3D scattering of obliquely incident plane SV waves by an alluvial valley embedded in a fluid-saturated,poroelastic layered half-space

The indirect boundary element method is used to study the 3D dynamic response of an infinitely long alluvial valley embedded in a saturated layered half-space for obliquely incident SV waves. A wave-number transform is first applied along the valley’s axis to reduce a 3D problem to a 2D plane strain problem. The problem is then solved in the section perpendicular to the axis of the valley. Finally, the 3D dynamic responses of the valley are obtained by an inverse wave-number transform. The validity of the method is confirmed by comparison with relevant results. The differences between the responses around the valley embedded in dry and in saturated poroelastic medium are studied, and the effects of drainage conditions, porosity, soil layer stiffness, and soil layer thickness on the dynamic response are discussed in detail resulting in some conclusions.     

Three-dimensional response of a layered cylindrical valley embedded in a layered half-space

An indirect boundary integral method to obtain the three-dimensional response of an infinitely long, layered, viscoelastic valley of arbitrary cross-section embedded in a layered viscoelastic half-space is presented. The valley is excited by homogeneous plane waves impinging at an oblique angle with respect to the axis of the valley. The method and associated computer programs are tested by comparison with available results in the limiting two-dimensional case of incidence normal to the axis of the valley. Additional comparisons with previous three-dimensional results obtained by a hybrid finite element-boundary integral method for cylindrical valleys subjected to obliquely incident waves show large differences. However, the results obtained here for an infinitely long valley appear to be in some agreement with earlier results for an elongated prolate semi-ellipsoidal valley and with results obtained by a discrete wavenumber boundary element approach. An extensive bibliography on the dynamic response of valleys is also presented.     

Oceanographer transform fault structure compared to that of surrounding oceanic crust: Results from seismic refraction data analysis

A high quality seismic refraction data set was collected near the intersection of the tranform portion of the Oceanographer Fracture Zone (OFZ) with the adjacent northern limb of the Mid-Atlantic Ridge spreading center (MAR). One seismic line was shot down the axis of the transform valley. Another was shot parallel to the spreading center, crossing from normal oceanic crust into the transform valley, and out again. This latter line was recorded by four Ocean Bottom Seismometers (OBSs) spaced along its length, providing complete reversed coverage over the crucial transform valley zone. Findings indicate that whereas the crust of the transform valley is only slightly thinner (4.5 km) compared to normal oceanic crust (5–8 km), the structure is different. Velocities in the range of 6.9 to 7.7. km/sec, which are characteristics of seismic layer 3B, are absent, although a substantial thickness (approximately 3 km) of 6.1–6.8 km/sec material does appear to be present. The upper crust, some 2 km in thickness, is characterized by a high velocity gradient (1.5 sec⁻¹) in which veloxity increases from 2.7 km/sec at the seafloor to 5.8 km/sec at the base of the section. A centrally-located deep of the transform valley has thinner crust (1–2 km), whereas the crust gradually thickens past the transform valley-spreading center intersection. Analysis of the seismic line crossing sub-perpendicular to the transform valley demonstrates abrupt thinning of the upper crustal section, and thickening of the lower crust outside of the trasform valley. In addition, high-velocity material seems to occur under the valley flanks, particularly the southern flanking ridge. This ridge, which is on the side of the transform opposite to the intersection of spreading ridge and transform, may be an expression of uplifted, partially serpentinized upper mantle rocks.     

Numerical simulation of channel pattern changes Part Ⅱ:Application in a conceptual channel

This paper presents simulated channel patterns for various scenarios in a conceptual alluvial valley by an improved two-dimensional (2-D) mathematical model described in the companion paper. Starting from the same initial channel, different channel patterns have been simulated over a real time period of 250 days for varied boundary and initial conditions, including the inlet water discharge and sediment load, initial valley slope, and erodibility of river banks. Impacts of these control factors are discussed, in terms of the longitudinal bottom profiles of simulated fluvial channels, the geometry of channel cross sections, and the water surface profiles in the conceptual river valley. Results suggest that the upper and lower parts of the same channel may have different planforms because the sediment transport conditions of the two parts differ greatly. Simulated causal relationship between control variables and channel patterns agrees qualitatively with known channel pattern theories.     

