Regular spacing of drainage outlets from linear fault blocks

Outlets of river basins located on fault blocks often show a regular spacing. This regularity is most pronounced for fault blocks with linear ridge crests and a constant half-width, measured perpendicular to the ridge crest. The ratio of the half-width of the fault block and the outlet spacing is used in this study to characterize the average shape (or spacing ratio) of 31 sets of drainage basins. These fault-block spacing ratios are compared with similar data from small-scale flume experiments and large-scale mountain belts. Fault-block spacing ratios are much more variable than those measured for mountain belts. Differences between fault-block spacing ratios are attributed to variability in factors influencing the initial spacing of channel heads and subsequent rates of channel incision during the early stages of channel network growth (e.g. initial slope and uplift rate, precipitation, runoff efficiency and substrate erodibility). Widening or narrowing of fault blocks during ongoing faulting will also make spacing ratios more variable. It is enigmatic that some of these factors do not produce similar variability in mountain belt spacing ratios. Flume experiments in which drainage networks were grown on static topography show a strong correlation between spacing ratios and surface gradient. Spacing ratios on fault blocks are unaffected by variations in present-day gradients. Drainage basins on the Wheeler Ridge anticline in central California, which have formed on surfaces progressively uplifted by thrust faulting during the last 14 kyr, demonstrate that outlet spacing is likely to be determined during the early stages of drainage growth. This dependency on initial conditions may explain the lack of correlation between spacing ratios of fault blocks and slopes measured at the present day. Spacing ratios determine the location of sediment supply points to adjacent areas of deposition, and hence strongly influence the spatial scale of lateral facies variations in the proximal parts of sedimentary basins. Spacing ratios may be used to estimate this length scale in ancient sedimentary basins if the width of adjacent topography is known. Spacing ratio variability makes these estimates much less precise for fault blocks than for mountain belts.     

Tectonic constraints on watershed development on frontal ridges: Mohand Ridge, NW Himalaya, India

Uplifting frontal ridges are one of the most conspicuous geomorphic features that mark the frontal parts of actively converging mountain belts. Growth of these ridges can lead to the simultaneous development of a drainage system that is defined by watersheds, stream network and long profiles of channels. In the present study, shape parameters of watersheds, stream network characteristics and pattern of network growth, shape of long profiles, and the SL index have been investigated in a part of NW Himalaya to understand the relationship between endogenic tectonic processes and exogenic fluvial processes. This explains the tectonic control on drainage systems in the uplifting frontal ridge. This watershed analysis was carried out using a Digital Elevation Model (DEM) and a number of anomalies have been identified and analysed. The most striking is the asymmetric development of watersheds on either side of an almost straight ridge crest. Watershed asymmetry along the ridge crest is characterized by larger area and less elongated watersheds in the southern flank (forelimb) in comparison to the northern flank (backlimb). Drainage network and long profile analysis establishes that the larger watershed area in the forelimb is due to dominance of headward erosion and its impact on drainage network growth. Dominance of headward erosion is due to slope variation in response to forelimb development along a fault-related fold. Even through, headward erosion has shifted the ridge crest; it is parallel with the trace of the Himalayan Frontal Thrust (HFT). The parallel ridge crest with reference to the HFT is indicative of the tectonic control of the HFT on the development of the watersheds. Hence, a well developed linkage between tectonic processes (fold development) and surface processes (headward erosion) is responsible for variation in watershed and drainage network pattern across the ridge crest. The study also investigates the role of planform ridge curvature on watershed development. The effect is more pronounced on an asymmetric ridge, such as the Mohand ridge, than on a symmetric ridge.     

Effect of a simple mountain range on underground seismic motion

Summary. The influence of a simple mountain range on seismic ground motion is studied. A two-dimensional model of the medium and vertically incident plane SH -waves are considered. Attention is devoted not only to the wavefield along the Earth's surface, but also within the medium. The wavefield is computed in two steps: (1) the computation of the impulse response by the finite-difference method, (2) the computation of the response to a time variation of the incident wave. approximately corresponding to a 'hundred-year' local earthquake at the site. Numerical results (the impulse response, the transfer function, the accelerograms, and their spectra) indicate strong spatial variability of the wavefield due to the topographical anomaly. The differences as large as 100 per cent in the peak amplitudes of the accelerograms, and of the order of 5 Hz in their predominant frequencies, at closely (∼ 200 m) spaced internal points of the medium have been found. Attention is focused also on the effect of causal absorption. Even unrealistically strong absorption seems to be unable to reduce significantly the spatial variability of the ground motion, caused by the topography. A variability like this. implying the occurrence of underground differential strains, might be of engineering importance in the antiseismic design of underground structures (tunnels, for example) in mountainous seismic regions. The ground-motion variability along the surface of the mountain is considerably smaller than within the medium.     

