Major Landslide Research Articles

Runoff and sediment yield from rural roads, trails and settlements in the upper Konto catchment, East Java, Indonesia

The potential importance of roads and settlements for the generation of storm runoff and sediment in tropical steeplands is increasingly recognised but rarely quantified. This paper presents runoff and sediment yield data for a cobbled and an unpaved road section, two large unbounded settlement plots, and several trails draining residential areas or terraced fields in a volcanic upland catchment in East Java. In addition, the sediment yield of major landslides associated with roads was quantified. The unpaved road section exhibited an average runoff coefficient of about 65% and yielded about 7 kg m − 2 yr − 1 of sediment. Both the runoff coefficient and the sediment yield for the cobbled road section (plus adjacent yards) were lower (38% and 1.9 kg m − 2 yr − 1 ). Sediment output from a 4160-m 2 hillside plot including a network of trails draining terraced fields (for which runoff and sediment outputs were shown to be negligible) was similar to that for the cobbled road (2–3 kg m − 2 yr − 1 depending on rainfall). However, a much higher value was obtained when the overall soil loss from the plot was expressed per square metre of trail surface area (ca. 42 kg m − 2 yr − 1 ) whereas the associated trail runoff coefficient was about 70%. The results obtained for several trails and large unbounded plots draining residential areas at two locations were less extreme (runoff coefficients of 24–43%; soil loss 1.3–3.5 kg m − 2 yr − 1 ). Landsliding occurred mainly at the end of the rainy season (March) and was estimated to have contributed ca. 2365 m 3 of sediment to the main road network during the 1988/89 wet season vs. ca. 905 m 3 in 1989/90 and only ca. 150 m 3 in 2000/01. It is concluded that, despite their relatively small areal extent (5% in the study area), rural roads, trails and settlements are significant producers of runoff and sediment at the catchment scale and should be included in watershed management programmes designed to reduce catchment sediment yields and reservoir siltation.

Landslide geomorphology of Cayton Bay, North Yorkshire, UK

SUMMARY The geomorphology of a large coastal landslide complex at Cayton Bay, North Yorkshire, is described. The area inland of the complex is occupied by a main road and a number of properties, so knowledge of current and possible future landslide behaviour is important for planning and risk assessment. Instability at the site is associated with a number of faults that bring argillaceous Upper Jurassic rocks to sea level. These soft rocks are overlain by more resistant sandstones. The sequence is capped by a thick and variable succession of glacial sediments, comprising tills with interbedded sand and gravel lenses that were deposited during the Dimlington Stadial of the Late Devensian. In connection with a study aimed at understanding cliff instability and recession, coastal erosion and the risk to people and assets at Cayton Bay, detailed geomorphological field mapping was conducted. This identified two major landslide systems, which include a periodically active mudslide complex at Cayton Cliff, recognized by shallow scarps and benches with occasional back-tilted blocks, and an area of dormant, deep-seated landslides at Tenants’ Cliffs, which includes a number of graben and horst structures. The origins of the landslides are unclear, but probably involved a variety of processes that led to a reduction in material shear strength or increases in pore water pressures. The original failure probably occurred in the middle Holocene, around 3000 years ago, when sea level reached its approximate current position. This would have initiated erosion of Jurassic argillaceous rocks (Oxford Clay) now exposed at sea level on the coast, and probable deep-seated failure of the overlying, more competent strata (Lower Calcareous Grit). Alternatively, it is possible that retreat of the Late Devensian ice sheet triggered instability, and that the landslide complex is a paraglacial landform produced during deglaciation, around 13 000 years BP. However sea levels were then lower so erosion of the Oxford Clay is unlikely to have been a factor in its development. Contemporary instability is likely to be due to the combined effects of coastal erosion and high groundwater levels, both of which are predicted to increase in the future due to the impact of climate change, implying an increased risk to coastal assets and a need to manage and mitigate such risks.

