月壳厚度横向变化对月震波传播影响的数值模拟研究

采用基于交错网格的二维伪谱与有限差分混合的方法, 根据现有的月球结构研究结果, 设计月壳?月幔分界面起伏的模型, 模拟浅震和深震月震波在不同起伏高度、宽度和位置情况下的传播, 讨论不同起伏高度、宽度和位置对月震信号传播的影响, 揭示月壳厚度横向变化对月震波形成强烈尾波的作用。结果表明: 起伏高度起主导作用, 主要影响尾波振幅和持续时间; 起伏宽度主要影响尾波振幅; 起伏位置的影响机制较为复杂, 不仅影响直达波到时, 还影响尾波振幅和持续时间。     

岩石层底部切向应力场及地球大地水准面异常

比较了两种不同方法,即Runcorn 方程和地幔对流模型,计算了作用于岩石层底部的切向应力场.讨论了在三种不同的边界模型下,核-幔边界对于地球大地水准面的贡献.结果表明:模型Ⅰ(自由核-幔边界;摩擦岩石层-地幔边界)和模型Ⅲ(刚性核-幔边界,摩擦岩石层-地幔边界)中,核幔边界对于地球大地水准面的贡献随球谐函数系数的增加而减少,在n≥2时可以忽略下边界的影响.而模型Ⅱ(摩擦核-幔边界,自由表面)中下边界影响不容忽视.证明使用Runcorn 方程计算岩石层底部的切向应力场是一个合理的近似.     

SS前驱震相有限频效应研究

首先, 根据有限频原理计算SS前驱震相的边界敏感核, 分析SS前驱震相对间断面起伏状况的敏感性。然后, 利用SPECFEM软件正演间断面存在起伏扰动下的SS前驱震相波形, 得到其到时扰动量, 与利用有限频敏感核计算得到的到时扰动量相比较, 说明有限频理论可以较好地解释SS前驱震相的波前愈合效应。最后, 利用边界敏感核反演界面起伏状况, 展示当考虑SS前驱震相的有限频效应时, 可以更好地恢复界面起伏的真实状况。研究结果为正确利用SS前驱震相反演地幔间断面起伏提供了必要的基础。     

中国东北上地幔各向异性研究

利用中国数字地震台网记录的地震波形,观测其穿过地球外核和地幔的SKS波分裂特征,确定了快波S波的振动方向和快慢波时间延迟。这种经过地幔传播的SKS震相的横波分裂主要来源于上地幔的各向异性。可以用应变引起的上地幔中矿物质的结晶优势排列来解释。结果表明,对于所分析的10个观测台站均发现了明显的横波分裂现象。时间延迟在0.4～1.8 s。在东部地区,快波振动方向与中国东北西太平洋俯冲带的板块相对运动方向基本一致,说明俯冲的太平洋板块地幔流对各向异性有重要意义。在西部地区,快波振动方向和时间延迟都具有较大差异,说明其总体形变的起因是复杂的,各向异性受到地幔流和历史构造运动的双重影响。     

中国东北上地幔各向异性研究

利用中国数字地震台网记录的地震波形,观测其穿过地球外核和地幔的SKS波分裂特征,确定了快波S波的振动方向和快慢波时间延迟。这种经过地幔传播的SKS震相的横波分裂主要来源于上地幔的各向异性。可以用应变引起的上地幔中矿物质的结晶优势排列来解释。结果表明,对于所分析的10个观测台站均发现了明显的横波分裂现象。时间延迟在0.4～1.8 s。在东部地区,快波振动方向与中国东北西太平洋俯冲带的板块相对运动方向基本一致,说明俯冲的太平洋板块地幔流对各向异性有重要意义。在西部地区,快波振动方向和时间延迟都具有较大差异,说明其总体形变的起因是复杂的,各向异性受到地幔流和历史构造运动的双重影响。     

