摘要:

An analysis of 179 mass movements on the North American Atlantic continental slope and upper rise shows that slope failures have occurred throughout the geographic extent of the outer margin. Although the slope failures show no striking affinity for a particular depth as an origination level, there is a broad, primary mode centered at about 900 m. The resulting slides terminate at almost all depths and have a primary mode at 1100 m, but the slope/rise boundary (at 2200 m) also is an important mode. Slope failures have occurred at declivities ranging from 1° to 30° (typically, 4°); the resultant mass movement deposits vary in width from 0.2 to 50 km (typically, 1–2 km) and in length from 0.3 to 380 km (typically, 2–4 km), and they have been reported to be as thick as 650 m. On a numeric basis, mass movements are slightly more prevalent on open slopes than in other physiographic settings, and both translational and rotational failure surfaces are common. The typical mass movement is disintegrative in nature. Open slope slides tend to occur at lower slope angles and are larger than canyon slides. Further, large‐scale slides rather than small‐scale slides tend to originate on gentle slopes (≈ 3–4°). Rotational slope failures appear to have a slightly greater chance of occurring in canyons, but there is no analogous bias associated with translational failures. Similarly, disintegrative slides seem more likely to be associated with rotational slope failures than translational ones and are longer than their nondisintegrative counterparts. The occurrence of such a variety of mass movements at low declivities implies that a regional failure mechanism has prevailed. We suggest that earthquakes or, perhaps in some areas, gas hydrates are the most likely cause of the slope failures.

ISSN:0360-8867    

DOI:10.1080/10641199109379879

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

ISSN:0360-8867    

DOI:10.1080/10641199109379878

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

GLORIA sidescan sonar surveys have shown that large landslides are ubiquitous around the submarine flanks of Hawaiian volcanoes, and GLORIA has also revealed large landslides offshore from Tristan da Cunha and El Hierro. On both of the latter islands, steep flanks formerly attributed to tilting or marine erosion have been reinterpreted as landslide headwalls mantled by younger lava flows. Large landslides have also been inferred from several oceanic islands elsewhere by other workers using different evidence, and we suggest that seacliffs previously attributed to marine erosion of many additional islands may instead be headwalls of still other landslides. These landslides occur in a wide range of settings and probably represent only a small sample from a large population. They may explain the large volumes of archipelagic aprons and the stellate shapes of many oceanic volcanoes. Large landslides and associated tsunamis pose hazards to many islands.

ISSN:0360-8867    

DOI:10.1080/10641199109379880

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

GLORIA sidescan sonar imagery and seismic‐reflection profiles show pervasive evidence for a wide variety of slides and slumps associated with the large canyons of the 1400‐km‐long Beringian margin. Styles of failure include mud and debris flows, slumps, and massive block slides. A 100‐km‐long shelf‐edge segment on the northern margin is characterized by a series of scalloped slide scars and incipient scars associated with sedimentary blocks, 1–2 km across, that seem to be the initiators of a series of small canyons. Some of the largest single slide masses, including huge blocks tens of kilometers wide, occur on the rise of the central margin. Sliding of these blocks may have initiated the incision of some of the world's largest submarine canyons, a prime example of which is the massive Zhemchug Canyon. Mass movement along the southern margin is widespread at the edges of Umnak Plateau. One mass failure, particularly well defined by GLORIA, is 55 km long. This slide and others along the plateau are associated with diapiric‐like structures indicative of relatively recent tectonism.

ISSN:0360-8867    

DOI:10.1080/10641199109379881

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Submarine slab slides occur in homoclinal sections of consolidated sediment and lithified strata on continental and insular slopes. They are characterized by distinctive structural and morphological features including complex scarps, sheet‐like or tabular form, proximity to intrastratal deformation, and highly comminuted, long‐runout debris sheets having distinct margins and flow forms. Such slides are difficult to explain by conventional concepts of slope stability that depend on Mohr‐Coulomb failure criteria unless conditions of very low effective stress are invoked. Geologic features suggest that some slab slides probably result from long‐term strength degradation of weak layers deep in the homoclinal section. Time‐dependent strain in clay‐rich layers can create potential slide surfaces of low factional strength. Competent layers are weak in tension and probably fragment in the first instance of, or even prior to, translation, and the allochthonous mass is readily transformed into a high‐momentum debris flow. The structure and geomorphology of slab slides provide important clues to their origin and behavior. Proper consideration of geological features and stress history of a failed homoclinal section can help determine the most likely mechanism of failure and the most useful model of kinematic behavior of slab sides.

ISSN:0360-8867    

DOI:10.1080/10641199109379882

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Closely spaced seismic‐reflection profiles across the eastern continental margin of Korea reveal that large‐scale slope failure scars occur ubiquitously on steep slopes (gradient, >5°) in water depths greater than 400 m. The slope failure scars are well defined with irregular scour planes and sometimes coalesce to form a large escarpment (maximum area, 275 km2). The failed material was deposited farther downslope as slide and mass‐flow deposits. These submarine slope failures are probably due to both seismicity associated with faulting and the steep gradient of the slope.