The mid-ocean ridge axial valley as a steady-state neck

In this paper the mid-ocean ridge axial valley is modelled as a steady-state lithospheric neck in which lithospheric stretching balances lithospheric accretion. Conversely, the axial high is a steady-state lithospheric bulge. The lithosphere is modelled as a thin plate with a Newtonian rheology. It is shown that an axial valley will occur if the rate of viscosity increase away from the ridge axis is faster than the rate at which accretion decreases. An axial high will occur if the opposite condition holds. This is consistent with the observation that axial valleys occur at low spreading rates and axial highs at high spreading rates. By fitting our model to profiles across the Mid-Atlantic Ridge and the East Pacific Rise and assuming the lithospheric thickness at the ridge axis to be 5 km, we find accretion widths of 6–8 km. We find the width over which there is a significant increase in lithospheric viscosity to be also 6–8 km.     

Evolution of rifted ocean ridges

A major question in seafloor tectonics has been, how does the 2-km-deep rift valley characteristic of slow-spreading ridges evolve into the relatively horizontal undulating relief of the rift mountains? Deep-tow studies of the Mid-Atlantic Ridge suggest that the primary mechanism for transformation of the rift valley topography is normal faulting along fault planes which dip away from the valley axis. The faulting occurs in a narrow zone just beyond the outer walls of the rift valley. This model allows for a steady-state evolution of the rift valley into the rift mountains in which the state of stress in the oceanic lithosphere continues to be in horizontal deviatoric tension throughout the entire process. Alternate mechanisms involving reverse faulting or regional tilt may be active but are found to be of less importance. Implications for various dynamic models of the rift valley are discussed.     

Spatial distribution of surface properties,runoff generation and landscape development in the Zin Valley Badlands,northern Negev,Israel

In?ltration tests, soil mapping and soil property analysis were used to assess the effect of within‐storm rainfall conditions on spatial patterns of surface characteristics relevant for runoff generation, continuity and erosion in the Zin Valley Badlands. Runoff and erosion differ strongly between ridges and slopes. Soils at both locations are susceptible to sealing, but on the sideslopes deep desiccation cracks inhibit continuous ?ow, even during high magnitude rainstorms. The discontinuous nature of runoff has a feedback on surface conditions. Erosion on the ridges maintains shallow soils prone to sealing while in?ltration and deposition on the sideslopes enhance soil depth, a prerequisite for stable desiccation cracks. Some runoff generated on the ridges is transmitted to the valley via rills. On straight sideslopes, rills are single and often discontinuous, indicating limited frequency of continuous runoff. Along concave valley heads, rill systems are well integrated and continuous, concentrating runoff and reducing in?ltration losses along slopes. The longitudinal, V‐shaped valley morphology of small catchments in the Zin Valley Badlands re?ects the long‐term effect of different erosion rates in valley heads and on sideslopes. Over time, valley incision lengthened the sideslopes, reducing the portion of annual rainfall that was runoff‐effective. Once sideslopes reached a critical length that inhibited frequent continuous ?ow, a colluvium with an increased in?ltration capacity developed, reducing runoff frequency even further. Consequently, erosion on the valley sideslopes decreased. Continuous ?ow from ridges to the valley channel remained more common in integrated rill systems in concavities and valley heads, leading to more erosion and retreat of the valley heads. The spatial patterns of runoff and erosion in the Zin Valley Badlands demonstrate that landscape development is strongly affected by processes that lead to differentiation of soil properties on hillslopes with uniform lithology. The patterns of surface characteristics and their role in landscape development are strongly dependent on rainfall conditions, highlighting the need for geomorphologists to identify the dynamic spatial and temporal scales relevant for landscape development. Copyright © 2004 John Wiley & Sons, Ltd.     

