pronounced reflectors (darker lines) were identified on the Chirp
profile and used to define seismic units (U1 to U5) (Fig. 4). The top unit (U1)
shows several internal parallel reflectors. It overlies a unit characterized by a strong
reflector at its base and only one discernable internal reflector (U2, subdivided into
U2a and U2b). The underlying unit (U3) does not show any internal reflector. These
top units (U1 to U3) appear to lie discordantly on the two deepest and concordant
units (U4 and U5). The Unit 4 shows the most pronounced pinch-out geometry but
the other units also have a tendency to thin-out seawards, and the reflectors to
become less distinct or merge together (dashed lines). Additionally, two acoustically transparent units are present, presumably corresponding to: (1) some reworked
sediments along the slope (gray) and (2) the SAR27 slide deposit (yellow), overlying a well-layered unit at the base of the slope.
Five CPTu were performed across the SAR27 slide. For each test, the corrected
cone resistance (qt) and excess pore pressure (Du) curves are plotted along the interpreted Chirp profile (Fig. 4). The correlation between depth (in metres) and time
(TWT in seconds) was done assuming a mean P-wave velocity of 1,550 m/s. All
CPTs have encountered premature refusal (before 30 m of penetration), indicating
that a layer with high resistance to compression was reached (black crosses at maximum CPTs depths on Fig. 4).
Fig. 4 Correlation between geotechnical and geophysical data. The CPT data are plotted along
SAR27 interpreted CHIRP profile assuming a wave velocity of 1,550 m/s (qt: corrected cone
resistance, Du: excess pore pressure)
548
A. Nouguès et al.
Levels characterized by high qt and low Du values on the CPTu profiles are
interpreted as silty sand to sand (Robertson et al. 1986) and classified as very loose
to loose (Baldi et al. 1986). Intervals with lower qt values and linear pore pressure
profiles correspond to clayey materials. The coarse-grained beds appear to correlate
with the high amplitude reflectors underlined on the Chirp profile (see CPT07-01
in Fig. 4). Due to the different vertical resolution of the Chirp profile (10–50 cm)
and of the CPTu measurement (5–10 cm), the high amplitude seismic reflectors
might correspond to several close qt peaks on the CPTu profiles (see colored CPTu
plots in Fig. 4). CPTu profiles acquired within the slide scar (CPT 07-02 and
08-03), show only very few qt peaks. However, each reflector correlates at least one
significant pressure (Du) drops (high permeability) indicating the presence of some
coarse-grained levels. Besides, the pink reflector (base of U2), which was strong
enough to stop prematurely CPT 07-01, is barely discernable when encountered by
CPT 07-02. When going seawards, the lateral decrease of the sand beds thickness
in addition to the decrease of the pressure confining those loose sandy layers have
probably prevent the CPTu cone to detect suitably the different reflectors.
3.4
Liquefaction Potential Assessment
It is possible to infer that liquefaction of sandy layers was a possible triggering
mechanism for the SAR27 slides based on the following facts: (1) the presence of
silty sand layers in a dominantly clayey lithology, (2) the recurrent regional seismicity and (3) the CPTu data showing possible ‘ghost’ loose sand layers (i.e. only
visible through their high permeability) when going seawards. We therefore
decided to assess the liquefaction potential of the SAR27 soil profile. The employed
method is a simplified procedure developed from empirical correlations between
soil resistance to liquefaction and CPTu tests (Youd et al. 2001). The resistance to
liquefaction or CRR (Cyclic Resistance Ratio) is derived from the cone resistance
corrected by the overburden effect and the fine-grained content. The CRR is then
compared to the CSR (Cyclic Stress Ratio), which represents the seismic solicitation (defined by a maximal acceleration amax). The Moment Magnitude (Mw) of the
earthquake is also taken into account.
From CPT 07-01, four loose silty-sandy layers were identified as potentially
liquefiable at 6.4, 7.5, 10.3 and 15.2 mbsf (Fig. 5). The comparison between the CRR
and CSR calculated for an expectable magnitude level (Mw = 7) reveals that liquefaction could occur for an amax of 0.1–0.15 g in the shallowest sand layers (corresponding
to the base of U1 and U2a). In order to cause liquefaction in the deepest layer (base
of U2b at 15 mbsf), accelerations of 0.2 g would be needed. For CPT 07-02, the
friction was not measured, preventing an accurate calculation of the liquefaction
potential for the two silty-sand levels at 17.25 and 18.75 mbsf (see CPT 07-02 on Fig.
