DEPOSITS FROM LANDSLIDES AND AVALANCHES TRIGGERED BY SEISMIC ACTIVITY IN SWEDISH LAPLAND

In the mountain area of Swedish Lapland, landforms with semicircular or horse‐shoe ridges encircling hummocky ground occur. They are interpreted as landslide or avalanche deposits directly upon, from or adjacent to down‐wasting ice. Since they are restricted to the vicinity of fault scarps in the Pärvie fault system the releasing factor is suggested to be displacement of bedrock blocks or related seismic activity.     

Insights into rock avalanche emplacement processes from detailed morpho‐lithological studies of the Tschirgant deposit (Tyrol,Austria)

Large, rapid rockslope failures generate deposits with complex morphologies due to a number of causal and influencing factors. To investigate these, we conducted a detailed case study at the carbonate Tschirgant deposit (Tyrol, Austria). It preserved evidence of simultaneous rock sliding (very large, coherent hummocks) and rock avalanche spreading (smaller, more scattered hummocks and ridges). Motion indicators, such as longitudinal ridges furthermore pinpoint the transition between linear sliding and radial spreading. The lithological distribution in the Tschirgant deposit shows that it retained source stratigraphy despite being split into two accumulation lobes by a high bedrock ridge. Furthermore, lithology had a very strong control on the final deposit morphology in that the different lithologic units form individual deposit surfaces. River erosion has created fortuitous outcrops that reveal the basal rock avalanche contact. The underlying valley‐fill sediments (substrates) have been intricately involved in shaping the rock avalanche morphology and, where entrained, highlight internal rock avalanche deformation features. This study shows that intrinsic dynamic properties of granular media (e.g. tendency for longitudinal alignments), emplacement mode, lithology (and source predisposition), runout path topography, and substrates form the quintet of causal factors that shape rock avalanche morphology. Copyright © 2015 John Wiley & Sons, Ltd.     

Surface exposure dating of the Veliki vrh rock avalanche in Slovenia associated with the 1348 earthquake

Over 30 samples from bedrock and boulders from the Veliki vrh rock avalanche have been collected for surface exposure dating. The limestone rocks have been radiochemically treated to isolate and determine long-lived ³⁶Cl by accelerator mass spectrometry. It could be shown that the Veliki vrh rock avalanche from the Košuta Mountain (Slovenia) event can be very likely linked to one of the major historical earthquakes in Europe happening on the 25^th of January 1348. Taken into account independently determined denudation rates, inherited ³⁶Cl originating from pre-exposure at shallow depths (20–55 m) could be calculated. The high amount of inherited ³⁶Cl, i.e. 17–46% of the total ³⁶Cl, makes this site not suitable for a precise determination of the ³⁶Cl production rate as it was originally anticipated. Veliki vrh is a “classic” rock avalanche of high velocity. The slope failed in the upper part with a translational slide predominantly along the bedding planes, whereas dynamic fragmentation is the cause for further crushing of the material and the long runout.     

滑坡-碎屑流物理模型试验及运动机制探讨

滑坡-碎屑流由于高速、远程的特点常常引发灾难性事故,其复杂的运动机制导致预测致灾范围非常困难。通过开展室内模型试验,研究了碎屑粒径、滑床糙率和挑坎对运动特性的影响。试验结果发现,滑坡碎屑运动距离受控于前端碎屑,且随着碎屑的粒径增大而增加,增加滑床糙率、挑坎均可使碎屑的运动距离减小。在前人研究成果的基础上结合碎屑材料的力学特性探讨了滑坡-碎屑流出现流态化的原因和高速远程机制,即高速运动中颗粒间的作用力远小于完整岩体,因此颗粒间的“黏聚力”不能维持滑坡体的整体性,同时致使滑坡体与滑床接触的过程中传递至滑坡体内部的摩阻力减少,从而导致碎屑滑坡的远程结果。     

Characteristics of wet-snow avalanche activity: 20 years of observations from a high alpine valley (Dischma,Switzerland)

The occurrence of wet-snow avalanches is, in general, poorly understood. For 20 years (winters of 1975–1976 to 1994–1995), the avalanche activity has been observed in the Dischma valley near Davos (Eastern Swiss Alps). The study area comprises a large starting zone of north-easterly aspect (2,300 m a.s.l.) with several avalanche paths. We have analyzed the occurrence data in combination with meteorological and snowpack data collected at an elevation of 2,090 m a.s.l. During the 20-year observation period, almost 800 wet-snow avalanches were observed, about 4.5 times more loose snow avalanches than slab avalanches. Considering both types of avalanches jointly, snow depth, precipitation and air temperature showed the highest correlation with avalanche activity. Most loose snow avalanches occurred when air temperature was high and/or after a precipitation period. Slab avalanches occurrence was primarily related to warm air temperatures and snowpack properties such as the isothermal state and the existence of capillary barriers. Radiation did not show up as a significant variable. The results suggest that in a transitional snow climate wet-snow avalanches are, as dry snow avalanches, often related to precipitation events, and that wet slab instability strongly depends on snowpack properties in relation to warming of the snowpack and melt water production.     