Landslide of Rishikesh‐Badrinath National Highway at Devprayag, District Tehri Garhwal, Uttaranchal (India)

Purpose – The purpose of this paper is to report on the landslide of Rishikesh‐Badrinath National Highway in Uttaranchal, India.Design/methodology/approach – The paper discusses causes, mitigation and preventive measures in and around the township of Devprayag.Findings – Apart from the major landslide events (such as Malpa and Okhimath in 1998) most landslides go unnoticed because the losses in most of the events are not great enough to catch the media attention. These are therefore often considered as stray events but the cumulative losses from these events are significant. Geological and tectonic evolution of the Himalayan terrain has resulted in large‐scale metamorphism, thrusting, faulting, folding and deformation of the rocks that have resulted in a terrain that is full of rocks that are highly fractured, jointed and sheared and thus in a fragile state. High relative relief, steep slopes, concentrated rainfall with high rates of surface flow together with the ongoing tectonism make the region highly ...

The 2004 Niigata Ken Chuetsu, Japan, Earthquake

The 23 October 2004 Niigata Ken Chuetsu, Japan, earthquake ( Mw=6.6) was the most significant earthquake to affect Japan since the 1995 Kobe earthquake, killing 48 people and injuring 4,160. Hundreds of major landslides destroyed entire upland villages and, together with permanent ground deformations, caused extensive damage to roads, rail lines and other lifelines, resulting in major economic disruption and an estimated cost of U.S. $40 billion. Two reconnaissance teams supported by EERI surveyed the earthquake damage soon after the event, employing new technologies such as handheld global positioning systems, satellite imagery, and light detection and ranging, which facilitated more rapid and detailed data collection than was possible in previous earthquake damage surveys.

The influence of landsliding on sediment supply and channel change in a steep mountain catchment

Rates of sediment supply by landsliding to an alluvial channel in a small catchment in central Switzerland were estimated over an 11-month study period. Fluvial sediment transport in the channel is independently monitored at the upstream and downstream ends of the study reach, yielding a unique opportunity to quantitatively compare the volume of sediment supplied to the channel with the volume in fluvial transport. Landslide-derived sediment discharge to the channel was greatest during the winter and spring months, while most of the fluvial sediment transport occurred during short, intense summer storms. Approximately 98 m 3 of sediment was delivered directly to the study reach by landsliding, ∼ 80 m 3 was transported into the reach from upstream, and ∼ 70 m 3 was transported out of the reach. Thus, there was a net accumulation of ∼ 100 m 3 of sediment during the 11-month study. Decadal-scale channel aggradation was independently assessed by comparing channel longitudinal profiles in 1993 and 2004. During this 11-year period, a total of ∼ 1500 m 3 of sediment has accumulated in the study reach. Aggradation has occurred largely in two broad zones that correspond with both the locations of major landslide complexes and reaches of high channel slope, indicating that hillslope sediment input left an imprint on the morphology of the channel bed that appears to be stable over at least decadal time scales.

Landslides in the Alaknanda Valley of Garhwal Himalaya, India

The occurrence of landslides is a recurring problem along the hill roads of Himalaya. Roads are often blocked due to landslides, which sometimes also leads to casualties. Identification of such unstable slopes is an imperative task to save life and property. The paper outlines features of major landslides along a National Highway in the Alaknanda valley of Garhwal Himalaya through photo documentation.

