中国大陆下扬子及邻区地壳结构基本特征:深地震测深研究综述

系统回顾了20世纪70年代以来在中国大陆下扬子及其邻区开展的深地震测深工程,总结了相关宽角反射/折射地震资料的波组特征及地壳结构的基本特征。下扬子及邻区深地震测深资料普遍具有较清晰的初至震波Pg、上地壳底界面反射波P1、中地壳底界面反射波P3、莫霍界面反射波Pm及上地幔顶部首波Pn,震相连续,易追踪对比。受浅表沉积盖层或基岩等的影响,Pg波常出现局部延迟滞后或走时超前现象。尤其是大别造山带个别炮点的反射能量较弱或同相轴扭曲、波形紊乱等,均与地壳内界面和莫霍面的深度突变或破碎有关。该区域地壳结构大致分为上、中、下3层,但视资料情况中地壳和下地壳又可进一步划分为2个亚层。华北地台和扬子地台地壳厚度30~36km,莫霍界面形态变化较缓,存在局部隆起,下地壳平均速度6.7±0.3km/s。但大别造山带下方地壳厚度32~41km,莫霍界面下凹且出现4~7km垂向错断,下地壳平均速度6.8±0.2km/s。     

First-order doubly-asymptotic formulation of the direct stiffness method for elastodynamic problems

A first-order formulation to analyze the dynamic response of layered soil profiles is presented as an alternative to the widely used second-order thin-layer method by the direct stiffness approach, including an efficient simulation of the underlaying elastic half-space. In contrast to the thin-layer method where response is expressed through a combination of second-order propagation modes, the proposed procedure uses first-order modal parameters that have the capacity to provide a good approximation in the complete wave number domain k, including the exact stiffness values for k=0 and k→∞, thus justifying its designation of doubly-asymptotic. This feature allows obtaining the exact soil profile response for static loads, while the proposed treatment of the elastic half-space reproduces naturally the radiation condition without a need of artificial damping. The capacity of the proposed formulation to solve elastodynamic problems is assessed by comparing its results with those of exact solutions available in the literature, and numerical solutions of rigid disks supported on the surface of different soil profiles.     

Experimental study of a stiff wave barrier in gelatine

Railway induced vibrations and re-radiated noise in buildings can be mitigated by means of wave barriers in the soil. Numerical simulations demonstrate that a stiff wave barrier, consisting of a material that is stiffer than the surrounding medium, can be very effective if the stiffness contrast between the barrier and the medium is sufficiently large. This paper presents results of a lab experiment that has been carried out to validate these findings, using gelatine instead of soil in order to reduce the wavelengths and thus the scale of the test setup. An expanded polystyrene beam is employed as wave barrier, while a non-contact measurement procedure is applied for visualizing the waves in the gelatine, based on reflections of a grid of laser rays. The experimental results are found to be in line with the numerical predictions, confirming the vibration reduction effectiveness of stiff wave barriers.     

垂直和倾斜断层围陷波的谱元法数值模拟及其波场特征对比

低速断层破碎带内传播的断层围陷波是断层内部信息的重要载体,研究其波场特征是反,演断层内部结构的关键。前人对围陷波的模拟主要针对垂直断层,但实际断层大部分是倾斜的。本文利用谱元法数值模拟地震波在垂直和倾斜断层区域的传播,并分别单独改变断层破碎带或断层两盘速度差异等参数,对比分析地震动在时域和频域发生的变化,研究这些参数对波场特征的影响。对比研究表明,当存在一定宽度的低速破碎带时,倾斜断层对近断层区域波场的影响和垂直断层相比在幅值分布上有比较显著的差异,尤其是当断层两盘同时还存在速度差异时;在倾斜和垂直断层破碎带上方,均能产生断层围陷波,其幅值特征差异不大,但在跨断层平面上的质点极化方式可能会存在差异:垂直断层围陷波质点极化方向表现为单一,而倾斜断层可能会出现多个方向。这些模拟结果有助于反演真实断层破碎带结构,也能够加深对近断层的强地面运动特征的认识。     

Numerical modelling of wave propagation in anisotropic soil using a displacement unit-impulse-response-based formulation of the scaled boundary finite element method

An efficient method for modelling the propagation of elastic waves in unbounded domains is developed. It is applicable to soil–structure interaction problems involving scalar and vector waves, unbounded domains of arbitrary geometry and anisotropic soil. The scaled boundary finite element method is employed to derive a novel equation for the displacement unit-impulse response matrix on the soil–structure interface. The proposed method is based on a piecewise linear approximation of the first derivative of the displacement unit-impulse response matrix and on the introduction of an extrapolation parameter in order to improve the numerical stability. In combination, these two ideas allow for the choice of significantly larger time steps compared to conventional methods, and thus lead to increased efficiency. As the displacement unit-impulse response approaches zero, the convolution integral representing the force–displacement relationship can be truncated. After the truncation the computational effort only increases linearly with time. Thus, a considerable reduction of computational effort is achieved in a time domain analysis. Numerical examples demonstrate the accuracy and high efficiency of the new method for two-dimensional soil–structure interaction problems.     

