Analysis of landslide frequencies and characteristics in a natural system,coastal British Columbia

Two hundred and one debris slides and debris ?ows were analyzed in a 286 km² study area on the west coast of Vancouver Island, British Columbia, Canada. The study area remains essentially untouched by humans and therefore affords a natural setting in which to examine slope processes. Landslides were identi?ed and characterized on aerial photographs from 1:15 000 to 1:31 680, and were then mapped and transferred to a GIS for analysis. Based on detailed landslide surveys, we propose a new method to accurately determine volume of landslides of this type by measured total area. Results indicate average denudation rates of 56 m³ y^?1 km^?2, and higher natural rates of failure than analogous regions in coastal British Columbia. In contrast, the landslide rates are substantially less than those from forested watersheds. Landslide distribution is spatially clustered in air photograph epochs, and we propose intense storm cells within regional events as the causal mechanism. Further, failures occurred preferentially over the West Coast Crystalline Complex (by 1·4 times), a metamorphic assemblage of gabbros, schists and amphibolites, but 1·5 times less often over the Island Plutonic Suite, a granitic intrusive formation. The former result represents a new ?nding, while the latter corroborates ?ndings of previous authors. We examined magnitude–frequency relationships of the data set and present for the ?rst time a strong argument that the rollover effect is not merely an artefact, but is instead a consequence of the physical characteristics of the landslides themselves. We subsequently analyzed magnitude–frequency relationships from two other complete data sets from coastal British Columbia and produced a family of curves corroborating this result. Copyright © 2004 John Wiley & Sons, Ltd.     

The effects of logging on frequency and distribution of landslides in three watersheds on Vancouver Island, British Columbia

Three hundred and sixty three landslides in three watersheds that totaled 382 km² were identified from air photographs, beginning at a date that preceded logging to the present. The three watersheds all lie on Vancouver Island; however, they have different precipitation regimes, topography, and amounts logged. Landslide areas in the watersheds varied in size from 200 m² to more than 1 ha. Nearly 80% of the landslides were debris slides; 15% were debris flows, and the remainder primarily rock falls. Following logging, the number of landslides increased substantially in all watersheds although the amount of increase was variable: approximately 11, 3, and 16 times in Macktush Creek, Artlish River, and Nahwitti River, respectively. Other analyses of changes in landslide density also produced highly variable results, with the number of landslides increasing between 2.4 and 24 times. Further, 2–12 times more landslides reached streams following logging activities. Densities for landslides impacting streams increased for the period of record from 1.5 to 10 times following logging activities. The densities were substantially greater where only landslides that reached streams since development began in a watershed were considered. Roads had the greatest spatial impact in the watersheds (compared to their total area), with frequencies determined to have increased by 27, 12, and 94 times for Macktush, Artlish, and Nahwitti, respectively. The results highlight the relative impact of roads and their role in slope stability.     

Work, persistence, and formative events: The geomorphic impact of landslides

Understanding the scale and frequency of physical processes that act upon and form the surface of the Earth is a fundamental goal of earth science. Here we determine the magnitudes of landslides that impact the landscape in terms of work, persistence, and formative events. A systematic analysis of rapid landsliding (the analysis did not consider creep and other slow semi-continuous processes) indicates that moderate-sized landslides do the most work transporting material on hillslopes. The work peak defines the moderate magnitude, and that magnitude varies based on local physiography and climate. Landslides that form the work peak are distinct from catastrophic landslides that are themselves formative and system resetting. The persistence time for debris slides/debris flows (P_DS) and rock slides/rock avalanches (P_RS) is calculated over six orders of magnitude. We consider an event catastrophic when it persists in the landscape, as described by a persistence ratio (P_F), an order of magnitude longer than the population of landslides that form the work peak.     

天然气水合物体系动态演化研究(Ⅱ):海底滑坡

天然气水合物被认为是大陆边缘沉积物强度变弱的一个因子，从而能解释大陆边缘海底滑坡的一些观测现象。天然气水合物的形成使沉积物强度增加，而其分解则使沉积物强度变弱。虽然无法直接观测沉积物中天然气水合物的活动过程与相应的海底滑坡，大量的背景资料表明，天然气水合物崩解常常有助于触发海底沉积物块体的运动。此外，大型滑塌可以释放大量的固态天然气水合物，水合物在水柱中上浮。大块天然气水合物可以在分解前到达海洋的上部层，一些甲烷可以直接进入大气中。本文综述与天然气水合物体系演化有关的海底滑坡的研究现状。     

藏南江孜盆地晚侏罗至早白垩世重力流沉积

西藏南部江孜盆地的沙拉岗矿区发育一套晚侏罗至早白垩世的碎屑流、滑动流与浊流等重力流沉积,主要由斜坡相碎屑岩夹硅质岩和灰岩构成。在这个斜坡背景中出现了上斜坡相,下斜坡相与海底扇相等3种类型的岩相组合。其中,上斜坡以各种规模的滑动沉积为特色,下斜坡以不同性质的碎屑流沉积为特征。海底扇具有完整的内扇、中扇与外扇组合,可划分出进积型和退积型两种序列类型,它们多半是由浊流形成的各种砂体组成的。     

