Ash-rich pyroclastic flows from the cataclysmic eruption of Mount Mazama (~7700 yr. B. P.), Cascade volcanic arc, Oregon, entered and blocked the narrow, bedrock-lined canyon of the Williamson River approximately 35 t...Ash-rich pyroclastic flows from the cataclysmic eruption of Mount Mazama (~7700 yr. B. P.), Cascade volcanic arc, Oregon, entered and blocked the narrow, bedrock-lined canyon of the Williamson River approximately 35 to 44 km from the source volcano. The blockage impounded a body of water which then released producing four stratigraphic units in the downstream debris fan. The four stratigraphic units are a boulder core comprised of locally sourced bedrock boulders and three sand-rich units including a fine-grained sand unit, a sandy pumice gravel (±basalt/hydrovolcanic tuff) unit, and a pumice pebble-bearing, crystal-rich sand unit. Hand-drilled auger holes up to ~1.6 m deep were used to obtain samples of the sand-rich units. Units were delimited using surface and down-hole observations, composition and texture, estimated density, statistical parameters of grain size, and vertical and lateral distribution of properties. Overtopping followed by rapid incision into the ash-rich pyroclastic flows progressively cleared the canyon, but a bedrock knickpoint near the head of the canyon limited the volume of debris available for transport to about 0.04 km<sup>3</sup> to 0.08 km<sup>3</sup>. Co-deposition of bedrock boulders and lithic-rich sand was followed by rapid deposition with minimal reworking of remobilized pyroclastics. Continued draining of the impounded lake sent hyperconcentrated flows onto the debris fan depositing pumice-rich gravels that graded upward to crystal-rich sands.展开更多
The Shenhu Submarine Canyon Group on the northern slope of the South China Sea consists of 17 slope-confined canyons,providing a good example for investigating their hosting sediments.Three drilling sites,including W0...The Shenhu Submarine Canyon Group on the northern slope of the South China Sea consists of 17 slope-confined canyons,providing a good example for investigating their hosting sediments.Three drilling sites,including W07,W18,and W19,have proven the occurrence of gas hydrate reservoirs in the inter-canyon area between canyons C11 and C12.Whereas,variations of the geomorphology and seismic facies analyzed by high-resolution 3D seismic data indicate that the gas hydrate-bearing sediments may form in different sedimentary processes.In the upper segment,a set of small-scale channels with obvious topographic lows can be identified,revealing fine-grained turbidites supplied from the shelf region during a very short-term sea-level lowstand.In the middle part,gas hydrate units at Site W07 showing mounded or undulation external configuration are interpreted as sliding sedimentary features,and those features caused by gravity destabilization were the main formative mechanism of gas hydrate-bearing sediments that were sourced from the upper segments.In contrast,for the canyon transition zone of lower segments between C11-C12 inter-canyon and C12 intra-canyon areas,where W18 and W19 sites are located,the gas hydratebearing sediments are deposited in the channelized feature in the middle to lower segment and slide erosive surface.Gas hydrate-bearing sediments of the lower segment were migrated through channelized features interconnecting with the middle to lower slope by gravity-driven flows.The majority of deposits tended to be furtherly moved by lateral migration via erosive surface created by sediment failed to intra-canyon area.The conclusion of this study may help better understand the interaction between the formation mechanism of gas hydrate-bearing sediments and the geomorphologic effects of inter-canyon areas.展开更多
Submarine canyon is an important channel for long-distance sediment transport, and an important part of deepwater sedimentary system. The large-scale Rizhao Canyons have been discovered for the first time in 2015 in t...Submarine canyon is an important channel for long-distance sediment transport, and an important part of deepwater sedimentary system. The large-scale Rizhao Canyons have been discovered for the first time in 2015 in the continental slope area of the western South China Sea. Based on the interpretation and analysis of multi-beam bathymetry and two-dimensional multi-channel seismic data, the geology of the canyons has however not been studied yet. In this paper, the morphology and distribution characteristics of the canyon are carefully described,the sedimentary filling structure and its evolution process of the canyon are analyzed, and then its controlling factors are discussed. The results show that Rizhao Canyons group is a large slope restricted canyon group composed of one east-west west main and nine branch canyons extending to the south. The canyon was formed from the late Miocene to the Quaternary. The east-west main canyon is located in the transition zone between the northern terrace and the southern Zhongjiannan Slope, and it is mainly formed by the scouring and erosion of the material source from the west, approximately along the slope direction. Its development and evolution is mainly controlled by sediment supply and topographic conditions, the development of 9 branch canyons is mainly controlled by gravity flow and collapse from the east-west main canyon. This understanding result is a supplement to the study of “source-channel–sink” sedimentary system in the west of the South China Sea, and has important guiding significance for the study of marine geological hazards.展开更多
Since 1950, 700 plots were established in the alpine and canyon region of western Sichuan. The distribution charac-teristics and the relationships between forest succession and environmental gradients were studied. Th...Since 1950, 700 plots were established in the alpine and canyon region of western Sichuan. The distribution charac-teristics and the relationships between forest succession and environmental gradients were studied. The results showed that the main tree species were Picea and Abies in this region, and there were more than 90 forest types. Abies forests mainly dis-tributed in the middle and upper reaches of rivers and their branches, and Picea forests mainly distributed in wide valleys and on half-shaded and half-sunny slopes. The natural regeneration was poor under primitive spruce and fir forest canopy, but was good in the spruce and fire forest gap. The relationship between forest succession and vertical gradient was closely related to the relationship between forest succession procession and plant synusia under primary forests. Human activities could promote and postpone succession process. The results of expanding regeneration were often influenced by topography, vegetation and wind direction.展开更多