Effect of morphology and discharge on hyporheic exchange flows in two small streams in the Cascade Mountains of Oregon,USA

Stream‐tracer injections were used to examine the effect of channel morphology and changing stream discharge on hyporheic exchange flows. Direct observations were made from well networks to follow tracer movement through the hyporheic zone. The reach‐integrated influence of hyporheic exchange was evaluated using the transient storage model (TSM) OTIS‐P. Transient storage modelling results were compared with direct observations to evaluate the reliability of the TSM. Results from the tracer injection in the bedrock reach supported the assumption that most transient storage in headwater mountain streams results from hyporheic exchange. Direct observations from the well networks in colluvial reaches showed that subsurface flow paths tended to parallel the valley axis. Cross‐valley gradients were weak except near steps, where vertical and cross‐valley hydraulic gradients indicated a strong potential for stream water to downwell into the hyporheic zone. The TSM parameters showed that both size and residence time of transient storage were greater in reaches with a few large log‐jam‐formed steps than in reaches with more frequent, but smaller steps. Direct observations showed that residence times in the unconstrained stream were longer than in the constrained stream and that little change occurred in the location and extent of the hyporheic zone between low‐ and high‐baseflow discharges in any of the colluvial reaches. The transient storage modelling results did not agree with these observations, suggesting that the TSM was insensitive to long residence‐time exchange flows and was very sensitive to changes in discharge. Disagreements between direct observations and the transient storage modelling results highlight fundamental problems with the TSM that confound comparisons between the transient storage modelling results for tracer injections conducted under differing flow conditions. Overall, the results showed that hyporheic exchange was little affected by stream discharge (at least over the range of baseflow discharges examined in this study). The results did show that channel morphology controlled development of the hyporheic zone in these steep mountain stream channels. Copyright © 2005 John Wiley & Sons, Ltd.     

LOCALLY GENERATED SURFACE WAVES IN SANTA CLARA VALLEY: ANALYSIS OF OBSERVATIONS AND NUMERICAL SIMULATION

The motions recorded by the Gilroy array of instruments on the surface of the Santa Clara Valley, California, during the 1989 Loma Prieta and 1984 Morgan Hill earthquakes are analysed for evidence of valley induced surface waves. The Santa Clara Valley extends in a NW–SE direction, south of the San Francisco Bay. The Gilroy linear array of instruments is an east–west alignment of stations crossing the Santa Clara Valley. Seismic refraction studies in the vicinity of the array indicate that the valley is wedge-shaped in cross-section with maximum thickness of the order of 1 km. Analysis of the recorded motions of the 1989 Loma Prieta earthquake reveal clear evidence of the fundamental and first and second higher modes of Rayleigh waves, while analysis of the recorded motions of the 1984 Morgan Hill earthquake shows, in addition to the above surface wave modes, the presence of the fundamental Love mode. Motions generated by the latter event were more complicated due to the presence of the low-velocity zone of the Calaveras fault, which traps and focuses seismic energy generated by slip on the fault, and leaks it to the surrounding medium in a rather complicated manner. The observed valley-induced surface waves are simulated using a hybrid numerical technique which combines the Boundary Integral Equation Method with the Finite Element Method. The mathematical formulation that we use has been developed for a class of cylindrical inclusions of infinite length, having an arbitrary cross-section, embedded in a homogeneous (or layered) half-space, subjected to plane waves impinging at an oblique angle with respect to the axis of the inclusion. Even though the model of the valley is two-dimensional, the response is three-dimensional and has the particular feature of repeating itself with a certain delay for different observers along the axis of the valley. This feature leads to a considerably simpler solution than that for a valley with a 3-D geometry.     