Dendrochronological reconstruction of snow avalanche activity in the Lahul Himalaya,Northern India

Mass wasting and avalanche events substantially impact the landscape morphology and consequently human habitation throughout the Himalaya. There is, however, a paucity of snow avalanche documentation for the region. The application of dendrochronologic research methods introduces a sensitive approach to document the recurrence of snow avalanche events in a region where historical records are either non-existent or difficult to access. An exploratory dendrochronologic study was undertaken in the Lahul Himalaya of Northern India during the summer of 2006. Included within the fieldwork was an assessment of avalanche track morphology to enable identification of the slope characteristics that might be associated with an increase in avalanche activity. Thirty-six trees growing on the Ratoli avalanche track were sampled. The oldest tree was a Cedrus deodara with a pith date of 1950. A tree-ring-derived avalanche response curve highlights four avalanche events that occurred from 1972 to 2006. The successful scientific results based on the application of the method used provide the basis for local planners to quantify slope failure hazards in forested areas throughout the western Himalaya.     

Vampire rock avalanches of January 2008 and 2003, Southern Alps, New Zealand

Rock avalanches fell from Vampire (2,645 m) Peak in the Southern Alps of New Zealand during January 2008. There were no direct witnesses, casualties or damage to infrastructure. Field observations indicate about 150,000 m³ (±50,000) of indurated greywacke collapsed retrogressively from a 73° slope between 2,380 and 2,520 m. Debris fell 800 m down Vampire’s south face and out 1.7 km across Mueller Glacier, with a 27.5° angle of reach. The resulting 300,000 m² avalanche deposit contains three distinct lobes. The national seismograph network recorded two pulses of avalanche-type shaking, equivalent in amplitude to a M _L 2.4 tectonic earthquake, for 60 s on Monday 7 January at 2349 hours (NZDT); then 45 s of shaking at M _L 2.5 on Sunday 13 January at 0923 hours (NZDT). Deposit lobes are inferred to relate directly with shaking episodes. The avalanche fell across the debris from an older avalanche, which was also unwitnessed and fell from a different source on Vampire’s south face between February and November 2003. The 2003 avalanche involved 120,000 m³ (±40,000) of interlayered sandstone and mudstone which collapsed from a 65° slope between 2,440 and 2,560 m, then fell 890 m down across Mueller Glacier at a 24° angle of reach. Prolonged above-freezing temperatures were recorded during January 2008, but no direct trigger has been identified. The event appears to be a spontaneous, gravitationally induced, stress failure.     

The Major Snow Avalanche Cycle of February 1986 in the Western United States

Snow avalanches are a significant hazard in mountainous environments around the world. This paper investigates the major February 1986 avalanche cycle that occurred in the western United States, and broadly analyzes the avalanche, snowpack, and weather conditions at twenty sites. These analyses suggest that the avalanche cycle resulted from the interaction of a relatively `normal' snowpack with an exceptional storm event, which was particularly noteworthy for the amount of precipitation it produced. Composited 500-hPa anomaly maps show the event resulted from an uncommonly persistent blocking pattern that resulted in a strong zonal flow and copious moisture being funneled over the western United States. Understanding severe and widespread avalanche cycles may improve our long-term forecasting of these events, and help mitigate theresulting avalanche activity.     

南极中山站地区冰崩概况

本文是根据１９９４年在南极中山站首次使用数字触发地震仪测量的冰崩，研究南极中山站地区冰崩规律及其与自然环境（温度、风速等）的关系，这对于考察船在南极的安全航行以及科考人员的人身安全都有极为重要的意义；并且对于南极冰川、冰山、冰盖等方面的科学研究有参考价值。结果表明，１～９月份冰崩次数曲线呈口朝上抛物线型变化，１０～１２月冰崩次数急剧增加。冰崩曲线与月最低气温、最高气温、月平均气温三者的拟合曲线吻合得很好，即冰崩次数随气温持续上升而增加，风速与冰崩的关系却与此相反     

The 1998 debris avalanche at Casita volcano, Nicaragua — investigation of structural deformation as the cause of slope instability using remote sensing

During Hurricane Mitch in 1998, a debris avalanche occurred at Casita volcano, Nicaragua, resulting in a lahar that killed approximately 2500 people. The failure that initiated the avalanche developed at a pre-existing cliff, part of the headwall of a gravitational slide of approximately 1.8 km² in plan view that cuts the southern flank of the volcano. Structural analysis, primarily based on a high-resolution DEM, has shown that this slide is caused by edifice deformation. Casita's eastern side is spreading radially outwards, forming a convex–concave profile and steepening original slopes. This deformation is possibly facilitated by millennia of persistent hydrothermal alteration of the volcano's core. The gravity slide has some typical features of smaller slumps, such as steep headwalls, an inner flatter area and a pronounced basal bulge fronted by thrusts. The headwall is the source of the 1998 avalanche, as well as several previous mass movements. Edifice deformation has led to extensive fracturing of the hydrothermally altered andesitic source rock, increasing instability further. Field evidence indicates that the gravity slide is still actively deforming, and with steep headscarps remaining, the hazard of future avalanches is increasing. The analysis presented here shows how small but highly damaging landslides can occur during the deformation of a volcanic edifice. We show that identification of instability is possible with remote sensing data and minimal reconnaissance work, implying the possibility of similar efficient and cost-effective analysis at other volcanoes known to host extensive hydrothermal systems. We demonstrate this with a simple structural analysis of two similar stratovolcanoes, Orosí (Costa Rica) and Maderas (Nicaragua).