Head Scarp Research Articles

Kinematics of an alpine retrogressive rockslide in the Japanese Alps

ABSTRACTThe Aresawa rockslide in the Japanese Alps experienced a partial collapse (5–10 × 105 m3) in 2004, followed by an accelerated downward movement of the head area rather than stabilization. This paper discusses the kinematics of a retrogressive rockslide area and factors promoting slope instability subsequent to the 2004 collapse, on the basis of geodetic surveys and meteorological observations from 2006 to 2010. The unstable area covers at least 2·3 × 104 m2 and consists of five active blocks. The main blocks are separated from the stable rock slope by pre‐existing ‘sackung features’ that follow the strike of the bedding plane. Shear zones below these sackung features have developed into slip planes potentially causing the next collapse. In particular, the propagation of a slip plane causing movement at 70 cm yr−1 in the head area is promoted by bidirectional movement. It consists of slow ‘dip‐slip’ movement in snow‐accumulating periods and rapid ‘side‐slip’ movement in snow‐melting and snow‐free periods, both of which occur on the same slip plane. This bidirectional movement appears to result from the combination of a loss of lateral support by the 2004 event and the immature state of the slip plane. The primary trigger of the acceleration in the unstable area is the stress release by the 2004 collapse. In addition, the presence of a potential slip plane below the sackung features also promotes slope instability, suggesting that sackung features behind a rockslide area may predefine the new head scarp of a forthcoming collapse. Copyright © 2012 John Wiley & Sons, Ltd.

Landslide susceptibility assessment in Limbe (SW Cameroon): A field calibrated seed cell and information value method

The dissected volcanic terrains around Limbe, SW Cameroon are frequently affected by small scale but destructive landslides. In this study, a raster-based data driven method involving seed cells is used to build a landslide susceptibility model for the Limbe area. Factors considered to be potential controls of slope failure within this area include slope gradient, rock type, distance from roads, slope orientation, mean annual precipitation, soil type, land cover type, stream density and distance from stream. 63 small to very small translational and rotational landslide scars were identified through extensive field work. Landslide data is randomly divided into a training (75%) and validation set (25%) and seed cells are generated by creating 25 m buffer zones around the head scarp of each scar. The quantitative relationship between landslide seed cells and the above-mentioned factors is established by a data driven approach to obtain weighted factor classes. Summing weighted factor layers, a continuous scale of susceptibility indices is obtained and reclassified into 5 susceptibility classes. Seed cells obtained from the validation data set were used to evaluate the quality of several models involving different controlling factors. Our preferred model combines the weight of 6 factors (i.e. slope gradient, land cover, mean annual precipitation, stream density, proximity to roads and slope orientation). 78% of the validation seed cells are located within the high to very high susceptibility class, which occupy 16.9% of the study area. The obtained susceptibility map is combined with the outline of urban areas and key infrastructures to evaluate zones that are vulnerable to the impact of future slope failures. Such an approach will assist civil protection and urban planning efforts in SW Cameroon. Language: en

The characteristics and failure mechanism of the largest landslide triggered by the Wenchuan earthquake, May 12, 2008, China

Strong earthquakes are among the prime triggering factors of landslides. The 2008 Wenchuan earthquake (Mw = 7.9) triggered tens of thousands of landslides. Among them, the Daguangbao landslide is the largest one, which covered an area of 7.8 km2 with a maximum width of 2.2 km and an estimated volume of 7.5 × 108 m3. The landslide is located on the hanging wall of the seismogenic fault, the Yingxiu–Beichuan fault in Anxian town, Sichuan Province. The sliding mass travelled about 4.5 km and blocked the Huangdongzi valley, forming a landslide dam nearly 600 m high. Compared to other coseismic landslides in the study area, the Daguangbao landslide attained phenomenal kinetic energy, intense cracking, and deformation, exposing a 1-km long head scarp in the rear of the landslide. Based on the field investigation, we conclude that the occurrence of the landslide is controlled mainly by the seismic, terrain, and geological factors. The special location of the landslide and the possible topographic amplification of ground motions due to the terrain features governed the landslide failure. The effects of earthquakes on the stability of slopes were considered in two aspects: First, the ground shaking may reduce the frictional strength of the substrate by shattering of rock mass. Second, the seismic acceleration may result in short-lived and episodic changes of the normal (tensile) and shear stresses in the hillshopes during earthquakes. According to the failure mechanism, the dynamic process of the landslide might contain four stages: (a) the cracking of rock mass in the rear of the slope mainly due to the tensile stress generated by the ground shaking; (b) the shattering of the substrate due to the ground shaking, which reduced the frictional strength of the substrate; (c) the shearing failure of the toe of the landslide due to the large shear stress caused by the landslide gravity; and (d) the deposition stage.