Diffraction of P, S and Rayleigh waves by three-dimensional topographies

The diffraction of P, S and Rayleigh waves by 3-D topographies in an elastic half-space is studied using a simplified indirect boundary element method (IBEM). This technique is based on the integral representation of the diffracted elastic fields in terms of single-layer boundary sources. It can be seen as a numerical realization of Huygens principle because diffracted waves are constructed at the boundaries from where they are radiated by means of boundary sources. A Fredholm integral equation of the second kind for such sources is obtained from the stress-free boundary conditions. A simplified discretization scheme for the numerical and analytical integration of the exact Green's functions, which employs circles of various sizes to cover most of the boundary surface, is used.
The incidence of elastic waves on 3-D topographical profiles is studied. We analyse the displacement amplitudes in the frequency, space and time domains. The results show that the vertical walls of a cylindrical cavity are strong diffractors producing emission of energy in all directions. In the case of a mountain and incident P, SV and SH waves the results show a great variability of the surface ground motion. These spatial variations are due to the interference between locally generated diffracted waves. A polarization analysis of the surface displacement at different locations shows that the diffracted waves are mostly surface and creeping waves.     

Numerical modelling of the gravity-induced destabilization of a slope: The example of the La Clapière landslide, southern France

A finite difference two-dimensional model with Hooke–Mohr–Coulomb properties and topography derived from the DEM are used to reproduce the La Clapière landslide. The principal factor defining the gravity-driven destabilization of the model is a gradual reduction in the cohesion. This reduction simulates a degradation of the material properties with time because of weathering/alteration processes. The inelastic deformation, fracturing, and faulting first occur at mountain scale and results in normal fault formation causing crest sagging. Later, the failure process is concentrated in the lower part of the slope and leads to the formation of a localized fault subparallel to the slope surface at a depth of ca. 100 m. This corresponds to the initiation of the La Clapière landslide and its propagation upslope. A slow crest sagging continues during the whole model evolution.     

Finite difference seismograms for laterally varying marine models

Summary. The method of finite differences is applied to the elastic wave equation to generate synthetic seismograms for laterally varying seafloor structures. The results are compared with borehole seismic data from the Gulf of California (Deep Sea Drilling Project Site 485) in which lines were shot over flat and rough topography. The significant new phenomenon observed in both the synthetic seismograms and the field data is the generation of a 'double head wave' due to the interaction of the incident wavefront with the side of a hill and the flat seafoor adjacent to the hill.
In these models the hills are on the order of a seismic wavelength in height and steep velocity gradients occur over distances comparable to wavelengths. Ray theoretical methods would not be suitable for studying such structures. True amplitude record sections are obtained by the finite difference method, which show for these models that the head wave generated at the flat seafloor adjacent to the hill is lower in amplitude than if the hill were not present and is lower in amplitude than the head wave generated at the hill.
A second feature which is important for borehole receivers is the existence of the 'direct wave root' in the upper basement. This energy occurs below the sharp interface when the direct wave impinges on the interface from above. There is no corresponding Snell's law ray path for this energy and the energy is evanescent with depth in the lower medium.
The properties of both the double head wave and the direct wave root are clearly demonstrated in the finite difference 'snapshot' displays.     

鄱阳湖湖滨沙山垄状地形的成因

江西星子县沙岭沙山位于鄱阳湖湖滨,临湖一侧发育顺盛行风方向延伸的垄状地形。研究从地貌学、沉积学和年代学的角度分析了垄状地形的成因。按照目前的风场计算出的沙丘走向与合成输沙方向近于垂直,说明这些垄状地形不是纵向沙丘。垄状地形内部发育平行层理,缺少滑动面,则进一步表明垄状地形并非沙丘。对垄状地形之间的沟谷形态观察后发现,这些沟谷与前丘上槽形风蚀坑形态颇为相似,且沙山的环境具备发育槽形风蚀坑的基本条件,所以我们将垄状地形之间的沟谷视为槽形风蚀坑。为确定风蚀坑的形成时代,我们在沙山上风区、下风区边缘选取了6个剖面,测得的28个光释光年龄显示,构成垄状地形的沙层堆积于约20kaBP以前的末次冰期。据此我们推测在冬季风最强的阶段(18-14kaBP),沙山由以前的风沙堆积转变为风沙侵蚀,产生了槽形风蚀坑。相邻槽形风蚀坑之间的条状突起就构成了垄状地形,因此星子县沙岭沙山临湖一侧的垄状地形属于风蚀地貌。     

