Representation of near-fault pulse-type ground motions

Near-fault ground motions with long-period pulses have been identified as critical in the design of structures. To aid in the representation of this special type of motion, eight simple pulses that characterize the effects of either the fling-step or forward-directivity are considered. Relationships between pulse amplitudes and velocity pulse period for different pulses are discussed. Representative ratios and peak acceleration amplification can exhibit distinctive features depending on variations in pulse duration, amplitude and the selected acceleration pulse shape. Additionally, response spectral characteristics for the equivalent pulses are identified and compared in terms of fixed PGA and PGV, respectively. Response spectra are strongly affected by the duration of pulses and the shape of the basic pulses. Finally, dynamic time history response features of a damped SDOF system subjected to pulse excitations are examined. These special aspects of pulse waveforms and their response spectra should be taken into account in the estimation of ground motions for a project site close to a fault.     

Increasing the vertical resolution of conventional sub‐bottom profilers by parametric equalization

Vertical resolution, i.e. the ability to resolve two close reflectors, is a crucial aspect of pulses used in geo‐acoustic exploration of sea sub‐bottoms. This paper deals with the problem of exploring the shallowest unconsolidated layers of the seafloor with conventional piezo‐electric sonar pulses. Such transducers do not have a sufficiently broad transmission response to enable them to radiate short high‐resolution pulses. Therefore, some kind of equalization process must be applied to broaden the transmission response. Here, inverse filtering is used to calculate the transducer driving waveform so that the subsequent acoustic pulse has a zero‐phase cosine‐magnitude nature. Within a specified bandwidth, this pulse has minimum length, i.e. maximum resolution. The method has been applied to compress the acoustic pulses radiated by two piezo‐electric transducers. In conventional performance, these transducers radiate narrowband pulses which contain several cycles at the natural resonance frequency. Under equalized driving, both transducers emit broadband pulses, with resolving power greatly increased, at the cost of some amplitude loss. That is, the pulses radiated by both transducers have been shortened from 1 ms (low‐frequency transducer) and 0.274 ms (high‐frequency transducer) in conventional performance to 0.13 ms and 0.038 ms in equalized mode, with amplitude losses of 33% and 56%, respectively. The great improvement in the resolution of this technique is demonstrated by comparing the synthetic echograms that should be obtained when exploring a wedge model using zero‐phase cosine‐magnitude pulses with conventional ping pulses.     

Near-fault ground motions with prominent acceleration pulses： pulse characteristics and ductility demand

Major earthquakes of last 15 years (e.g., Northridge 1994, Kobe 1995 and Chi-Chi 1999) have shown that many near-fault ground motions possess prominent acceleration pulses. Some of the prominent ground acceleration pulses are related to large ground velocity pulses, others are caused by mechanisms that are totally different from those causing the velocity pulses or fling steps. Various efforts to model acceleration pulses have been reported in the literature. In this paper, research results from a recent study of acceleration pulse prominent ground motions and an analysis of structural damage induced by acceleration pulses are summarized. The main results of the study include: (1) temporal characteristics of acceleration pulses; (2) ductility demand spectrum of simple acceleration pulses with respect to equivalent classes of dynamic systems and pulse characteristic parameters; and (3) estimation of fundamental period change under the excitation of strong acceleration pulses. By using the acceleration pulse induced linear acceleration spectrum and the ductility demand spectrum, a simple procedure has been developed to estimate the ductility demand and the fundamental period change of a reinforced concrete (RC) structure under the impact of a strong acceleration pulse.     

