下地幔底部小尺度不均一速度结构研究

发布时间：2018-06-04 13:42

  本文选题：核幔边界 + 下地幔底部　； 参考：《中国科学院广州地球化学研究所》2017年博士论文

【摘要】：作为地球内部结构最复杂、动力学过程最活跃的区域之一,核幔边界及其附近结构一直是地球科学研究的热点。其中,核幔边界是地球内部物质成分变化最剧烈的界面,是固态的硅质地幔与液态的铁质外核相接触的分界面。而其下地幔底部作为一种“热-化学”边界层,可能是地幔对流过程中俯冲板块的最后归宿以及地幔柱上涌的起点,同时也是地核和地幔物质与能量交换的重要场所。虽然全球尺度的层析成像模型已经揭示出该区域大尺度的不均一性速度结构,但是由于分辨率不足的原因,无法得到一些小尺度结构的图像。而小尺度结构的地震学特性对理解下地幔的动力学演化、物质成分组成具有十分重要的指示作用。因此,在本论文中我们选取了PKP前驱波以及PKKP衍射波作为研究手段,对产生这些地震信号的下地幔底部的异常结构体进行了研究,并且探讨其背后的地球动力学意义。首先,本文选取了发生在勘察加半岛和阿留申群岛的地震,利用南极台网记录到的PKP前驱波信息,通过偏移成像以及对前驱波振幅的拟合,发现产生这些前驱波的强散射体结构位于太平洋异常北部的高速异常体中,大体分布在30-40°N之间,深度在核幔边界附近400公里范围内。同时这些散射体的结构存在着横向不均一性:中部区域(160-180°E)的散射体呈现出1.0~1.2%的P波速度扰动,而两侧区域(140-160°E,180-200°E)的速度扰动只有0.5%。利用ScS-S相对走时残差的方法对同样的区域进行分析,得到了剪切波速度结构:中间区域的S波速度异常为2.0~3.0%,而两侧的速度异常为1.0~1.5%。这些散射体结构可能代表了古俯冲板块在核幔边界处的残余,而这种散射强度的横向不均一性变化可能有两种原因导致:(1)不同的俯冲板块来源;(2)同一板块的内部组分在分离过程中产生的不同物质成分。其次,本文综合分析了澳大利亚ASAR以及WRA台阵所记录的发生在南美洲西海岸的地震产生的PKP前驱波的信息。利用台阵处理方法分析了相关性较好的前驱波的慢度信息,该结果表明在澳大利亚东南部以及太平洋东部的下地幔都存在着散射体结构。而利用AxiSEM方法进行波形模拟的结果进一步表明,在太平洋异常的东部边界则存在着小尺度的、块状的超低速区异常结构。这些超低速区P波的速度扰动在-6~-10%,厚度变化在20~40公里、宽度大约为30公里。然后利用我们开发的偏移成像方法得到了太平洋东部下方核幔边界处的散射区域分布图像,散射强度最大的散射体中心位于(-89°E,-10°N),该结果也与波形模拟的结果相吻合。对于产生PKP前驱波的超低速区的来源,则可能是部分熔融或者化学成分异常所导致的。最后,本文选取了发生在太平洋西岸的地震事件,利用美国USArray记录到的PKKPab衍射波,采用拉东变换的方法得到了PKKPab衍射波的射线参数信息,并将其转换为下地幔底部采样区域的速度扰动值。研究结果表明在非洲LLSVP的西北边界以及苏门答腊岛的南部,其下方核幔边界处的速度扰动为-4%~-8.5%,可能暗示了低速区/超低速区的存在。此外,在北大西洋下方,衍射波的速度变化可能暗示了离LLSVP边界的远近对于超低速区速度变化的影响。通过对小尺度散射体以及超低速区的研究,本文认为这两者可能存在着成因上的联系。具体来说,板块在俯冲到核幔边界的过程中,分离的洋壳会形成小尺度的散射体分布在下地幔,正是这些小尺度的散射体产生了本文观测到的随机扰动的PKP前驱波。同时这些洋壳物质可能会导致超低速区的形成。
[Abstract]:As one of the most complex and dynamic regions of the earth's internal structure, the boundary of the nuclear mantle and its adjacent structures have always been the hot spots in the earth science research. Among them, the boundary of the core mantle is the most intense interface of the earth's internal material components, and the interface between the solid silicon mantle and the liquid iron outer core. The bottom as a "thermal chemical" boundary layer may be the final destination of the subducted plate and the starting point of the upwelling of the mantle plume during the convection of the mantle, and it is also an important place for the exchange of material and energy of the earth's core and mantle. Because of the lack of resolution, some small scale structures can not be obtained. The seismological characteristics of the small scale structure are very important to the understanding of the dynamic evolution of the mantle and the composition of the material components. Therefore, in this paper, we selected the PKP precursor wave and the PKKP diffraction wave as the research means. The anomalous structures at the bottom of the lower mantle at the bottom of these seismic signals are studied and the Geodynamic Significance behind them is discussed. First, this paper selects the earthquakes occurring in the Kamchatka Peninsula and Aleutian Islands, using the PKP precursors recorded by the Antarctic network, and through the migration imaging and the fitting of the amplitude of the precursors. The strong scatterers of these precursors are located in the anomalous body of abnormal high speed in the north of the Pacific Ocean, which are generally distributed between 30-40 N and 400 km near the boundary of the nuclear mantle. At the same time, the structure of these scatterers has a transverse