The dynamic stress intensity factor (DSIF) and the scattering of SH wave by circle canyon and crack are studied with Green's function. In order to solve the problem, a suitable Green's function is constructed...The dynamic stress intensity factor (DSIF) and the scattering of SH wave by circle canyon and crack are studied with Green's function. In order to solve the problem, a suitable Green's function is constructed first, which is the solution of displacement fields for elastic half space with circle canyon under output plane harmonic line loading at horizontal surface. Then the integral equation for determining the unknown forces in the problem can be changed into the algebraic one and solved numerically so that crack DSIF can be determined. Last when the medium parameters are altered, the influence on the crack DSIF is discussed partially with the displacement between circle canyon and crack.展开更多
The wave propagation behavior in an elastic wedge-shaped medium with an arbitrary shaped cylindrical canyon at its vertex has been studied.Numerical computation of the wave displacement field is carried out on and nea...The wave propagation behavior in an elastic wedge-shaped medium with an arbitrary shaped cylindrical canyon at its vertex has been studied.Numerical computation of the wave displacement field is carried out on and near the canyon surfaces using weighted-residuals(moment method).The wave displacement fields are computed by the residual method for the cases of elliptic,circular,rounded-rectangular and flat-elliptic canyons,The analysis demonstrates that the resulting surface displacement depends,as in similar previous analyses,on several factors including,but not limited,to the angle of the wedge,the geometry of the vertex,the frequencies of the incident waves,the angles of incidence,and the material properties of the media.The analysis provides intriguing results that help to explain geophysical observations regarding the amplification of seismic energy as a function of site conditions.展开更多
This paper reports a series solution of wave functions for two-dimensional scattering and diffraction of plane SH waves induced by a symmetrical V-shaped canyon with different shape ratios. A half-space with a symmetr...This paper reports a series solution of wave functions for two-dimensional scattering and diffraction of plane SH waves induced by a symmetrical V-shaped canyon with different shape ratios. A half-space with a symmetrical V-shaped canyon is divided into two sub-regions by using a circular-arc auxiliary boundary. The two sub-regions are represented by global and local cylindrical coordinate systems, respectively. In each coordinate system, the wave field satisfying the Helmholtz equation is represented by the separation of variables method, in terms of the series of both Bessel functions and Hankel functions with unknown complex coefficients. Then, the two wave fields are described in the local coordinate system using the Graf addition theorem. Finally, the unknown coefficients are sought by satisfying the continuity conditions of the auxiliary boundary. To consider the phase characteristics of the wave scattering, a parametric analysis is carried out in the time domain by assuming an incident signal of the Ricker type. Surface and subsurface transient responses demonstrate the characteristics and mechanisms of wave propagating and scattering.展开更多
This paper presents a 2.5D scattering of incident plane SV waves by a canyon in a layered half-space by using the indirect boundary element method （IBEM）. A free field response analysis is performed to provide the d...This paper presents a 2.5D scattering of incident plane SV waves by a canyon in a layered half-space by using the indirect boundary element method （IBEM）. A free field response analysis is performed to provide the displacements and stresses on the boundary of the canyon where fictitious uniform moving loads are applied to calculate the Green＇s fi.mctions for the displacements and stresses. The amplitudes of the loads are determined by the boundary conditions. The free field displacements are added to the fictitious uniform moving loads induced displacements and the total response is obtained. Numerical calculations are performed for a canyon with homogenous and in one layer over bedrock. The effects of the thickness and stiffness of the layer on the amplification are studied and discussed.展开更多
This paper presents an indirect boundary integration equation method for diffraction of plane P waves by a two-dimensional canyon of arbitrary shape in poroelastic half-space. The Green＇s functions of compressional a...This paper presents an indirect boundary integration equation method for diffraction of plane P waves by a two-dimensional canyon of arbitrary shape in poroelastic half-space. The Green＇s functions of compressional and shear wave sources in poroelastic half-space are derived based on Biot＇s theory. The scattered waves are constructed using the fictitious wave sources close to the boundary of the canyon, and magnitude of the fictitious wave sources are determined by the boundary conditions. The precision of the method is verified by the satisfaction extent of boundary conditions, the comparison between the degenerated solutions of single-phased half-space and the well-known solutions, and the numerical stability of the method.展开更多