ISSN:0360-8867    

DOI:10.1080/10641199109379883

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

A variety of slope failure features have been identified in the vicinity of the Hellenic arc. In prodelta environments small‐scale block slumps are the predominant products of shelf instabilities on relatively low angle slopes (0.5–2°), while relatively larger scale rotational and translational sides, as well as debris and turbidity flows, were observed on steeper slopes (2–8°). The shear plane is believed to be the gas‐charged sediments and/or the fine‐grained (and low shear strength) basal sedimentary layer of the prodelta sediment sequences. A series of rotational slide blocks were also observed within the shelf‐edge Late Pleistocene prodelta sequence. These failures were initiated soon after the sequence was deposited but are presently inactive. Large‐scale translational slides and slumps were detected, particularly in the fore‐arc continental slopes, associated with major active fault zones. Pleistocene interglacial hemipelagic mud sheets or sapropelic layers may act as failure planes within these slopes. Cyclic loading induced by earthquake activity is the principal cause of the undrained slope failures in the investigated area of the northeastern Mediterranean Sea. Storm‐wave loading or tsunamis could be additional factors for inducing failure of the upper slope, particularly during low sea‐level stands. Infinite‐slope stability analysis indicates that, in general, slopes greater than I ° are unstable in areas in which the expected peak ground accelerations exceed 13–15% g.

ISSN:0360-8867    

DOI:10.1080/10641199109379884

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The stability of seafloor sediments constitutes an important consideration in planning and design of various offshore facilities. The stresses induced either by waves during a storm or by shaking during an earthquake may cause shear failure in ocean floor sediments leading to landslides. In this paper, the basic elements of analysis for submarine landslides are presented. The loadings associated with both wave action and earthquakes are treated as random processes and some of the soil parameters are also considered to be random variables. The analyses for different modes of seafloor instability are formulated in a stochastic framework, which provides the basis for the evaluation of risk of submarine landslides in a given offshore region. Numerical results of some example analyses are also presented.

ISSN:0360-8867    

DOI:10.1080/10641199109379885

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The Beaufort Sea continental slope is disrupted by a belt of massive bedding‐plane slides and rotational slumps. This zone coincides with a region of sediment containing gas hydrate, an inclusion compound of gas and water. Quantitative studies suggest that elevated pore‐fluid pressures generated as a by‐product of gas hydrate disassociation during repeated episodes of eustatic sea level lowering during Pleistocene time were a major cause of these slides. Eustatic sea level fall causes reduced pressures acting on seafloor sediment. In oceanic areas underlain by sediment with gas hydrate, the reduction of sea level initiates disassociation along the base of the gas hydrate, which, in turn, causes the release of large volumes of gas into the sediment and creates excess pore‐fluid pressures and reduced slope stability. A quantitative approach was taken to predict excess pressures at the base of the gas hydrate zone, and the stability of the overlying sediment for a variety of sediment types. Our studies suggest that during eustatic sea level fall, fluid diffusion properties dominate the disassociation process in fine‐grained marine sediment. Slope failure appears likely for this sediment type on moderate slopes unless pressures can be adequately vented away from the gas hydrate base. These models give credence to the theory that fluctuations in global climate, manifest in periodic Pleistocene eustatic‐sea level regressions, likely triggered seafloor landslides on the continental slope of the Beaufort Sea and other margins where gas hydrate is present in seafloor sediment.

ISSN:0360-8867    

DOI:10.1080/10641199109379886

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The concept of the steady‐state of deformation can be applied to predicting the ultimate form a landslide will take. The steady‐state condition, defined by a line in void ratio‐effective stress space, exists at large levels of strain and remolding. Conceptually, if sediment initially exists with void ratio‐effective stress conditions above the steady‐state line, the sediment shear strength will decrease during a transient loading event, such as an earthquake or storm. If the reduced shear strength existing at the steady state is less than the downslope shear stress induced by gravity, then large‐scale internal deformation, disintegration, and flow will occur. If sediment exists at a state that is on or below the steady‐state line, disintegration and flow will typically not occur. Confirming these concepts, studies of subaerial landslides show an association between disintegrative flows and void ratio‐effective stress states above the steady‐state line. Nondisintegrative landslides are associated with void ratio‐effective stress states on or below the steady‐state line. A subsequent investigation of three submarine landslides also confirmed these concepts. One limited‐deformation slump had an initial state coinciding with the steady‐state line, and a second limited‐deformation slump had an initial state above the steady‐state line but gravitational shear stresses less than the steady‐state shear strength. A third failure involved sediment with an initial state above the steady‐state line and initial gravitational shear stresses greater than the steady‐state shear strength. A disintegrative flow was the result. The flow stopped near a change in slope where the gravitational shear stress fell below the steady‐state shear strength.

ISSN:0360-8867    

DOI:10.1080/10641199109379887

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor