Large Mass Movements Research Articles

A large and active debris-rockslide in the Central Andes of Argentina (30.26°S): Morphometry and triggering mechanisms

A large (>0.1 km2) and complex mass movement in the Central Andes of Argentina (final portion of Cordillera de Olivares, Frontal Cordillera), was studied to identify the triggering factors and understand their relationship with geomorphic, cryogenic and climatic dynamics. This debris-rockslide is composed of clast supported blocks of Permian–Triassic volcanic breccias. In order to characterize this feature, high resolution satellite imagery interpretation was carried out, together with the study of the landslide detachment zones and landslide bodies. These debris-rockslide events could have originated as a consequence of the combination of internal slow deformation and fragmentation under periglacial conditions, followed by a sudden collapse of the rock mass. Pre- and post-slide digital elevation models (DEMs) were created from topographical data with the help of a Geographic Information System (GIS) tool. Approximately 14.89 M m3 of rock and debris travelled nearly 2 km from an elevation of 5023 m–4325 m asl. Although usually the origin of such catastrophic movements is related to seismically active areas with earthquakes whose magnitude frequently exceed Ms 6, our hypothesis is that this debris-rockslide event has a climatic origin caused by large snow accumulations during winters and subsequent fast meltdown processes during spring, which would have facilitated the sliding. The paper outlines the important role that snowmelt can play in the genesis and evolution of rock displacements and the importance of meteorological data, seismic catalogues, historical aerial photography and satellite images in geomorphological back-analysis.

Geology of the Brenner Pass-Fortezza transect, Italian Eastern Alps

We present a 1:30,000 geological map resulting from detailed geostructural surveys carried out along the Italian segment of the design corridor for the Brenner Pass railway base tunnel (BBT), extending from Fortezza (Italy) to Innsbruck (Austria). The map covers the southern part of the Austroalpine-Penninic collisional wedge, the Periadriatic Fault System, associated Oligocene igneous bodies (Periadriatic magmatism) and part of the Southalpine basement. The Penninic Zone in the western Tauern Window is represented by the double domal structure of the Europe-derived Tux and Venediger-Zillertal basement and cover nappe system, capped by the ophiolitic Glockner nappe. The overlying Austroalpine nappe system is here represented by the polymetamorphic Merano-Mules basement and minor cover sheets. The Southern Alps domain includes the Bressanone Granite and pre-granite quartz-phyllites. Four Alpine ductile deformation phases have been recognized, followed by ductile-brittle shear zones, and finally brittle deformations along faults with extensional and strike-slip kinematics. The Quaternary is characterized by glacial deposits, large gravitational mass movements and landslides.

MAY ’68 AND THE ETHICAL TURN IN FRENCH THOUGHT

It has become somewhat of a cliché to refer to May 1968 in France as an “interpretation in search of an event.” As many critics have pointed out, this designation fails to account for the real significance of the “events,” for the fact that May ’68 was one of the largest mass movement in French history—numerically, geographically, and sociologically—and one of the most acute political crises in postwar France. Yet, to refer to May ’68 as an interpretation does capture the extent to which the historiography of May ’68 has appeared, almost from the beginning, indistinguishable from its history, or, as Kristin Ross has put it, the extent to which May ’68 cannot be considered separately from its memory. At the risk of reducing some of these rich historiographical debates, we could say that, broadly speaking, two main issues have divided historians, philosophers, political theorists, and sociologists over the past forty years. The first has to do with the scope of the events, the degree to which the student leaders, the workers on strike, or the cultural legacy “mattered” more, or whether any of it mattered at all. The second line of contention has centered on the legacy of May ’68. According to some, May ’68 represented the acceleration of capitalism and modernization and it inaugurated an era of individualism and/or narcissism and political disengagement. In the eyes of these critics, ’68 was thus ultimately a conservative, or at least a libertarian, revolution. According to others, this was instead a truly radical period—socially, culturally, and politically—and this radicalism began to run out of steam towards the late 1970s. Instead of revolution, many intellectuals who had participated in May ’68 began to talk about ethics, morality, human rights, and the rule of law. Not surprisingly, commentators have once again differed on their reading of this “turn,” either condemning it or celebrating it, depending on their political bent. This “condemning” position has been articulated particularly forcefully by Kristin Ross in her excellent May ’68 and Its Afterlives. According to Ross, the “turn to ethics” marked a conscious “retreat from politics . . . that distorted not only May's ideology but much of its memory as well.” Ross's book is thus devoted to the analysis of this political radicalism—a radicalism that she situates in the union of intellectual contestation and workers’ struggle around what she calls “a polemics of equality”—and to the critique of its betrayal after 1976. On the other side of the spectrum, historians such as Tony Judt and Sunil Khilnani have praised this “ethical turn” as the moment in which French intellectuals were finally able to exorcise the (communist) revolutionary ideal that had haunted them throughout the twentieth century and to embrace liberalism.

Large scale mass movements triggered by the Kashmir earthquake 2005, Pakistan

The SPOT image analysis in Muzaffarabad Azad Kashmir, northwest Himalayas, Pakistan reveals that the Kashmir earthquake 2005 triggered a number of coseismic mass movements along the hanging wall block of the Muzaffarabad Fault. The Neelidandi and Langarpura rock falls have been identified as two major reactivated mass movements with an estimated volume of 3.1 × 106 m3 and 5.76 × 106 m3, respectively. The Neelidandi and Langarpura mass movements were initiated during earthquake in the direction of northwest-southeast extension and northeast-southwest directed thrusting, respectively. The Neelidandi rock fall occurred in sheared cherty dolomites and limestones of the Cambrian Muzaffarabad Formation, whereas the Langarpura rock fall occurred in alternating clays, shales, claystones, siltstones and sandstones of the Miocene Murree Formation. These rock units along the fault are highly fractured and jointed. The geotechnical maps and geological longitudinal profiles show the relationship between the geometrical characteristics and mechanism of these mass movements. Their characteristics were analyzed according to the role of topographic, seismic, geological and tectonic factors. The steep topography, sheared rocks, lithology, coseismic uplift and strong ground shaking of the hanging wall block along Muzaffarabad Fault facilitated the gravity collapse of these mass movements.

Meandering of Tributaries of the Tigris River Due to Mass Movements within Iraq

Tigris and Euphrates are the main rivers in Iraq. The former has 5 main tributaries while the latter has no tributaries within Iraq. All these rivers exhibit meandering and/or shifting of their courses due to many reasons; such as mass movements and alluvial fan's development. The meandering and shifting of river courses due to mass movements is dealt with in this study. Many examples are given from different parts of Iraq within different rivers and streams. In each case, the geology of the involved area is described; the reasons of the shifting and/or meandering of the river course are given. GIS technique was used to determine the coverage of the involved area. Landsat, Google Earth and DEM images were used to indicate the suffered areas from shifting and meandering of the rivers and streams. When possible, age of the shifting and/or meandering of the river and stream courses was estimated; using exposure age determination; mainly depending on the presence of river terraces, erosional forms, vegetation cover and the maturity of the involved area. Some of the given examples of shifting of river courses were found to be still active; others are inactive. In many cases, indications for very large mass movement phenomena were seen. Moreover, some of the large involved areas are highly populated, others includes few houses and/ or farms. The majority of the meanders are developed due to mass movement phenomena; others are caused by alluvial fans and/or tectonic features, lime plunging of anticlines.

Paleostress analysis of a gigantic gravitational mass movement in active tectonic setting: The Qoshadagh slope failure, Ahar, NW Iran

Abstract For better understanding of the dynamics of large slope failures, the slope-scale stress field reconstructions are eligible. A new approach of paleostress analysis of large gravitational mass movements, based on the multiple inverse method in 9D space and processed by a non-commercial code MARK2010, is demonstrated on the case of a gigantic gravitational slope failure in active tectonic setting at Mount Qoshadagh in NW Iran. The entire slope failure represents a very deep-seated gravitational slope deformation with relatively short displacement, roto-translational base and an estimated volume of about 83–110 km3 which is of the last glacial age. This gigantic slope failure has developed in an active-tectonic setting along the Qoshadagh Fault System (or called South Ahar Fault), which caused fast uplift of the entire mountain range of Qoshadagh, and it was probably seismically triggered. Using this paleostress approach, we were able to distinguish the mass-movement stress phase of NW compression in the rockslide's zone of compression and NW–SE extension in the upper part from a variety of other 8 regional-tectonic and 2 subsequent gravitational phases. Maximum principal stress of the slope-failure phase was in its compression zone sub-parallel to the estimated displacement vector; in the head it was sub-vertical. The paleostress analysis has been proved as a useful tool for reconstructing the stress field evolution of large slope failures.

Recurrence analysis of the mass movement activity at Stambach (Austria) based on radiocarbon dating

The Stambach mass movement (Austria) is a large and deep-seated mass movement in the Austrian Alps. It consists of a complex and compound mass movement system. The latest major reactivation of the Stambach mass movement was initiated in 1982 by rock fall activity that triggered an earth flow, which transformed into a mud flow. Six sediment cores were taken along the entire earth flow body showing a complex mixture of rock fall blocks and earth flow material. Whenever the earth flow was active, numerous wooden remains were buried within the flow mass. Thirteen of these remains sampled from the sediment cores were radiocarbon dated. The results indicate that the first activation of the Stambach mass movement occurred at least around 9750–9900cal BP, followed by at least three further events during the Holocene, around 6310–5650, 2320–1880, and 1600–1180cal BP. Accumulation of toppled rock towers in the head area of the earth flow, followed by a sudden collapse and saturated, undrained loading of the earth flow body, is the main trigger for activating the earth flow. These long lasting preparatory processes make it difficult to determine certain recurrence intervals. However, our data show that the Stambach mass movement was (and most probably still can be) reactivated after more than 3000years of dormancy.

Interplay between mass movement and fluvial network organization: An example from southern Apennines, Italy

Relationships between landslides and drainage network arrangement have been investigated by classical and quantitative geomorphological analysis in an intermountain valley (i.e. the upper valley of the Sinni River) of the southern Italian Apennines. The 120km2-wide test-area is characterized by very low grade metamorphic rocks (mainly shale) and associated crystalline-metamorphic rocks which underwent high exhumation and uplift rates during Quaternary times. We recognized and mapped 534 landslides through the stereoscopic interpretation of one set of 20 panchromatic aerial photographs 1:33,000 scale. Landslides were classified according to both type of movement and relative age. We detected and mapped 35 new landslides after a long rainfall period during the winter 2008–2009. The percentage of area affected by landslides (DL) of the area has a high value (about 37%). Analysis of distribution of landslides areas within different age classes suggests that recent/present-day landslide distribution is strongly influenced by the legacy of widespread large and ancient landslides, probably generated within a different morpho-climatic setting. A quantitative geomorphological analysis of the drainage network has been performed to evaluate the influence of landslide processes on morphometric properties of the drainage network. In particular, the degree of hierarchical organization of the drainage network is assessed through the hierarchical anomaly density, a measure of the number of anomalous confluences (i.e. channels of u order not flowing in channels of order u+1) in drainage basins. On the basis of statistical relationships between morphometric indices of the drainage network and DL, we argue that the activity of more recent mass movements rather than very old landslides can act as a disturbing element within the development of the drainage network, generating anomalous confluences of the 1st order. Empirical relationships between suspended sediment yield and some geomorphic parameters of the drainage network allowed the estimation of short-term denudation rates in the study area. The comparison between uplift and denudation rates suggests that the landscape system has not reached a steady state between uplift and erosion, in which tectonic uplift can represent a disturbing element within the recent morphogenesis. On the basis of the data presented, (large) mass movements in the study area are mainly linked to the relief growth promoted by Quaternary tectonics, whereas litho-structural and morpho-climatic factors assume a minor role.

Owyhee River intracanyon lava flows: Does the river give a dam?

Rivers carved into uplifted plateaus are commonly disrupted by discrete events from the surrounding landscape, such as lava flows or large mass movements. These disruptions are independent of slope, basin area, or channel discharge, and can dominate aspects of valley morphology and channel behavior for many kilometers. We document and assess the effects of one type of disruptive event, lava dams, on river valley morphology and incision rates at a variety of time scales, using examples from the Owyhee River in southeastern Oregon. Six sets of basaltic lava flows entered and dammed the river canyon during two periods in the late Cenozoic ca. 2 Ma–780 ka and 250–70 ka. The dams are strongly asymmetric, with steep, blunt escarpments facing up valley and long, low slopes down valley. None of the dams shows evidence of catastrophic failure; all blocked the river and diverted water over or around the dam crest. The net effect of the dams was therefore to inhibit rather than promote incision. Once incision resumed, most of the intracanyon flows were incised relatively rapidly and therefore did not exert a lasting impact on the river valley profile over time scales >10 6 yr. The net long-term incision rate from the time of the oldest documented lava dam, the Bogus Rim lava dam (≤1.7 Ma), to present was 0.18 mm/yr, but incision rates through or around individual lava dams were up to an order of magnitude greater. At least three lava dams (Bogus Rim, Saddle Butte, and West Crater) show evidence that incision initiated only after the impounded lakes filled completely with sediment and there was gravel transport across the dams. The most recent lava dam, formed by the West Crater lava flow around 70 ka, persisted for at least 25 k.y. before incision began, and the dam was largely removed within another 35 k.y. The time scale over which the lava dams inhibit incision is therefore directly affected by both the volume of lava forming the dam and the time required for sediment to fill the blocked valley. Variations in this primary process of incision through the lava dams could be influenced by additional independent factors such as regional uplift, drainage integration, or climate that affect the relative base level, discharge, and sediment yield within the watershed. By redirecting the river, tributaries, and subsequent lava flows to different parts of the canyon, lava dams create a distinct valley morphology of flat, broad basalt shelves capping steep cliffs of Tertiary sediment. This stratigraphy is conducive to landsliding and extends the effects of intracanyon lava flows on channel geomorphology beyond the lifetime of the dams.

The interaction between active normal faulting and large scale gravitational mass movements revealed by paleoseismological techniques: A case study from central Italy

Paleoseismological techniques have been applied to characterize the kinematic behaviour of large-scale gravitational phenomena located in proximity of the seismogenic fault responsible for the Mw 7.0, 1915 Avezzano earthquake and to identify evidence of a possible coseismic reactivation. The above mentioned techniques were applied to the surface expression of the main sliding planes of the Mt. Serrone gravitational deformation, located in the southeastern border of the Fucino basin (central Italy). The approach allows us to detect instantaneous events of deformation along the uphill-facing scarp. These events are testified by the presence of faulted deposits and colluvial wedges. The identified and chronologically-constrained episodes of rapid displacement can be probably correlated with seismic events determined by the activation of the Fucino seismogenic fault, affecting the toe of the gravitationally unstable rock mass. Indeed this fault can produce strong, short-term dynamic stresses able to trigger the release of local gravitational stress accumulated by Mt. Serrone's large-scale gravitational phenomena.The applied methodology could allow us to better understand the geometric and kinematic relationships between active tectonic structures and large-scale gravitational phenomena. It would be more important in seismically active regions, since deep-seated gravitational slope deformations can evolve into a catastrophic collapse and can strongly increase the level of earthquake-induced hazards.

Slope instability induced by volcano-tectonics as an additional source of hazard in active volcanic areas: the case of Ischia island (Italy)

Ischia is an active volcanic island in the Gulf of Naples whose history has been dominated by a caldera-forming eruption (ca. 55 ka) and resurgence phenomena that have affected the caldera floor and generated a net uplift of about 900 m since 33 ka. The results of new geomorphological, stratigraphical and textural investigations of the products of gravitational movements triggered by volcano-tectonic events have been combined with the information arising from a reinterpretation of historical chronicles on natural phenomena such as earthquakes, ground deformation, gravitational movements and volcanic eruptions. The combined interpretation of all these data shows that gravitational movements, coeval to volcanic activity and uplift events related to the long-lasting resurgence, have affected the highly fractured marginal portions of the most uplifted Mt. Epomeo blocks. Such movements, mostly occurring since 3 ka, include debris avalanches; large debris flows (lahars); smaller mass movements (rock falls, slumps, debris and rock slides, and small debris flows); and deep-seated gravitational slope deformation. The occurrence of submarine deposits linked with subaerial deposits of the most voluminous mass movements clearly shows that the debris avalanches impacted on the sea. The obtained results corroborate the hypothesis that the behaviour of the Ischia volcano is based on an intimate interplay among magmatism, resurgence dynamics, fault generation, seismicity, slope oversteepening and instability, and eruptions. They also highlight that volcano-tectonically triggered mass movements are a potentially hazardous phenomena that have to be taken into account in any attempt to assess volcanic and related hazards at Ischia. Furthermore, the largest mass movements could also flow into the sea, generating tsunami waves that could impact on the island’s coast as well as on the neighbouring and densely inhabited coast of the Neapolitan area.

Combining digital elevation model analysis and run-out modeling to characterize hazard posed by a potentially unstable rock slope at Turtle Mountain, Alberta, Canada

Turtle Mountain in Alberta, Canada has become an important field laboratory for testing different techniques related to the characterization and monitoring of large slope mass movements as the stability of large portions of the eastern face of the mountain is still questionable. In order to better quantify the volumes potentially unstable and the most probable failure mechanisms and potential consequences, structural analysis and runout modeling were preformed. The structural features of the eastern face were investigated using a high resolution digital elevation model (HRDEM). According to displacement datasets and structural observations, potential failure mechanisms affecting different portions of the mountain have been assessed. The volumes of the different potentially unstable blocks have been calculated using the Sloping Local Base Level (SLBL) method. Based on the volume estimation, two and three dimensional dynamic runout analyses have been performed. Calibration of this analysis is based on the experience from the adjacent Frank Slide and other similar rock avalanches. The results will be used to improve the contingency plans within the hazard area.

Physical modelling of rainfall‐ and snowmelt‐induced erosion of stony slope underlain by permafrost

AbstractPhysical modelling experiments have been carried out in a cold room to test on a small scale, the effects of water supply during the thaw of an experimental slope with permafrost. Permafrost was maintained at depth and a thin active layer was frozen and thawed from the surface. Data from the experiments relate to two different conditions, first with moderate rainfall, and second with heavy rainfall during the thaw period.When moderate rainfall is applied during thaw phases, the experimental slope is slightly degraded. At the scale of the experiment, erosion processes involve frost jacking of the coarse blocks, frost creep and gelifluction that induce slow and gradual down slope displacements of the active layer, but also small landslides leading to large but slow mass movements with short displacements. Changes in experimental slope morphology are marked by the initiation of a small‐scale drainage network and the development of a little crest line which shows a progressive upslope migration. With such boundary conditions, there is not enough water supply to evacuate downslope the whole of the eroded material and a topographic smoothing is observed.When heavy rainfall is applied during thaw periods, rapid mass wasting (small mud‐flows and debris flows) become prominent. Slope failures are largely controlled by the water saturation of the active layer and by the occurrence of steeper slopes. At the scale of the experiment, rates of erosion and maximum incision increase by about 100% leading to significant slope degradation with marked and specific scars comparable to gullying. These morphological changes are dependant on both the size and the frequency of catastrophic events. These experiments provide detailed data that could improve the knowledge of the physical parameters that control the initiation, at a small‐scale, of erosion processes on periglacial slopes with a thin active layer and/or with thin cover of mobilizable slope deposits. Copyright © 2010 John Wiley & Sons, Ltd.

Influence of the antiformal setting on the kinematics of a large mass movement: the Passo Vallaccia, eastern Italian Alps

The article examines the influence of antiformal settings on the evolution of alpine slopes using 2D numerical modelling. The northern slope of the Upper Vizze Valley, which follows a huge antiform involving meta-granitic and meta-sedimentary rocks, is affected by a complex gravitational phenomenon mainly controlled by the attitude of the foliation. A preliminary geological model based on field surveys was refined using the results of 2D distinct element numerical modelling. In the crown area, the sub-horizontal meta-sedimentary sequence at the fold hinge is cut by sharp planes, whilst in the middle zone the sequence was passively dragged downwards and finally, in the lowest part of the slope, corresponding to the steeply dipping antiformal flank, the deformation propagated across the entire meta-sedimentary succession through strain partitioning along multiple shear surfaces.

A geologic study of the Juan del Grijalva landslide, the most important mass movement during the last century in Mexico

On November 4th 2007, along the Grijalva River in the state of Chiapas, Mexico, has occurred one of the largest landslides ever known. This landslide, known as Juan del Grijalva, destroyed the town of the same name, killing 20 people, and moved 55 million cubic meters of rock and debris down slope to completely block the Grijalva River. In order to understand the characteristics and factors that triggered the Juan del Grijalva landslide, geologic studies were conducted at the site. The results indicate that the landslide was composed of a lithologic sequence of thin-bedded shales and thin to medium-thick-bedded sandstones. This was faulted into several blocks dipping in the same sense as the mass movement. The main triggering factor was the increment of the pore pressure into the lithologic unit due to water saturation after 5 days of heavy rain before the incident. According to records from the last century, the Juan del Grijalva mass movement represents one of the largest mass movements recorded all over the world. The risk conditions of the area after the landslide lead to the rapid construction of an artificial channel to drain the accumulating mass of water upstream and therefore prevent a future catastrophic inundation down stream.

From the Idea of Self-Management to Capitalism: The Characteristics of the Polish Transformation Process

The Polish “Solidarity” was the largest independent self-governing mass movement in the Eastern European region, which after the transition fell apart. This article is seeking to answer the question; how could pure market capitalism also be established in Poland, which fully contradicted the “Solidarity” self-governing socialism theory? How was it that such a large mass movement could disappear under these new circumstances? The Polish opposition activists (Kuron, Modzelewski, Kolakowski, Brus, Michnik) wanted to establish a “self-governing socialism” together with the working class from the middle of the 1960s. In the years 1980–81 this was also established for a short period during the “Solidarity”. After the introduction of Martial law, due to the world economic system, the indebtedness of socialist economics and the pressure of international financial institutions it was impossible to resolve the crisis in a “socialist way”. In addition to this the revolutionary changes within the industry scattered t...

Description and analysis of the debris flows occurred during 2008 in the Eastern Pyrenees

Abstract. Rainfall-triggered landslides taking place in the Spanish Eastern Pyrenees have usually been analysed on a regional scale. Most research focussed either on terrain susceptibility or on the characteristics of the critical rainfall, neglecting a detailed analysis of individual events. In contrast to other mountainous regions, research on debris flow has only been performed marginally and associated hazard has mostly been neglected. In this study, five debris flows, which occurred in 2008, are selected; and site specific descriptions and analysis regarding geology, morphology, rainfall data and runout were performed. The results are compared with worldwide data and some conclusions on hazard assessment are presented. The five events can be divided into two in-channel debris flows and three landslide-triggered debris flows. The in-channel generated debris flows exceeded 10 000 m3, which are unusually large mass movements compared to historic events which occurred in the Eastern Pyrenees. In contrast, the other events mobilised total volumes less than 2000 m3. The geomorphologic analysis showed that the studied events emphasize similar patterns when compared to published data focussing on slope angle in the initiation zone or catchment area. Rainfall data revealed that all debris flows were triggered by high intensity-short duration rainstorms during the summer season. Unfortunately, existing rainfall thresholds in the Eastern Pyrenees consider long-lasting rainfall, usually occurring in autumn/winter. Therefore, new thresholds should be established taking into account the rainfall peak intensity in mm/h, which seems to be a much more relevant factor for summer than the event's total precipitation. The runout analysis of the 2008 debris flows confirms the trend that larger volumes generally induce higher mobility. The numerical simulation of the Riu Runer event shows that its dynamic behaviour is well represented by Voellmy fluid rheology. A maximum front velocity of 7 m/s was back-analysed for the transit section and even on the fan velocities larger than 2 m/s were obtained. This preliminary analysis of the major Eastern Pyrenean debris flows represents the first background for future studies. Additional research on other events is necessary to support the results presented herein, and to properly assess and reduce hazard related to debris flows.

Évolutions socioreligieuses en Inde.

Prenant acte du long mouvement de la Vague safran (1983-2005), cet article analyse les nouvelles tendances de l’hindouisme culturel et national en Inde. Le surgissement de nouveaux « gourous thérapeutiques » dans le Nord ; le développement d’une mouvance tentée par l’action armée au Maharashtra et l’avancée de la « théologie de la prospérité » au Gujarat sont tour à tour mis en scène. Enrichissement, violence et santé : de nouvelles inflexions des « modernités » au Sud ?

２００８年岩手・宮城内陸地震に伴う地表地震断層―震源過程および活断層評価への示唆―

The Mjma 7.2 (Mw 6.9) Iwate-Miyagi Nairiku earthquake struck mountainous regions east of volcanic front in northern Honshu. To understand the relation between coseismic surface deformation and the seismogenic faulting and to have lessons in the long-term earthquake forecasting, we have performed urgent field investigations immediately after the main shock, while fragile structure and surface geomorphic features were fresh. More than 13 fault-rupture observations suggest that the estimated total length of the tectonic ground breakages reaches ∼20 km even though their locations are spotty rather than continuous along the entire trend. Contractional features such as thrust fault exposures, flexure, tilting, and buckling deformations predominate on the rupture zone, which is consistent with the reverse faulting under the WNW-ESE compressional stress field in northern Honshu. Such shortening features as well as vertical displacements were visible on cultural features such as concrete, asphalt paved roads, sidewalks, guardrails, drainage ditches, and rice paddies. Amounts of vertical offset and horizontal shortening measured using such cultural piercing points are mostly smaller than 50 cm (∼1 m of net slip). Fractures with such small slip, in turn, would not have been noticeable and may reflect the spotty distribution of the ruptures. Meanwhile, near the southern end of the rupture zone, fault structure and slip sense become complex and measured offsets are exceptionally large. We found a E-W-striking ∼1-km-long continuous rupture involved with 4-to-8-m dextral and 2-to-4-m vertical offsets of a paved road, trails, and rills near a massive giant landslide at the northern rim of the Aratozawa dam reservoir. Terrestrial LiDAR (Light Detection and Ranging) measurements together with our field observations reveal typical features of strike-slip faulting such as mole tracks, fissures, pressure ridges, bulges, and shutter ridges as well as the offset rills and ridges. Detail mapping of the ruptures suggests that this strike-slip dominant fault is a lateral ramp or tear fault that connects two distinct NNE-trending thrust faults, although we cannot rule out the possibility of a large mass movement due to gravitational force to induce such large displacements without suffcient geodetic and geologic data. The mapped zone of the ruptures approximately locates along the central part of the surface projection of a ∼40-km-long west-dipping source fault and associated aftershock zone. It also well corresponds to an asperity estimated from seismic and geodetic inversions, particularly southern end of the ground breakage zone. However, from the viewpoint of the long-term predictability, the surface fracturing occurred where none of active faults was previously mapped. Although several active geomorphic strands are likely to have reoccupied with the 2008 event, they are more spotty than the 2008 ruptures. Thus it would not have allowed us to properly evaluate size of the shock and entire extension of the rupture.

Late Quaternary mass movement events in Lake El′gygytgyn, North‐eastern Siberia

AbstractLake El′gygytgyn is situated in a 3·6 Myr old impact crater in North‐eastern Siberia. Its sedimentary record probably represents the most complete archive of Pliocene and Quaternary climate change in the terrestrial Arctic. In order to investigate the influence of gravitational sediment transport on the pelagic sediment record in the lake centre, two sediment cores were recovered from the lower western lake slope. The cores penetrate a sub‐recent mass movement deposit that was identified by 3·5 kHz echo sounding. In the proximal part of this deposit, deformed sediments reflect an initial debris flow characterized by limited sediment mixture. Above and in front of the debrite, a wide massive densite indicates a second stage with a liquefied dense flow. The mass movement event led to basal erosion of ca 1 m thick unconsolidated sediments along parts of its flow path. The event produced a suspension cloud, whose deposition led to the formation of a turbidite. The occurrence of the turbidite throughout the lake and the limited erosion at its base mainly suggest deposition by ‘pelagic rain’ following Stokes’ Law. Very similar radiocarbon dates obtained in the sediments directly beneath and above the turbidite in the central lake confirm this interpretation. When applying the depositional model for the Late Quaternary sediment record of Lake El′gygytgyn, the recovered turbidites allow reconstruction of the frequency and temporal distribution of large mass movement events at the lake slopes. In total, 28 turbidites and related deposits were identified in two, 12·9 and 16·6 m long, sediment cores from the central lake area covering approximately 300 kyr.