Characteristics of near-fault ground motion containing velocity pulses

There are many reports about the research on near-fault velocity pulses, which focus on the generation of velocity pulse and simplify the velocity pulse so as to be used in the seismic design of structure, However few researches have put emphasis on the characteristics of near-fault ground motions containing velocity pulses, especially the characteristics relevant with the design response spectrum prescribed by the code. Through collection of a large number of near-fault records containing velocity pulses, the response spectra and the characteristic periods of records containing no pulses are compared with those of records containing pulses. Response spectra of near-fault records are compared with standard spectra given by code; furthermore, the response spectra and the characteristic periods of each earthquake are compared with that given by code. The result shows that at long periods （longer than 1.5 s）, the response spectrum of pulse-containing records is bigger than the response spectrum of no-pulse-containing records; when the characteristic period of near-fault records is calculated, the method that does not fix frequency is more reasonable because the T1 and T2 have a lagging tendency; regardless of the site Ⅰ and site Ⅱ, the characteristic period of pulse-containing records is over twice bigger than the characteristic period given by the code,     

Interstory drift ratio of building structures subjected to near-fault ground motions based on generalized drift spectral analysis

This paper focuses on the interstory drift ratio (IDR) demands of building structures subjected to near-fault ground motions having different impulsive characteristics based on generalized interstory drift spectral analysis. The near-fault ground motions considered include the idealized simple pulses and three groups of near-fault ground motions with forward directivity pulses, fling-step pulses and without velocity pulse. Meanwhile, the building systems are equivalently taken as shear-flexural beams with representative lateral stiffness ratios. The IDR distribution of continuous beams subjected to three groups of near-fault ground motions is acquired. It is illustrated that the maximum IDR shifts from the upper half to the lower half of buildings with an increase in lateral stiffness ratio. For long-period systems, the average IDR under impulsive ground motions is significantly greater than that under non-pulse motions. Finally, for moment-resisting frame buildings the forward directivity pulses amplify the drift response of higher modes, while the fling-step pulses excite primarily their contribution in the first mode and generate large deformation in the lower stories. The essential reason for this phenomenon is revealed according to the distinct property of near-fault impulsive ground motions and generalized drift spectral analysis.     

The first electric field pulse of cloud and cloud-to-ground lightning discharges

In this study, the first electric field pulse of cloud and cloud-to-ground discharges were analyzed and compared with other pulses of cloud discharges. Thirty eight cloud discharges and 101 cloud-to-ground discharges have been studied in this analysis. Pulses in cloud discharges were classified as ‘small’, ‘medium’ and ‘large’, depending upon the value of their relative amplitude with respect to that of the average amplitude of the five largest pulses in the flash. We found that parameters, such as pulse duration, rise time, zero crossing time and full-width at half-maximum (FWHMs) of the first pulse of cloud and cloud-to-ground discharges are similar to small pulses that appear in the later stage of cloud discharges. Hence, we suggest that the mechanism of the first pulse of cloud and cloud-to-ground discharges and the mechanism of pulses at the later stage of cloud discharges could be the same.     

沉放深度对气枪震源激发信号影响的试验研究

以2014年11月福建街面水库气枪震源激发试验观测数据为研究对象,利用基于互相关的时延估计法对不同沉放深度及不同水深的气枪激发展开研究,分析沉放深度及激发环境水深对气枪震源激发信号到时的影响。结果显示:激发位置不变,气枪沉放深度在8—30 m范围内变化时,震中距为13 km的测震台站记录到的气枪信号的首脉冲到时差异很小,而气泡脉冲到时则有明显差异,且随着沉放深度的增加,气泡脉冲信号到时提前,8 m沉放深度与30 m沉放深度气泡脉冲的到时差近80 ms,这种变化与布设在库底的海底地震仪记录到的信号变化相一致,分析认为这种变化与气枪沉放深度增加引起的气枪激发信号的周期减小有关;在不同水位开展的相同沉放深度的激发信号差异不大。因此,应用气枪震源监测地壳介质变化时需注意气枪震源沉放深度变化所带来的影响。      

Superfine time structure and polarization of hard X-ray emission during solar flares: Simulation of isolated pulses

A superfine time structure has been detected recently via hard X-ray satellite observations during solar flares. Some pulses are hundreds of milliseconds in length. Time series are sharply nonstationary sequences of overlapping pulses. We interpreted a similar time structure in a model of the nonstationary kinetics of accelerated electron beams. In this work, we present calculation results of the degree of polarization of the hard X rays of solar flares with millisecond pulses. The dependence of the polarization degree on the plasma concentration, radiation energy, and the observation angle has been derived from calculations with the thick-target nonstationary kinetics model for rectangular and triangular individual pulses of accelerated electrons in the region of injection with the angular dependence cos²ⁿ θ. It is shown that the degree of polarization does not exceed 78% and decreases down to 50–65% 20 s after the beginning of the injection.     

