基于EMD和VARMA模型的地震动仿真

通过EMD方法将地震动分解成若干固有模态函数,提出了用固有模态函数的时变VARMA建模实现地震动仿真的思路。算例分析表明,该方法充分利用了固有模态函数的特性,解决了直接基于ARMA或VARMA模型建模的仿真方法所面临的模型判阶的难题,并可同时考虑地震动的强度和频率非平稳特性,使仿真地震动与实际地震动在能量时频分布特性上具有较好的一致性且样本统计性较好,弹性及弹塑性反应谱拟合精度较高。     

Modelling particle residence times in agricultural river basins using a sediment budget model and fallout radionuclide tracers

Contemporary patterns in river basin sediment dynamics have been widely investigated but the timescales associated with current sediment delivery processes have received much less attention. Furthermore, no studies have quantified the effect of recent land use change on the residence or travel times of sediment transported through river basins. Such information is crucial for understanding contemporary river basin function and responses to natural and anthropogenic disturbances or management interventions. To address this need, we adopt a process‐based modelling approach to quantify changes in spatial patterns and residence times of suspended sediment in response to recent agricultural land cover change. The sediment budget model SedNet was coupled with a mass balance model of particle residence times based on atmospheric and fluvial fluxes of three fallout radionuclide tracers (⁷Be, excess ²¹⁰Pb and ¹³⁷Cs). Mean annual fluxes of suspended sediment were simulated in seven river basins (38–920 km²) in south‐west England for three land cover surveys (1990, 2000 and 2007). Suspended sediment flux increased across the basins from 0.5–15 to 1.4–37 kt y^‐1 in response to increasing arable land area between consecutive surveys. The residence time model divided basins into slow (upper surface soil) and rapid (river channel and connected hillslope sediment source area) transport compartments. Estimated theoretical residence times in the slow compartment decreased from 13–48 to 5.6–14 ky with the increase in basin sediment exports. In contrast, the short residence times for the rapid compartment increased from 185–256 to 260–368 d as the modelled connected source area expanded with increasing sediment supply from more arable land. The increase in sediment residence time was considered to correspond to longer sediment travel distances linked to larger connected source areas. This novel coupled modelling approach provides unique insight into river basin responses to recent environmental change not otherwise available from conventional measurement techniques. © 2014 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.     

Aeolian sediment fingerprinting using a Bayesian mixing model

Identifying sand provenance in depositional aeolian environments (e.g. dunefields) can elucidate sediment pathways and fluxes, and inform potential land management strategies where windblown sand and dust is a hazard to health and infrastructure. However, the complexity of these pathways typically makes this a challenging proposition, and uncertainties on the composition of mixed‐source sediments are often not reported. This study demonstrates that a quantitative fingerprinting method within the Bayesian Markov Chain Monte Carlo (MCMC) framework offers great potential for exploring the provenance and uncertainties associated with aeolian sands. Eight samples were taken from dunes of the small (~58 km²) Ashkzar erg, central Iran, and 49 from three distinct potential sediment sources in the surrounding area. These were analyzed for 61 tracers including 53 geochemical elements (trace, major and rare earth elements (REE)) and eight REE ratios. Kruskal–Wallis H‐tests and stepwise discriminant function analysis (DFA) allowed the identification of an optimum composite fingerprint based on six tracers (Rb, Sr, ⁸⁷Sr, (La/Yb)_n, Ga and δCe), and a Bayesian mixing model was applied to derive the source apportionment estimates within an uncertainty framework. There is substantial variation in the uncertainties in the fingerprinting results, with some samples yielding clear discrimination of components, and some with less clear fingerprints. Quaternary terraces and fans contribute the largest component to the dunes, but they are also the most extensive surrounding unit; clay flats and marls, however, contribute out of proportion to their small outcrop extent. The successful application of these methods to aeolian sediment deposits demonstrates their potential for providing quantitative estimates of aeolian sediment provenances in other mixed‐source arid settings, and may prove especially beneficial where sediment is derived from multiple sources, or where other methods of provenance (e.g. detrital zircon U–Pb dating) are not possible due to mineralogical constraints. Copyright © 2017 John Wiley & Sons, Ltd.     

Coupled motions of the inner core and possible geomagnetic implications

As the inner core is a good electrical conductor any ambient magnetic field would diffuse into it on a time scale long compared to several thousand years, and conversely be frozen there on shorter time scales. From the observations that the dipole component of the Earth's magnetic field has been inclined persistently to the spin axis over hundreds of thousands of years, and that the dipole drifts and decays significantly more slowly than the nondipole field, it is suggested that the external dipole is simply a manifestation of a field frozen in an inclined inner core. It is shown that the much neglected gravitational restoring torque can be significant for an inclined inner core, so much so that its motion is in the main determined by gravity, with electromagnetic and inertial coupling effects being of secondary importance. A regular precession of the inner core is shown to be possible where its spin axis drifts westward relative to the mantle with a period of ～ 7000 y. Some preliminary calculations of the possible motions of a gravitationally coupled mantle-inner core system are shown.     