VEGETATION DYNAMICS IN THE DRY VALLEYS OF YUNNAN,CHINA, DURING THE LAST 150 YEARS: IMPLICATIONS FOR ECOLOGICAL RESTORATION

In southwestern China, there are many opinions about past environmental conditions and the historical drivers of change, but little direct evidence. Such is the case in Dry Valley ecosystems, where current conservation programs appear to be based more on subjective perceptions than scientific understanding. Our research objective was to develop an historical profile—the description and explanation of environmental change—for the Dry Valleys to see if today's perceptions about past change are supported by data. Our study area encompassed northwestern Yunnan and adjacent Sichuan and Tibet. Specifically, we assessed vegetation dynamics by comparing historical records from 1868-1949 to conditions in 2001-2005. We used two sources to establish baseline conditions from which to assess change: 28 historical publications and 56 old photographs for which we rephotographed the modern scene. Our major finding is that there has been no substantial change in the cover of Dry Valley vegetation during the last 150 years. We also found that the aerial coverage of crop fields has decreased and major landslide activity was caused by chronic geological instability and infrastructure development. Overall, we found that comparisons of written and photographic records between 1868 and 2005 do not support many common assumptions regarding environmental degradation in the Dry Valleys, upon which current ecological restoration programs are based.

Analysis of the peat slide at Pollatomish, County Mayo, Ireland

A major landslide event occurred at Pollatomish, County Mayo, Ireland in September 2003, during a period of intense rainfall. It comprised about 30 significant individual longitudinal planar type slides of peat and weathered rock. Relatively simple limit state stability analyses, using the method of slices and an infinite slope analysis, were used to model the slide and it was found that the features observed on site could easily be reproduced. These included confirmation that thin layers of peat could be stable on steep slopes but the margin of safety reduces rapidly under elevated pore pressure conditions. As was observed in the field, the analyses suggested the most vulnerable zone was the upper layer of weathered rock but that slides could occur in the peat if its thickness was appreciable. Careful site characterization is vital in such studies. Here efforts have been made to understand the effect of fibers on the peat strength and some sensitivity analyes have been performed to assess the critical engineering parameters of the peat.

The landslide hazard and human vulnerability in La Paz City, Bolivia

Landslides in the city of La Paz, Bolivia are complex in space and time. Their distribution within the city is differentiated by geographical variations in slope gradient, the nature of overlying surface deposits, and drainage density patterns. When mapped, the distribution of the most landslide‐prone locations in the city coincides with the most mobile surface deposits on the higher and steeper slopes of the city. The timing of landslides is triggered when slope materials become saturated with moisture by rainfall, stream water, water seepage from high surrounding water tables, and from domestic sources. Landslide frequencies over the last 40 years show the role of inter‐annual variations in summer rainfalls, especially the late summer period (Jan–Mar), and rising water tables connected to Lake Titicaca. Associations with ENSO events, both warm (El Nino) and cold (La Nina), prove negative, pointing to the importance of moisture sources from the north, south and east (Amazon) but not from the west (Pacific). The most vulnerable group exposed to the landslide hazard comprise the inhabitants of the self‐built informal housing areas who occupy the more elevated steeper slopes of the northern part of the city. But societal vulnerability to earth movement in the city is widespread and interconnected. Higher income groups in the southern part of the city are affected by slower, if less dramatic, forms of land failure, and rich housing in downstream areas of the city are impacted by flash flooding encouraged by deforestation on the northern hill slopes where the poor live. Two case studies highlight the vulnerability and plight of the inhabitants of the self‐built settlements in the aftermath of a major landslide, and a brief discussion of hazard mitigation and the importance of building hazard resilient communities concludes the paper.

Contribution of deep‐seated bedrock landslides to erosion of a glaciated basin in southern Alaska