面波有限频率法3-D灵敏度核函数的计算

高分辨率面波成像应克服射线理论的高频近似假设,考虑有限频率效应.该文给出了一种计算面没有限频率3-D灵敏度核函数的方法.该方法可直接计算球壳层状地球模型中面波振型特征值和特征函数,而不再需要采用球化平变换来近似计算球壳层状地球模型中面波频散参数.作者计算了相位延迟、振幅扰动和群到时延迟的3-D灵敏度核函数,并对面波有限频率效应进行了讨论.分析结果显示:面波相位延迟的3-D灵敏度核函数同2-D体波走时的核函数形状类似,说明面波具有二维传播特性.对相速度数据而言,Fresnel带方法适用于低频面波的反演,射线方法适用于高频面波的反演,而对于中等频率的面波反演应采用有限频率方法.频率的商低是相对于所要取得的分辨率而言的.在反演地壳上地幔细结构方面,群频散数据更具有优越性;然而,群到时延迟核函数的极强的旁瓣可能使得反演变得不稳定.     

地幔柱的发生机制研究——以Ferrar为例

利用三维数值模拟方法,根据地幔对流控制方程,以180 Ma喷发的Ferrar大火成岩省(LIP)为例,在模型中引入Pangea超大陆、大型横波低速带(LLSVPs)和Pangea超大陆边缘的俯冲带,模拟地幔对流过程,研究其对应地幔热柱从地球内部热边界层(例如核幔边界)的生成过程,并讨论导致该地幔柱产生的相关因素.结果表...     

激波和爆震波在U形管中传播过程模拟

为缩短爆震室轴向尺寸,并满足爆燃波向爆震波转变(DDT)或激波向爆震波转变(SDT)距离的需求,采用曲管爆震室代替直管爆震室来研究曲管中的爆震特性。应用数值方法对激波在U形管中传播、加速过程,SDT过程以及爆震波在U形管中的传播过程进行了模拟。U形管的曲率直径和入射激波强度对SDT有很大影响,较小的曲率直径对SDT有较大的促进作用;在入射激波速度大于960 m/s,曲率直径小于78 mm时,激波可以成功向爆震波转变;爆震波在弯管段形成后,向上游和下游两个方向传播,传播的速度和压力均不同;向上游传播的爆震波可以显示出"三波点"的运动轨迹;弯曲光管对爆震波在短距离内形成是有利的,可以用来作为预爆管。     

下扬子区深部热状态与岩石圈厚度

根据下扬子地区区域构造，地温场和大地热流密度的分布特征，结合岩石热性参数测试结果和该区的地表分层模型，确定了地幔热流密度分布特征和壳幔边界温度以及居里等温面的埋藏深度。根据于玄武岩固相线公式，估算了上地幔介质部分熔融开始的深度。     

分数导数黏弹性标准线性固体半无限长杆中纵波的传播

在半无限黏弹性长杆中,波在传播过程中波阵面强度会降低,其强度的衰减与黏弹性材料的性质有关.为了分析波在黏弹性杆件传播过程中波速和衰减因子的变化情况,基于分数阶导数理论,分析了黏弹性标准固体模型相关参数对波速和衰减因子的影响,所建模型能够较好地描述出纵波在传播过程中波的相速度和衰减因子的变化.在黏弹性标准固体模型复柔量的基础上,分析了分数导数黏弹性微分算子的阶数、角频率与延迟时间数对波相速度和衰减因子的影响.结果表明:随着延迟时间值的增大,纵波在黏弹性线性标准固体半无限长杆中的传播相速度不断地增大,而衰减因子不断减小;随着分数导数微分算子的增大,纵波传播相速度和衰减因子呈现出先增大后减小的趋势.     

弥散黏滞性波动方程的吸收边界算法

针对地震波数值模拟中因计算区域有界而产生的人工边界反射问题,提出了一种弥散黏滞性波动方程的吸收边界算法.在所研究的有界计算区域周围加入合适的吸收层,使得吸收层内的地震波随传播距离指数衰减而达到吸收边界反射的目的.利用傅里叶变换求得无界空间该波动方程的解;引入衰减函数构建有界空间相应的辅助方程,合理地选择衰减函数使得计算区域内辅助方程的解为原方程的解,而边界吸收层内的解呈指数衰减;运用有限差分法在均匀介质和均匀层状介质中求解弥散黏滞性波动方程,采用该方法对边界进行处理并与未加边界处理的波场快照和地震记录进行对比.实验结果表明,所提方法处理后的波场快照几乎无边界反射波存在,在空间位置为x=147 m、z=747 m的地震记录中,0.5s处边界反射波的振幅趋于0.所提方法也适用于声波方程和Stokes方程.     