This paper presents a step-by-step procedure using the three-dimensional boundary element approach to study the behavior of semi-circular canyons under seismic shear waves. The boundary element code TDASC allows utili...This paper presents a step-by-step procedure using the three-dimensional boundary element approach to study the behavior of semi-circular canyons under seismic shear waves. The boundary element code TDASC allows utilization for various canyon geometries, evaluation of concurrent seismic waves and calculation of the ground motions on canyons due to an excitation at any arbitrary point of the incident field. Considering the widening ratio of the canyon(including prismatic, semi-prismatic and non-prismatic canyons), wave characteristics(wavelength, dimensionless period, direction) and maximum amplification pattern, the solution was applied to carry out a series of parametric studies. It was shown that canyon form can significantly affect the displacement amplification, especially at the points located on its edges. By increasing the wave dimensionless frequency(η > 1), the amplification pattern becomes more complex. On the basis of the results from a variety of considered cases, a new expression has been presented for the limiting wavelength beyond which the widening of the canyon will not have a major effect on the displacement amplification. To verify the reliability of the proposed approach, the obtained results, expressed in terms of displacement amplitude, were compared with those from the available published literature and a reasonably good agreement was observed.展开更多
The questions on how vortices are constructed and on the relationship between the flow patterns and concentration distributions in real street canyons are the most pressing questions in pollution control studies. In t...The questions on how vortices are constructed and on the relationship between the flow patterns and concentration distributions in real street canyons are the most pressing questions in pollution control studies. In this paper, the very large eddy simulation (VLES) and large eddy simulation (LES) are applied to calculate the flow and pollutant concentration fields in an urban street canyon and a cross-road respectively. It is found that the flow separations are not only related to the canyon aspect ratios, but also with the flow velocities and wall temperatures. And the turbulent dispersions are so strongly affected by the flow fields that the pollutant concentration distributions can be distinguished from the different aspect ratios, flow velocities and wall temperatures.展开更多
To evaluate the importance of the canyon topography effects on large structures, based on a rigid frame bridge across a 137-m-deep and 600-m-wide canyon, the seismic response of the canyon site is analyzed using a two...To evaluate the importance of the canyon topography effects on large structures, based on a rigid frame bridge across a 137-m-deep and 600-m-wide canyon, the seismic response of the canyon site is analyzed using a two-dimensional finite element model under different seismic SV waves with the assumptions of vertical incidence and oblique incidence to obtain the ground motions, which are used as the excitation input on the pier foundations of the bridge with improved large mass method. The results indicate that canyon topography has significant influences on the ground motions in terms of inci- dent angle. The peak ground acceleration values vary greatly from the bottom of the canyon to the upper comers. Under ver- tical incident SV waves, at the upper comers of canyon the peak ground accelerations greatly increase; whereas the peak ground accelerations diminish at the bottom comers of canyon. Under oblique incident SV waves, the shaking of the canyon slope perpendicular to the incidence direction is much more severe than that of the opposite side of canyon. And the ground surface has been characterized by larger deformations in the case of oblique incident waves. It is also concluded that the low piers and frame of the continuous rigid frame bridge ape more sensitive to the multi-support seismic excitations than the flexible high piers. The canyon topography as well as the oblique incidence of the waves brings the continuous rigid frame bridge severe responses, which should be taken into account in bridge design.展开更多
This paper investigates in detail the nature of diffraction of plane P waves around a canyon in poroelastic half-space, and studies the effects of incident frequency, drainage condition, porosity, etc, on the diffract...This paper investigates in detail the nature of diffraction of plane P waves around a canyon in poroelastic half-space, and studies the effects of incident frequency, drainage condition, porosity, etc, on the diffraction of waves. It is shown that the surface displacement amplitudes of the drained case are close to those of the undrained case, however, the surface displacement amplitudes of the dry case are very different from those of the saturated （either drained or undrained） cases. There are large phase shift between the dry case and the saturated cases, as well as slightly longer resultant wavelengths for the undrained case than those for the drained case and longer resultant wavelengths for the drained case than those for the dry case. For small porosity the surface displacement amplitudes for the saturated cases are almost identical to those for the dry case; while for large porosity, the effect of drainage condition becomes significant, and the surface displacement amplitudes for the undrained case are larger than those for the drained case. As the incident frequency increases, the effect of porosity becomes significant, and more significant for the undrained case than that for the drained case. As the porosity increases, the pore pressures increase significantly but their oscillations become smoother. As the incident frequency increases, the pore pressures become more complicated.展开更多
Dating of monazite grains preserved within foliations defining FIAs(foliation inflection/intersection axes preserved within porphyroblasts)have provided a detailed chronology of deformational and metamorphic events du...Dating of monazite grains preserved within foliations defining FIAs(foliation inflection/intersection axes preserved within porphyroblasts)have provided a detailed chronology of deformational and metamorphic events during a regional～1700 Ma orogeny,in Big Thompson region of the Colorado Rockies in the USA.FIAs controlled dating of monazite grains preserved within multiple-foliations of garnet and staurolite porphyroblast have complemented and potentially revolutionized all traditional dating approaches available.With this technique it is possible to accurately differentiate the展开更多
