PRODELTA SLOPE STABILITY AND ASSOCIATED COASTAL
HAZARDS IN TECTONICALLY ACTIVE MARGINS: GULF OF
CORINTH (NE MEDITERRANEAN)
V. LYKOUSIS, D. SAKELLARIOU, G. ROUSSAKIS
National Centre for Marine Research, Ag. Kosmas, 166.04 Hellinikon, Athens, Greece
Abstract
Submarine prodelta instabilities appear as a sequence of rotational block slumps,
restricted to the Upper Holocene silty clay deposits (unit A). The individual slump
blocks are about 80-150m long and 5-15m in thickness. Slip zones are associated with
the deeper Lower Holocene and/or methane gas-charged sediments (unit B). Mud flows
appear towards the steeper prodelta slopes. Calculation of slope stability with the
Normalised-Soil-Parameter (NSP) method of the normally consolidated prodelta
sediments (Lower Holocene unit B), indicates that instabilities could be induced by
earthquake ground accelerations from 0.13-0.14 g. Somewhat higher accelerations
(0.27-0.30 g) are able to initiate mass failures in overconsolidated prodelta sediments
(Upper Holocene unit A). These estimations are in accordance with the observed
sediment mass wasting processes. Since the regional expected peak ground acceleration
range between 0.10-0.15 g. the prodelta slopes in W. Greece are highly unstable.
Keywords: Submarine slide, slope stability, coastal collapse, tsunamis, W. Greece
1. Introduction
The marine prodelta sequences are the predominant modern unconsolidated sediments,
deposited on continental shelves. The marine prodelta deposits consist mainly of silty
clays to clayey silts, developed in a prismatic-lobate external configuration. Due to their
unconsolidated state, the prodelta sedimentary sequences are prone to sediment failures
and various types of instabilities (Prior and Coleman, 1982). Low strengths associated
with prodelta sediments are the direct result of the rate of deltaic sedimentation,
especially towards the upper delta slope near the distributaries (Terzaghi, 1956).
Since the 1960s, the offshore oil and gas industry has experienced directly the effects of
sediment mass movement of prodelta sediments. Movement of permanent platforms,
jack - up rigs, together with repeated pipeline and cable breaks, have been reported often
for the area offshore of the Mississippi delta; these were associated with cyclic wave
loading during the passage of various Hurricanes (Bea and Audibert, 1980).
Nevertheless, it is accepted generally that the cyclic loading from seismic ground
accelerations (seismic activity) is the world-wide predominant factor affecting offshore
slope instabilities (Lee and Edwards, 1986).
The western part of Greece (mainland, coastal areas and islands) is, seismically, the
most active area in Europe, due to its proximity to the Hellenic Trench. The Hellenic
Arc and Trench system constitutes an island arc, related to the subduction of the African
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Lykousis et al.
Figure 1. Map of the gulfs of patras and Corinth with the drainage pattern of the surrounding land areas.
The main rivers are marked in bold. Dotted lines mark the location of the profiles shown in Fig 2 and 3.
plate beneath the European plate. The regional tectonics involves complex slip patterns
across the boundaries of several microplates (McKenzie, 1972; Mascle and Martin,
1990) implying high seismicity. Intense shallow earthquake motions, particularly
pronounced in W. Greece, are expected to generate peak ground accelerations of 1530%g (Makropoulos and Burton, 1985). Within this region, sediment mass failures
during the late century are responsible for numerous cable breaks (Heezen et al., 1966).
The purpose of this paper is the study of the types of instabilities in prodelta sediments
subjected to cyclic loading, the identification of their geotechnical character and the
investigation of the possible failure mechanisms. The Gulfs of Patras and Corinth (Fig.
1) were selected as case studies.
2. Methodology
High resolution seismic reflection profiles (3.5kHz, O.R.E. LTD) were used to identify
the prodelta sediment sequences and the associated instabilities. Short sediment cores (2
- 3m) were recovered using a BENTHOS INSTR. gravity corer. The entire field work
was carried out from R/V AEGAEO.