Composite rock slope kinematics at the current Randa instability, Switzerland, based on remote sensing and numerical modeling

Kinematic analysis of slope instabilities in brittle rock is crucial for understanding the reaction of the rock mass to external forcing factors. In steep terrain, inaccessibility often limits collection of relevant data and remote sensing techniques must be applied. This is the case at the current Randa rock slope instability in southern Switzerland, where a total volume of about 5–6 million m 3 moves at a rate of up to 30 mm/yr. A large portion of the unstable rock mass is exposed in an 800 m high inaccessible cliff; the main scarp of the May 1991 rock slope failure. Between 2005 and 2007, a comprehensive suite of remote sensing techniques, including photogrammetry, LiDAR, and GB-DInSAR, was combined with 3D geodetic measurements to characterize the rock mass structure and displacement patterns. Photogrammetry and LiDAR data were measured simultaneously from a helicopter using a system allowing for oblique view angles, which provided optimal observations of the steep rock cliff. We used these datasets to map large-scale structures and extract their orientation and minimum persistence, as well as to characterize the 1991 failure scarp. The northern part of the May 1991 failure surface shows a transition from stepped planar sliding at the base, to failed rock bridges in the center, to tensile failure close to the vertical head scarp. Kinematic analysis of the discontinuity sets in the currently moving rock mass shows that both toppling and translational sliding are feasible failure mechanisms. Toppling is more likely for steep faces above 2200 m, whereas translational failure is more likely in the lower portion of the instability. Interpretation of GB-DInSAR displacement maps revealed similar kinematic behavior, and also allowed identification of a basal rupture zone and lateral release plane bounding the instability. Displacement vectors derived from geodetic surveying provided new insights into the 3D kinematic behavior of the instability. All information extracted from different data sets were integrated in a conceptual model, which was then investigated with 2D numerical simulation using the discontinuum code UDEC. The numerical models were able to reproduce the hypothesized kinematic behavior well.

Monitoring rapid head scarp movement in an alpine rockslide

The Aresawa rockslide in the Japanese Alps, which partially collapsed in May 2004, produced a number of new tension cracks in the head area. This paper discusses the dynamics of the rock slope and controls on the surface velocity, based on the results of on-site monitoring of the surface movement and meteorological parameters (air and ground surface temperatures, precipitation and snow depth). The rock mass deformation progressed at ca. 60 cm yr − 1 mainly along a slip plane dipping downslope at ca. 50°. The surface velocity of the slipping rock mass showed a significant seasonal variation in response to the water infiltration. The velocity was small (≤ 1 mm day − 1 ) in winter. Subzero air temperatures and a heat-insulating snow cover, and resulting growth of seasonal frost, prevent water infiltration into the bedrock and contribute to the rock slope stability. Most of the movement in this period is attributed to gravitational deformation originating from the overburden pressure. In contrast, the surface velocity increased (up to 10 mm day − 1 ) during snow-melting and snow-free periods. Snowmelt and rainfalls promote water infiltration into the bedrock, destabilizing the rock slope and accelerating the rock slip. Such a rapid movement is attributed to water-induced sliding superimposed on gravitational deformation.

Reconstructing the geochronological evolution of large landslides by means of the trenching technique in the Yesa Reservoir (Spanish Pyrenees)