Oblique wave diffraction by a flexible floating structure in the presence of a submerged flexible structure

Expansion formulae associated with the interaction of oblique surface gravity waves with a floating flexible plate in the presence of a submerged horizontal flexible structure are derived using Green’s integral theorem in water of finite and infinite water depths. The associated Green’s functions are derived using the fundamental solution associated with the reduced wave equation. The integral forms of the Green’s functions and the velocity potentials are advantageous over the eigenfunction expansion method in situation when the roots of the dispersion relation coalesce. As an application of the expansion formulae, diffraction of oblique waves by a finite floating elastic plate in the presence of a submerged horizontal flexible membrane is investigated in water of finite depth. The accuracy of the numerical computation is demonstrated by analysing the convergence of the complex amplitude of the reflected waves and the energy relation. Effect of the submerged membrane on the diffraction of surface waves is studied by analysing the reflection and transmission coefficients for various parametric values. Further, the derivation of long wave equation under shallow water approximation is derived in a direct manner in the appendix. The concept and methodology can be easily extended to deal with acoustic wave interaction with flexible structures and related problems of mathematical physics and engineering.     

基于各向异性分析的微地震震源矢量场重建和裂缝解释

针对微地震裂缝解释的复杂性,从震源矢量场的重建开始研究,在研究VTI介质速度模型各向异性条件下的走时和透射系数的变化特征基础上,形成了各向异性条件下的群、相速度及透射系数的计算方法.针对多级检波器水平分量朝向的多向性特点,提出了多级检波器水平分量的偏振分析方法,得到了完整的水平特征矢量,克服了单级检波器水平分量偏振分析构建特征矢量信息不全的问题,形成了高精度微地震事件定位方法,实现各向异性VTI介质速度模型的高斯束微地震格林函数正演模拟.利用格林函数模拟场、观测记录场,从构建完整场研究入手,重建震源矢量场.根据重建的震源矢量场,提出了裂缝解释的全新的系列方法,包括单条裂缝、裂缝网络的解释方法.通过实际资料的测试分析,验证了研究技术的实用性.     

Impact of sea-level rise and coral mortality on the wave dynamics and wave forces on barrier reefs

A one-dimensional wave model was used to investigate the reef top wave dynamics across a large suite of idealized reef-lagoon profiles, representing barrier coral reef systems under different sea-level rise (SLR) scenarios. The modeling shows that the impacts of SLR vary spatially and are strongly influenced by the bathymetry of the reef and coral type. A complex response occurs for the wave orbital velocity and forces on corals, such that the changes in the wave dynamics vary reef by reef. Different wave loading regimes on massive and branching corals also leads to contrasting impacts from SLR. For many reef bathymetries, wave orbital velocities increase with SLR and cyclonic wave forces are reduced for certain coral species. These changes may be beneficial to coral health and colony resilience and imply that predicting SLR impacts on coral reefs requires careful consideration of the reef bathymetry and the mix of coral species.     

台风莫拉克登陆期间的中尺度波动特征分析

利用波作用理论对台风莫拉克登陆期间的降水进行诊断分析。结果表明:波作用密度异常能在一定程度上指示暴雨雨区发展移动,其异常值的空间分布能够反映雨区上空动力场和热力场的典型垂直结构特征。台风内中尺度波动与暴雨落区宏观上具有一定联系。为了详细研究台风内部的波动特征,利用WRF(Weather Research and Forecasting model)模式模拟的高分辨率资料对台风登陆过程中波动特征进行分析,低波数波动的传播主导强对流的非对称分布,2波在登陆初期对对流分布起着关键作用,中尺度波动中同时存在涡旋罗斯贝波以及重力惯性波的特征,对登陆期间涡旋混合的现象有重要作用。     

The Impact of Orographically-Induced Gravity Wave on the Diurnal Cycle of Rainfall over Southeast Kalimantan Island

Precipitation measurements from the Tropical Rainfall Measuring Mission (TRMM) satellite indicate that the southeastern area of Kalimantan (Borneo) Island receives much less rainfall than elsewhere on the island during the period from July to October.Results from sur-face meteorological observations show that the diurnal cycle of rainfall differs greatly between the eastern and western coasts of the island.Rainfall on the western coast of the island is frequent in the afternoon and evening,whereas almost all rainfall on the eastern coast occurs in the morning.Meanwhile,the Global Positioning System (GPS)-derived precipitable water (PW) on the eastern coast shows a substantial decrease in moisture in the af-ternoon and evening.Numerical experiments with a mesoscale model reveal that gravity waves driven by di-urnal heating of the elevated land surface of the moun-tains on Sulawesi Island,which lies approximately 300 kilometers to the east of Kalimantan Island,significantly affect the diurnal cycle of rainfall over southeast Kali-mantan Island.