【作者】 戴志阳；

【导师】 刘斌；

【作者基本信息】 中国科学技术大学 ， 固体地球物理， 2008， 博士

【摘要】 位于地幔底部数百公里的D”层是固体地幔和液态外核的动力学、热学和化学边界,它控制着CMB物质、能量的交换,对地幔对流、板块的运动、磁场的变化有关键性的影响。地震学研究表明D”层具有明显的地震各向异性。地震各向异性是理解地球动力学过程的重要工具,携带了大量的关于固体地球演化与动力学过程的潜在信息。在合适的条件下,地震各向异性可以为采用其它方法（如层析成像）观测不到的地幔对流提供地震学上的指示。因此,对D”层地震各向异性位置、强度和方向的研究有助于深化对核幔边界附近区域物理性质的认识。西太平洋地区是地球上现今构造活动非常活跃、复杂的地区之一,其深地幔中可能存在非常复杂的结构,并且和中国大的地质活动背景有关。因此西太平洋下D”层地震各向异性的研究对于认识该区域的地球动力学过程及地球演化具有十分重要的意义。首先,本文利用1994-2007年间发生在汤加-斐济及周边地区的146个地震,由IRIS36个台站记录到的数据和2000-2005年发生在东亚北部及北太平洋地区的26个地震,由IRIS的18个台站记录到的数据,使用ScS-S的相对走时分析方法研究了西太平洋下D”层的剪切波速度各向异性。我们得到了512个ScS波径向分量和横向分量的分裂时间,并计算了D”层的地震波速度各向异性强度。发现沿近北-南方向传播的地震波和沿近北西-南东方向传播的地震波产生的剪切波分裂模式差别较大:前者以V_ScSH＞V_ScSV的分裂为主,后者以V_ScSH＜V_ScSV(V_ScSH和V_ScSV分别表示水平和垂直极化的ScS震相的速度)的分裂为主,并且后者的各向异性强度明显要大于前者。整体而言,剪切波分裂呈现出V_ScSH＜V_ScSV的模式,并且V_ScSH＜V_ScSV的剪切波分裂主要分布在研究区的中东部,V_ScSH＜V_ScSV的剪切波分裂主要位于研究区的西南部。ScS的分裂时间值从-4.08s到4.53s,绝对值平均值分别为1.26s;各向异性强度值为从-1.96%到2.55%,绝对值平均值为0.61%;分裂时间的分布和各向异性强度的分布趋势一致。用S-ScS相对走时分析方法研究D”层各向异性可以非常方便地消除震源一侧和台站一侧各向异性的影响,但是只能在径向和横向分量上测量,从而只能检测具有垂直对称轴的横向各向同性。如果要探测具有更一般形式的各向异性,则需要采用更复杂的算法。为此,本文利用来自发生在汤加-斐济俯冲区及周围地区的15个地震,由IRIS的8个台站记录到的数据,利用旋转相关法获取了对西太平洋下D”层采样的ScS震相的分裂参数-快分量的极化方向和快慢分量的延迟时间,并将其结果与相对走时方法的结果进行了比较。发现快波的极化方向（以正北方向为参考,顺时针方向为正）变化很大,从4°变化到170°（存在180°整数倍的不确定性）;快轴方向与径向的夹角从7.48°变化到73.86°,大部分分裂结果的夹角小于45°,说明研究区的大部分区域有V_ScSV＞V_ScSH;时间延迟从0.2s变化到3.9s,平均值为2.2s;其各向异性的强度值从0.11%变化到2.01%,平均值为1.06%。旋转相关方法与相对走时方法得出的结果对于V_ScSV和V_ScSH相对大小的判断有75%相符,两者得出的延迟时间与各向异性强度存在一些差别。由于互相关计算中波形窗口的选择和时间平移等会引起误差,并且上地幔各向异性校正存在很大的不确定性,D”层剪切波各向异性的旋转相关方法分析结果可能会存在较大的误差,但是它可以在更大的方位角范围内探测快波的极化方向。地震各向异性是地球内部动力学过程的反映,利用地震各向异性的测量可以推测地幔不同部分的主要变形机制和流动特性。但是,地震各向异性的解释非常复杂,解释结果常常互相矛盾。由于震源与接收台站的地理局限性,目前所有的D”层各向异性研究的方位覆盖采样都是有限的,有限的方位覆盖很难区分D”层不同的各向异性模式,因此由分裂测量到D”层各向异性的解释必须作一些必要的假设。基于对西太平洋下D”层采样的剪切波分裂观测数据,本文详细讨论了西太平洋下D”层各向异性可能的流变场机制:西太平洋下D”层内垂直上升流应占支配地位;水平流动构造也可能同时存在,但尺度相对小一些,并主要分布在研究区的西南面。研究区下可能存在上升流所致的不均匀性物质定向排列成的垂直组构;下地幔物质的晶格优选方位也是一种可能的机制;研究区中可能存在偏离水平面的流动,使各向异性晶体或不均匀性物质定向排列形成方位各向异性,可以解释成具有倾斜对称轴的横向各向同性。（Mg,Fe）SiO₃后钙钛矿可能是D”层的主要矿物,具有明显的弹性各向异性,为解释D”层地震波各向异性等现象提供了一种新的途径。后钙钛矿滑动系和所导致的晶格优选方位对于理解观测到的各向异性非常重要,但是人们对于后钙钛矿主滑移系的性质有不一致的认识。因此,针对复杂的D”层应力环境中晶体特征与应力场和晶轴取向的密切关系,采用第一原理模拟三轴应力场中的MgSiO₃后钙钛矿相的弹性性质,利用最小能量原理获得稳定的晶体空间取向类型。D”层的晶体取向和应力差对晶体性质产生明显的影响,其能量、弹性常数和波速随晶体取向和应力差的变化而变化;晶体a轴取向平行于最大压缩方向,以b轴为垂直对称轴的三轴应力场中的剪切波横向各向异性大于静水应力场中的剪切波横向各向异性,随着应力差的增加,具有最小能量的晶体空间取向类型比其他取向类型更为稳定,结果支持以（010）面为主滑移面的观点。在D”层垂直上升流区域,[010]水平取向时具有V_SH＜V_SV的各向异性;在水平流区域,[001]和[010]垂直取向均可产生V_SH＞V_SV的各向异性。更多还原

【Abstract】 D" layer, the lowermost few hundred kilometers of the mantle, serves as a dynamical, thermal and chemical boundary layer between the solid mantle and the liquid outer core. Heat, angular momentum, and possible some materials are exchanged across the core-mantle boundary （CMB） and this layer is postulated to influence mantle convection, the plate motion and the earth’s magnetic field. Seismological studies indicate the presence of anisotropy in D" layer. Seismic anisotropy is an important tool in understanding dynamic processes in the Earth and carries potential information on the evolution and dynamics of the solid Earth. In the favorable conditions it can provide a seismic signature to mantle flow invisible to other methods such as tomography. Therefore the location, orientation, and magnitude of seismic anisotropy in D" layer is helpful to constrain how chemical and melt heterogeneity or anisotropic minerals are oriented by patterns of flow near CMB. The western Pacific is one of the active and complex tectonic regions presently in the Earth and may have extremely complicated structure in the deep mantle. Therefore study of the D" layer beneath the western Pacific is critical to understand the deep structure and dynamic process of this region.At first, Using seismic shear phases from 146 Tonga-Fiji and its adjacent region events during 1994 and 2007 recorded by 36 stations of the Incorporated Research Institute for Seismology （IRIS） broadband arrays, and from 26 northeast Asia and north Pacific events during 2000 and 2005 recorded by 18 stations of IRIS, we studied the shear wave anisotropy in D" layer beneath the western Pacific utilizing the ScS-S differential travel time method. We obtained 512 splitting time values between the radial and transverse components of ScS wave and calculated the anisotropy strength. The pattern of the shear wave splitting is different between the seismic waves propagating in the direction of N-S and that in the direction of NW-SE: The former mainly involve major V_ScSH> V_ScSV (V_ScSH is velocity of horizontally polarized ScS wave, V_ScSV is velocity of vertically polarized ScS wave) splitting value, the latter mainly contain the V_ScSH < V_ScSV value, and the anisotropy strength of the latter is obviously larger than that of the former. On the whole, the large majority of shear waves involve the pattern of V_ScSH < V_ScSV value. The shear wave splitting with V_ScSH < V_ScSV is focused on the central and eastern part of the study area, while the shear wave splitting with V_ScSH < V_ScSV is mainly distributed in the southwestern part of the study area. The splitting time values of ScS wave range from -4.08s to 4.53s with an average absolute value of 1.26s. The strength of anisotropy varies from -1.96% to 2.55% with an average absolute value of 0.61%. The distribution trend of the splitting time values and the anisotropy strength are consistent.Using the ScS-S differential time method can remove the effect of upper mantle anisotropy conveniently and reveal the anisotropy in D" layer preferably. But the splitting observation measure is limited on the radial and the transverse components, thus attempting to examine transverse isotropy with a vertical axis of symmetry （VTI）. A more complex method must be used to resolve more general forms of anisotropy. Therefore, using seismic shear phases from 15 Tonga-Fiji and its adjacent region events recorded by 8 stations of IRIS, we obtained the splitting parameters （i.e. the polarization direction of the fast wave and the time delay between the separated fast and slow waves） of the ScS phase in D" layer beneath the western Pacific utilizing the rotating-correlation method and compared the result with that of ScS-S differential time method. The polarization direction （given by azimuth from north） of the fast wave vary greatly from 4°to 170°（note: it has n times 180 degree ambiguity）. The angles between the fast direction and the radial direction range from 7.48°to 73.86°, and most of the angles are less than 45°, suggesting that there is a pattern of V_ScSV>V_ScSH in most of the study area. The delay time is 0.2s-3.9s with an average value of 2.2s and the strength of anisotropy is 0.11%-2.01% with an average value of 1.06%. The estimation about the relative size of V_ScSV and V_ScSH from the two methods has 75% similarity, but their delay time and anisotropy strength has some difference. Due to the uncertain of the upper mantle anisotropy correction and the computing errors of the correlation value from the selection of the waveform windows and the shift of time, there may be larger errors in the anisotropy result using the rotating-correlation method. But it can examine the polarization direction of fast wave at a wider variety of azimuths.Seismic anisotropy may reveal the dynamical process of the Earth interior, by which the dominant deforming mechanism and rheology property of the different layer of the mantle can be inferred. But it is very complex to interpret the seismic anisotropy results and there are often contrary explanations. Due to geographical limitations in the distribution of earthquake sources and seismic sensors, at present, none of the deep mantle anisotropy studies has significant azimuthal raypath sampling. Limited azimuth coverage makes it difficult to distinguish one anisotropy pattern from another. Therefore assumptions are necessary to proceed from measurements to interpretations. Based on the observation and analyse of the shear wave splitting, we inferred the possible rheological field mechanism for the D" layer beneath the western Pacific. In this area, the vertical upwelling flow is expected to be dominant. The horizontal flow structures may exist but the magnitude may be relatively small and mainly located at the southwestern part of the study area. There may be vertical fabrics formed by the aligned heterogeneous materials resulting from the ascending flow. Lattice preferred orientation （LPO） of the lower mantle minerals in this region is a possible mechanism for the observed anisotropy too. Additionally, flow out of horizontal plane may also exist and align anisotropic crystals or heterogeneous materials to form azimuthal anisotropy which can be explained as transverse isotropy with a tilted axis of symmetry.（Mg, Fe） SiO₃ post-perovskite may be the main mineral phase in D" layer and is remarkably anisotropic. It can provide a new approach for explain the seismic observations such as the seismic anisotropy in D" layer. Therefore, the slip systems of those phases and resultant LPO are important for understanding the observed seismic anisotropy. But the nature of the dominant slip system for post-perovskite phase has yet to be clarified. Considering the complex stress environment of D" layer and properties of minerals associated with the stress field and the orientation of crystallographic axes, single-crystal energy in triaxial stress field was calculated to study elastic constants, single crystal and aggregated acoustic velocities. The conditions were found under which energy is a minimum. The calculations show that the orientation of crystallographic axes and the differential stress significantly affect on the properties of the post-perovskite mineral. The a-axis tend to align paralleling to the maximal compression direction, and transverse anisotropy in shear wave velocity with the b-axis as vertical symmetry axis is larger than that under the condition in hydrostatic stress field. The crystal orientation type with minimum energy becomes more stable than the other orientation type with the differential stress increasing. The results also support （010） plane as the dominant slip plane. There is anisotropy with V_SH< V_SV in the upwelling region of the D" layer when [010] axis orients horizontally. While in the horizontal flow region, vertically oriented [001] and [010] axes both result in anisotropy with V_SH> V_SV.更多还原