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1.
山地和平原地形在破坏性地震中对震害和地震动有很大影响,不同地形对地震动有不同的地形效应。为研究山地和平原地形对地震动的影响规律,基于集集地震后续M L6.6余震的强震观测记录,对位于不同地形且与震中位于同一直线上的四个强震台记录处理计算,从波形特征、频谱分析、加速度反应谱计算、与规范谱比较等方面进行对比分析,完成长周期地震动反应谱分布的计算。研究表明,山地地形对地震动的高频成分有地形放大效应,沉积平原对地震动的低频长周期部分有放大作用,并且长周期反应谱的峰值区域始终位于沉积平原中部。平原上强震记录产生的长周期放大系数谱已超出规范谱,沉积平原上长周期建筑的抗震设防应引起重视。 相似文献
2.
Leon Scheiber Oliver Lojek Axel Götschenberg Jan Visscher Torsten Schlurmann 《地球表面变化过程与地形》2021,46(2):478-489
Underwater dunes are a morphological feature that are explored by marine scientists and coastal engineers alike. This study presents new methodologies in order to simplify bedform identification and morphodynamic analyses. Specifically, subaqueous compound dunes are decomposed with a simple yet extensive tracking algorithm, which relies on a repeated evaluation of unfiltered bed elevation profiles according to five predefined length classes. In a second step, morphological trends are assessed in the form of bed migration rates, bed slope asymmetries and net sediment changes, in which all parameters are referred to equidistant sections of the examined fairway stretch. This integrated approach not only avoids the challenges in weighting the varying size and abundance of dunes of different scales but also ensures comparability between dune-specific and areal parameters, which significantly improves the interpretation of the morphological setting as a whole. The developed methods are applied to the Outer Jade fairway, an anthropogenically influenced and regularly maintained waterway in the German Bight, and allow scrutiny of spatio-temporal trends in this region. Based on a unique data set of 100 sequential high-quality echo-sounding surveys, various types of bedforms are identified, comprising large-scale primary as well as superimposing secondary dunes that are assumed to interfere with each other. Temporal trends show a long-term rise of the troughs of major bedforms and constant maximum crest elevations near the official maintenance depth, which matches the observed long-term aggradation of sediments. The spatial distribution of integrated morphodynamic parameters reflects a previously described zone of primary dune convergence and facilitates the precise localization of this geophysical singularity. The presented findings both confirm the robustness of the proposed methodologies and, in return, enhance the understanding of morphological processes in the Outer Jade. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd. 相似文献
3.
Couplet-style stratifications refer to the sedimentary sequences that consist of alternating coarse-grain-dominated bed and fine-grain-dominated bed, with or without a sandwiched middle-sized bed. The formation mechanism is complicated due to the interplays of varying driving force （s）, sediment supply ＆ transport and topographic configurations. This paper presents a comprehensive overview of such characteristic bedforms, which emerge during the transport and deposition of non-uniform sediments. The leading formative models include water-stage variation, gravel-overpassing process, superimposition of bedload sheets and avalanching process （inverse grading）. Each process might produce similar or distinct sedimentological features with respect to grading, matrix content, grain attitude, bounding faces between beds and internal longitudinal/transverse geometry. The couplet-style stratified strata might play an active role in landscape evolution. 相似文献
4.