Detailed geomorphological mapping has revealed that the slopes upstream of the Yesa Dam are affected by several flysch landslides. The largest landslides correspond to La Refaya rotational slide (11 million m 3), situated just upstream of the spillway and El Vertedero translational slide (6 million m 3), located 1.5 km upstream of the dam. A portion of El Vertedero landslide (3 million m 3) was reactivated in 2006 due to overloading caused by an earth heap. A higher earth dam will be built on the downstream side of the existing gravity concrete dam to raise the maximum water level of the reservoir by 23 m. The retrodeformation analysis of the deposits exposed in two trenches excavated in the upper part of these landslides together with the obtained absolute ages (C14 and OSL) have provided information on the kinematics and evolution of these slope movements. The main findings derived from the 45 m long trench dug in the closed depression developed at the foot of the head scarp of La Refaya landslide include: (1) The landslide is older than 12 ka and has undergone a cumulative rotation higher than 17°, yielding a mean rotation rate of about 1.4°/millennium. (2) Most of the rotation (14°) occurred before 5.8 ka and according to the trench stratigraphy the kinematics of the landslide has been dominated by progressive displacement; no evidence of catastrophic reactivation has been found. The 45 m long trench excavated across the scarp and bench situated at the head of the non reactivated portion of El Vertedero translational slide reveals that: (1) The landslide is older than 38 ka. (2) The trenched portion has undergone at least one reactivation subsequent to 20 ka and has remained inactive for a long time as suggested by the highly degraded bedrock scarp, with a slope of 18° compared to the 60° dip of the underlying failure plane. Based on this study, corrective measures with an estimated cost of 11.5 million euros have been designed for this slope. The investigation illustrates the usefulness and cost-effectiveness of the trenching technique allied with geochronological methods to obtain information on the precise limits, age, failure mechanism, kinematics (progressive vs. episodic displacement, movement rate) and chronological evolution of landslides.

Size distribution of submarine landslides along the middle continental slope of the East China Sea

This paper investigates the size distribution of submarine landslides on the middle continental slope of the East China Sea (ECS) using the size of the landslide source regions. Geomorphometric mapping is used to identify 102 mass movements from multibeam bathymetric data and to extract morphological information about the head scarps and side walls. These mass movements have areas ranging between 0.06 km2 and 15.51 km2 and volumes between 0.002 km3 and 2 km3. The area vs volume relationship of these failure scarps is approximately linear, suggesting a fairly uniform failure thickness in each event with scarce deep excavating landslides. The cumulative area distribution of the slope failures can be described by an inverse power law. The submarine landslides on the mid-ECS continental slope could be considered as a large-scale self-organizing system because they have the characteristics of a dissipative system in a critical state.

Rincon Mountain megaslide: La Conchita, Ventura County, California

The 1995 and 2005 landslides in the 200-m high sea cliff above the community of La Conchita, California, are known to be part of a reactivated Holocene prehistoric landslide. We propose that the prehistoric Holocene slide is part of a much larger, several hundred million cubic meter late Pleistocene slide complex composed of upper slumps and lower flows, informally termed as the Rincon Mountain megaslide. An approximate age of 30 ka for the Rincon Mountain landslide is derived based on a 25-m high fault scarp produced in earthflow deposits that overlie the megaslide deposits and a known rate of faulting (~ 0.8 m/ky). Geomorphic evidence for the megaslide includes a prominent 100-m high amphitheater-shaped head scarp, back-tilted landslide benches, hummocky topography, and numerous smaller landslides and earthflow deposits. Geologic evidence includes deposits composed of slide breccia with fragments of the late Pleistocene (45 ka) emergent marine platform and terrace deposits displaced several tens of meters. Isolated parts of the Rincon Mountain landslide are active in the La Conchita area, but no evidence exists that the entire slide mass is moving as a unit. Landslides from the 200-m high slope behind La Conchita will reoccur and future development on the proposed Rincon Mountain slide should be very carefully evaluated to avoid reducing slope stability and reactivation of the megaslide.

Stratigraphic controls on a salt-withdrawal intraslope minibasin, north-central Green Canyon, Gulf of Mexico: Implications for misinterpreting sea level change