Based on the interpretation of high resolution 2D/3D seismic data, sedimentary filling characteristics and full- filled time of the Central Canyon in different segments in the Qiongdongnan Basin of northwestern South ...Based on the interpretation of high resolution 2D/3D seismic data, sedimentary filling characteristics and full- filled time of the Central Canyon in different segments in the Qiongdongnan Basin of northwestern South China Sea have been studied. The research results indicate that the initial formation age of the Central Canyon is traced back to 11.6 Ma （T40）, at which the canyon began to develop due to the scouring of turbidity currents from west to east. During the period of 11.6-8.2 Ma （T40-T31）, strong downcutting by gravity flow occurred, which led to the formation of the canyon. The canyon fillings began to form since 8.2 Ma （T31） and were dominated by turbidite deposits, which constituted of lateral migration and vertical superposition of turbidity channels during the time of 8.2-5.5 Ma. The interbeds of turbidity currents deposits and mass transport deposits （MTDs） were developed in the period of 5.5-3.8 Ma （T30-T28）. After then, the canyon fillings were primarily made up of large scale MTDs, interrupted by small scale turbidity channels and thin pelagic mudstones. The Central Canyon can be divided into three types according to the main controlling factors, geomorphology-controlled, fault-controlled and intrusion- modified canyons. Among them, the geomorphology-controlled canyon is developed at the Ledong, Lingshui, Songnan and western Baodao Depressions, situated in a confined basin center between the northern slope and the South Uplift Belt along the Central Depression Belt. The fault-controlled canyon is developed mainly along the deep-seated faults in the Changchang Depression and eastern Baodao Depression. Intrusion-modified canyon is only occurred in the Songnan Low Uplift, which is still mainly controlled by geomorphology, the intrusion just modified seabed morphology. The full-filled time of the Central Canyon differs from west to east, displaying a tendency of being successively late eastward. The geomorphology-controlled canyon was completely filled before 3.8 Ma （T28）, but that in intrusion-modified canyon was delayed to 2.4 Ma （T27） because of the uplifted southern canyon wall. To the Changchang Depression, the complete filling time was successively late eastward, and the canyon in eastern Changchang Depression is still not fully filled up to today. Difference in full-filled time in the Central Canyon is mainly governed by multiple sediment supplies and regional tectonic activities. Due to sufficient supply of turbidity currents and MTDs from west and north respectively, western segment of the Central Canyon is entirely filled up earlier. Owing to slower sediment supply rate, together with differential subsidence by deep-seated faults, the full-filled time of the canyon is put off eastwards gradually.展开更多
This paper presents 2.5D scattering of incident plane SH waves by a canyon in layered half-space by the indirect boundary element method (IBEM). The free field response is carried out to give the displacements and str...This paper presents 2.5D scattering of incident plane SH waves by a canyon in layered half-space by the indirect boundary element method (IBEM). The free field response is carried out to give the displacements and stresses on the line which forms boundary of the canyon. The fictitious uniform moving loads are applied to the same line to calculate the Green's functions for the displacements and stresses. The amplitudes of the loads are determined by the boundary conditions. The displacements due to the free field and from the fictitious uniform moving loads have to be added to obtain the whole motion. The numerical results are carried out for the cases of a canyon in homogenous and in one layer over bedrock. The results show that the 2.5D wave scattering problem is essentially different from the 2D case, and there exist distinct differences between the wave amplification by a canyon in layered half-space and that in homogeneous half-space. The reasons for the distinct difference are explored, and the effects of the thickness and stiffness of the layer on the amplification are discussed.展开更多
The process of mass movements and their consequent turbidity currents in large submarine canyons has been widely reported, however, little attention was paid to that in small canyons. In this paper, we document mass m...The process of mass movements and their consequent turbidity currents in large submarine canyons has been widely reported, however, little attention was paid to that in small canyons. In this paper, we document mass movements in small submarine canyons in the northeast of Baiyun deepwater area, north of the South China Sea （SCS）, and their strong effects on the evolution of the canyons based on geophysical data. Submarine canyons in the study area arrange closely below the shelf break zone which was at the depth of -500 m. Within submarine canyons, seabed surface was covered with amounts of failure scars resulted from past small-sized landslides. A complex process of mass transportation in the canyons is indicated by three directions of mass movements. Recent mass movement deposits in the canyons exhibit translucent reflections or parallel reflections which represent the brittle deformation and the plastic deformation, respectively. The area of most landslides in the canyons is less than 3 km2. The trigger mechanisms for mass movements in the study area are gravitational overloading, slope angle and weak properties of soil. Geophysical data indicate that the genesis of submarine canyons is the erosion of mass movements and consequent turbidity currents. The significant effects of mass movements on canyon are incision and sediment transportation at the erosion phases and fillings supply at the fill phases. This research will be helpful for the geological risk assessments and understanding the sediment transportation in the northern margin of the SCS.展开更多
Water tank experiments are carried out to investigate the convection flow induced by bottom heating and the effects of the ambient wind on the flow in non-symmetrical urban street canyons based on the PIV (Particle Im...Water tank experiments are carried out to investigate the convection flow induced by bottom heating and the effects of the ambient wind on the flow in non-symmetrical urban street canyons based on the PIV (Particle Image Visualization) technique. Fluid experiments show that with calm ambient wind, the flows in the street canyon are completely driven by thermal force, and the convection can reach the upper atmosphere of the street canyon. Horizontal and vertical motions also appear above the roofs of the buildings. These are the conditions which favor the exchange of momentum and air mass between the street canyon and its environment. More than two vortices are induced by the convection, and the complex circulation pattern will vary with time in a wider street canyon. However, in a narrow street canyon, just one vortex appears. With a light ambient wind, the bottom heating and the associated convection result in just one main vortex. As the ambient wind speed increases, the vortex becomes more organized and its center shifts closer to the leeward building.展开更多