河北昌黎黄金海岸横向沙脊表面的粒度分布模式

河北昌黎黄金海岸自然保护区内的横向沙脊规模高大、形态典型,对其表面采集的47个沙丘表面沉积物粒度样品的分析结果,该海岸横向沙脊表面粒度总体上是中砂、分选好、对称和中等峰态,但脊顶、坡部及坡脚的粒度分异明显,自沙脊两侧坡脚向脊顶粒径变细、分选变好,向陆背风坡的细化与分选优于向海迎风坡,脊顶粒径最细,向陆背风坡脚最粗,是一种新型的沙丘表面粒度分布模式。该粒度分布模式是该区域主风向与强风向交替变化及其风速差异、沙脊高大且两侧不对称等组合作用的结果。     

山地绿洲边界层特征的数值模拟

使用美国NCAR新版MM5V35非静力平衡模式, 采用三重嵌套的降尺度方法, 通过设计理想的地形和山地绿洲, 模拟研究了山地绿洲环流及边界层特征。山地绿洲的地面感热通量和潜热通量与周围沙漠差异较大, 对形成沙漠绿洲环流有明显贡献。在无山地绿洲的情况下, 地形仅对感热通量有影响, 对潜热通量影响不大。山地绿洲可以使绿洲上空的干冷气流通过下沉气流向下层输送, 在山地绿洲上空形成下沉干冷气流的边界层特征, 它可以抑制由地形形成的向上输送的暖湿气流, 改变地形形成的边界层特征。地形引起的环流是低层复合, 高空辐散。绿洲引起的环流是低空辐散, 高空复合。两种环流呈完全相反的情况。山地绿洲的气候效应可以抑制山地地形的气候效应。     

海岸横向沙脊的移动与形态变化——以河北昌黎黄金海岸横向沙脊为例

采用RTKGPS测量技术与方法,2006～2008年连续三年共9次对我国海岸沙丘主要分布区域之一的河北昌黎黄金海岸翡翠岛的一个典型海岸横向沙脊进行了三个横剖面的高精度观测。海岸横向沙脊高程、断面及其典型位置的观测结果表明,观测期间海岸横向沙脊脊顶和背风坡坡脚夏秋季东移、冬春季节西移,脊顶总体向西移动,年均最大移距1．8...     

Assessment of geomorphic hazards in connection with permafrost occurrence in the Zugspitze area (Bavarian Alps, Germany)

In the Zugspitze area (Bavarian Alps, Germany), permafrost conditions are present in limestone bedrock and in regolith. Distribution is strongly dependent on topography in the east–west oriented mountain crest with steep north- and south-facing slopes. Numerous structures mainly for tourist purposes (cable car and recreation buildings, ski-lift masts, rack-railway tunnel, tunnel with supply facilities) are situated in the area, and several of them are placed on ground with permafrost. Results from a temperature measurement programme and distribution modelling show that for some of these constructions, the effects of permafrost degradation have to be considered in terms of stability of the foundations.The permafrost limit is close to the summit crest, and therefore, stability evaluations for the constructions in this area have to bear in mind the possible warming or even melting of ice within the bedrock crevasses caused by climate warming. Stability of the foundations as well as stability of rock walls in this area will probably be affected by a shifting of the permafrost limit. Constructions in the Zugspitzplatt area are already affected by the melting ground ice, and stabilizing measures have to be evaluated for several foundations where subsidence is likely to occur.Besides the local results, the study provides for the first time data on permafrost distribution in the northern Alpine margin based on standard methods of BTS measurements and numerical modelling.     