heterogeneity: the scatterer of the middle region (160-180 E) shows the P wave velocity disturbance of 1.0~1.2%. The velocity disturbances in the two sides (140-160 E, 180-200 E) are only analyzed by 0.5%. using ScS-S relative travel time residuals, and the shear wave velocity structure is obtained. The S wave velocity anomaly in the middle region is 2.0~3.0%, and the velocity anomaly on both sides is 1.0~1.5%., which may represent the ancient subduction plate in the region. The remnants of the core mantle boundary, and the transverse heterogeneity of this scattering intensity may have two reasons: (1) different subduction plate sources; (2) the internal components of the same plate in the separation process of the different material components. Secondly, this paper synthetically analyses the occurrence of Australia ASAR and WRA array in South America. The information of the PKP precursors produced by the earthquake in the west coast. By using the array processing method, the slowness information of the better precursors is analyzed. The results show that there is a scatterer structure in the lower mantle in southeastern Australia and the east of the Pacific. The results of the waveform simulation using the AxiSEM method further indicate that in Taiping. The eastern boundary of the oceanic anomaly has a small scale, massive ultra low velocity region abnormal structure. The velocity of P waves in these ultra low velocity regions is disturbed at -6~-10%, the thickness varies at 20~40 km, and the width is about 30 km. Then we use the migration imaging method we developed to get the scattered region distribution image at the eastern part of the Pacific Ocean. The center of the scatterer with the largest scattering intensity is located at (-89 E, -10 N). The result is also consistent with the result of the waveform simulation. The source of the ultra low velocity region of the PKP precursor may be caused by the partial melting or the abnormal chemical composition. Finally, this paper selects the earthquake events in the West Bank of the Pacific Ocean, using the American USArray record. The PKKPab diffraction wave recorded is obtained by using the method of Radon transform to obtain the ray parameters of the PKKPab diffraction wave and transform it into the velocity perturbation value in the bottom sampling area of the lower mantle. The results show that the velocity perturbation at the boundary of the nuclear mantle beneath the north-west boundary of the African LLSVP and the south of Sumatra island is -4%~-8.5%. It implies the existence of the low velocity region / ultra low velocity region. In addition, the velocity variation of the diffraction wave below the North Atlantic may imply the effect of the distance from the LLSVP boundary to the velocity variation in the ultra low velocity region. By the study of the small scale scatterers and the ultra low velocity regions, this paper considers that the two may have a genetic link. During the process of subduction to the boundary of the nuclear mantle, the separated oceanic crust will form a small scale scatterer in the lower mantle. It is these small scatterers that produce the PKP precursor of random perturbations observed in this paper.
【学位授予单位】：中国科学院广州地球化学研究所
【学位级别】：博士
【学位授予年份】：2017
【分类号】：P315.2
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本文编号：1977567