Catastrophic events often interrupt long-term Earth surface processes. In coastal areas, although millennial-scale trends of climatic and sea-level changes determine the trajectory of sedimentary landform evolution, storm and/or tsunami activity can cause abrupt changes in depositional conditions that may alter the long-term sedimentary processes. Here, we report a sedimentary hiatus that is widely observed from the late Holocene sedimentary sequence at the seabed along the southeast China coast. This hiatus was discovered by close temporal sedimentary and radiocarbon dating analyses of a seabed sedimentary sequence in the mouth area of the Pearl River estuary. The results suggest that a couple of metres in the middle to late Holocene sediment at the seabed were eroded by a catastrophic event happening c. 1000–800 cal. Years BP. In theory, a mega-tsunami generated from the Manila Trench could have caused such erosion at the seabed, but there is a lack of direct geological and historical evidence to support such a hypothesis. Much more likely, a super-typhoon struck the coast and caused the erosion. This hypothesis is strongly supported by the region's historical and geological records, which suggest a period characteristic of intense typhoons ranging from the Medieval Warm Period to the climate transition phase (c. 1000–600 cal. Years BP). During the subsequent Little Ice Age, deposition of sandy sediment continued, suggesting frequent but weaker typhoon activity. Over the past two centuries the deposition of sandy sediment and gravels began, implying the beginning of a phase of intensifying typhoon conditions, possibly a result of the recent warming climate. © 2020 John Wiley & Sons, Ltd. 相似文献
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Side scan sonar records, sediment textural characteristics, and in-situ field observations were used to study gravelly and pebbly dunes and sorted bedforms on the inner shelf of Marettimo Island, along the northwestern Sicilian shelf. The dunes are composed of coarse sands, gravels and pebbles (D50: 2–16 mm), displaying a symmetrical shape with a wavelength in the range of 1–2.5 m and a height of 0.15–0.30 m. The bedforms are distributed in a patchy pattern in a depth range of 10–50 m, and are described for the first time on a Mediterranean inner shelf. Sorted bedforms are linear morphological features developed almost perpendicular to the coast in the eastern sector of the island between 15 and 50 m water depth. Bottom shear stresses required for sediment entrainment and the generation of the shallower dunes can be reached during strong storms (Hs=5–6 m; Tp=9–11 s), which are not common in the Mediterranean Sea. However, wave storm events recorded in the study area during the last 17 years are not able to generate the coarsest and deeper dunes, suggesting that the stirring mechanism for dune formation is associated with severe storms that have a recurrence interval of more than 17 years. The long-term stability of the coarse bedforms is supported by the permanence of sorted bedforms without significant morphological changes for long periods (>13 years). Therefore, it is shown that processes forming coarse bedforms can occur in tideless and moderate-energy settings like those of the Mediterranean continental shelves, although the morphological features are probably less dynamic and remain unaltered for longer periods than on higher-energy shelves. 相似文献
7.
The concept of entropy originates from thermodynamic theory where entropy represents the turbidness of molecular movement. If the status of the molecular movement in a thermodynamic system is more disordered, the entropy of the system is larger. Wiggins[1… 相似文献
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Experiments are conducted in a laboratory flume on the propagation of a surface wave against unidirectional flow with a sediment bed. This article presents the spatial variation of bedforms induced by the wave-blocking phenomenon by a suitably tuned uniform fluid flow and a counter-propagating wave. The occurrence of wave-blocking is confirmed by finding a critical wave frequency in a particular flow discharge in which the waves are effectively blocked and is established using the linear dispersion relation. The purpose of this work is to identify wave-blocking and its influence on the development of bedforms over the sediment bed. Interestingly bedform signatures are observed at a transition of bedforms in three zones, with asymmetric ripples having a steeper slope downstream face induced by the incoming current, followed by flat sand bars beneath the wave-blocking zone and more symmetric ripples below the wave-dominated region at the downstream. This phenomenon suggests that the sediment bed is segmented into three different regions of bed geometry along the flow. The deviations of mean flows, Reynolds stresses, turbulent kinetic energy, and power spectral density due to the wave-blocking phenomenon are presented along the non-uniform flow over sediment bed. The bottom shear stress, bed roughness and stochastic nature of the bedform features are also discussed. The results are of relevance to engineers and geoscientists concerned with contemporary process as well as those interested in the interpretation of palaeoenvironmental conditions from fossil bedforms. © 2019 John Wiley & Sons, Ltd. 相似文献
9.