AbstractLandslides represent a key component of catchment‐scale denudation, though their relative contribution to the erosion of glaciated basins is not well known. Bedrock landslide contribution was investigated on the surface of one of eleven glaciers on a glaciated ridge in the Chugach‐St Elias Range of southern Alaska, where the debris from four major landslides is easily distinguished from moraines and other supraglacial material. A series of aerial and satellite photos from 1972 to 2000 and field observations in 2001 and 2002 indicate that three of four landslides have fallen onto the surface of the glacier since about 1978. The landslides, which originated from the steeply dipping (60–70°) bedrock walls, were deposited onto the glacier in the ablation zone and are currently being transported downstream supraglacially. Individual glacial valleys with topographic relief of ∼400 m are cut into high‐grade metamorphic rock characterized by a steep north‐dipping foliation and fractured by numerous large joints. Measurements of landslide area and average thickness obtained from high‐resolution survey data indicate a total landslide volume of ∼2·3 × 105 m3. This volume suggests a basin‐averaged erosion rate from landslides of 0·48 mm a−1. An overall basin‐scale erosion rate of 0·7 to 1·7 mm a−1 can be inferred, but depends on the percentage of the total‐basin sediment yield contributed by supraglacial sources. A mean rockwall retreat rate of 6·7 mm a−1 is calculated and is considerably higher than published rates, which range from 0·04 to 4·0 mm a−1. Controls on landslide generation include seismicity, freeze–thaw processes, topography, rock strength, and debuttressing. It is likely all of these factors contribute to failure, although the primary controls for the landslides in this study are thought to be rock strength and topography. The absence of landslides on ten of the eleven glaciers on this ridge is attributed to landslide magnitude–frequency relationships and short temporal scale of this study. Large‐volume bedrock landslides (>100 000 m3) may have low frequency, occurring less than once in a 55‐year time frame. Copyright © 2005 John Wiley & Sons, Ltd.

The submarine flanks of Anatahan Volcano, commonwealth of the Northern Mariana Islands

The submarine flanks of Anatahan volcano were surveyed comprehensively for the first time in 2003 and 2004 with multibeam and sidescan sonar systems. A geologic map based on the new bathymetry and backscatter data shows that 67% of the volcano's submarine flanks are covered with volcaniclastic debris and 26% is lava flows, cones, and bedrock outcrops. The island of Anatahan is only 1% of the volume of the entire volcano, which has a height from its submarine base of 3700 m and an average diameter of ∼35 km. NE Anatahan is a prominent satellite volcano located 10 km NE of the island, but it is only 6% of Anatahan's volume (40 km 3 vs. 620 km 3). Seventy-eight submarine eruptive vents are mapped associated with lava flows and cones between depths of 350 and 2950 m, and 80% of these vents are located in a cluster on the east flank of the volcano. The distribution of cones and lava flows vs. depth suggests a possible change in eruptive style from explosive to effusive between 1500 and 2000 m. Eruptive vents below 2000 m have produced mostly lava flows. There is no evidence of major landslides on the submarine flanks of Anatahan volcano, in contrast to many basaltic islands and seamounts, suggesting that mass wasting at felsic oceanic arc volcanoes may be characterized by sediment flows of unconsolidated volcaniclastic debris instead of mass movements of relatively large intact blocks.

Monitoring the morphological evolution of the Sciara del Fuoco during the 2002–2003 Stromboli eruption using multi-temporal photogrammetry

After a period of anomalous activity affecting the Volcano of Stromboli (Aeolian volcanic arc, Italy), the “Sciara del Fuoco” slope, situated on the north–east flank of the island, was affected by major landslides on December 30, 2002. Recent lava accumulations starting from the beginning of the eruption (December 28, 2002) and a portion of the subaerial and submarine deposits were detached. As a result, tsunami waves several meters high affected the coasts of the island. After the event, monitoring activities, coordinated by the Italian Civil Protection Department, included systematic photogrammetric surveys. The digital photogrammetric technique was adopted to extract high-resolution digital elevation models and large-scale orthophotos. The comparison between the data collected before the eruption and that acquired on January 5, 2003, together with bathymetric data, allowed to define the geometry and to estimate the volume of the surfaces involved in the landslides. The following 13 photogrammetric surveys (from January to September 2003) enabled the monitoring of the continuous and relevant morphological changes induced by both the lava flow and the evolution of the instability phenomena. The method adopted for the data analysis and the results obtained are described in the paper.