文摘Ash-rich pyroclastic flows from the cataclysmic eruption of Mount Mazama (~7700 yr. B. P.), Cascade volcanic arc, Oregon, entered and blocked the narrow, bedrock-lined canyon of the Williamson River approximately 35 to 44 km from the source volcano. The blockage impounded a body of water which then released producing four stratigraphic units in the downstream debris fan. The four stratigraphic units are a boulder core comprised of locally sourced bedrock boulders and three sand-rich units including a fine-grained sand unit, a sandy pumice gravel (±basalt/hydrovolcanic tuff) unit, and a pumice pebble-bearing, crystal-rich sand unit. Hand-drilled auger holes up to ~1.6 m deep were used to obtain samples of the sand-rich units. Units were delimited using surface and down-hole observations, composition and texture, estimated density, statistical parameters of grain size, and vertical and lateral distribution of properties. Overtopping followed by rapid incision into the ash-rich pyroclastic flows progressively cleared the canyon, but a bedrock knickpoint near the head of the canyon limited the volume of debris available for transport to about 0.04 km<sup>3</sup> to 0.08 km<sup>3</sup>. Co-deposition of bedrock boulders and lithic-rich sand was followed by rapid deposition with minimal reworking of remobilized pyroclastics. Continued draining of the impounded lake sent hyperconcentrated flows onto the debris fan depositing pumice-rich gravels that graded upward to crystal-rich sands.
基金Sopported by the Guangdong Province Marine Economic Development(Six Major Marine Industries)Special Fund Project(No.[2021]58)the Southern Marine Science and Engineering Guangdong Laboratory(Zhuhai)(No.SML2021SP307)+2 种基金the Innovation Group Project of Southern Marine Science and Engineering Guangdong Laboratory(Zhuhai)(Nos.311020003,31102004)the Guangdong Basic and Applied Basic Research Foundation(Nos.2019A1515012017,2019A1515010786)the“Fourteenth FiveYear Plan”Prospective Basic Major Scientific and Technological Projects of CNPC(No.2021DJ4901)。
文摘The Shenhu Submarine Canyon Group on the northern slope of the South China Sea consists of 17 slope-confined canyons,providing a good example for investigating their hosting sediments.Three drilling sites,including W07,W18,and W19,have proven the occurrence of gas hydrate reservoirs in the inter-canyon area between canyons C11 and C12.Whereas,variations of the geomorphology and seismic facies analyzed by high-resolution 3D seismic data indicate that the gas hydrate-bearing sediments may form in different sedimentary processes.In the upper segment,a set of small-scale channels with obvious topographic lows can be identified,revealing fine-grained turbidites supplied from the shelf region during a very short-term sea-level lowstand.In the middle part,gas hydrate units at Site W07 showing mounded or undulation external configuration are interpreted as sliding sedimentary features,and those features caused by gravity destabilization were the main formative mechanism of gas hydrate-bearing sediments that were sourced from the upper segments.In contrast,for the canyon transition zone of lower segments between C11-C12 inter-canyon and C12 intra-canyon areas,where W18 and W19 sites are located,the gas hydratebearing sediments are deposited in the channelized feature in the middle to lower segment and slide erosive surface.Gas hydrate-bearing sediments of the lower segment were migrated through channelized features interconnecting with the middle to lower slope by gravity-driven flows.The majority of deposits tended to be furtherly moved by lateral migration via erosive surface created by sediment failed to intra-canyon area.The conclusion of this study may help better understand the interaction between the formation mechanism of gas hydrate-bearing sediments and the geomorphologic effects of inter-canyon areas.
基金The Major Special Project of Guangdong Provincial Laboratory of Southern Marine Science and Engineering(Guangzhou) under contract No. GML2019ZD0207the National Natural Science Foundation of China under contract No. U20A20100the China Geological Survey Project under contract Nos DD20221712, DD20221719 and DD20191002。
文摘Submarine canyon is an important channel for long-distance sediment transport, and an important part of deepwater sedimentary system. The large-scale Rizhao Canyons have been discovered for the first time in 2015 in the continental slope area of the western South China Sea. Based on the interpretation and analysis of multi-beam bathymetry and two-dimensional multi-channel seismic data, the geology of the canyons has however not been studied yet. In this paper, the morphology and distribution characteristics of the canyon are carefully described,the sedimentary filling structure and its evolution process of the canyon are analyzed, and then its controlling factors are discussed. The results show that Rizhao Canyons group is a large slope restricted canyon group composed of one east-west west main and nine branch canyons extending to the south. The canyon was formed from the late Miocene to the Quaternary. The east-west main canyon is located in the transition zone between the northern terrace and the southern Zhongjiannan Slope, and it is mainly formed by the scouring and erosion of the material source from the west, approximately along the slope direction. Its development and evolution is mainly controlled by sediment supply and topographic conditions, the development of 9 branch canyons is mainly controlled by gravity flow and collapse from the east-west main canyon. This understanding result is a supplement to the study of “source-channel–sink” sedimentary system in the west of the South China Sea, and has important guiding significance for the study of marine geological hazards.