Prediction of sediment transport in the swash-zone by using a nonlinear wave model

In the present work, the sediment transport rate in the swash zone is estimated using a Boussinesq model coupled with a porous flow model in order to incorporate the infiltration–exfiltration effects. The transport rate is estimated from a modified Meyer–Peter and Müller formula using different values the multiplier C for uprush and backwash to incorporate the observed higher transporting efficiency of the uprush due to the presence of pre-suspended sediment from the bore collapse. Alternatively, the depth-integrated transport equation for suspended sediment is solved. The comparison between model results and experimental data shows that both approaches are able to give satisfactory results.     

Estimating sediment deposition volume in a reservoir using the physiographic soil erosion-deposition model

A traditional way to estimate the amount of sediment deposition in a reservoir is simply based on the variation in the elevation measurements made at different times.In the present study,a physiographic soil erosion-deposition(PSED) model is applied to.Tseng-Wen reservoir watershed to compute the discharge and sediment concentration into the reservoir,and the deposition volume.The PSED model is verified using three storm events and then compared to the historic data recorded at the hydrological stations in the reservoir watershed,which shows close agreement between the simulated and recorded values of the flow discharge into the reservoir and the corresponding sediment yield. The numerical simulation was also conducted for all rainfall events from 1996 to 2001 to estimate the annual sediment deposition volume in the reservoir.Similarly,the simulated result was compared to that obtained from the cross-section elevation measurement.It is found that some measured data points had large uncertainty or were questionable.On the contrary,the model results and the measured data that renders the model credible.Therefore,regardless of the estimation for the flow discharge into the reservoir induced by a single rainfall event,the sediment yield,or the sediment deposition,the PSED model has successfully demonstrated its ability.     

Assessing sediment transport dynamics from energy perspective by using the instrumented particle

Sediment transport has been extensively studied. There is still a need to learn more about the mechanisms that make bed particles move, which is caused by turbulent flow in the low transport stages(above the motion threshold and below continuous transport). This work is focused on the use of an advanced tool to obtain a better perception of sediment transport dynamical methods: an instrumented particle equipped with a micro-electromechanical systems(MEMS) sensor. Particle transport experiments w...     

Simulation of digital earthquake accelerograms using the inverse discrete Fourier transform

Acceleration time histories of horizontal earthquake ground motion are obtained by inverting the discrete Fourier transform, which is defined by modelling the probability distribution of the Fourier phase differences conditional on the Fourier amplitude. The Fourier amplitude spectrum is modelled as a scaled, lognormal probability density function. Three parameters are necessary to define the Fourier amplitude spectrum. They are the total energy of the accelerogram, the central frequency, and the spectral bandwidth. The Fourier phase differences are simulated conditional on the Fourier amplitudes. The amplitudes are classified into three categories: small, intermediate and large. For each amplitude category, a beta distribution or a combination of a beta distribution and a uniform distribution are defined for the phase differences. Seven parameters are needed to completely define the phase difference distributions: two for each of the three beta distributions, and the weight of the uniform distribution for phase differences corresponding to small Fourier amplitudes. Approximately 300 uniformly processed horizontal ground motion records from recent California earthquakes are used to develop prediction formulas for the model parameters, as well as to validate the simulation model. The moment magnitude of the earthquakes ranges from 5.8 to 7.3. The source to site distance for all the records is less than 100 km. Copyright © 2002 John Wiley & Sons, Ltd.     

Simulation of broadband seismic ground motions at dam canyons by using a deterministic numerical approach

As a deterministic numerical approach for simulation of earthquake ground motions, the spectral element method (SEM) is applied to generate a broadband acceleration array for dam-canyons instead of the traditional empirical or stochastic methods. Specifically, the SEM analysis model with an extra fine mesh is used for the Pacoima Canyon to simulate the entire path starting from earthquake source rupture via the propagation medium to the local site. The source and the 3D earth model (velocity structure) are validated through the modeling of the Newhall earthquake on 28 October 2012 at a frequency of up to 8 Hz. Subsequently, the San Fernando earthquake records on 13 January 2001 are further used to study the effects of propagation path in simulation. Finally, the spatially varying ground motions at the Pacoima Canyon are obtained for different source mechanisms. The results show that the source mechanism and the local site topography significantly affect the distribution of the peak accelerations along the canyon.     