Landslides and spreading of oceanic hot-spot and arc shield volcanoes on Low Strength Layers (LSLs): an analogue modeling approach

During their growth and evolution, oceanic hot-spot and arc shield volcanoes are destabilized and often dismantled by landslides. The origin of these phenomena can be found in the structure of volcanic islands marked by several low strength horizons such as a marine substratum, hyaloclastites, hydrothermally altered rocks, or volcaniclastic deltas, that weaken the edifice. We have tested the role of these Low Strength Layers (LSLs) with analogue modeling. We conclude that destabilization on these levels can lead to landslides or gravitational spreading. In our experiments, slides generated are only slump-type events, but they can evolve, in nature, into debris avalanches through mechanisms that could not be modeled in the laboratory. The role of fan-shaped volcaniclastic deltas included in the edifice structure is critical to slump formation. This process is more efficient if deltas are associated with another upper LSL, such as weak hyaloclastites or hydrothermally altered rocks. In this case, recurrent events generating collapses with increasing width are observed. Gravitational spreading of a volcanic island occurs when it is underlain by a ductile marine substratum, or includes an intra-edifice unconstrained hyaloclastic layer, 1/3 as thick as the height of the subaerial edifice. Spreading is characterized by edifice stretching and subsidence and does not generate major landslides. We also show that gravitational spreading on a marine substratum tends to inhibit slide generation, especially when the thickness of the sedimentary layer is larger than 1/10 of the edifice total height. However, if this value is lower, spreading occurs later and slumps can develop initially. We describe natural examples of oceanic hot-spot or arc shield volcanoes affected by both spreading and landslides, such as Reunion Island, Hawaii Island, James Ross Island (Antarctica) and Martinique Island (Antilles). The main structural features on these volcanoes can be explained solely by deformation on LSLs. We therefore propose a general model for the destabilization of volcanic islands that is applicable to all shield-like edifices.

Slope failure in underconsolidated soft soils during the development of a port in Tianjin, China. Part 1: Field investigation

This paper presents the field investigation of a major landslide that occurred in a newly dredged slope for port development in the city of Tianjin, northern China. The dredged slope comprised underconsolidated soft clay of recent marine and river delta deposits. The landslide consisted of a number of individual slides that occurred sequentially and extended retrogressively and laterally into reclaimed land. It lasted for about 1 h, occupying a plan area that is about 200 m long and 150 m wide in the reclaimed land. The failed soil mass was estimated to have a volume of between 700 000 and 800 000 m3. Part of the failed soil mass slipped into a newly dredged open space in the sea. The distance of the debris travel into the berth was about 80 m. The debris surface had a gentle slope of about 3.0°. The angle from the dredged slope crest to the far edge of the debris was about 6.8°. A common rupture surface was found underneath the debris and above the in situ marine mud. It controlled the inland extension of the slope failure. The landslide occupied an inclined area of between 27 000 and 30 000 m2, with an overall slope angle of about 3–4°. A kinematic model is proposed to explain the extension of the landslide into the reclaimed land. Findings of this field investigation are further used in the companion paper for a theoretical investigation of the mechanism of the landslide. The engineering approach and results presented in this study could be useful in the design and construction of dredged slopes in underconsolidated soft soils. Key words: port development, land reclamation, dredge excavation, landslide investigation, soft soils, case studies.

Slope failure in underconsolidated soft soils during the development of a port in Tianjin, China. Part 2: Analytical study