基金This article was supported by State Tenth Five-Year Plan Project (2001BA510B0105) and the Project for Pioneering New Knowledge from Chinese Academy of Sciences (KZCX2-SW-319).
文摘Since 1950, 700 plots were established in the alpine and canyon region of western Sichuan. The distribution charac-teristics and the relationships between forest succession and environmental gradients were studied. The results showed that the main tree species were Picea and Abies in this region, and there were more than 90 forest types. Abies forests mainly dis-tributed in the middle and upper reaches of rivers and their branches, and Picea forests mainly distributed in wide valleys and on half-shaded and half-sunny slopes. The natural regeneration was poor under primitive spruce and fir forest canopy, but was good in the spruce and fire forest gap. The relationship between forest succession and vertical gradient was closely related to the relationship between forest succession procession and plant synusia under primary forests. Human activities could promote and postpone succession process. The results of expanding regeneration were often influenced by topography, vegetation and wind direction.
文摘The dynamic stress intensity factor (DSIF) and the scattering of SH wave by circle canyon and crack are studied with Green's function. In order to solve the problem, a suitable Green's function is constructed first, which is the solution of displacement fields for elastic half space with circle canyon under output plane harmonic line loading at horizontal surface. Then the integral equation for determining the unknown forces in the problem can be changed into the algebraic one and solved numerically so that crack DSIF can be determined. Last when the medium parameters are altered, the influence on the crack DSIF is discussed partially with the displacement between circle canyon and crack.
文摘The wave propagation behavior in an elastic wedge-shaped medium with an arbitrary shaped cylindrical canyon at its vertex has been studied.Numerical computation of the wave displacement field is carried out on and near the canyon surfaces using weighted-residuals(moment method).The wave displacement fields are computed by the residual method for the cases of elliptic,circular,rounded-rectangular and flat-elliptic canyons,The analysis demonstrates that the resulting surface displacement depends,as in similar previous analyses,on several factors including,but not limited,to the angle of the wedge,the geometry of the vertex,the frequencies of the incident waves,the angles of incidence,and the material properties of the media.The analysis provides intriguing results that help to explain geophysical observations regarding the amplification of seismic energy as a function of site conditions.
基金National Natural Science Foundation of China Under Grant No.51278382
文摘This paper reports a series solution of wave functions for two-dimensional scattering and diffraction of plane SH waves induced by a symmetrical V-shaped canyon with different shape ratios. A half-space with a symmetrical V-shaped canyon is divided into two sub-regions by using a circular-arc auxiliary boundary. The two sub-regions are represented by global and local cylindrical coordinate systems, respectively. In each coordinate system, the wave field satisfying the Helmholtz equation is represented by the separation of variables method, in terms of the series of both Bessel functions and Hankel functions with unknown complex coefficients. Then, the two wave fields are described in the local coordinate system using the Graf addition theorem. Finally, the unknown coefficients are sought by satisfying the continuity conditions of the auxiliary boundary. To consider the phase characteristics of the wave scattering, a parametric analysis is carried out in the time domain by assuming an incident signal of the Ricker type. Surface and subsurface transient responses demonstrate the characteristics and mechanisms of wave propagating and scattering.
基金National Natural Science Foundation of China Under Grant No.50908156 and 50978183Tianjin Natural Science Foundation Under Grant No. 07JCZDJC10100
文摘This paper presents a 2.5D scattering of incident plane SV waves by a canyon in a layered half-space by using the indirect boundary element method （IBEM）. A free field response analysis is performed to provide the displacements and stresses on the boundary of the canyon where fictitious uniform moving loads are applied to calculate the Green＇s fi.mctions for the displacements and stresses. The amplitudes of the loads are determined by the boundary conditions. The free field displacements are added to the fictitious uniform moving loads induced displacements and the total response is obtained. Numerical calculations are performed for a canyon with homogenous and in one layer over bedrock. The effects of the thickness and stiffness of the layer on the amplification are studied and discussed.
基金support from the Program for New Century Excellent Talents in University (NCET-05-0248)the Key Program for Applied Basic Research of Tianjin Municipality (07JCZDJC10100)
文摘This paper presents an indirect boundary integration equation method for diffraction of plane P waves by a two-dimensional canyon of arbitrary shape in poroelastic half-space. The Green＇s functions of compressional and shear wave sources in poroelastic half-space are derived based on Biot＇s theory. The scattered waves are constructed using the fictitious wave sources close to the boundary of the canyon, and magnitude of the fictitious wave sources are determined by the boundary conditions. The precision of the method is verified by the satisfaction extent of boundary conditions, the comparison between the degenerated solutions of single-phased half-space and the well-known solutions, and the numerical stability of the method.