The effect of the atmosphere on the propagation of high-power ultrabroadband microwave emission pulses

The effect of the atmosphere on the propagation of single successive pulses of the ultrabroadband microwave emission has been calculated in detail. It has been indicated that nonlinear effects mainly influence the propagation of the LF pulse part in the absence of divergence, which substantially affects the propagation of the following pulses.     

Transient and permanent shear strains in a building excited by strong earthquake pulses

Nonlinear waves in a building with bi-linear constitutive law are described for excitation by vertically propagating S-wave pulses. Conditions that lead to the nonlinear deformation are described in terms of dimensionless amplitudes and wavelengths of these pulses. It is shown that a building can fail during the first passage of the incident wave (during a time shorter than the travel time from the bottom to the top of the building). Peak amplitudes of (1) transient strains, (2) permanent strains, and (3) the peak ductility in the building are described in terms of (a) the amplitudes of incident pulses and (b) the places where the pulses occur in the building.     

Asymmetrical pulses, the periodicity and synchronization of low frequency microseisms

Seismic records of eight IRIS broadband stations were analyzed at distances of 70 to 7160 km from the magnitude 8.3 Hokkaido earthquake of September 25, 2003. The stations situated in the subduction zone recorded asymmetrical microseismic pulses lasting 3–10 min a few days before the earthquake. No such pulses were observed in the records of the stations situated outside the subduction zone. Similar pulses were also recorded before the magnitude 7.8 Kronotskii, Kamchatka earthquake of 1997. The pulses are hypothesized to have been caused by creeping movements. Synchronous oscillations of microseismic noise with periods of 1–3 h were recorded as far as 3000 km from the Hokkaido earthquake a few days before it occurred. The noise coherence measure increased for stations closer to the epicenter. The question of the source of this coherence remains open. These effects belong to the class of those occurring in dissipative metastable systems; parts of the terrestrial lithosphere during the precursory periods of seismic catastrophes seem to be such systems.     

近断层地震动作用下土质边坡动力响应研究

根据汶川地震中滑坡多沿断层分布的特点,利用FLAC有限差分软件建立了一个土质边坡动力数值分析模型,分别研究了具有向前方向性效应、滑冲效应和无速度脉冲的近断层地震动作用下边坡的动力响应.结果表明:含速度脉冲地震动对边坡的破坏作用远强于无速度脉冲地震动,且具有滑冲效应的地震动引起的边坡动力响应稍大于向前方向性效应地震动.近断层地震作用下,边坡水平方向绝对峰值加速度分布存在沿高程放大效应.具有向前方向性效应的地震动会增强边坡加速度的高程放大效应而具有滑冲效应的地震动则在一定程度上削弱了这种放大效应,且边坡中下部绝对峰值加速度值相对于向前方向性效应地震动和无速度脉冲地震动引起的绝对峰值加速度值较大.     

近断层速度脉冲型地震动相关问题研究

近断层速度脉冲型地震动研究对揭示建筑结构的破坏机理、开展抗震设防以及抗震设计具有重要价值。首先,对速度脉冲成因进行了系统的总结,并探讨了区分方向性效应速度脉冲和滑冲效应速度脉冲的思路;其次,系统地介绍了近断层速度脉冲的识别方法,评述了各种脉冲识别方法的优缺点;然后,基于速度脉冲特性,探讨了前方向性效应对速度脉冲特性的影响以及速度脉冲对反应谱的放大作用;最后,对速度脉冲型地震动输入方法以及对结构响应研究进行了系统总结,探讨了速度脉冲型地震动输入的关键问题。基于丰富的理论研究,未来对于速度脉冲型地震动研究工作应当充分结合实际工程需求,推进理论成果的规范标准化与工程实践。      

Stick-slip unstable shear failure as a source of high frequency elastic radiation