地震极震区岩体滑坡滑动面振动破坏形成与稳定性评价的探讨

汶川Ms8.0级地震表明:极震区地质灾害最严重,滑坡密度、规模和分布面积等远大于该区以外区域。对极震区岩石坡体稳定性与滑坡发生的预测评价,必须考虑极震区地震波的特点与岩体的地质结构及其弱层面力学强度。已有方法只考虑水平加速度的作用对极震区可能得不到符合实际的结果。根据岩体结构力学观点,结合岩石强度分析,提出极震区发生滑坡主要是坡体中的弱层弱面发生了张破坏或剪破坏以及两种破坏的结合—张剪破坏。极震区地震波入射地表角度大,必须应用合向量作用的地震力来评价坡体的稳定性,同时分别考虑P波和S波的作用。对两种常见的单一弱层面和双弱层面组合的简单地质模型,根据极限平衡原理、定义了5种破坏类型在指定方向的临界破坏加速度,只要代入地震作用方位角与岩体弱层面的几何参数及其力学强度,就能计算出不同破坏类型的临界加速度,然后用实际地震加速度转换成指定方向的加速度,与临界加速度进行比较,即可判断滑动面形成破坏的力学类型与坡体稳定性。强调了这种方法只有在详细工程地质工作的基础上才能获得好的结果。     

地球内核平动振荡的地震激发

本文利用球对称、非自转、弹性和各向同性地球模型(SNREI)理论模拟计算了地球内核平动振荡的地震激发.以2004年12月26日苏门答腊大地震为例,讨论震源机制解(标量地震矩、走向、倾角、滑动角和深度)对内核平动振荡振幅激发的影响;基于全球21个Mw8.0级以上的大地震,分别计算10个数据资料较好的超导重力台站理论上可以接收到的内核平动振荡信号的频率域振幅.结果表明标量地震矩对内核平动振荡振幅的影响最大,走向、倾角、滑动角和深度对内核平动振荡振幅也有一定影响,但是影响相对较小;不同区域获得的由大地震引起的内核平动振荡信号的幅度存在显著差异,此结果为频率域多台站加权迭积提供了计算基础.另外,只有武汉台站接收到的2011年日本Tohoku Mw9.1地震激发的内核平动振荡的振幅值达到了地球表面高精度、高灵敏度的超导重力仪检测水平,振幅值为0.0103 nm·s-2.结果说明地震激发的内核平动振荡的信号极其微弱,信号几乎淹没在背景噪声中,必须利用多台站迭积法才有可能将信号提取出来.     

机载LiDAR技术在地质灾害监测中的应用研究

利用成熟的技术路线和地质灾害监测点激光点云的特征,以及地裂缝的地形深度急剧变化规律,采取一种改进Morove算子的兴趣点提取方法,根据高程变化特征提取地裂缝;根据滑坡体的地形非连续性,在三角形生长算法基础上,采用异变点不参与构网的方法,使滑坡体与原山体地形分离,监测出山体滑坡地质灾害点,为地质灾害的监测提供新的思路和方法。     

High-resolution seismic-reflection investigation of the northern Gulf of Mexico gas-hydrate-stability zone

We recorded high-resolution seismic-reflection data in the northern Gulf of Mexico to study gas and gas-hydrate distribution and their relation to seafloor slides. Gas hydrate is widely reported near the seafloor, but is described at only one deep drill site. Our data show high-reflectivity zones (HRZs) near faults, diapirs, and gas vents and interbedded within sedimentary sections at shallow depth (<1 km). The HRZs lie below the gas-hydrate-stability zone (GHSZ) as well as within the zone (less common), and they coincide with zones of shallow water-flows. Bottom simulating reflections are rare in the Gulf, and not documented in our data.We infer HRZs result largely from free gas in sandy beds, with gas hydrate within the GHSZ. Our estimates for the base BHSZ correlate reasonably with the top of HRZs in some thick well-layered basin sections, but poorly where shallow sediments are thin and strongly deformed. The equivocal correlation results from large natural variability of parameters that are used to calculate the base of the GHSZ. The HRZs may, however, be potential indicators of nearby gas hydrate. The HRZs also lie at the base of at least two large seafloor slides (e.g. up to 250 km²) that may be actively moving along decollement faults that sole within the GHSZ or close to the estimated base of the GHSZ. We suspect that water/gas flow along these and other faults such as ‘chimney’ features provide gas to permit crystallization of gas hydrate in the GHSZ. Such flows weaken sediment that slide down salt-oversteepened slopes when triggered by earthquakes.     

Characteristics of the Shetland Islands (UK) peat slides of 19 September 2003

An extreme rainfall event over the southern Shetland Islands in northern Scotland, UK, on 19 September 2003, triggered at least 20 significant peat slides and at least 15 smaller landslides of varying types. The peat slides were examined and surveyed to characterise and explain the distinctive morphological features that were produced. The failures varied in size from 0.4 to 7.3 ha (2,300 to 59,000 m³ displaced volumes of peat) and involved blanket peat up to 3 m deep and slope gradients as low as 4°. Almost all of the failure surfaces were located at the peat–mineral interface. The morphological features included large areas (up to 0.5 ha) of intact peat that moved without breaking up, linear compression and thrust features and unusual occurrences of mineral debris. These features suggest peat of high tensile strength throughout its depth and the generation of high and sometimes artesian water pressures at the base of the peat during the event. However, the variations between peat slides highlight some of the difficulties of trying to assess the susceptibility of blanket peat to failure without full knowledge of the local peat geotechnical properties and structural features within the peat mass.