4). However, sediment levels deeper than 15 m are rarely observed to liquefy
(Krinitzsky et al. 1993). Others CPTu could not provide results in terms of liquefaction potential as no sand layers were recognized on tip cone resistance profiles.
Detailed Analysis of a Submarine Landslide
549
Fig. 5 Comparison between the CSR
calculated for several maximal accelerations
(amax) and the CRR computed from CPT 07-01
4
Discussions and Conclusion
In the upper part of the scar, numerous extensional faults and terraces indicate that
the upper, mostly cohesive sediments (Unit 1 and 2) only experienced moderate
displacements, whereas the smoother morphology observed downslope together
with evidence of a mass-deposit attests of enhanced sediment disruption.
Geometrically, the base of the Unit 3 (light blue horizon) appears to be the basal
slip plane of the slide. The base of the Unit 2 (pink horizon) is interpreted as a
secondary slip plane, where the tilted blocks are probably rooted in. SAR27 slide
is therefore inferred to be a translational slide with multiple slip planes.
As shown by the CPT-based liquefaction analysis, the loose silty-sand layers
found in cores from the SAR27 slide area are potentially liquefiable for seismic
accelerations (0.1–0.2 g) that are expectable over the margin: Pelaez Montilla et al.
(2003) calculated a regional expectable PGA of 0.15 g for a return period of 475
years and Laouami et al. (2006) showed from strong motion recordings of the 2003
Boumerdes earthquake that site effects could locally lead PGAs reaching 0.6 g. An
earthquake and related liquefaction can therefore be considered as very probable
triggering mechanisms of the SAR27 slide. However, several predisposing or
aggravating factors are assumed to have played a major role in the slide initiation.
One of the main predisposing factors is the lithology, composed of successive
loose sandy layers interbedded within impermeable clayey intervals. As demonstrated by Yang and Elgamal (2002), the clay layers increase sands vulnerability to
liquefaction by trapping excess pore pressure and hindering its dissipation.
Biscontin and Pestana (2006) also discussed the fact that the presence of these
coarse-grained levels could lead to a delayed failure due to the impeded migration
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Fig. 6 Summary of the triggering and evacuation processes (a) expected to have generated
SAR27 slide and its present morphology (b) reconstructed from the SAR and CHIRP data
of excess pore pressure after the end of shaking. Once liquefaction has been triggered by an earthquake, low static forces in a gentle slope can be sufficient to cause
significant displacements.
In addition, the layers’ thinning seawards implies a decrease of the sand levels
burial depth and a correlative lateral increase of their liquefaction potential (see
Sultan et al. 2008). Consequently, liquefaction was more likely to occur: (1) in the
superficial, unconfined sand beds (bases of U1 and U2) but with few consequences
on the general slope stability and (2) in the deepest sand beds (merged bases of U3
an U4) when they become shallower seawards, leading to sediment destabilization
from the base of the slope (Fig. 6a).
The sliding scenario for the SAR27 slide involves: (a) predisposing factors,
essentially due to the lithology and the layers geometry; (b) a triggering factor, very
probably being an earthquake, with subsequent localized liquefaction and (c) a
retrogressive failure and evacuation process inferred from the slide architecture
including two modes of sediment transport: (1) a deep-based slide with moderate
to high disruption as attested by the high water contents of the deposit and its
smooth morphology and (2) a shallower instability in the upper layers marked by
extension and moderate displacements.
Even if the SAR27 slide has a fresh morphological expression on the seafloor,
the 15–20 cm thick clayey sediment present on top of the slide deposit in the interface core is likely a hemipelagic drape implying that would imply that SAR27 slide
is older than the 2003 earthquake. Further detailed work on core dating is needed
to ascertain this assumption, or tentatively link the SAR 27 slide to an historic event.
Detailed Analysis of a Submarine Landslide
551
In addition, the search for slides showing indubitable evidences of the 2003 event
as a triggering mechanism remains open (Dan et al. this volume).
Acknowledgments The first author thanks Fugro-France for the time allocated to write the
paper. We thank the captain and crew of the R/V L’Atalante during PRISME 2007 cruise.
IFREMER and ANR projects (ISIS and DANACOR) are acknowledged for the material and
financial supports. S. Migeon and H. Haflidason are acknowledged for their constructive reviews
and comments. This work is dedicated to Bruno Savoye, whose memory keeps giving us the will
to go ahead and always progress.
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