This paper presents results of a field study designed to examine the structure of flow over mobile and fixed bedforms in a natural stream and to compare the results with findings of previous laboratory studies within the framework of double time–space averaging approach. Measurements of turbulence were obtained in a small river in Illinois, USA, over a fine spatial grid of sampling points above a mobile sandy bedform and its artificially moulded replica. Flow structure over the artificial bedform is similar to that observed in laboratory studies, but is markedly different from the flow structure over natural bedforms. These differences are most pronounced in the roughness sublayer, whereas flow in the logarithmic layer over natural and artificial sand waves is fairly similar and exhibits spatial uniformity. The double time–space averaged distributions of turbulence statistics conform to the multilayer model of flow structure over bedforms. Mean velocity distributions indicate neither classical flow recirculation nor substantial reduction of velocities in the lee of bedform crests. However, vertical patterns of turbulence statistics over depth suggest that stacked wakes similar to those observed in laboratory studies exist above the bedforms. Thus, despite the absence of flow separation, wake development seems to be induced by the systematic influence of upstream bedforms on the vertical structure of turbulence. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
10.
2011年,作者在贺兰山中段腰坝配件厂地区进行1∶5万区域地质调查时发现了震旦系盖帽白云岩,进而研究了其沉积特征、碳氧同位素组成和时代,并与该区紫花沟兔儿坑组白云岩、华南和华北地台南缘相应地层进行对比.结果表明,盖帽白云岩为含陆源细碎屑白云岩,与兔儿坑组白云岩相近;其δ13C值为-4.7‰～0.79‰,且自下而上呈下降趋势;其时代与华南震旦系下、中部相当,与华北地台南缘震旦系罗圈组的沉积特征和古地理环境相近;与全球Gaskiers冰期有相关性.这对华北地台的古构造、古地理、古气候及动物演化“源头”的研究有重要意义. 相似文献
11.
沉积微相划分及有利沉积微相带筛选是油气田地质研究的基础工作,本文针对陕北斜坡东南部下寺湾油田研究区延9储层分流河道骨架砂体识别及有利沉积微相带分布研究,提出了基于地质历史时期沉积水动力能量分析的划相方法.该方法有效的避免了薄互层砂体累加厚度对沉积微相划分的干扰,可以在平面上快速准确的评价划分沉积能量等级,并筛选有利沉积相带.特别是在平面上井数较多,地质资料繁多的情况下可以利用沉积水动力定量参数快速确定不同沉积微相带边界.总体而言,该方法理论基础充分,且具有较高的实用性和可操作性. 相似文献
12.
Leonardo S. R. Mocitaiba Edson E. S. Sampaio Olivar A. L. de Lima 《Studia Geophysica et Geodaetica》2017,61(4):801-825
The time variation of a source of alternating current produces an electromagnetic coupling between the transmitting and receiving systems in induced electrical polarization and resistivity surveys that employ horizontal electrical sounding. The coupling alters, sometimes significantly, the resistivity and chargeability values and, consequently, compromises the interpretation of the data. This work develops the analysis of this coupling on the time domain for both a homogeneous Earth and an Earth constituted of several homogeneous horizontal layers. Based on the results, it evaluates the theoretical variation of the voltage and compares it to observed data of an induced electrical polarization and resistivity survey with horizontal electrical sounding. The comparative study of the voltage decay curves assesses the contribution of the electromagnetic coupling to the values of the chargeability and apparent resistivity as a function of the bipole length and the period of the source current. Besides that, the respective pseudosections have delineated the horizontal and vertical variations of those two electrical properties, providing the interpretation of the geoelectrical section. This research is important for the oil industry because it may furnish information that help to: (i) define the structure of the sedimentary strata; (ii) estimate the clay content of the sandstones; and (iii) detect the possible presence of metallic sulphide halos at fractured zones of the sediments above the oil reservoir. It may be adapted to analysis of EM coupling in both resistive and induced electric well logs. 相似文献
13.