Three-dimensional seismic data from the Fuji basin, a salt-controlled intraslope minibasin in north-central Green Canyon, Gulf of Mexico, reveal complex interactions between gravity- and suspension-driven sedimentation. Seismic volumes for late Pleistocene (470 ka) to Holocene fill within the Fuji basin consist of approximately 45% mass transport complexes (MTCs), 5% channelized sandy turbidites, and 50% hemipelagites and muddy turbidites. At least ten MTCs within the Fuji basin flowed radially toward its depocenter, either from basin flanks (i.e., intrabasinal) or as a result of larger-scale salt motion (i.e., extrabasinal). Sediment transport directions are inferred on the basis of elongate basal incisions and smaller-scale scours, head scarps, fold orientation within the complexes, and stratigraphic thinning trends at downdip margins. An amalgamated set of three channelized sandy turbidite complexes less than 350 m (1148 ft) thick and 3 km (1.8 mi) across represents the main sand delivery pathway into the Fuji basin. These deposits are thought to be due to shelf bypass, and possibly, to proximity to the Pleistocene shoreline. Hemipelagites and muddy turbidites are homogeneous, and their thickness is relatively consistent at basin scale. This facies represents background sedimentation.A process-driven model has been developed involving halokinetic autocyclicity as the primary control on sedimentation in the Fuji basin. Passive salt motion accounts better for both the directions of sediment transport and the frequency of late Pleistocene–Holocene MTCs than currently popular eustatic and steady-state bathymetric models. The conclusion is significant in casting doubt on the generally assumed importance of eustasy in controlling off-shelf lowstand sedimentation and in implying marked variations in stratigraphic details at length scales of less than 10 km (6.2 mi).

Decadal-scale analysis of ground movements in old landslides in western Belgium

Summary. More than 150 large deep-seated landslides have been mapped in the Flemish Ardennes. Slope instabilities that have occurred in this hilly area of western Belgium during the last decades correspond to ground movements within these pre-existing landslides. In order to identify the mechanisms and controlling factors of these ground movements, a good knowledge of their spatial and temporal distribution is critical. 13 landslides affecting two hills were investigated based on several DTMs extracted by aerial stereophotogrammetry and spanning the 1952–1996 period. Vertical ground displacements were measured at each pixel by DTM subtraction with a confidence value of 0.70 m. Horizontal displacements were also estimated within the landslides and along the head scarps through topographical profiles. Most observed movements displayed patterns typical of rotational landslides. Vertical and horizontal displacements vary in magnitude both spatially and temporally, with respective ranges – 7.4 m–+ 3.8 m and 0–14 m. Many displacements are materialized in the field. We distinguished two kinds of slope processes, corresponding to either reactivation at a deeper level or shallower motion. The former re-uses pre-exiting surfaces of rupture located at depths of ~ 15– 20 m and is associated with the largest subsidence and uplift. They are also smaller reactivations confined at the landslide head. The other displacements consist in (1) earth flows occurring in the zone of accumulation sometimes as a consequence of large upslope reactivations, and (2) small failures occurring randomly. While most movements were triggered by intense rainfall, their spatial and temporal distribution is strongly related with the nature of the vegetation cover and the human activity.

CRE dating on the head scarp of a major landslide (Séchilienne, French Alps), age constraints on Holocene kinematics

Cosmic Ray Exposure (CRE) dating applied to the active Séchilienne landslide (Romanche valley, Belledonne Massif, French Alps) provides information about its Holocene dynamics from initiation to present day activity. Glacier retreat at 1100 m a.s.l. is estimated at 16.6 ± 0.6 10Be ka from glacially polished bedrock samples, with total deglaciation of the valley achieved at least by 13.3 ka. Application of the CRE method along vertical profiles sampled with a high spatial resolution of 3 m on the head scarp yields: (1) an initiation of the rock-slope failure at 6.4 ± 1.4 10Be ka and (2) a continuous rock-slope failure activity with a mean head scarp exposure rate of 0.6 cm/yr. The data suggest an increase of the head scarp exposure rate between 2.3 and 1.0 ka. After this acceleration phase, the exposure rate is similar to that obtained by the present day monitoring data over 20 years in the depletion zone (1.3 cm/yr). Since the failure initiation occurred more than 5400 yr after the total deglaciation of the valley the slope failure does not appear as an immediate consequence of debutressing in the Romanche valley. This result is consistent with studies of other large alpine rockslides in the Alps. Failure initiation of the Séchilienne landslide occurred during the Holocene Climatic Optimum, a hot and wet period. The temperature and precipitation changes of this climatic optimum seem to have a worsening effect at the regional scale to trigger large mass wasting in this glacial alpine valley.