文摘This paper presents a step-by-step procedure using the three-dimensional boundary element approach to study the behavior of semi-circular canyons under seismic shear waves. The boundary element code TDASC allows utilization for various canyon geometries, evaluation of concurrent seismic waves and calculation of the ground motions on canyons due to an excitation at any arbitrary point of the incident field. Considering the widening ratio of the canyon(including prismatic, semi-prismatic and non-prismatic canyons), wave characteristics(wavelength, dimensionless period, direction) and maximum amplification pattern, the solution was applied to carry out a series of parametric studies. It was shown that canyon form can significantly affect the displacement amplification, especially at the points located on its edges. By increasing the wave dimensionless frequency(η > 1), the amplification pattern becomes more complex. On the basis of the results from a variety of considered cases, a new expression has been presented for the limiting wavelength beyond which the widening of the canyon will not have a major effect on the displacement amplification. To verify the reliability of the proposed approach, the obtained results, expressed in terms of displacement amplitude, were compared with those from the available published literature and a reasonably good agreement was observed.
基金This research was supported by the National Natural Science Foundation of China under Grant Nos.40405004 and 40233030.
文摘The questions on how vortices are constructed and on the relationship between the flow patterns and concentration distributions in real street canyons are the most pressing questions in pollution control studies. In this paper, the very large eddy simulation (VLES) and large eddy simulation (LES) are applied to calculate the flow and pollutant concentration fields in an urban street canyon and a cross-road respectively. It is found that the flow separations are not only related to the canyon aspect ratios, but also with the flow velocities and wall temperatures. And the turbulent dispersions are so strongly affected by the flow fields that the pollutant concentration distributions can be distinguished from the different aspect ratios, flow velocities and wall temperatures.
基金supported by National Natural Science Foundation of China (No. 50708100)National Science and Technology Support Project of China (No. 2006BAC13B02)partially by Basic Research Program of Institute of Mechanics Engineering, China Earthquake Administration (No. 2007B02)
文摘To evaluate the importance of the canyon topography effects on large structures, based on a rigid frame bridge across a 137-m-deep and 600-m-wide canyon, the seismic response of the canyon site is analyzed using a two-dimensional finite element model under different seismic SV waves with the assumptions of vertical incidence and oblique incidence to obtain the ground motions, which are used as the excitation input on the pier foundations of the bridge with improved large mass method. The results indicate that canyon topography has significant influences on the ground motions in terms of inci- dent angle. The peak ground acceleration values vary greatly from the bottom of the canyon to the upper comers. Under ver- tical incident SV waves, at the upper comers of canyon the peak ground accelerations greatly increase; whereas the peak ground accelerations diminish at the bottom comers of canyon. Under oblique incident SV waves, the shaking of the canyon slope perpendicular to the incidence direction is much more severe than that of the opposite side of canyon. And the ground surface has been characterized by larger deformations in the case of oblique incident waves. It is also concluded that the low piers and frame of the continuous rigid frame bridge ape more sensitive to the multi-support seismic excitations than the flexible high piers. The canyon topography as well as the oblique incidence of the waves brings the continuous rigid frame bridge severe responses, which should be taken into account in bridge design.
基金support from the Program for New Century Excellent Talents in University (NCET-05-0248)the Key Program for Applied Basic Research of Tianjin Municipality (07JCZDJC10100)
文摘This paper investigates in detail the nature of diffraction of plane P waves around a canyon in poroelastic half-space, and studies the effects of incident frequency, drainage condition, porosity, etc, on the diffraction of waves. It is shown that the surface displacement amplitudes of the drained case are close to those of the undrained case, however, the surface displacement amplitudes of the dry case are very different from those of the saturated （either drained or undrained） cases. There are large phase shift between the dry case and the saturated cases, as well as slightly longer resultant wavelengths for the undrained case than those for the drained case and longer resultant wavelengths for the drained case than those for the dry case. For small porosity the surface displacement amplitudes for the saturated cases are almost identical to those for the dry case; while for large porosity, the effect of drainage condition becomes significant, and the surface displacement amplitudes for the undrained case are larger than those for the drained case. As the incident frequency increases, the effect of porosity becomes significant, and more significant for the undrained case than that for the drained case. As the porosity increases, the pore pressures increase significantly but their oscillations become smoother. As the incident frequency increases, the pore pressures become more complicated.
