使用数值模型研究俯冲板块动力学的进展

板块构造理论的核心是俯冲板块的动力学问题,可以通过地震学、矿物物理和岩石地球化学、地质构造和沉积学以及数值模拟对其进行多学科综合研究.其中数值模拟通过建立理论模型并使用数值方法模拟俯冲板块的动力学效应,直观地给出了定量化的俯冲板块的演化过程和地表响应,并用以解释其他学科研究所得到的观测和实验结果.因此使用数值模型进行定量化模拟是研究俯冲板块动力学的最重要的手段之一.文章综述了使用数值模型研究俯冲板块动力学多个方面在近期取得的进展,包括板块构造的起源和俯冲板块的启动过程,俯冲板块的热结构,以及俯冲板块在上地幔、地幔过渡带和下地幔的主要动力学表现.使用数值模型得到的结果是基于质量、动量和能量守恒方程的理论结果,必须综合使用其他各学科的研究成果对模拟结果进行比对和验证,以最终理解俯冲板块的动力学过程.随着计算能力的大幅提升和模拟方法的不断进步,数值模拟方法将为研究包括俯冲板块动力学在内的地球科学前沿问题提供更为准确和高效的手段.     

地震激发的地球自由振荡研究综述

地球自由振荡的研究已成为探查地球内部结构的重要手段之一.影响地球自由振荡的因素有很多,如地球物理学结构横向不均匀性及径向不均匀性、地球自转、地球椭率、地球内部各向异性、非弹性性以及震源机制性质,还有地球外界如引潮力等.本文详细介绍和总结了近几十年来关于地球自由振荡理论研究、实验及观测资料方面的研究,重点是数值模拟方法方面的研究工作.通过观测和理论分析对比,以数值实验的方式推测地球内部更详细的结构分布,如地球内部密度分布以及波速度等特性;并从弹性波理论出发结合高性能并行计算对全球自由振荡过程进行数值模拟研究.     

地幔柱数值模型的研究进展

地幔柱模型作为地球内部对流系统的重要组成部分,可与板块构造理论相媲美.它们共同构成了地球内部物质和能量循环的重要过程.自从地幔柱模型提出以来,地震学、高温高压矿物学、地质学以及地球动力学数值模拟对其进行了多学科的综合研究.其中数值模拟通过建立理论模型来研究地幔柱的动力学效应,定量化地给出了地幔柱的演化过程以及地球内部和表层的响应,并用以解释其他学科的观测和实验结果.因此,地球动力学数值模拟是研究地幔柱动力学最重要的手段之一.文章总结了近几十年地幔柱数值模型的研究进展,包括地幔柱的起源和存在的证据,地幔柱与岩石圈、洋中脊、俯冲带和地幔过渡带的相互作用以及地幔柱与大型剪切波低速区的关系.数值模型得到的理论结果必须与其他各学科的观测结果进行对比和验证才能真正理解地幔柱的动力学过程.随着数值模拟方法的发展和计算能力的提高,地球动力学数值模拟将为研究包括地幔柱动力学在内的地球科学前沿问题提供更为准确和高效的手段.     

地电阻率交流观测中信号检测方法研究

目前我国地电阻率观测台站受地铁、 轻轨等干扰的情况越来越多, 严重影响地电阻率观测数据质量. 地电阻率交流观测方法被认为是一种可以解决此类干扰问题的有效方法. 本文首先介绍该观测方法的理论基础; 然后采用数值模拟方法对实际观测过程进行模拟, 给出在不同参数条件下, 分别采用数字滤波器法、 频谱分析法和相关检测法进行信号检测所得到的相对误差和相对均方差结果; 最后, 通过对3种信号检测方法进行比较和分析, 给出频谱分析法在地电阻率交流观测中可以作为最佳信号检测方法的建议, 为地电阻率交流观测系统的研制提供理论参考.      

地下实验室与深地环境下的地球物理观测

地下实验室具有良好的低本底环境条件,为许多前沿学科研究提供了重要的实验场所,已经成为一个国家关键性的科学研究平台.目前国际上很多国家利用山体中的隧道空腔或地下矿井建立起众多的地下实验室,其中大多数进行天体物理和粒子物理实验与研究,部分同时进行着地球科学、岩体力学、地下生物等学科的相关实验.本文在介绍国际上一些重要地下实验室的基础上,着重对其在地球物理方面的观测与实验研究成果进行介绍;针对深部地下实验室所具有的“超静”与“超净”环境,阐述深部地下环境多种物理场观测所能达到的测量指标;针对深部地下实验室可实现的超高精度观测,讨论深地地球物理观测对目前仪器科学、装备与观测技术提出的挑战,及其将应对解决的诸多地球物理领域关键基础性科学问题.     