Investigation of the nature of slip surface using geochemical analyses and 2-D electrical resistivity tomography: a case study from Lapseki area, NW Turkey

The nature and subsurface structure of the slip surface of a landslide was studied on the basis of geochemical analyses and 2-D electrical resistivity tomography (ERT) survey. Head scarp and lateral slip surfaces of the landslide marked by clear slickensided shear planes were composed of the average amounts of clayey silt (32.5%) and sand (67.5%). Energy dispersive X-ray spectroscopy (EDX) data revealed the enrichment of Si (23.24%), Fe (12.2%), Al (9.51%) and C (8.34%) in the elemental composition of the disturbed slip surface. From X-ray diffractometry (XRD) data, six main clay types were determined, such as Volkonskoite, Halloysite, Ferrosilite, Saponite, Illite and Nontronite. The ERT survey displayed that the landslide developed as a reactivation on the upper part of an old landslide body.

Geomorphological investigation of the excavation‐induced dündar landslide, bursa — turkey

.This paper discusses the occurrence and development of the excavation‐induce deep‐seated landslide, which took place near Dündar village, located west of Orhaneli town in northwestern Turkey. The event occurred in the Bursa‐Orhaneli lignite field, which has been actively operating since 1979. Due to undermining of a gently inclined slope (10°) to extract a coal seam, primary tension cracks, which were precursors of the movement, were first observed in the northern head area in mid‐ to late October 2003. This movement happened simultaneously with precipitation that was significantly above long‐term average measured at a nearby climatology station (Keles). This precipitation amount is characterized statistically by a significant standardized anomaly of 1.6. The majority of the monthly precipitation total in October 2003, which mainly consisted of rain showers and thunderstorms, occurred in the last week of the month. By April 2004, rotational failure continued intermittently. After a relatively wet (rainy and snowy) period from January 2004 to April 2004, the main rotational slump occurred in late April 2004, causing the entire destruction of Dündar village's cemetery. Daily climatic and synoptic meteorological data have proved that heavy showers in late April may had triggered the last slump by producing rain showers of 19.3 mm and 19.9 mm daily total on 27 and 28 April 2004, respectively.Field observations carried out along the main head scarp have shown that the slope failure was facilitated by a pre‐existing normal fault with an east‐ west direction and 80° dip. Grain‐size analysis showed that the failure occurred on clayey silt, which forms 55% of the slip surface material. Based on the evidence from X‐ray fluorescence and energy dispersive X‐ray spectroscopy results, smectite‐type clay ‐ a product of the chemical weathering of tuff ‐ was the main constituent of the slip surface material. The landslide occurred over an area of 600 m × 650 m with a total volume of 8775 000 m3. Approximately 28 hectares of farm land were entirely destroyed and the excavated coal seam was buried. The mining operation was moved to 100 m north of the landslide area near Gümüşpınar village. From morphological evidence, it is concluded that excavation activities caused the failure to extend in more than one direction as an enlarging sliding mechanism; this produced a high landslide risk for Gümüşpınar village, where the most significant normal fault with a 75 m vertical displacement in a coal‐bearing sequence is found in the lignite field.

Ferguson rock slide buries California State Highway near Yosemite National Park

During spring 2006, talus from the toe area of a rock-block slide of about 800,000 m3 buried California State Highway 140, one of the main routes into heavily-visited Yosemite National Park, USA. Closure of the highway for 92 days caused business losses of about 4.8 million USD. The rock slide, composed of slate and phyllite, moved slowly downslope from April to June 2006, creating a fresh head scarp with 9–12 m of displacement. Movement of the main rock slide, a re-activation of an older slide, was triggered by an exceptionally wet spring 2006, following a very wet spring 2005. As of autumn 2006, most of the main slide appeared to be at rest, although rocks occasionally continued to fall from steep, fractured rock masses at the toe area of the slide. Future behavior of the slide is difficult to predict, but possible scenarios range from continued scattered rock fall to complete rapid failure of the entire mass. Although unlikely except under very destabilizing circumstances, a worst-case, rapid failure of the entire rock slide could extend across the Merced River, damming the river and creating a reservoir. As a temporary measure, traffic has been rerouted to the opposite side of the Merced River at about the same elevation as the buried section of Highway 140. A state-of-the-art monitoring system has been installed to detect movement in the steep talus slope, movement of the main slide mass, local strong ground motion from regional earthquakes, and sudden changes in stream levels, possibly indicating damming of the river by slide material.