南黄海盆地是大型海相中、古生界和陆相中、新生界两期叠合型盆地,本文根据最新地震资料结合钻井资料及与海陆地质资料的对比研究对盆地内地震层序进行了划分,得到了陆相中、新生界盆地基底即中、古生界海相盆地的顶界埋深、三叠系和上二叠统的残留厚度,推断了陆相层基底地层分布,并根据磁力异常资料推测了海相中、古生界盆地结晶基底埋深,推断了海相中、古生界整体残留情况.下三叠统青龙组和上二叠大隆及龙潭组在南黄海盆地南部坳陷及勿南沙隆起广泛存在,而在北部坳陷的分布则狭窄,中部隆起则由于隆起和剥蚀作用导致这两套地层几乎没有残余.对中-古生界海相盆地和中-新生界陆相盆地的基底特征进行了比较,认为海相中、古生界在南黄海地区区域性存在,海相层厚度分布特征受基底起伏控制,同时受到印支板块运动的影响,中部隆起区是海相中、古生界比较稳定的地区.下古生界可能在南黄海盆地广泛分布,但受资料限制,还难以获知其残余地层的特征. 相似文献
14.
Denis-Didier Rousseau 《中国科学D辑(英文版)》2006,49(7):724-730
The Qinan Miocene loess-soil sequence (~22―6.2 Ma)[1] discovered from the Gansu Group[2] several years ago extends the well-known Quaternary loess-soil sequence and the late Tertiary Red Clay se-quence of the Loess Plateau to early Miocene epoch. Most recently, the Dongwan late Miocene–Pliocene loess-soil sequence (~7.1―3.5 Ma)[3] further extends the upper limit of the reported Qinan Miocene loess-soil sequence into the Pliocene. These extensions have great potentials for the study o… 相似文献
15.
This Commentary introduces a Special Issue of Earth Surface Processes and Landforms dedicated to the memory of Stephen Coleman (1966–2012) who sadly passed away on July 23, 2012. Stephen was an incredible enthusiast for the study of bedforms and made seminal contributions to our knowledge and understanding of flow, sediment transport and bedform dynamics. Stephen presented a keynote address at MARID 3, which has led to this Special Issue (see Coleman SE, Nikora VI. 2011. Fluvial dunes: initiation, characterization, flow structure. Earth Surface Processes and Landforms 36 : 39–57. DOI. 10.1002/esp.2096) and was always keen to discuss research, from theory to techniques to experiments. He was always generous with his time and touched many with the influence of his thoughts and intellect. Our community has lost an outstanding scientist and Stephen will be greatly missed by us all. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
16.
大型克拉通内部沉积盆地基底组构与盆地起源和油气聚集有密切关系.除了钻井岩心及周缘露头研究可提供盆地结晶基底类型证据外,反射地震信号记录了沉积盆地起源时期的有关大地构造作用信息,盆地基底组构可通过记录长度大的反射地震剖面研究.2007年中石化在塔里木盆地将1400 km的地震剖面接收记录从6 s加长到12 s,为研究克拉通盆地结晶基底的组构和类型提供了难得的第一手资料.这篇文章主要介绍这次调查的反射地震剖面,讨论深反射地震数据处理的关键技术,展示塔里木盆地巴楚—塔中地区的四条12 s反射地震剖面,并对反射剖面的散射模式作初步分析.本次调查表明,将地震剖面接收记录从6 s加长到12 s,采集处理的成本只增加了3％左右,但是可为研究克拉通盆地结晶基底和上地壳不均匀性提供很有价值的第一手资料. 相似文献
17.