Experimental study of near-bed concentration and sediment vertical mixing parameter for vertical concentration distribution in the surf zone

Two formulae for the near-bed concentration(C_a) and the sediment vertical mixing parameter(m) are established based on a large scale wave flume experiment.The advantage of the new formulae is that the turbulent kinetic energy induced by wave breaking can be taken into account;the formula for C_a is in terms of the near-bed,time-averaged turbulent kinetic energy,and the formula for m is in terms of depth-and time-averaged turbulent kinetic energy.A new expression for suspended sediment load also is established by depth integration of the vertical distribution of the suspended sediment concentration obtained on basis of the new formulae.Equation validation is done by comparing the predicted C_a and m to measurements for different types of waves(regular wave,wave group,and irregular wave),and good agreement is found.The advantages of the proposed formulae over previous formulae also are discussed.     

基于有限断层动力学模型的逆断层—三维层状沉积盆地地震动模拟

近断层效应使得沉积盆地对地震动放大效应更为复杂。本文针对逆断层发震下三维层状沉积盆地地震反应,基于波动谱元法,采用有限断层动力学模型,模拟断层动力破裂、地壳层地震波传播和层状沉积盆地对地震波散射全过程。在此基础上,对比分析了层状和均质沉积盆地对近断层地震动放大效应的影响,讨论了不同断层倾角下层状沉积盆地地震动加速度特性。结果表明：层状沉积盆地PGA空间分布与均质沉积盆地存在较大差异,由于近断层效应和盆地效应,层状沉积盆地地表局部范围竖向PGA大于水平向PGA;90°断层倾角下层状沉积盆地地表地震动放大范围与60°断层倾角结果明显不同,主要集中在盆地中心区域和断层附近,且幅值远小于60°断层倾角下结果;沿断层走向,盆地内地表地震动加速度峰值对应时刻较盆地外延后。     

Predicting the equilibrium bed slope in natural streams using a stochastic model for incipient sediment motion

In this paper, a method to predict the equilibrium bed slope in natural streams based on the incipient motion criterion is proposed. The method is based on the criterion suggested by Gessler to simulate the grain size distribution of the armour coat using the concept of critical shear stress of a sediment mixture. In particular, a different expression of the probability for a single particle size to be part of the armour coat is firstly proposed; then, a simple two‐steps criterion is suggested to estimate the safety factor required by the proposed approach. The method is applied in three different Italian regions (Calabria, Basilicata, and Tuscany) and required several field campaigns involving the survey of 251 stream reaches upstream of grade‐control structures. The area including five Calabrian streams is firstly used to test the suitability of the two‐steps approach. In this area, considering the detailed information about the particle size of bed material, the effect of using a simplified grain size distribution for each torrent is also checked. The method is then applied within the two additional areas (Basilicata and Tuscany) in order to check the geographic influence on the prediction of the stable longitudinal profile. A comparison between measured and estimated values of the equilibrium bed slope showed a good agreement for the Calabrian streams, where the number of the investigated stream reaches is greater, and satisfactory results for the torrents located in Basilicata and Tuscany where the field data set is more limited. The overall results encourage the extension of the proposed method to additional field data and suggest the use of this approach by hydraulic designers in order to stabilize the longitudinal profile of natural streams. Copyright © 2011 John Wiley & Sons, Ltd.     

主动底辟盐构造的二维离散元模拟

盐岩底辟构造是一类与油气藏关系极为密切的构造类型,是当前国外构造地质研究和油气勘探的热点.目前盐岩底辟构造的研究仍处于描述性阶段,对盐及其相关构造动力学演化的研究还很薄弱.对于油气而言,盐岩底辟构造既提供了油气运移的驱动力和运移通道,又可以产生丰富的圈闭.离散元模拟是构造地质地球物理研究中一种新的动力学模拟方法,作者通过构建离散元模型,模拟了主动底辟盐构造断裂系统的演化过程,结果表明,该断裂系统的发育主要经历了对称变形、不对称变形、整体破裂三个阶段.研究结果揭示了盐岩底辟构造的动力学演化过程,有利于提高对盐岩底辟构造相关油气藏的勘探和预测.     

Validation of elastic wave measurements of rock fracture compliance using numerical discrete particle simulations

We test various methods of quantifying the compliance of single and multiple rock fractures from synthetic ultrasonic data. The data are generated with a 2D discrete particle scheme which has previously been shown to treat fractures in agreement with linear-slip theory. Studying single fractures, we find that delays derived from peak amplitudes do not correspond to group delays, as might be expected. This is due to waveform distortion caused by the frequency-dependent transmission across the fracture. Instead the delays correspond to an expression for phase delays, which we derive from linear-slip theory. Phase delays are a unique function of compliance, whereas group delays are non-uniquely related to compliance. We believe that this property of group delays has hindered the wider application of deriving fracture compliances from traveltimes. We further show that transmission coefficients derived from waveform spectra yield more accurate fracture compliances than those obtained from ratios of signal peak amplitudes. We also investigate the compliance of a set of parallel fractures. Fracture compliance can only be determined from transmission coefficients if the fracture spacing is so large that the first arriving pulse is not contaminated by reverberations. In the case of contamination the direct measurement of group or phase delays is not practical. However, we demonstrate that in such cases of strong waveform distortion the coda wave interferometry method is very effective for determining relative fracture compliance. First break delays in the fracture set data are related to those observed in single fracture simulations. This means that fracture set compliance can be estimated from first break data if used together with numerical simulations.     