Engineering geology and future stability of the El Risco landslide, NW-Gran Canaria, Spain

A 200 m long segment of the only main road in NW-Gran Canaria is built on landslide deposits near the village of El Risco. Structural mapping and analysis of the topography reveal that the N–S striking landslide head scarp is the upper part of a sub-circular failure surface. The southern side of the landslide is delimited by a much older E–W strike-slip fault. Prior to pavement resurfacing in 2006, cracks in the road tarmac at the northern and southern sides of the landslide suggested ongoing creep movement. Slope stability analyses suggest that peak ground acceleration (PGA) was the most likely trigger for the initial failure.

Importance of aeolian processes in the origin of the north polar chasmata, Mars

A detailed examination of the location and orientation of sand dunes and other aeolian features within the north polar chasmata indicates that steep scarps strongly influence the direction and intensity of prevailing winds. These steep scarps are present at the heads and along the margins of the north polar chasmata. Topographic profiles of the arcuate head scarps and equator-facing wall of Chasma Boreale reveal unusually steep polar slopes ranging from ∼6°–30°. The relatively steep-sloped (∼8°), sinuous scarp at the head of two smaller chasmata, located west of Chasma Boreale, exhibits an obvious resistant cap-forming unit. Scarp retreat is occurring in places where the cap unit is actively being undercut by descending slope winds. Low-albedo surfaces lacking sand dunes or dust mantles are present at the base of the polar scarps. A ∼100–300 m deep moat, located at the base of the scarps, corresponds with these surfaces and indicates an area of active aeolian scour from descending katabatic winds. Small local dust storms observed along the equator-facing wall of Chasma Boreale imply that slope wind velocities in Chasma Boreale are sufficient to mobilize dust and sand-sized particles in the Polar Layered Deposits (PLD). Two amphitheater forms, located above the cap-forming unit of the sinuous scarp and west of Chasma Boreale, may represent an early stage of polar scarp and chasma formation. These two forms are developing within a younger section of polar layered materials. The unusually steep scarps associated with the polar chasmata have developed where resistant layers are present in the PLD, offering resistance during the headward erosion and poleward retreat of the scarps. Steep slopes that formed under these circumstances enhance the flow of down-scarp katabatic winds. On the basis of these observations, we reject the fluvial outflood hypothesis for the origin of the north polar chasmata and embrace a wind erosion model for their long-term development. In the aeolian model, off-pole katabatic winds progressively remove materials from the steep slopes below chasmata scarps, undermining resistant layers at the tops of scarps and causing retreat by headward erosion. Assuming a minimum age for the onset of formation of Chasma Boreale (10 5 yr) results in a maximum volumetric erosion rate of 0.35 km 3 yr −1 . Removal of this volume of material from the equator-facing wall and head scarps of chasma would require a rate for scarp retreat of 0.5 m yr −1 .

High-resolution repeat bathymetric mapping using a mid-size autonomous underwater vehicle

Shipborne bathymetric surveys south of Santa Barbara, CA indicate that massive slope failures have occurred along the northern flank of the Santa Barbara Basin. This region is seismically active and has the highest sedimentation rates along the California coast due to rapid erosion of the Transverse Ranges. A set of geodetic monitoring systems is being developed to study the character and deformation of the ocean bottom in this region, particularly that of a prominent east-west trending crack aligned with the head scarp of one of the underwater landslides. One component is a high-resolution repeat bathymetric mapping system composed of a multibeam sonar mounted inside a Bluefin 21 AUV. Navigation to the decimeter level is provided by an inertial navigation system and accurate depth sensor, and by a long baseline (LBL) acoustic transponder system previously developed to monitor tectonic plate motions. The quality of the data, particularly that from the high-precision LBL system, is enhanced significantly by the decrease in the radiated acoustic and vibration noise of the AUV to levels at, or below, ocean background noise levels through modifications to the AUV propulsion system. [Work sponsored by British Petroleum and the Office of Naval Research.]

Examples of multiple rock-slope collapses from Köfels (Ötz valley, Austria) and western Norway

This paper describes the effects of large massive rock-slope failures on subsequent slope stability. Three examples of large rock-slope failures from the Austrian Alps and Norway demonstrate that failure increases the probability of further collapses. At Köfels, Austria, a Holocene rock-slope failure several km 3 in size filled the Ötz valley. The morphology of the deposits indicates that at least one subsequent failure occurred along the head scarp of the first failure, most likely a slide of similar size. Debris of the second landslide slid over the older deposits, forming the famous Köfels frictionite. At least three rock-slope failures, all of them in excess of 10 6 m 3, occurred from the same mountainside within the last 6000–8000 years at Tafjord in western Norway. The most recent of these failures in 1934, triggered a destructive tsunami. Five large failures with volumes ≥ 50,000m 3 occurred at Ramnefjell, Norway within 50 years; two of them caused considerable damage and a large number of death due to the formation of destructive tsunamis. Two-dimensional finite element models of rock-slope stability before and after the Köfels and Tafjord landslides show that massive rock-slope failures produce: (a) irregular slopes, parts of which are as steep or steeper than the slope before failure and which represent new zones of instability; and (b) zones of weakness related to slow slope deformation and related cracking.

