(Table 1) Experimental shear strength values for IODP Exp315 and Exp316 holes

We investigate the mechanics of slope failures on the Nankai accretionary complex offshore Japan in the vicinity of a major out-of-sequence thrust fault (termed the "megasplay"). Incorporating laboratory-measured shear strength of slope sediments sampled during Integrated Ocean Drilling Project (IODP) Expeditions 315 and 316 with local seafloor slope angles from bathymetric data and constraints on in-situ effective stress conditions from drilling, we find that slopes in the study area are stable and submarine landslides are not expected to occur under static conditions. In order to assess the possibility of slope failure triggered by coseismic rupture of the megasplay fault, we use empirical relations for strong ground motion attenuation from earthquakes with Mw 6-9. We find that the slope sediments should be stable based on computations from one model, developed from a catalog of worldwide subduction zone earthquakes (Youngs et al., 1997, doi:10.1785/gssrl.68.1.58). However, using a different model developed primarily from a catalog of crustal earthquakes in Japan (Kanno et al., 2006, doi:10.1785/0120050138), we find that slopes should be unstable for earthquakes 8 <= Mw <= 9, and possibly unstable for events with 6 <= Mw < 8, depending on the proximity of rupture to the seafloor. Considering limitations of the models and geologic observations of slope failure recurrence, the true slope stability is likely to be in between the predictions of the two models, and we suggest that it may be modulated by long-term pore pressure fluctuations.

Consolidation properties and strength characteristics of ODP Site 204-1244 sediments

Eight whole-core samples from Ocean Drilling Program Site 1244, Hydrate Ridge, Cascadia continental margin, were provided to Massachusetts Institute of Technology (Cambridge, Massachusetts, USA) for geotechnical characterization. The samples were collected from depths ranging from 5 to 136 meters below seafloor (mbsf). Seven of the eight whole-core samples were located within the gas hydrate stability zone, whereas the eighth sample was located in the free gas zone. Atterberg limits testing showed that the average liquid limit of the soil is 81% and the average plastic limit is 38%, giving an average plasticity index of 43%. The liquid limit is sensitive to oven drying, shown by a drop in liquid limit to 64% when tests were performed on an oven-dried sample. Loss on ignition averages 5.45 wt%. Constant rate of strain consolidation (CRSC) tests were performed to obtain the compression characteristics of the soil, as well as to determine the stress history of the site. CRSC tests also provided hydraulic conductivity and coefficient of consolidation characteristics for these sediments. The compression ratio (Cc) ranges from 0.340 to 0.704 (average = 0.568). Cc is fairly constant to a depth of 79 mbsf, after which Cc decreases downhole. The recompression ratio (Cr) ranges from 0.035 to 0.064 (average = 0.052). Cr is constant throughout the depth range. In situ hydraulic conductivity varies between 1.5 x 10**-7 and 3 x 10**-8 cm/s and shows no trend with depth. Ko-consolidated undrained compression/extension (CKoUC/E) tests were also performed to determine the peak undrained shear strength, stress-strain curve, and friction angle. The normalized undrained strength ranges from 0.29 to 0.35. The friction angle ranges from 27 to 37. Because of the limited amount of soil, CRSC and CKoUC/E tests were also conducted on resedimented specimens.

Laboratory investigation of flow structure and resistance of high- and low-angle dunes

A prominent control on the flow over subaqueous dunes is the slope of the downstream leeside. While previous work has focused on steep (~30°), asymmetric dunes with permanent flow separation, little is known about dunes with lower lee-slope angles for which flow separation is absent or intermittent. Here, we present a laboratory investigation where we systematically varied the dune lee-slope, holding other geometric parameters and flow hydraulics constant, to explore effects on the turbulent flow field and flow resistance. Three sets of fixed dunes (lee-slopes of 10°, 20° and 30°) were separately installed in a 15 m long and 1 m wide flume and subjected to 0.20 m deep flow. Measurements consisted of high-frequency, vertical profiles collected with a Laser Doppler Velocimeter (LDV). We show that the temporal and spatial occurrence of flow separation decreases with dune lee-slope. Velocity gradients in the dune leeside depict a free shear layer downstream of the 30° dunes and a weaker shear layer closer to the bed for the 20° and 10° dunes. The decrease in velocity gradients leads to lower magnitude of turbulence production for gentle lee-slopes. Aperiodic, strong ejection events dominate the shear layer, but decrease in strength and frequency for low-angle dunes. Flow resistance of dunes decreases with lee-slope; the transition being non-linear. Over the 10°, 20° and 30° dunes, shear stress is 8%, 33% and 90 % greater than a flat bed, respectively. Our results demonstrate that dune lee-slope plays an important, but often ignored role in flow resistance.