Simulation of earthquake ground motions using autoregressive moving average (ARMA) models

Parsimonious representations of recorded earthquake acceleration time series are obtained by fitting stationary autoregressive moving average models after a variance-stabilizing transformation. Simulated acceleration series are then constructed by generating realizations from the fitted stationary models and applying the reverse transformation. As demonstrated on three components of a typical series, the response spectra for the observed and simulated series show good agreement for periods of less than eight seconds. Further, the model parameters for the three components are very similar, suggesting a consistency which could be useful for identifying site-specific characteristics.     

Assessment of runoff and sediment yield in the Miyun Reservoir catchment by using SWAT model

The Soil and Water Assessment Tool (SWAT) was used to simulate the transport of runoff and sediment into the Miyun Reservoir, Beijing in this study. The main objective was to validate the performance of SWAT and the feasibility of using this model as a simulator of runoff and sediment transport processes at a catchment scale in arid and semi‐arid area in North China, and related processes affecting water quantity and soil erosion in the catchment were simulated. The investigation was conducted using a 6‐year historical streamflow and sediment record from 1986 to 1991; the data from 1986 to 1988 was used for calibration and that from 1989 to 1991 for validation. The SWAT generally performs well and could accurately simulate both daily and monthly runoff and sediment yield. The simulated daily and monthly runoff matched the observed values satisfactorily, with a Nash‐Sutcliffe coefficient of greater than 0·6, 0·9 and a coefficient of determination 0·75, 0·9 at two outlet stations (Xiahui and Zhangjiafen stations) during calibration. These values were 0·6, 0·85 and 0·6, 0·9 during validation. For sediment simulation, the efficiency is lower than that for runoff. Even so, the Nash‐Sutcliffe coefficient and coefficient of determination were greater than 0·48 and 0·6 for monthly sediment yield during calibration, and these values were greater than 0·84 and 0·95 during validation. Sensitivity analysis shows that sensitive parameters for the simulation of discharge and sediment yield include curve number, base flow alpha factor, soil evaporation compensation factor, soil available water capacity, soil profile depth, surface flow lag time and channel re‐entrained linear parameter, etc. Copyright © 2009 John Wiley & Sons, Ltd.     

Simulation of spatially-distributed copper pollution in a large river basin using the ECOMAG-HM model

The semi-distributed physically-based model ECOMAG-HM was developed to simulate cycling of heavy metals in large river basins: on the surface, and in soil, groundwater and river water. The model was applied to study the spatial distribution and temporal dynamics of copper concentrations in watercourses of the Nizhnekamskoe Reservoir watershed in Russia. This watershed is characterized by high background concentrations of heavy metals due to wide occurrence of ore deposits and considerable concentrations of ore-parent elements in rocks. The model was found to adequately reproduce the spatial variation of the mean annual copper concentrations at different monitoring points of the river network. The mean annual specific copper washoff, with the surface and subsurface components of river runoff, and the total copper washoff from the watershed into the river network were calculated and mapped. The contributions of natural and anthropogenic factors to river water pollution by copper were evaluated.     

Simulation of individual particle movement in a gravel streambed

A two‐dimensional simulation model of travel distances of individual particles in a gravel‐bed river is presented. The model is based on a number of rules, which include particle size, entrainment, trajectory, distance of movement and entrapment. Particle interactions are controlled by resistance fields defined about each obstacle and critical elevation defined in the model. Resistance fields, particle dropping and critical elevation rules control particle interactions. The interaction rules cause the particles to develop pebble clusters, stone cells and transverse structures (transverse ribs). The simulated travel distances of individual particles are consistent with reported field results. Individual particle travel distances were simulated using two different models; one without interactions between the individual particles and the stationary bed and one with interactions. The case without interactions demonstrates the random nature of sediment transport, and narrow ranges of travel distances. Wider ranges of travel distances, similar to those for natural situations, were obtained for the cases with interactions. The more intense the interaction between the mobile stones and the stationary ones, the wider the range of distances of travel for a given particle size. Modelling the mean travel distance yielded a result similar to that published previously, which was based on empirical data. Well developed bed‐surface structures were obtained for relatively poorly sorted sediment with intense interactions between particles. Transverse structures developed when relatively large particles were allowed to move. Copyright © 2002 John Wiley & Sons, Ltd.