Submarine landslides in the Santa Barbara Channel as potential tsunami sources

Abstract. Recent investigations using the Monterey Bay Aquarium Research Institutes (MBARI) Remotely Operated Vehicles (ROVs) "Ventana" and "Tiburon" and interpretation of MBARI's EM 300 30 kHz multibeam bathymetric data show that the northern flank of the Santa Barbara Basin has experienced massive slope failures. Of particular concern is the large (130 km2) Goleta landslide complex located off Coal Oil Point near the town of Goleta, that measures 14.6-km long extending from a depth of 90 m to nearly 574 m deep and is 10.5 km wide. We estimate that approximately 1.75 km3 has been displaced by this slide during the Holocene. This feature is a complex compound submarine landslide that contains both surfical slump blocks and mud flows in three distinct segments. Each segment is composed of a distinct head scarp, down-dropped head block and a slide debris lobe. The debris lobes exhibit hummocky topography in the central areas that appear to result from compression during down slope movement. The toes of the western and eastern lobes are well defined in the multibeam image, whereas the toe of the central lobe is less distinct. Continuous seismic reflection profiles show that many buried slide debris lobes exist and comparison of the deformed reflectors with ODP Drill Site 149, Hole 893 suggest that at least 200 000 years of failure have occurred in the area (Fisher et al., 2005a). Based on our interpretation of the multibeam bathymetry and seismic reflection profiles we modeled the potential tsunami that may have been produced from one of the three surfical lobes of the Goleta slide. This model shows that a 10 m high wave could have run ashore along the cliffs of the Goleta shoreline. Several other smaller (2 km2 and 4 km2) slides are located on the northern flank of the Santa Barbara Basin, both to the west and east of Goleta slide and on the Conception fan along the western flank of the basin. One slide, named the Gaviota slide, is 3.8 km2, 2.6 km long and 1.7 km wide. A distinct narrow scar extends from near the eastern head wall of this slide for over 2km eastward toward the Goleta slide and may represent either an incipient failure or a remnant of a previous failure. Push cores collected within the main head scar of this slide consisted of hydrogen sulfide bearing mud, possibly suggesting active fluid seepage and a vibra-core penetrated ~50 cm of recent sediment overlying colluvium or landslide debris confirming the age of ~300 years as proposed by Lee et al. (2004). However, no seeps or indications of recent movement were observed during our ROV investigation within this narrow head scar indicating that seafloor in the scar is draped with mud.

Slope stability evaluation using Back Propagation Neural Networks

The Yudonghe landslide, located in western Hubei Province of China, consists of eastern and western subunits as well as a main landslide mass with upper and lower slip surfaces. As an important landslide close to Shuibuya Dam on the Qing River, its stability is crucial, as the slide might reactivate because of a change in ground-water level caused by filling of the Shuibuya Reservoir. Existing weakness zones, growth of ruptures, the downslope attitude of geologic strata, and water infiltration, which reduced the strength of rocks and soils, have been found to be the most important factors contributing to the Yudonghe landslide. With regard to the landslide processes, it can be noted that the original large-scale slide activity was due to erosion by the Qing River, the second sliding resulted from the fall of blocks from the head scarp, and the final activity was the growth of the eastern and western secondary slides. A base failure was the main type of slope movement, however, it was obvious that more than one sliding event occurred, as inferred from striations and fractures detected by microstructure analysis of soils along the failure surfaces. Slope instability was evaluated by the method of Back Propagation Neural Networks (BPNN), in which a four-layer BPNN model with five input nodes, two hidden layers, and two output nodes was constructed using a training data set of landslide samples throughout the Qing River area. The predicted results of this analysis showed that the factor of safety was 1.10, which indicates that the Yudonghe landslide is currently in a marginally stable condition.