This paper presents the analytical study of a major landslide that occurred in a newly dredged slope during port development in the city of Tianjin, northern China. The slope comprised mainly underconsolidated and submerged soft soils. The landslide consisted of a number of individual slides that occurred sequentially and extended retrogressively and laterally into the reclaimed land. The paper proposes a slope stability assessment methodology for the examination of such slope failures, with retrogressive and lateral extension into reclaimed land. Both the total and the effective stress approaches are adopted in the slope stability assessment. Eight different sets of soil shear strength parameters are used in the assessment. They were determined by using different methods in the laboratory and in the field or by back analysis. The assessment results indicate that the total stress approach, together with undrained vane strength values, gives the best result in the stability assessment of the original dredged slope and the subsequent failed slopes. The paper further examines the factors that might have caused or triggered the occurrence of the landslide. Backfilling of a thick general soil layer and a sand cushion for vacuum preloading of the hydraulic fill in the reclaimed land might have rendered the entire slope marginally stable. Lowering of the tidal level might also have had an effect in triggering the landslide. The engineering approach and results presented in this study could be useful in the design and construction of dredged slopes in underconsolidated soft soils.Key words: land reclamation, dredge excavation, slope stability assessment, landslide investigation, marine mud, case studies.

Landslides During the Earthquakes on May 26 and July 26, 2003 in Miyagi, Japan

We conducted a site investigation for two major landslides : Dateshita landslide, Tsukidate-cho during the earthquake on May 26, 2003; and Nishisaruta landslide, Kanan-cho during the earthquake on July 26, 2003. In addition, we examined physical and mechanical soil properties and performed preliminary numerical simulations. The subsurface soil of the gentle slope of Dateshita landslide with an angle of about 7° was a fill with pyroclastic sediments. The structure of the fill was very loose, but the unsaturated soil remained stable with high suction. The landslide occurred during or immediately after the principle motion of the earthquake. The slide mass behaved as a mudflow. Then the collapsed soil easily fluidized with cyclic shear. It is likely that the saturated fill liquefied during the earthquake. Moreover, it is possible that the unsaturated fill fluidized, losing the initial shear strength during cyclic shear induced by the earthquake. The Nishisaruta landslide with similar magnitude and configuration to the Dateshita landslide occurred a few minutes after the principle motion of the main shock. Rainfall was an important feature that exacerbated the Nishisaruta landslide, whereas no rainfall had been observed for a week before the Dateshita landslide. The subsurface soil of slope of the Nishisaruta landslide was a fill with fine-graded sand, which originated from sandstone on the hill. The upper portion of the slope that lost shear strength because of liquefaction descended along the slope, and spread with high water content on the lower rice field. Numerical simulations have suggested that the saturated fill liquefied during the main shock. Residual excess pore pressure induced by the foreshock affected the slope's stability.

A preliminary engineering geological study in the northern part of the Taplejung Window, far eastern Lesser Nepal Himalaya

In the northern part of the Taplejung area, two tectonic units are recognised. These are the Higher Himalayan and Lesser Himalayan Sequences separated by the Main Central Thrust (MCT). The Lesser Himalayan Sequence is exposed in the Taplejung Window represented by Taplejung Formation, Mitlung Augen Gneiss, and Lingkhim Schist from the bottom to top. The main rock types in the Lesser Himalayan Sequence are phyllite, schist, metasandstone, quartzite and augen gneiss. Three Proterozoic granitic bodies are intruded into Taplejung Formation: Tamor River Granite, Amarpur Granite and Kabeli Khola Granite. The rocks of the Higher Himalaya on the hanging wall of the MCT are composed of kyanite-sillimanite bearing banded gneiss, orthogneiss, migmatite, quartzite.
 Three major landslides in the area, viz., Hangdewa, Hireba, and Paire lie along the valley slope sides of the Tamor River. Stream erosion and incision, presence of highly weathered rocks, surface and subsurface drainage are responsible in activating the landslides. Due to the presence of mainly weathered phyllites, favourable structural orientations and wet cultivation practice in the area have contributed to the development of these landslides. The Hangdewa and Hireba landslides have endangered the villages in between them and ultimately the Suketar airport. The landslides are gradually destroying more and more areas of cultivated land and property and increased loss of human lives.
 Excessive anthropogenic interference in the form of development activities like unplanned urbanization, deforestation and unfavorable agricultural practices have greatly aggravated the situation. The intensity of damages amplifies during the late part of monsoon each year. The study suggests that further detailed surface and subsurface investigation of the landslides are needed to prevent further loss of lives and property.