Seismological constraints on a possible plume root at the core-mantle boundary

Recent seismological discoveries have indicated that the Earth's core-mantle boundary is far more complex than a simple boundary between the molten outer core and the silicate mantle. Instead, its structural complexities probably rival those of the Earth's crust. Some regions of the lowermost mantle have been observed to have seismic wave speed reductions of at least 10 per cent, which appear not to be global in extent. Here we present robust evidence for an 8.5-km-thick and approximately 50-km-wide pocket of dense, partially molten material at the core-mantle boundary east of Australia. Array analyses of an anomalous precursor to the reflected seismic wave ScP reveal compressional and shear-wave velocity reductions of 8 and 25 per cent, respectively, and a 10 per cent increase in density of the partially molten aggregate. Seismological data are incompatible with a basal layer composed of pure melt, and thus require a mechanism to prevent downward percolation of dense melt within the layer. This may be possible by trapping of melt by cumulus crystal growth following melt drainage from an anomalously hot overlying region of the lowermost mantle. This magmatic evolution and the resulting cumulate structure seem to be associated with overlying thermal instabilities, and thus may mark a root zone of an upwelling plume.     

Deformation of the lowermost mantle from seismic anisotropy

The lowermost part of the Earth's mantle-known as D″-shows significant seismic anisotropy, the variation of seismic wave speed with direction. This is probably due to deformation-induced alignment of MgSiO(3)-post-perovskite (ppv), which is believed to be the main mineral phase present in the region. If this is the case, then previous measurements of D″ anisotropy, which are generally made in one direction only, are insufficient to distinguish candidate mechanisms of slip in ppv because the mineral is orthorhombic. Here we measure anisotropy in D″ beneath North and Central America, where material from subducting oceanic slabs impinges on the core-mantle boundary, using shallow as well as deep earthquakes to increase the azimuthal coverage in D″. We make more than 700 individual measurements of shear wave splitting in D″ in three regions from two different azimuths in each case. We show that the previously assumed case of vertical transverse isotropy (where wave speed shows no azimuthal variation) is not possible, and that more complicated mechanisms must be involved. We test the fit of different MgSiO(3)-ppv deformation mechanisms to our results and find that shear on (001) is most consistent with observations and the expected shear above the core-mantle boundary beneath subduction zones. With new models of mantle flow, or improved experimental determination of the dominant ppv slip systems, this method will allow us to map deformation at the core-mantle boundary and link processes in D″, such as plume initiation, to the rest of the mantle.     

低频声源海底地震波的时域合成波形分析

在频域快速场模型的基础上进行逆傅里叶变换,得到海底地震波的时域合成波形计算方法,并对不同浅海环境参数下的合成波形进行了计算.结果表明:海底地震波的波动成分为海底表面波和简正波;当声源频率低于简正波最小截止频率时,不存在无衰减传播的简正波,其波动成分主要为海底表面波;随着声源频率增大,各阶简正波逐渐被激发且逐渐增强;声源深度越接近于海底,越有利于海底地震波的激发,波动成分中表面波越发占据主要地位;与硬质海底相比,软质海底更有利于低频声源激发海底地震波,但不利于较高频率声源海底地震波的激发.     

Seismic detection of folded, subducted lithosphere at the core-mantle boundary

Seismic tomography has been used to infer that some descending slabs of oceanic lithosphere plunge deep into the Earth's lower mantle. The fate of these slabs has remained unresolved, but it has been postulated that their ultimate destination is the lowermost few hundred kilometres of the mantle, known as the D' region. Relatively cold slab material may account for high seismic velocities imaged in D' beneath areas of long-lived plate subduction, and for reflections from a seismic velocity discontinuity just above the anomalously high wave speed regions. The D' discontinuity itself is probably the result of a phase change in relatively low-temperature magnesium silicate perovskite. Here, we present images of the D' region beneath the Cocos plate using Kirchhoff migration of horizontally polarized shear waves, and find a 100-km vertical step occurring over less than 100 km laterally in an otherwise flat D' shear velocity discontinuity. Folding and piling of a cold slab that has reached the core-mantle boundary, as observed in numerical and experimental models, can account for the step by a 100-km elevation of the post-perovskite phase boundary due to a 700 degrees C lateral temperature reduction in the folded slab. We detect localized low velocities at the edge of the slab material, which may result from upwellings caused by the slab laterally displacing a thin hot thermal boundary layer.     