Summary The results of laboratory measurements on square perspex models with a stress concentrator under uniaxial compression are presented. An attempt was made to interpret these results in order to determine the mutual relations between the mechanical parameters characterizing the treated models, including their focal zones, parameters of model loading, parameters of seismogenic displacement and parameters of radiated elastic pulses. Particular attention was given to the study of slip displacement, slip velocity, rupture velocity, nucleation points of rupture propagation, nucleation points of elastic radiation and to the frequency analysis of radiated pulses. Measurements of the displacement in the focal zone enabled us to determine the source function, to construct theoretical seismograms in a far field and to compare them with the real pulses from ultrasonic transducers located there. This allowed testing theoretical and experimental approaches to the study of how slip displacement, slip velocity and rupture velocity are related to the parameters of radiated pulses.     

A method to extract successive velocity pulses governing structural response from long-period ground motion

A series of relatively long-period velocity pulses appearing in the later part of ground motion, which is the characterization of far-source long-period ground motions in basin (“long-period ground motion” for short), is mainly influenced by focal mechanism, basin effect, and dispersion. It was supposed that the successive low-frequency velocity pulses in long-period ground motion caused the resonance of long-period structures in basin, which are of special concern to designers of super high-rise buildings. The authors proposed a wavelet-based successive frequency-dependent pulse extraction (WSFPE) method to identify and extract these pulses with dominant period of interest from long-period ground motions. The pulses extracted by using two frequently used methods (zero-crossing analysis, empirical mode decomposition) were compared to the pulses extracted by using WSFPE. The results demonstrate that the WSFPE provides higher resolution in time–frequency domain than the other two methods do. The velocity pulses extracted by using WSFPE are responsible for the resonance and maximum response of structure subjected to long-period ground motions. WSFPE can be used to make a better understanding of long-period ground motions and to promote the formation of long-period ground motion model which will help the seismic design of long-period structures built in sedimentary basin.     

MÉLANGES MAURICE PARDÉ

Abstract

Existing models for generating synthetic daily streamflow data are unsuitable for reproducing the predominant features of daily flows, the rising and recession of flood flows, the peaks of the floods, the volume of the waves and the range. In this paper, a model is presented which is able to reproduce the important features of daily flows. In the model the measured record of daily data is assumed to be the output of a linear system. The input of the system consists of pulses, occurring on certain days. The pulses are convoluted with the system function in order to produce the output. The form of the system function depends on the magnitude of the output. First, the days on which pulses occur, the magnitude of pulses, and the form of the system function as a function of the system output, are determined. Subsequently, a model was developed for the generation of the pulses. The model consists of a combination of two processes. Using a Markov chain model, the sequence of dry and wet days (days with and without pulses) is generated. Thereafter, a pulse of certain magnitude is assigned to each wet day. A modified first-order autoregressive process is used to produce these correlated pulses. The random components of the pulses are taken from a transformed exponential distribution. The periodicity of the flows within the year is reproduced by using different model parameters for each month of the year. The model yields good results for small and medium size basins, especially as far as peak flows, the volume of the waves, and the range are concerned. A sequence of daily flows from at least 20 years is required for input data.     

大容量气枪震源子波激发特性分析

大容量气枪水库激发作为陆地震源的可行性与有效性已经得到成功验证.为进一步提高气枪震源激发效果,本文通过水库气枪激发试验对单枪容量为2000 in³的气枪震源激发子波特征及规律进行了研究.依据近场水听器和远场短周期地震仪记录数据,分析气枪震源沉放深度、工作压力等不同激发条件对压力脉冲和气泡脉冲的影响.有助于人们根据不同尺度地下结构探测对震源激发信号的要求,调整气枪激发参数和激发环境,获得最佳激发效果.试验结果表明：（1）沉放深度对压力脉冲波形影响较小,其优势频率不随沉放深度而改变;（2）随着沉放深度从5 m增加到11 m,气泡脉冲的优势频率由5 Hz增加至7 Hz,其最大振幅亦近线性递增;（3）工作压力越大,激发压力脉冲能量越强,而对气泡脉冲的影响主要体现在主频降低.适合远距离深穿透地下结构探测的低频信号主要来自大容量气枪所激发气泡的反复振荡,由于气枪振荡过程非常复杂,本文通过较为简洁的数学和物理模型进行了解释.     