Alluvial fan development in Alpine areas is often affected by catastrophic sedimentary processes associated with extreme ?oods events, causing serious risks for people living on the fans. Hazard assessment in these areas depends on proper identi?cation of the dominant sedimentary processes on the fans. Data from a set of 209 alluvial fans from the central Alps of Italy are presented in this paper and analysed with the help of various statistical techniques (linear regression, principal components analysis, cluster analysis, discriminant analysis and logistic regression). First, we used modern sedimentary facies and historical records (?ood events since 15th century), to distinguish between the two dominant sedimentary processes on alluvial fans: debris ?ows and stream?ows. Then, in order to analyse the main controls on past and present fan processes, 36 morphological, geological and land‐use variables were analysed. As with observations for arid‐environment fans, catchment morphology is the most in?uential factor in the study area, whereas geology and land use are minor controls. The role of climatic change and landsliding within the catchments also seems to be very important and is discussed. Statistical techniques also help in differentiating groups of alluvial fans by sets of controlling factors, including stage and type of evolution. Finally, by using discriminant analysis and logistic regression, we classi?ed alluvial fans according to the dominant sedimentary process, with a success rate ranging between 75 and 92 per cent. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
18.
A previously unknown field of large‐scale sedimentary bodies has been mapped and studied on the continental shelf off the Cape Trafalgar near the Strait of Gibraltar with particular emphasis on the relationship between large‐scale sediment bodies and the superimposed bedforms. This study is based on a grid of 975 km of high‐resolution seismic profiles collected at water depths ranging between 15 and 60 m. High variability of large‐scale sedimentary bodies is attributed to the complex interaction of hydrodynamic agents. The most prominent sedimentary features are sand banks and ridges that indicate long‐term southwest and southward‐directed sediment transport patterns, possibly due to the interplay of two dominant current systems flowing southward and westward. These sediment bodies evolve laterally to distinct external geometries, such as sand shoals in shallow water and sand sheets in the vicinity of larger sand banks that indicate moderate current velocities. In addition, pre‐existing physiography is considered to play a role in the generation of certain sediment bodies, developed over inclined surfaces or confined laterally by elevations. Relationships between superimposed bedforms (mostly very large dunes) and underlying sediment bodies vary across the study area. Most superimposed bedforms occur over the complex mosaic of sediment banks and sheets, suggesting the interaction of several high‐energy currents with different directions, such as tidal and/or wind‐driven currents. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
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A series of independent faulted basins developed in the present middle reaches of the Yellow River during late Cenozoic, among which the Sanmen Lake Basin is located in the east edge of the Loess Plateau, a transitional zone between the second and third macromorphological step of China. The thick strata of the Sanmen Group deposited in the large basin. The Sanmen Group is a perfect place for the study on paleoenvironmental change, tectono-climatic cycles as well as the formation and evolution of the Yellow River. In this paper, the paleoenvironmental changes, regional tectonic movement and the evolutionary process of the Sanmen Lake Basin during the past 5 Ma were reconstructed based on the analysis of paleomagnetic stratigraphy, pollen, TOC and carbonate content from the Huangdigou outcrop near the Sanmenxia Reservoir, Pinglu County, Shanxi Province. The sedimentary records from the outcrop indicate that the basin was first formated by fault activity at about 5.4 MaBP, and after the strong tectonic movement at 3.6 MaBP the lake enlarged and the rainfall of summer monsoon increased. There was no great climatic transition near 2.6 MaBP, corresponding to the bottom age of loess in the Loess Plateau. After Olduvai event (about 1.77 MaBP) the Picea and Abies were presented in the sediments, which indicates a colder climate. The tectonic movement at 1.2 MaBP caused the light angular discordance between the upper and lower Sanmen Group. The sedimentary records show a cold and wet climate during the prosperous periods of loess accumulation such as L15, L9, L6. The tectonic intensification periods of the Sanmen Basin correspond with the tectonic movements in the Qinghai-Xizang Plateau chronologically. The earliest age of the outflow from the Paleo-Sanmen Lake or the partly cutting off of the Sanmenxia Gorge was about 0.41- 0.35 MaBP. The age of cutting thoroughly the Sanmenxia Gorge by the Yellow River and the disappearance of the Paleo-Sanmen Lake was about 0.15 MaBP, which symbolized the formation of the present Yellow River and had an important influence on the environmental and morphological evolution in the middle and lower reaches of the Yellow River. 相似文献