Chemistry of various minerals of altered basalts from ODP Hole 111-504B

Basalts recovered from Hole 504B during ODP Leg 111 are more or less altered, but there is no sign of strong shear stress or widespread penetrative deformation; hence, they retain well their primary (igneous) structures and textures. The effect of alteration is recognized as the partial or total replacement of primary minerals (olivine, clinopyroxene, and plagioclase) by secondary minerals and as the development of secondary minerals in open spaces (e.g., veins, fractures, vugs, or breccia matrix). The secondary minerals include zeolite (laumontite and stilbite), prehnite, chlorite, epidote, Plagioclase (albite and/or oligoclase), amphibole (anthophyllite, cummingtonite, actinolite, and hornblende), sodic augite, sphene, talc, anhydrite, chalcopyrite, pyrite, Fe-Ti oxide, and quartz. Selected secondary minerals from several tens of samples were analyzed by means of an electron-probe microanalyzer; the results are presented along with brief considerations of their compositional features. In terms of the model basaltic system, the following two types of low-variance (three-phase) mineral assemblages were observed: prehnite-epidote-laumontite and prehnite-actinolite-epidote; both include chlorite, albite and/or oligoclase, sphene, and quartz. The mineral parageneses delineated by these low-variance mineral assemblages suggest that the metamorphic grade ranges from the zeolite facies to the prehnite-actinolite facies. The common occurrence of prehnite indicates that greenschist facies conditions were not attained even in the deepest level of Hole 504B, which, in a strict sense, contradicts the previous interpretation that the lower portion of Hole 504B suffered greenschist facies alteration.

Slope stability analyses based on sediment cores of the Ligurian Margin, Southern France

Submarine slope failures of various types and sizes are common along the tectonic and seismically active Ligurian margin, northwestern Mediterranean Sea, primarily because of seismicity up to ~M6, rapid sediment deposition in the Var fluvial system, and steepness of the continental slope (average 11°). We present geophysical, sedimentological and geotechnical results of two distinct slides in water depth >1,500 m: one located on the flank of the Upper Var Valley called Western Slide (WS), another located at the base of continental slope called Eastern Slide (ES). WS is a superficial slide characterized by a slope angle of ~4.6° and shallow scar (~30 m) whereas ES is a deep-seated slide with a lower slope angle (~3°) and deep scar (~100 m). Both areas mainly comprise clayey silt with intermediate plasticity, low water content (30-75 %) and underconsolidation to strong overconsolidation. Upslope undeformed sediments have low undrained shear strength (0-20 kPa) increasing gradually with depth, whereas an abrupt increase in strength up to 200 kPa occurs at a depth of ~3.6 m in the headwall of WS and ~1.0 m in the headwall of ES. These boundaries are interpreted as earlier failure planes that have been covered by hemipelagite or talus from upslope after landslide emplacement. Infinite slope stability analyses indicate both sites are stable under static conditions; however, slope failure may occur in undrained earthquake condition. Peak earthquake acceleration from 0.09 g on WS and 0.12 g on ES, i.e. M5-5.3 earthquakes on the spot, would be required to induce slope instability. Different failure styles include rapid sedimentation on steep canyon flanks with undercutting causing superficial slides in the west and an earthquake on the adjacent Marcel fault to trigger a deep-seated slide in the east.

Results of analysis and model simulation of the bed profiles

Bedforms such as dunes and ripples are ubiquitous in rivers and coastal seas, and commonly described as triangular shapes from which height and length are calculated to estimate hydrodynamic and sediment dynamic parameters. Natural bedforms, however, present a far more complicated morphology; the difference between natural bedform shape and the often assumed triangular shape is usually neglected, and how this may affect the flow is unknown. This study investigates the shapes of natural bedforms and how they influence flow and shear stress, based on four datasets extracted from earlier studies on two rivers (the Rio Paraná in Argentina, and the Lower Rhine in The Netherlands). The most commonly occurring morphological elements are a sinusoidal stoss side made of one segment and a lee side made of two segments, a gently sloping upper lee side and a relatively steep (6 to 21°) slip face. A non-hydrostatic numerical model, set up using Delft3D, served to simulate the flow over fixed bedforms with various morphologies derived from the identified morphological elements. Both shear stress and turbulence increase with increasing slip face angle and are only marginally affected by the dimensions and positions of the upper and lower lee side. The average slip face angle determined from the bed profiles is 14°, over which there is no permanent flow separation. Shear stress and turbulence above natural bedforms are higher than above a flat bed but much lower than over the often assumed 30° lee side angle.

(Table 1) Experimental details and results

Factor-of-safety analyses of submarine slope failure depend critically on the shear strength of the slope material, which is often evaluated with residual strength values and for normally consolidated sediments. Here, we report on direct measurements of both shear strength and cohesion for a quartz-clay mixture over a wide range of overconsolidation ratios (OCRs). For normally consolidated sediment at low stresses, cohesion is the dominant source of shear strength compared to friction. Significant increases in peak shear strength occur for OCR > 4, and the primary source of this strength increase is due to increased cohesion, rather than friction. The proportion of added shear strength due to cohesion depends log-linearly on the OCR. We show that at shallow depths where OCR values can be high, overconsolidated clays can be stronger than pure or nearly pure quartz sediments, which are cohesionless under near-surface conditions. Our data also suggest that areas which have experienced significant unroofing due to previous mass movements are less likely to experience subsequent failure at shallow depths due to increased peak strength, and if failure occurs it is expected to be deeper where the OCR is lower. In seismically active areas, this is one potential explanation for the general observation of lower slope failure recurrence compared to rates expected from triggering due to local earthquakes.