Identification of acceleration pulses in near-fault ground motion using the EMD method

In this paper, response spectral characteristics of one-, two-, and three-lobe sinusoidal acceleration pulses are investigated, and some of their basic properties are derived. Furthermore, the empirical mode decomposition （EMD） method is utilized as an adaptive filter to decompose the near-fault pulse-like ground motions, which were recorded during the September 20, 1999, Chi-Chi earthquake. These ground motions contain distinct velocity pulses, and were decomposed into high-frequency （HF） and low-frequency （LF） components, from which the corresponding HF acceleration pulse （if existing） and LF acceleration pulse could be easily identified and detected. Finally, the identified acceleration pulses are modeled by simplified sinusoidal approximations, whose dynamic behaviors are compared to those of the original acceleration pulses as well as to those of the original HF and LF acceleration components in the context of elastic response spectra. It was demonstrated that it is just the acceleration pulses contained in the near-fault pulse-like ground motion that fundamentally dominate the special impulsive dynamic behaviors of such motion in an engineering sense. The motion thus has a greater potential to cause severe damage than the far-field ground motions, i.e. they impose high base shear demands on engineering structures as well as placing very high deformation demands on long-period structures.     

基于背向散射脉冲传播时间统计特性的随机界面参数反演研究

研究了利用背向散射脉冲传播时间统计特性反演随机界面参数的问题.首先扩展了Fuks & Godin关于随机界面背向散射脉冲传播时间统计特性理论,给出了当随机界面高度-斜率统计相关情形下,最早返回的两个脉冲传播时间及其时延的概率密度函数（PDF）表达式.进而提出了一种基于背向散射脉冲传播时间统计特性的随机界面参数反演算法：采用遗传算法匹配前两个背向散射脉冲传播时间及其时延的PDF实现随机界面高度—斜率相关系数的绝对值|ρ|和无量纲参数T的反演.最后利用参数的后验概率分析反演结果的不确定性.数值模拟结果表明：该算法可有效反演随机界面的参数,参数|ρ|相对于T反演精度较高;|ρ|和T之间存在较强的参数耦合.与前人认为可忽略高度—斜率相关性（|ρ|）的观点不同：此类反演问题中应同时考虑|ρ|和T.     

Gravel‐bed river evolution in earthquake‐prone regions subject to cycled hydrographs and repeated sediment pulses

Sediment often enters rivers in the form of sediment pulses associated with landslides and debris flows. This is particularly so in gravel‐bed rivers in earthquake‐prone mountain regions, such as Southwest China. Under such circumstances, sediment pulses can rapidly change river topography and leave the river in repeated states of gradual recovery. In this paper, we implement a one‐dimensional morphodynamic model of river response to pulsed sediment supply. The model is validated using data from flume experiments, so demonstrating that it can successfully reproduce the overall morphodynamics of experimental pulses. The model is then used to explore the evolution of a gravel‐bed river subject to cycled hydrographs and repeated sediment pulses. These pulses are fed into the channel in a fixed region centered at a point halfway down the calculational domain. The pulsed sediment supply is in addition to a constant sediment supply at the upstream end. Results indicate that the river can reach a mobile‐bed equilibrium in which two regions exist within which bed elevation and surface grain size distribution vary periodically in time. One of these is at the upstream end, where a periodic discharge hydrograph and constant sediment supply are imposed, and the other is in a region about halfway down the channel where periodic sediment pulses are introduced. Outside these two regions, bed elevation and surface grain size distribution reach a mobile‐bed equilibrium that is invariant in time. The zone of fluctuation‐free mobile‐bed equilibrium upstream of the pulse region is not affected by repeated sediment pulses under the scenarios tested, but downstream of the pulse region, the channel reaches different fluctuation‐free mobile‐bed equilibriums under different sediment pulse scenarios. The vertical bed structure predicted by the simulations indicates that the cyclic variation associated with the hydrograph and sediment pulses can affect the substrate stratigraphy to some depth. Copyright © 2017 John Wiley & Sons, Ltd.