构造物理模拟与数值模拟技术的对比与结合

详细分析了应力场数值模拟与物理模拟的实验过程,对比两技术之间实验条件、模型转换、实验结果等方面具有的优势及适用条件,提出两者相结合进行构造成因机制分析的方法,并以高邮凹陷南断阶为例进行了说明.研究认为:数值模拟与物理模拟技术具有各自的优势,前者得到的是变形初期的应力分布结果,主要用于成因解释和变形预测,而物理模拟成果主要反应整个变形过程的构造演化;南断阶的断层分布范围受张应力控制、断层走向受剪应力影响,竹墩和许庄地区具有相似的应力分布和断层形成顺序,故断层组合及成因相似,断裂带由张应力作用产生的NE向断层及张剪应力作用下产生的NNE向、EW向共轭断层共同组成.     

瑞利面波二维物理模型的数值模拟研究

在瞬态多道瑞利面波的理论和模拟研究中,常将三维地质模型简化为二维模型来处理,以此降低分析问题的难度和节约建模成本,然而常规的实验室二维物理模型难以实现.本文基于有限元数值模拟技术对不同模型厚度的数值模型激发的声波波场进行了数值模拟,研究和分析了具有不同菲涅尔带半径R厚度的三维均匀半空间和横向非均匀模型的瑞利面波波场和频散特征,并与对应的二维数值模拟结果进行对比分析.研究表明,在模型厚度小于最小第一菲涅尔带半径R条件下,三维物理模型实验结果与二维数值模拟模型实验结果基本一致.因此,在实际物理模拟实验中,可用厚度小于最小第一菲涅尔带半径R的三维物理模型近似二维模型.本文最后以数值模拟确定的合理模型厚度制作物理模型进行实验,验证了本文提出的问题和解决方案的可行性.     

地震短期预测方法物理机理的研究途径

通过实验室研究与野外观测实验分析，可以揭示典型短期前兆现象的动力机制和响应条件。基于特殊条件岩石力学实验结果，给出典型地震活动图像产生的条件及其物理机制。进行多种类型、不同尺度动力作用的野外试验，对流体前兆成因过程的机理进行揭示和解释。进行地电阻率、地电场的典型观测研究，建立异常识别的综合分析方法等。这些研究工作是探索地震短期预测方法物理机理的有效途径。     

利用对称四极横向剖面法探测走滑断层的应用

从走滑断层难以探测的实际出发,讨论了其物性特征;通过对低阻板模型的物理模拟、数值模拟以及对实地观测资料的分析,研究了对称四极纵、横向剖面法视电阻率曲线的变化特征和差异. 结果表明,横向剖面法相对纵向剖面法异常幅度明显增加,可对地震活动断层进行更为有效的探测和定位. 这一研究为走滑断层探测提供了新的思路. 同时,利用对称四极横向剖面法可以解决走滑断层难以探测这一技术难题.      

一种超声——声发射复合传感器

地震学的研究工作中,除了对大量的地震现象进行深入的实际观测和再进一步从理论上探讨外,在实验室里,模拟地壳深部条件,进行岩石力学试验、模型试验也是一个可行的方法,它是检验地震理论的一个很好的手段。 在水库地震研究过程中,也是一样。模拟试验是其中一个不可缺少的部分。在试验过程中不但要用声速法测定在不同荷载作用下试件的动力学参数来间接了解试件的物理     

Mean-field concept and direct numerical simulations of rotating magnetoconvection and the geodynamo

Mean-field theory describes magnetohydrodynamic processes leading to large-scale magnetic fields in various cosmic objects. In this study magnetoconvection and dynamo processes in a rotating spherical shell are considered. Mean fields are defined by azimuthal averaging. In the framework of mean-field theory, the coefficients which determine the traditional representation of the mean electromotive force, including derivatives of the mean magnetic field up to the first order, are crucial for analyzing and simulating dynamo action. Two methods are developed to extract mean-field coefficients from direct numerical simulations of the mentioned processes. While the first method does not use intrinsic approximations, the second one is based on the second-order correlation approximation. There is satisfying agreement of the results of both methods for sufficiently slow fluid motions. Both methods are applied to simulations of rotating magnetoconvection and a quasi-stationary geodynamo. The mean-field induction effects described by these coefficients, e.g., the α-effect, are highly anisotropic in both examples. An α²-mechanism is suggested along with a strong γ-effect operating outside the inner core tangent cylinder. The turbulent diffusivity exceeds the molecular one by at least one order of magnitude in the geodynamo example. With the aim to compare mean-field simulations with corresponding direct numerical simulations, a two-dimensional mean-field model involving all previously determined mean-field coefficients was constructed. Various tests with different sets of mean-field coefficients reveal their action and significance. In the magnetoconvection and geodynamo examples considered here, the match between direct numerical simulations and mean-field simulations is only satisfying if a large number of mean-field coefficients are involved. In the magnetoconvection example, the azimuthally averaged magnetic field resulting from the numerical simulation is in good agreement with its counterpart in the mean-field model. However, this match is not completely satisfactory in the geodynamo case anymore. Here the traditional representation of the mean electromotive force ignoring higher than first-order spatial derivatives of the mean magnetic field is no longer a good approximation.     