Grain-size analysis was performed using the Sedigraph laser technique (Micrometrics
5100), whilst selected samples were analysed for organic carbon and total carbonate
content with a Fisons EA 1108 CHN Analyser. Geotechnical classification tests were
performed downcore, according to British Standards 1377 (1975). Consolidation tests
were performed in undisturbed samples, utilising back-pressured, fixed-ring odometers
(Silva et al., 1981). Graphical procedures were used to determine the coefficient of
consolidation (logarithm of time) and the preconsolidation stress (maximum past effective
stress, exerted on the sediments) (Casagrande, 1936).
Slope stability was estimated under undrained conditions utilising the NSP approach
(Lee and Edwards, 1986). This is a sophisticated approach, incorporating more
geotechnical parameters (including cyclic shear strength tests, consolidation tests etc)
into the evaluation of slope stability, compensating partly for the effects of disturbance
Prodelta slope stability
435
during sediment coring. Static and cyclic triaxial compression and extension tests were
run under undrained conditions, using computer controlled GDS triaxial
instrumentation; this was in sediment samples taken from about 1 m sub-bottom depth
Figure 2. 3.5kHz profile off the Evinos river, in the Gulf of Patras. Typical small-scale rotational
instabilities and mudflows within the Upper Holocene unit B are recognised. The Lower Holocene unit
B (failure zone?), the Late Pleistocene transgressive unit C and gassy sediments are shown, as well.
of an Upper Holocene muddy section. These tests determine the susceptibility of the
sediment to strength loss, during repeated external (wave and/or earthquake) loading.
The samples were consolidated to stress levels identical to those imposed on the static
triaxial samples. In this way the shear strength of sediment under cyclic loading
conditions can be evaluated, relative to the predetermined static undrained shear
strength. Axial loads were applied sinusoidally with a frequency of 0.01-0.03 Hz until,
either an extentional or compressional strain of 15% was reached. Lee and Edwards
(1986) describe details of these procedures.
3. Results and discussion
3.1 PRODELTA SLOPE INSTABILITIES
Evidence of sediment instability is common in the majority of almost all low-angle (0.5
- 2o) modern prodelta slopes of the seismically active continental slopes of W. Greece
(Kyparisiakos and Messiniakos Gulfs, Gulf of Patras, Gulf of Corinth). In these areas of
normally consolidated prodelta deposits (sedimentation rates from 3 - 5m/ka), slope
failures manifest themselves as shallow seated arcuate or listric gravity faults, with
small vertical displacement (Fig. 2). These faults are slip planes associated with a
sequence or peripheral (rotational) block slumps restricted to the Upper Holocene
(upper stratified layers - A). The individual slump blocks are about 80-150m long and
have slip planes extending to a mean depth of 10m. The deeper, transparent (reflectionfree) Lower Holocene muddy layers (B), together with the parallel Upper Pleistocene
transgressive sequences (C), are mostly unaffected. These observations imply a basal
436
Lykousis et al.
shear plane of Lower Holocene muddy and/or methane-gas-charged sediments, these are
expected to have lower shear strength from the overlying muddy-silty stratified layers.
The basal muddy layer could be compared with the “weak” sediment zone (with values
of undrained shear strength as low as 4.8kPa), measured 15m below the seabed within
Figure 3. Air gun 10in3 profile off the Meganitis river in the Gulf of Corinth. Small scale gravity faults
landward the shelf edge indicates retrogressive failure processes. Slide scarp and slump debris in the
middle and lower slope are also shown.
Mississippi prodelta deposits (Whelan et al., 1976). Toward the upper and steeper (2-3o)
part of the prodelta slopes, the presence of near-bottom chaotic reflectors, in water
depths of 15 - 40m, is indicative of an active sub-surface mudflow, of up to 5m in
thickness (Fig. 2). This set of deformational features resembles contemporaneous faults
and slumps, observed on the Adriatic shelf (Correggiari et al., 2001), the shelf off the
South Pass of the Mississippi Delta (Coleman and Prior, 1981), near Eureka, California
(Lee et al., 1981) and in the Gulf of Alaska (Carlson et al., 1980). Similar failure
processes (rotational block slumps) have been observed also within Upper Pleistocene
prodelta sequences on the shelf-edge of the Northwest Aegean margin (Thermaikos
Gulf, Lykousis and Chronis, 1989).