文摘Dating of monazite grains preserved within foliations defining FIAs(foliation inflection/intersection axes preserved within porphyroblasts)have provided a detailed chronology of deformational and metamorphic events during a regional～1700 Ma orogeny,in Big Thompson region of the Colorado Rockies in the USA.FIAs controlled dating of monazite grains preserved within multiple-foliations of garnet and staurolite porphyroblast have complemented and potentially revolutionized all traditional dating approaches available.With this technique it is possible to accurately differentiate the
基金The National Natural Science Foundation of China under contract Nos 41372112 and 91028009the National Key Projects of Oil and Gas under contract No.2011ZX05025-002-02the Open Fund of Key Laboratory of Tectonics and Petroleum Resources(China University of Geosciences),Ministry of Education under contract No.TPR-2012-05
文摘Based on the interpretation of high resolution 2D/3D seismic data, sedimentary filling characteristics and full- filled time of the Central Canyon in different segments in the Qiongdongnan Basin of northwestern South China Sea have been studied. The research results indicate that the initial formation age of the Central Canyon is traced back to 11.6 Ma （T40）, at which the canyon began to develop due to the scouring of turbidity currents from west to east. During the period of 11.6-8.2 Ma （T40-T31）, strong downcutting by gravity flow occurred, which led to the formation of the canyon. The canyon fillings began to form since 8.2 Ma （T31） and were dominated by turbidite deposits, which constituted of lateral migration and vertical superposition of turbidity channels during the time of 8.2-5.5 Ma. The interbeds of turbidity currents deposits and mass transport deposits （MTDs） were developed in the period of 5.5-3.8 Ma （T30-T28）. After then, the canyon fillings were primarily made up of large scale MTDs, interrupted by small scale turbidity channels and thin pelagic mudstones. The Central Canyon can be divided into three types according to the main controlling factors, geomorphology-controlled, fault-controlled and intrusion- modified canyons. Among them, the geomorphology-controlled canyon is developed at the Ledong, Lingshui, Songnan and western Baodao Depressions, situated in a confined basin center between the northern slope and the South Uplift Belt along the Central Depression Belt. The fault-controlled canyon is developed mainly along the deep-seated faults in the Changchang Depression and eastern Baodao Depression. Intrusion-modified canyon is only occurred in the Songnan Low Uplift, which is still mainly controlled by geomorphology, the intrusion just modified seabed morphology. The full-filled time of the Central Canyon differs from west to east, displaying a tendency of being successively late eastward. The geomorphology-controlled canyon was completely filled before 3.8 Ma （T28）, but that in intrusion-modified canyon was delayed to 2.4 Ma （T27） because of the uplifted southern canyon wall. To the Changchang Depression, the complete filling time was successively late eastward, and the canyon in eastern Changchang Depression is still not fully filled up to today. Difference in full-filled time in the Central Canyon is mainly governed by multiple sediment supplies and regional tectonic activities. Due to sufficient supply of turbidity currents and MTDs from west and north respectively, western segment of the Central Canyon is entirely filled up earlier. Owing to slower sediment supply rate, together with differential subsidence by deep-seated faults, the full-filled time of the canyon is put off eastwards gradually.
基金supported by National Natural Science Foundation of China (Nos. 50908156 and 50978183)
文摘This paper presents 2.5D scattering of incident plane SH waves by a canyon in layered half-space by the indirect boundary element method (IBEM). The free field response is carried out to give the displacements and stresses on the line which forms boundary of the canyon. The fictitious uniform moving loads are applied to the same line to calculate the Green's functions for the displacements and stresses. The amplitudes of the loads are determined by the boundary conditions. The displacements due to the free field and from the fictitious uniform moving loads have to be added to obtain the whole motion. The numerical results are carried out for the cases of a canyon in homogenous and in one layer over bedrock. The results show that the 2.5D wave scattering problem is essentially different from the 2D case, and there exist distinct differences between the wave amplification by a canyon in layered half-space and that in homogeneous half-space. The reasons for the distinct difference are explored, and the effects of the thickness and stiffness of the layer on the amplification are discussed.
基金The National Science and Technology Major Project under contract No.2011ZX05056-001-02
文摘The process of mass movements and their consequent turbidity currents in large submarine canyons has been widely reported, however, little attention was paid to that in small canyons. In this paper, we document mass movements in small submarine canyons in the northeast of Baiyun deepwater area, north of the South China Sea （SCS）, and their strong effects on the evolution of the canyons based on geophysical data. Submarine canyons in the study area arrange closely below the shelf break zone which was at the depth of -500 m. Within submarine canyons, seabed surface was covered with amounts of failure scars resulted from past small-sized landslides. A complex process of mass transportation in the canyons is indicated by three directions of mass movements. Recent mass movement deposits in the canyons exhibit translucent reflections or parallel reflections which represent the brittle deformation and the plastic deformation, respectively. The area of most landslides in the canyons is less than 3 km2. The trigger mechanisms for mass movements in the study area are gravitational overloading, slope angle and weak properties of soil. Geophysical data indicate that the genesis of submarine canyons is the erosion of mass movements and consequent turbidity currents. The significant effects of mass movements on canyon are incision and sediment transportation at the erosion phases and fillings supply at the fill phases. This research will be helpful for the geological risk assessments and understanding the sediment transportation in the northern margin of the SCS.
文摘Water tank experiments are carried out to investigate the convection flow induced by bottom heating and the effects of the ambient wind on the flow in non-symmetrical urban street canyons based on the PIV (Particle Image Visualization) technique. Fluid experiments show that with calm ambient wind, the flows in the street canyon are completely driven by thermal force, and the convection can reach the upper atmosphere of the street canyon. Horizontal and vertical motions also appear above the roofs of the buildings. These are the conditions which favor the exchange of momentum and air mass between the street canyon and its environment. More than two vortices are induced by the convection, and the complex circulation pattern will vary with time in a wider street canyon. However, in a narrow street canyon, just one vortex appears. With a light ambient wind, the bottom heating and the associated convection result in just one main vortex. As the ambient wind speed increases, the vortex becomes more organized and its center shifts closer to the leeward building.