Control Volume Method for the Thermal Convection Problem in a Rotating Spherical Shell: Test on the Benchmark Solution

The development of the control volume method for the thermal convection problem in a rotating spherical shell is presented. In contrast to the spectral methods, commonly used in geodynamo simulations, the control volume method belongs to the class of grid methods (the solution is approximated by a set of discrete values in physical space). In the present paper we concentrate on some problems of convergence and stability of the method. Case 0 of the numerical dynamo benchmark (Christensen et al., 2001, Phys. Earth Planet. Inter., 128, 25-34) was used to check the correctness of our computer code. The results demonstrate good convergence to the suggested standard solution.     

The Karlsruhe Dynamo Experiment. A Mean Field Approach

At the Forschungszentrum Karlsruhe an experiment is in preparation which it is hoped, in view of the geodynamo and other cosmic dynamos, that a homogeneous dynamo will be demonstrated and investigated. This experiment is discussed within the framework of mean-field dynamo theory. Results are presented concerning kinematic cylindrical mean-field dynamo models reflecting some features of the experimental device, as well as results of detailed calculations of the -effect that apply to arbitrarily high magnetic Reynolds numbers. On this basis estimates of the excitation conditions of the dynamo are given and predictions concerning the geometrical structure of the generated magnetic fields are made.     

The Dynamical Effects of Hyperviscosity on Numerical Geodynamo Models

The most fundamental difficulty in the construction of an Earth-like dynamo model is associated with the constraint caused by the rapid rotation of the Earth. To stabilise numerical codes, many workers have introduced hyperviscosity into the governing equations. One of the major effects introduced by hyperviscosity is to offset the rotational constraint, and, consequently, to alter the key dynamics of an Earth-like dynamo. In this paper, an Earth-like convection model with or without the presence of an imposed magnetic field is investigated with or without the effect of hyperviscosity. A nonlinear dynamo model with the mean field approximation is also used to examine the dynamical effect of hyperviscosity. The results suggest that great care should be taken when hyperviscosity is employed in geodynamo models.     

A generalized two-disk dynamo model

Abstract

A generalized two-disk dynamo model is considered that includes mechanical friction; this model is intended to simulate in its broad character the behavior of the geodynamo. Fixed points, limit cycles and chaotic attractors are located for different input parameters of the model. The chaotic regimes are of several kinds as are the “routes to chaos”. Several approximate models, helpful for studying the dynamo are discussed. A number of essential differences from the well-known Rikitake dynamo are demonstrated.     

Geomagnetic Reversals and Dynamo Bursts in a Simple Geodynamo Model

The question of what exactly happens with the geodynamo process during the reversal of a geomagnetic field is studied in a simple geodynamo model. The geodynamo action is described by the so called dynamo number characterizing the joint action of the main drivers of the geomagnetic field, i.e., the differential rotation and mirror–asymmetric convection. In mirror-asymmetric convection, for instance, in the northern hemisphere, there are more right vortices than left vortices, whereas in the southern hemisphere, there are more left vortices than right vortices. The effect of the magnetic field on the flow is described by the suppression of the mirror asymmetry: due to this suppression, e.g., in the northern hemisphere, the excess of right vortices over left vortices decreases. It is also assumed that due to this suppression, the mirror asymmetry can change its sign; i.e., the number of left vortices in the northern hemisphere can become larger than the number of right vortices. Correspondingly, the dynamo number can also change its sign. It is shown that the short-term changes of the sign of the dynamo number are responsible for the very short time span accommodating the reversal, when compared to the interval between the reversals.     

Small Scale Kinematic Dynamos: Beyond the α-Effect

In geodynamo simulations which simulate the generation of an axial dipolar magnetic field, the generation mechanism appears to be adequately described as an α²-dynamo with an anisotropic α-effect. The anisotropy in the α-effect favors an equatorial dipole field, however, which calls into question the interpretation in terms of an α²-dynamo. It is shown in this article with kinematic dynamo calculations and exemplary velocity fields with an anisotropic α-effect that both types of dipolar magnetic field can be generated. Two examples of working dynamos in a sphere with flows with zero α-effect are also provided.     

Discussion on ‘relaxation method for pounding action between adjacent buildings at expansion joint’ by H. Takabatake,M. Yasui,Y. Nakagawa and A. Kishida

The paper under discussion presents a detailed study on the reduction of pounding force on buildings due to expansion joints being filled with rubber. From shake table experiments and numerical simulations, the authors of the paper concluded that the rubber can reduce the maximum pounding force and hence the pounding damage to buildings. However, the writers of this short communication observed some significant issues in the experimental results as well as the numerical simulations. These observations are presented and raise questions about the validity of the results and the subsequent conclusions. Copyright © 2014 John Wiley & Sons, Ltd.     

Convection driven dynamos

Various models of thermal convection in rapidly rotating fluids permeated by strong magnetic fields are discussed. Particular attention is paid to the possibility that the magnetic field can be maintained by dynamo action rather than by externally applied electric currents. Two dynamo models are given particular attention. They are the plane layer model of Childress and Soward (1972) and the annulus model of Busse (1975). Though these models do not totally resolve the geodynamo problem, they do highlight important features of hydromagnetic dynamos. As a result some speculations are made about the true character of the geodynamo.