Earthquake rupture dynamics frozen in exhumed ancient faults

Most of our knowledge about co-seismic rupture propagation is derived from inversion and interpretation of strong-ground-motion seismograms, laboratory experiments on rock and rock-analogue material, or inferred from theoretical and numerical elastodynamic models. However, additional information on dynamic rupture processes can be provided by direct observation of faults exhumed at the Earth's surface. Pseudotachylytes (solidified friction-induced melts) are the most certain fault-rock indicator of seismicity on ancient faults. Here we show how the asymmetry in distribution and the orientation of pseudotachylyte-filled secondary fractures around an exhumed fault can be used to reconstruct the earthquake rupture directivity, rupture velocity and fracture energy, by comparison with the theoretical dynamic stress field computed around propagating fractures. In particular, the studied natural network of pseudotachylytes is consistent with a dominant propagation direction during repeated seismic events and subsonic rupture propagation close to the Rayleigh wave velocity.     

Study on crustal, lithospheric and asthenospheric thickness beneath the Qinghai-Tibet Plateau and its adjacent areas

Based on the results of pure dispersions of Rayleigh wave tomography in the Qinghai-Tlbet Plateau and its adjacent areas, tsklng S wave velocities from previous linear inversion as the initial model, using the simulated annealing algorithm, a nonlinear simultaneous inversion has been carried out for S wave velocity and thickness of different layers, including the crust, the lithosphere and the asthenosphere. The results indicate： The crustal thickness shows strong correlation with geology structures sketched by the sutures and major faults. The crust is very thick in the Qinghal-Tibet Plateau, varying from 60 km to 80 kin. The Ilthospherlc thickness in the Qinghai-Tibet Plateau Is thinner （130-160 kin） than Its adjacent areas. And two blocks can be recognized, divided by an NNE strike boundary running between 90°E-92°E inside the plateau. Its asthenosphere is relatively thick, varies from 150 km to 230 kin, and the thickest area is located in the western Qiangtsng. India has a thinner crust （32-38 kin）, a thicker lithosphere of 190 km and a rather thin asthenosphere of only 60 kin. Sichuan and Tarlm basins have the crust thickness less than 50 kin. Their Iithospheres are thicker than the Qinghai-Tibet Plateau, and their asthenospheres are thinner. A discussion has been made on the character and formation mechanism of the typical crust-mantle transition zone in the western Qiangtsng block.     

声波在液浸多层多孔介质中的反射和透射

根据Biot多孔介质声学理论和多孔介质界面条件,从理论和实验上研究了声波在液浸层状多孔介质中的反射和透射.从导出的计算公式可知,声波的反射、透射系数与声波的频率、入射角及各层多孔介质参数(厚度、孔隙度、弹性参数等)有关.对水浸单层多孔介质板的参数进行的计算表明,当介质板厚与声波波长呈一定比例时,反射系数、透射系数能取得极值.用实验测量方法对这一结论进行了验证,实验结果与理论预测结果相吻合.     

基于无线电掩星观测的火星电离层观测研究进展

火星电离层早期的观测数据非常少,除了Viking登陆器对火星电离层的在位测量外,火星电离层的主要物理信息是通过掩星观测方法得到的.近年来,Mars Global Surveyor和Mars Express轨道器通过掩星观测的方法对火星的上层大气和电离层进行了长期的观测,得到了大量的火星电离层电子密度廓线资料.火星电离层受到来自太阳EUV和X射线辐射、太阳风、太阳耀斑、中性大气、表面壳磁场、宇宙射线、流星等多种因素的影响,使其结构发生瞬态或季节性的变化.本文介绍了行星无线电掩星探测的基本原理和技术特点,回顾了国内外科学家们基于已有的火星掩星观测数据（主要是Mars GlobalSurveyor和Mars Express）在火星电离层研究中的一些最新科学成果,并详细介绍了火星电离层的结构和火星夜间电离层的主要特征.