The westerly fluctuation and water vapor transport over the Qilian-Heihe valley

The westerly fluctuation and the atmospheric water vapor transport over the Qilian-Heihe valley are analyzed and the results show that, in the water vapor transport stream field from Jun to September, this valley is in the westerly stream and the water vapor comes from westerlies water transport via the Black Sea and the Caspian Sea. The net water vapor transport is less net import and different from most areas of the northwest China. The interannual changes in water vapor transport over the valley arise from the westerly fluctuation, and have a positive relationship to the interannual changes in westerly wind speed. The cold air actions from the Mongol low pressure are the primary system that controls the westerly water vapor transport. Its action chain is that, the Mongol low pressure is strengthened → the circulation meridionality will be increased → the cold air will move southwards → the westerly will be stronger → the wind convergence of direction and speed will be stronger → the water vapor convergence transport will be increased → the local water vapor content will be increased. The interannual changes in atmospheric water vapor transport over the valley rely mainly on the convergence transport, but the effect of advection transport is less. The interannual changes of strong or weak westerly affect mainly the convergence transport, and then make the atmospheric water vapor net transport increase or decrease over the Qilian-Heihe valley.     

The westerly fluctuation and water vapor transport over the Qilian-Heihe valley

The westerly fluctuation and the atmospheric water vapor transport over the Qilian-Heihe valley are analyzed and the results show that, in the water vapor transport stream field from Jun to September, this valley is in the westerly stream and the water vapor comes from westerlies water transport via the Black Sea and the Caspian Sea. The net water vapor transport is less net import and different from most areas of the northwest China. The interannual changes in water vapor transport over the valley arise from the westerly fluctuation, and have a positive relationship to the interannual changes in westerly wind speed. The cold air actions from the Mongol low pressure are the primary system that controls the westerly water vapor transport. Its action chain is that, the Mongol low pressure is strengthened → the circulation meridionality will be increased → the cold air will move southwards → the westerly will be stronger → the wind convergence of direction and speed will be stronger → the water vapor convergence transport will be increased → the local water vapor content will be increased. The interannual changes in atmospheric water vapor transport over the valley rely mainly on the convergence transport, but the effect of advection transport is less. The interannual changes of strong or weak westerly affect mainly the convergence transport, and then make the atmospheric water vapor net transport increase or decrease over the Qilian-Heihe valley.     

城市密集建筑群对沉积谷地地震动放大效应的影响

采用有限元模拟方法建立了建筑群-沉积谷地二维模型,并在土体截断边界上施加粘弹性人工边界,在频域与时域中对比分析此体系和单独沉积谷地的地震反应,观察地震时沉积谷地与建筑群之间的动力相互作用规律。分析表明,沉积谷地中建筑群对谷地本身的地震反应具有显著影响。入射波频率较低时,由于共振效应的存在,在部分区域处建筑群-沉积谷地体系的地表位移响应幅值会大于单独沉积谷地,但随着入射波频率的增加,建筑群的存在又会对地震反应产生明显的减弱效果;建筑群对谷地的影响还与建筑高度和建筑间距有关,且不同位置处的响应也存在很大差异。计算结果可为沉积谷地中设防烈度的设置以及工程抗震设计提供部分理论依据。     

Application of electrical and electromagnetic methods to study sedimentary covers in high mountain areas

The results of geophysical studies conducted with selected electrical and electromagnetic methods in the Kondratowa Valley in the Tatra Mountains (the Carpathian Mountains, Poland) are presented in the article. The surveys were performed with the following methods: electrical resistivity tomography (ERT), georadar (GPR) and conductivity meter (CM). The objective of the noninvasive geophysical measurements was to determine the thickness of the Quaternary postglacial sediments that fill the bottom of the valley and to designate the accumulation of boulders deposited on Quaternary sediments. The results of ERT surveys conducted along the axis of the valley allowed to determine the changeability of the thickness of the postglacial sediments and allowed to designate a few areas of occurrence of boulders. The ERT, GPR and CM surveys conducted across the valley allowed to designate with high accuracy the thickness of the accumulation of boulders sliding down the valley bottom from the couloirs surrounding the valley.     