3.2 ASSOCIATED COASTAL HAZARDS
The scarps of the observed prodelta failures are located very close to the coast (50300m) and in fairly shallow water depths (15-50m). They appear usually in the steep
slopes of the Gulf of Corinth, where significant upslope (landward) retrogressional
sliding is often (Fig. 3). These retrogressional processes are responcible for the
Prodelta slope stability
437
generation of coastal collapses, observed often during strong seismic shocks. Extensive
retreat of coastal strips has been reported during 373 BC and 1817, 1861, 1965 and 1995
AD (Schmidt 1875; Papazachos & Papazachos; Mouyaris et al. 1992; Lekkas et al.
1996;) in the Western Gulf of Corinth (Aegialia-Eratini region). The dimensions of the
retreated coastal strips are in the order of several kilometers in length and tens to
hundreds of meters in width.
Slump-induced tsunamis, not accompanied necessarily by coastal retreat, constitute an
additional significant coastal hazard. Catastrophic tsunamis, some of them with human
losses, were observed during 373 BC, 1748, 1817, 1861, 1965 and 1995 (Galanopoulos
(1966; Papadopoulos & Chalkis 1984 and Lykousis et al. 1995); these were not always
related directly to specific earthquakes. The wave height of these tsunsmis ranged from
3-7m, except in the case of 373 BC (submergence of ancient city Eliki), where the wave
high was estimated to be around 20m.
3.3 SLOPE STABILITY
The NSP slope stability method has been used successfully, to identify the failure
conditions in slides and mudflows, in the seismically active slopes of the western US
continental margin offshore California and Alaska (Lee et al., 1999). In Europe, the
method has been applied to the analysis of slope stability of the Ebro passive margin in
the Western Mediterranean (Baraza et al., 1990) and in the Gulf of Cadiz (Atlantic
Spanish coasts) (Lee and Baraza, 1999).
The relative stability of slopes undergoing seismic loading can be assessed, by the NSP
method, considering the following equation:
Kc = ( ¶ [AcArUS(OCR)m – sin ]
(Lee and Edwards, 1986)
:KHUH7DEOHV DQG,,
kc is the critical ground earthquake acceleration, capable of inducing slope instability;
is the ratio of submerged, to total bulk density, of the sediment;
Ac is a factor for correcting laboratory shear strengths to anisotropic (real) consolidation
conditions;
Ar is the factor for degrading static shear strengths for cyclic loading;
U is the degree of consolidation - this factor equals 1, since the bottom sediments are
normally consolidated to slightly overconsolidated;
S is the ratio of the undrained shear strength to consolidation stress, for normal
consolidation - this ratio was calculated from the triaxial tests;
OCR is the overconsolidation ratio, calculated for the lower part of the Holocene layer
(around 1 m bsb);
m is a sediment constant, relating overconsolidated normalised strength to normally
consolidated normalized strength (typically equal to about 0.8, according to Ladd et al.,
1977), but can be measured from specific isotropic consolidation tests, in which a
known OCR is induced followed by graphical estimations (Mayne, 1985). Note: in the
tests described here, m was estimated to be 0.78; this is slightly higher than the value
calculated for the W. Mediterranean (Ebro) margin (0.76)(Baraza et al., 1990); and
is the mean slope angle, ranging from 0.5-30.
Lykousis et al.
438
Laboratory analysis of a series of short sediment cores from prodelta deposits was
revealed slight differences in the geotechnical character of seismic stratigraphic units A
relatively to B. The unit A representing Upper Holocene clayey silts-silty clays display
high water content (40-90 %), low bulk density (1.4-1.8 g cm-3) and relatively low
values of undrained shear strength (3-20 kPa). These values exhibits gradual and almost
linear increase or decrease with depth (bulk density, shear strength and water content,
Atterberg limits respectively). The lower unit B (Lower Holocene silty clays) indicates
similar values of geotechnical properties with unit A. The water content is relatively
high (40-80 %) and the bulk density slightly lower (1.3-1.6 g cm-3) due to their finer
texture in respect to unit A. The low values of shear strength (3-8 kPa) reflect, as well,
mainly textural differences. Important differences was observed in the consolidation
characteristics due to higher silt content in the unit A (significantly higher
overconsolidation ratio of unit A).
The various calculated Normalised Soil Parameters in prodelta sediments and used in
this study are presented in Table I
Table 1. Prodelta slope stability using Normalised Soil Parameters (NSP).