风向对半固定沙垄表面风速影响——以古尔班通古特沙漠为例

利用DETI可移动测风系统对古尔班通古特沙漠半固定沙垄表面风速进行实地观测,获取了主要风季不同风向条件下沙垄表面7个典型部位距离地表20 cm、40 cm、60 cm、100 cm和200 cm 5个高度的风速系列数据,系统研究风向对半固定沙垄表面坡面风速及风速廓线的影响。结果表明,迎风坡的气流加速和背风坡的风速降低现象在实际观测中得到证实,但受风向的影响甚大。垄顶风速放大率随入射角的增大呈指数关系递增,而背风坡风速占垄顶风速的比率则随入射角的增大呈线性关系递减。大角度入射气流速度变化主要受控于沙垄形态,小角度入射气流速度变化主要受控于地表植被状况。无论气流以何种角度入射,距沙垄表面20 cm高度气流的加速均较其他高度缓和。沙垄底部和坡中下部的风速廓线变化趋势基本一致,且呈较好的对数拟合关系;两坡中上部和垄顶部的对数拟合方程相关系数偏小;受回旋涡流的影响,风速廓线在背风坡上部有明显偏折,初步断定涡流中心在距地表40 cm左右的高度。     

Climatic influences on Holocene variations in soil erosion rates on a small hill in the Mojave Desert

A detailed study of a small hill in NE Mojave Desert in eastern California was conducted to elucidate the effect of climate on the variations in soil erosion rates through Holocene. Field surveys and sampling were carried out to obtain information on topography, geomorphology, soil and vegetation conditions, seismic refraction, sediment deposition, and hillslope processes. Integration of this information allowed reconstruction of the hill topography at the end of the Pleistocene, deduction of the evolution of the hill from the end of the Pleistocene to the present, and estimation of total soil losses resulting from various hillslope processes. The estimates are consistent with the premise that early Holocene climate change resulted in vegetation change, soil destabilization, and topographic roughening. Current, very slow, hillslope transport rates (e.g., 5 mm ky⁻¹ by rodent burrowing, a presently important transport form) appear inconsistent with the inferred total soil loss rate (31 mm ky⁻¹). Packrat midden studies imply that the NE Mojave Desert experienced enhanced monsoonal precipitation in the early Holocene, presumably accentuating soil loss. Water erosion on one slope of the hill was simulated using Water Erosion Prediction Project (WEPP), a process-based erosion model, using 4 and 6 ky of precipitation input compatible with an appropriate monsoonal climate and the present climate, respectively. The WEPP-predicted soil losses for the chosen slope were compatible with inferred soil losses. Identification of two time periods within the Holocene with distinct erosion characteristics may provide new insight into the current state of Mojave Desert landform evolution.     

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.     

Some remarks on the Gaussian beam summation method

Summary. Recently, a method using superposition of Gaussian beams has been proposed for the solution of high-frequency wave problems. The method is a potentially useful approach when the more usual techniques of ray theory fail: it gives answers which are finite at caustics, computes a nonzero field in shadow zones, and exhibits critical angle phenomena, including head waves. Subsequent tests by several authors have been encouraging, although some reported solutions show an unexplained dependence on the 'free' complex parameter ε which specifies the initial widths and phases of the Gaussian beams.
We use methods of uniform asymptotic expansions to explain the behaviour of the Gaussian beam method. We show how it computes correctly the entire caustic boundary layer of a caustic of arbitrary complexity, and computes correctly in a region of critical reflection. However, the beam solution for head waves and in edge-diffracted shadow zones are shown to have the correct asymptotic form, but with governing parameters that are explicitly ε-dependent. We also explain the mechanism by which the beam solution degrades when there are strong lateral inhomogeneities. We compare numerically our predictions for some representative, model problems, with exact solutions obtained by other means.     

Computation of wave fields in inhomogeneous media — Gaussian beam approach

Summary. An asymptotic procedure for the computation of wave fields in two-dimensional laterally inhomogeneous media is proposed. It is based on the simulation of the wave field by a system of Gaussian beams. Each beam is continued independently through an arbitrary inhomogeneous structure. The complete wave field at a receiver is then obtained as an integral superposition of all Gaussian beams arriving in some neighbourhood of the receiver. The corresponding integral formula is valid even in various singular regions where the ray method fails (the vicinity of caustic, critical point, etc.). Numerical examples are given.     