Pro‐glacial soil variability and geomorphic activity – the case of three Swiss valleys

Soils in pro‐glacial areas are often approached from a chronosequence viewpoint. In the chronosequence approach, the objective is to derive rates of soil formation from differences in properties between soils of different age. For this reason, in chronosequence studies, soils are sampled in locations that are assumed geomorphically stable and that have different age. As a result, these studies do not necessarily yield a complete view of soil variability in pro‐glacial areas, and may miss important relations between geomorphology and soil development. In this contribution, we present new soil observations from three closely related pro‐glacial areas in Switzerland. These observations were intended to get closer to a complete view of soil variability, and to assess impacts from factors other than time on soil development. About 40 soils were visited in each pro‐glacial valley in a combined design‐convenience sampling scheme and described in the field. Linear modelling was used to assess effects of time and topographic factors on soil properties. The time since glacial retreat turned out to rarely explain more than half of the variation in soil properties, and a linear model combining effects of time and topographic variables explained typically about half of the variation in each pro‐glacial valley. Models differed and were not transferable between valleys. Apparently, time and the present‐day shape of the landscape combined are insufficient information to accurately predict soil properties. Field evidence points to the importance of the geomorphic history and regime of the valleys as a reason for this. Copyright © 2014 John Wiley & Sons, Ltd.     

Spreading of Stratified Currents in Lowland Reservoirs

The results of comprehensive hydrophysical studies of gradient and density-induced gravitational currents with thermal stratification in valley reservoirs are considered. The lengthwise and vertical development of currents as well as their evolution is considered. Local swells are detected in the near-bed nepheloid layer and clouds of suspended materials are found to be ejected from the density-induced current near abrupt changes in the bed relief. The near-bed density-induced current is found to strongly interact with the drift current in the presence of inner wave and variable-direction wind. The measured vertical profiles of velocity in the sections along the axis of the current and the longitudinal distributions of the average flow velocity and suspended material concentration are found to agree with model calculations.     

A combined central- and fissure-type phonolitic volcano in western Kenya

The Nyamaji volcano is a small eruptive complex of late Miocene age associated with the nearby Usaki ijolite and Sokolo carbonatite intrusion of Homa Bay in the Kavirondo Rift valley of Kenya. It is probably a satellite volcano to the major volcanic structure of Kisingiri - Rangwa which lies 25 km to the west. The Nyamaji volcanic complex is composed of agglomerates, breccias and tuffs erupted from a central vent, whilst at much the same time lavas were extruded from fissures which are now occupied by dykes. These two contemporaneous events gave rise to an interdigitated sequence of pyroclastic deposits and effusive lavas. The pyroclastic rocks of Vulcanian origin cover an area at least 30 km² in extent, are poorly bedded, and usually are about 25 m (80ft.) thick though they often thin to zero over topographic highs in the pre-existing landscape. At Nyamaji itself, the Strombolian style pyroclastic pile exceeds 330 m (1100 ft.) in thickness over an area of 1 km², and this marks the position of the original central vent. The fragmental material in the pyroclastic rocks includes ijolite, phonolite, nephelinite, trachyte, carbonatite, granite, and feldspathic and aegirine-bearing fenites; the matrix is sometimes calcareous, sometimes feldspathic. Nephelinitic lavas occur amongst the lowest lavas, but the lavas above are nearly all phonolitic. The oldest dykes are nephelinitic and are rare; the youngest dykes are phonolitic and are abundantly exposed. Both lavas and dykes contain xenoliths similar to those in the pyroclastic rocks. A series of volcanic plugs pierce the lavas. These plugs, mostly non-conduit type, average 200–500 m diameter, are mainly composed of glassy to very fine-grained phonolites, and show good flow structures. The plugs, especially those near the Ruri hills, tend to lie along N - S and E - W lines. The majority of the dykes also lie along these directions. The dominant structural directions within the nearby Usaki ijolite complex and the Wasaki carbonatite are also N - S and E - W, respectively. These directions are quite different from the axis of the Kavirondo rift valley which here is NE - SW, and from the strike of the Precambrian basement. The Nyamaji volcanic structure differs from nearly all the other East African volcanoes by its dominant phonolitic petrochemistry.     