Sediment
sequence
A
B
Sampling
depth (cm)
100
100
Water
content (%)
40
50
S
Ar
OCR
Ac
m
0.42
0.32
0.88
0.52
0.64
0.91
2
1
0.76
0.94
0.74
0.78
Four values of kc were calculated using the slope angle, the geotechnical characteristics
(mean values) and the consolidation state of the three cores from the upper (unit A) and
lowermost slope (distal bottomsets-unit B). The calculations were repeated for the
maximum and minimum slope angles estimated in prodelta slopes (Table II). These
values of kc clearly indicate that prodelta slope gradient up to 30 is not a significant
factor affecting slope stability in both units. The significant differences in the values of
kc that was calculated for the units A and B mostly reflect the consolidation state of
each unit, textural and water content variations. The kc calculated values of 0.27-0.30 g
for the unit A are appreciably higher than the regionally expected (0.16-0.18 g) and
consequently is appeared to be relatively stable. The lower kc values that estimated for
the unit B (0.13-0.14 g) indicate that this is a weak zone regarding of sediment slope
stability and could be regarded as a potential slide plane. This is in accordance with the
high resolution seismic profiling analysis (see 3.1. Prodelta slope instabilities). These
values are very much comparable with the estimated kc values for the N. Aegean upper
and lower slope (0.16-0.17 g and 0.29-0.30 g respectively) (Lykousis et al., 2002) and
comparable with the kc values that were calculated for the S. Aegean slopes
(Perissoratis and Papadopoulos, 1999). The application of these kc values of the Lower
Holocene unit B (potential failure zone) on a regional basis indicates that the prodelta
slopes in W. Greece are highly unstable.
Table 2. Seismically induced ground acceleration able to cause failure (kc).
Sediment sequence
A
B
Max / min Slope angle (o)
0.5 / 3
0.5 / 3
Kc (g’s)
0.296 / 0.278
0.141 / 0.129
Prodelta slope stability
439
4. Conclusions
Evidence of instability is common, in almost all the modern prodelta slopes of the
seismically active continental margins of W. Greece. In low-angle (0.5-3o) prodelta
slope, sediment failures has appeared as shallow-seated arcuate or listric gravity faults,
with small vertical displacement. These faults are slip planes, associated with a
sequence or peripheral (rotational) block slumps restricted to the Upper Holocene
(upper stratified layers). The individual slump blocks are about 80-150m wide; they
have slip planes extending to a mean depth of 10m. The deeper, transparent (reflectionfree) Lower Holocene muddy layers, together with the parallel Late Pleistocene
transgressive sequences, are mostly unaffected. These observations imply a basal shear
zone of Lower Holocene muddy and/or methane-gas-charged sediments, which are
expected to have lower shear strength than the overlying muddy-silty stratified layers
(Upper Holocene). In the steeper (2-6o) prodelta slopes, translational sliding is the
prominent instabilities associated with retrogressional failure processes. These active
processes have caused significant coastal collapse-retreat, with important human and
economic impact. Prodelta failures that occurred in the last 0.15 ka in the Gulf of
Corinth have destroyed telecommunication cables and initiated destructive tsunami
waves.
The regional prodelta slope stability was calculated using the Normalised-SoilParameter (NSP) method (Lee et al., 1986). The calculated values of the earthquake
ground acceleration, able to generate failures (kc) of the prodelta slopes (0.13-0.14 g)
are lower than those due to earthquake motion expected (0.16-0.18 g); consequently
these are in agreement with the observed slope failures. The application of these kc
values on a regional basis in the W. Greece marginal slopes indicates that the normally
consolidated prodelta sediments in this area are highly unstable. The Lower Holocene
sediment sequence (B) and the associated gas charged sediments should be regarded as
potential failure zone. This is in agreement with the observed instabilities in the
investigated areas, where the expected ground accelerations varies from 0.15-0.20 g.
Overconsolidated prodelta sediments was appeared relatively stable, since the calculated
ground accelerations able to trigger mass failures range from 0.27-0.30 g.
5. Aknowledgements
The authors are greatful to Prof. M.B. Collins and Dr. C. Perissoratis for their
constructive comments on the original manuscript submitted. The officers and crew of
the R/V AEGAEO are also greatfully aknowledged.
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