The extent and significance of mass-movements in Eastern Africa: case studies of some major landslides in Uganda and Kenya

The East African region has experienced major landslides in recent years. These landslides have caused many fatalities and injuries, loss of many hectares of productive farmlands and destruction to infrastructure such as roads, railways and bridges. The warm and wet climate of the landslide-prone regions causes rapid weathering and produces a regolith weaker than the underlying rock with an interface between the two layers. This interface serves as the most common plane along which landslides are initiated once it becomes saturated. Landslides in the region are associated with steep topography, human activities such as deforestation, overgrazing, and unplanned farming on steep slopes and are induced by earthquakes and high intensity of rainfall. The landslide-prone areas are agriculturally very productive and the inhabitants depend on agriculture for their livelihood. The areas also contribute substantially to the national food reserve. The landslides are therefore a burden to the economies of the individual farmers and national governments of the region.

The impact of landslides in the Umbria region, central Italy

Abstract. The Umbria Region of Central Italy has a long history of mass movements. Landslides range from fast moving rock falls and debris flows, most abundant in mountain areas, to slow moving complex failures extending up to several hectares in the hilly part of the Region. Despite landslides occurring every year in Umbria, their impact remains largely unknown. We present an estimate of the impact of slope failures in the Umbria region based on the analysis of a catalogue of historical information on landslide events, a recent and detailed regional landslide inventory map, and three event inventories prepared after major landslide triggering events. Emphasis is given to the impact of landslides on the population, the transportation network, and the built-up areas. Analysis of the available historical information reveals that 1488 landslide events occurred at 1292 sites in Umbria between 1917 and 2001. In the same period 16 people died or were missing and 31 people were injured by slope movements. Roads and railways were damaged by slope failures at 661 sites, and 281 built-up areas suffered landslide damage. Three event inventories showing landslides triggered by high intensity rainfall events in the period 1937–1941, rapid snow melting in January 1997, and earthquakes in September–October 1997, indicate the type, abundance and distribution of damage to the population, the built-up areas and the transportation network caused by typical landslide-triggering events. Analysis of a geomorphological landslide inventory map reveals that in some of the municipalities in the region total landslide area exceeds 25%. Of the more than 45 700 landslide areas shown in the geomorphological inventory map, 4115 intersect a road or railway, and 6119 intersect a built-up area. In these areas slope failures can be expected during future landslide triggering events.

Susceptibility of mid‐ocean ridge volcanic islands and seamounts to large‐scale landsliding

With a view to assessing the incidence of large‐scale landsliding, a morphologic database was created for volcanic islands and seamounts on young oceanic lithosphere. The database included 44 mid‐ocean ridge seamounts, Jasper seamount and the islands of Ascension, Bouvet, Guadalupe, and several of the Galapagos and Azores Islands, supplemented with published reports from a further five volcanic edifices. The data reveal that major landslides are common on edifices taller than 2500 m but are rare in shorter edifices, implying a threshold of instability at around 2500 m. A number of causes of this threshold are discussed. For example, many structures taller than 2500 m are, or were originally, volcanic islands, and therefore their flanks probably include extensive weak hyaloclastite built up from lava‐sea interactions around coasts. Compaction of hyaloclastites in larger edifices lead to regions of low permeability, which may help to explain the more deeply seated slope failures. It is intriguing that the threshold also coincides with the edifice height at which volcanic ridges become observable, a stage at which dike intrusion is probably common because slope oversteepening or excess pore pressure associated with dike intrusions have been proposed elsewhere as landslide triggers. The current indications of landsliding reveal no observable relation to rainfall, as landslides occur equally in wet and dry climates, and no relation to tectonic setting, as there are relatively few major landslides in the seismically active Azores group.