腾格里沙漠东南缘格状沙丘粒度特征与成因探讨*

腾格里沙漠南缘格状沙丘表面沉积物的系统采样分析结果表明,粒径与分选参数在沙丘断面具有规律变化是形态-气流相互作用的结果.文中根据沙丘形态和气流状况,对格状沙丘成因及类型归属等问题作了探讨.认为格状沙丘发育在双向低风能环境,其中主梁为主风(西北风)作用下形成的横向沙丘,副梁则发育在主梁基础上并受制于主风和次主风(东北风)交替作用,在形态,沉积和动力学方面具有纵向要素特征.因此,该区格状沙丘属纵向要素叠置于横向沙丘之上的一种复杂型沙丘.     

Shear-wave polarizations on a curved wavefront at an isotropic free surface

Summary. We present polarization diagrams of the particle motions at the free surface of an isotropic half-space generated by incident shear waves from a local buried point source. The reflectivity technique is used to calculate synthetic seismograms from which the particle motions are plotted. The particle motions are examined over a range of epicentral distances in a uniform isotropic half-space for different source frequencies and polarization angles, and for different Poisson's ratios. The particle motions due to a curved wavefront possess different characteristics from those generated by plane wavefronts at corresponding incidence angles. A curved wavefront generates a local SP -phase: a P -headwave which propagates along the free surface, and arrives shortly before the direct S -wave. These two arrivals give rise to cruciform particle motions in the sagittal and horizontal planes, which could be misinterpreted as anisotropy-induced shear-wave splitting. An examination of the particle motion in the transverse plane, mutually orthogonal to the sagittal and horizontal planes, can be used to discriminate between isotropic and anisotropic interpretations. The amplitude of the SP -phase is enhanced when it propagates in a low-velocity surface layer overlying the source layer, and may then become the dominant phase on radial-component seismograms. The presence of even a single surface layer may introduce considerable complexity into the seismogram, and we examine the effects of layer thickness, velocity contrast, and source depth on the corresponding polarization diagrams. Reliable information on the source and propagation path characteristics of shear waves from a buried local point source can only be obtained from free-surface records if they are recorded within a very limited epicentral distance range.     

Gaussian beams, complex rays, and the analytic extension of the Green's function in smoothly inhomogeneous media

Summary. Some relations between Gaussian beams, complex rays and the analytic extension of the Green's function in smoothly inhomogeneous media are shown in this paper. It is found that: (1) a single Gaussian beam is a paraxial approximation of the analytical extension of the ray-approximated Green's function in smoothly inhomogeneous media by putting the source point into a complex space. The Gaussian beam approximation of the Green's function has an advantage in computational efficiency and stability and can avoid the singularity problems at caustics, but also introduces a parabolic approximation to the wavefront and an angle-dependent amplitude damping. Therefore the validity of the Gaussian beam approximation should be checked using other methods. (2) Complex-ray tracing, which does not involve the paraxial approximation, can also avoid the singularity problemsm though without the computational efficiency. Therefore, it should be used to verify the Gaussian beam approximation, whenever possible. (3) The decomposition of a plane wave into an ensemble of Gaussian beams is equivalent to approximating the Green's function (the kernel of the ray-Kirchhoff method) with a single Gaussian beam. This introduces a parabolic approximation to the wavefront and a Gaussian windowing for arrival angles which may cause some problems in modelling wave propagation and scattering and has no advantages over other methods. (4) The representation of a point source field by a superposition of Gaussian beams, on the other hand, is equivalent to approximating the Green's function with a bundle of overlapped Gaussian beams. This representation is similar to a Maslov uniform asymptotic representation. It has no caustics and has improved accuracies at the caustics for quasi-plane waves compared to the extended WKBJ method.     

基于第二次冰川编目数据的中国冰川高度结构特征分析

主要基于第二次中国冰川编目数据,计算和分析中国西部各个山脉冰川面积随高度分布特征、冰川平衡线高度场的分布规律和积累区比率的分布特征。结果表明：① 各个山脉冰川面积随高度分布呈近似正态分布,冰川最大面积所占总面积的百分比与冰川分布高度差的比值可以作为描述冰川面积随高度分布的形状参数;② 平衡线高度的分布特征受气候和地形影响,由南向北逐渐降低,由东向西逐渐升高;西北和南部高大山脉边缘比较密集,青藏高原内部比较稀疏。③ 冰川积累区比率的分布特征与水汽、地形和物理冰川属性有关。各大山脉外侧和海洋型冰川区积累区比率较小（<0.5）,山脉内侧及高原内陆地区和极大陆型冰川区的积累区比率较大（>0.7）。