Detailed streamflow data for understanding hydrologic responses in the Logan River Observatory

The Logan River watershed, located in Northern Utah, USA, consists of a relatively pristine, mountainous area that drains to a lower elevation, valley area influenced by both urban development and agriculture. The Logan River Observatory has been collecting aquatic (streamflow and water quality) and climate data throughout the Logan River watershed since 2014. While streamflow measurements are commonly made at the outlets of research watersheds, the Logan River watershed consists of diverse hydrologic, topographic, and geologic settings that require a detailed understanding of streamflow variability over time at many locations. Here, we illustrate: (a) the importance of collecting streamflow time series throughout complex watersheds, and (b) how simple flow balances can provide much needed hydrologic insight into the locations and timing of gains and losses over reaches to guide future investigations.     

The character and age structure of valley fills in upper Wolumla Creek catchment,south coast,New South Wales,Australia

Extensive valley fills at the base of the escarpment in upper Wolumla Creek, on the south coast of New South Wales, Australia, have formed from a combination of ‘cut and fill’ processes. The valley fills comprise series of alternating, horizontally bedded sand and mud units, reflecting reworking of detritus from deeply weathered granites of the Bega Batholith. Sand units are deposited as sand sheets or splays on floodplain surfaces or in floodouts that form atop intact valley fill surfaces downstream of discontinuous gullies. Alternatively, sands are deposited from bedload and form bars or part of the valley floor within channel fills. Organic-rich mud units are deposited from suspension in swamps or in seepage zones at the distal margin of floodouts. Within 5 km of the escarpment, valley deposits grade downstream from sand sheet and splay deposition in floodouts, to mud deposition in swamp and seepage zones. Radiocarbon dates indicate that virtually the entire valley fill of upper Wolumla Creek was excavated prior to 6000 years BP . Remnant terraces are evident at valley margins. The valley subsequently filled between 6000 years BP and 1000 years BP producing valley fills around 12 m deep, but no greater than 300 m wide. Reincision into the valley fill, on a scale smaller than the present incision phase, is indicated at around 1000 years BP , following which the channel refilled. Portion plans dated from 1865 refer to the study area as ‘Wolumla Big Flat’, and show large areas of swampy terrain, suggesting that the valley fill had re-established by this time. Within a few decades of European settlement the valley fill incised once more. Upper Wolumla Creek now has a channel over 10 m deep and 100 m wide in places, draining a catchment area of less than 20 km². © 1998 John Wiley & Sons, Ltd.     

Scattering of obliquely incident seismic waves by a cylindrical valley in a layered half-space

Three-dimensional scattering of seismic waves by a cylindrical alluvial valley embedded in a layered half-space is investigated by using the combination of the boundary integral representation and the finite element method. The surface displacements due to incident plane harmonic body waves (P, SV and SH) propagating at an arbitrary angle to the axis of the cylindrical valley are evaluated numerically for two semi-elliptical alluvial valleys. The presence of the layer is found to have a strong effect on the amplification of the surface displacements in some cases. The three-dimensional motion seems to be quite critical and may cause large amplification. The surface ground motion becomes significant when compared with corresponding free-field motion as the wavelengths become comparable to the characteristic length of the valley. The maximum amplification always occurs atop the valley. Numerical results show that the amplitude and the amplification pattern of the surface displacement strongly depend upon the frequency, the angle and the type of the incident waves.     

瑞雷波斜入射下层状半空间中沉积谷地周围的三维散射研究

针对层状半空间中沉积谷地对斜入射瑞雷波的三维散射问题, 采用直接刚度法计算自由场波场, 以层状半空间中移动斜线均布荷载动力格林影响函数求解三维散射波场, 建立了求解该问题的间接边界元方法. 通过与已有结果的比较, 验证了该方法的正确性, 并以均匀半空间以及弹性基岩上单一土层场地中沉积谷地为例进行了计算分析. 研究结果表明: 层状半空间与均匀半空间中沉积谷地对瑞雷波的散射存在显著差别; 层状半空间中瑞雷波的振动模态对沉积附近位移幅值有着重要影响; 土层刚度和厚度等参数也对沉积附近位移幅值大小及空间分布有着显著的影响.