Fine-scale structure of the mid-mantle characterised by global stacks of PP precursors
Fine-scale structure of the mid-mantle characterised by global stacks of PP precursors
复制标题
DOI：
10.1016/j.epsl.2017.05.027
复制
发表时间：
2017-08
影响因子：
5.3
通讯作者：
H. Bentham;S. Rost;M. Thorne
中科院分区：
文献类型：
--
作者：
H. Bentham;S. Rost;M. Thorne
俯冲带可能是地幔成分不均匀性的主要来源，它可能保存了俯冲历史和地幔对流过程的记录。与地幔不均匀性相关的精细尺度结构可以利用作为尾波到达或作为许多体波到达之前的能量到达的散射地震波场来研究。在这项研究中，我们分析的前体PP通过创建堆栈记录在全球分布的站。我们从193个台站记录的600个地震中创建了与PP到达对齐的5°距离箱（范围为70-120°）叠加，共叠加了7320个地震记录。由于直接P到达后的能量，P尾波，干扰PP前体，我们通过减去最佳拟合指数曲线来抑制P尾波。由此产生的堆栈表明，PP前体相关的散射在地幔中的异质性是存在的所有距离。横向变化的探讨，通过生产两个区域堆栈横跨大西洋和太平洋半球，但我们发现这两个地区之间的missory签名只有微不足道的差异。这两个地区的相似性表明，混合良好的俯冲物质可以生存在上地幔和中地幔深度。为了描述在地幔中的散射波场，我们比较全球堆栈使用蒙特卡洛声子散射技术产生的合成地震图。我们提出了一个最佳拟合的分层非均匀性模型，BRT 2017，其特征是一个三层地幔，背景非均匀性强度（λ = 0.8%）和1000 km和1800 km之间的非均匀性强度增加的深度间隔（λ = 1%）。地幔不均匀性的尺度长度为8 km。由于8公里尺度的地幔不均匀性可能与俯冲洋壳有关，在中地幔深度检测到的不均匀性增加可能与由于粘度增加而导致的板块停滞有关，这支持了最近的观测结果，即由于铁自旋在1000公里深处的转变而导致地幔粘度增加。
Subduction zones are likely a major source of compositional heterogeneities in the mantle, which may preserve a record of the subduction history and mantle convection processes. The fine-scale structure associated with mantle heterogeneities can be studied using the scattered seismic wavefield that arrives as coda to or as energy preceding many body wave arrivals. In this study we analyse precursors to PP by creating stacks recorded at globally distributed stations. We create stacks aligned on the PP arrival in 5° distance bins (with range 70–120°) from 600 earthquakes recorded at 193 stations stacking a total of 7320 seismic records. As the energy trailing the direct P arrival, the P coda, interferes with the PP precursors, we suppress the P coda by subtracting a best fitting exponential curve to this energy. The resultant stacks show that PP precursors related to scattering from heterogeneities in the mantle are present for all distances. Lateral variations are explored by producing two regional stacks across the Atlantic and Pacific hemispheres, but we find only negligible differences in the precursory signature between these two regions. The similarity of these two regions suggests that well mixed subducted material can survive at upper and mid-mantle depth. To describe the scattered wavefield in the mantle, we compare the global stacks to synthetic seismograms generated using a Monte Carlo phonon scattering technique. We propose a best-fitting layered heterogeneity model, BRT2017, characterised by a three layer mantle with a background heterogeneity strength (ϵ= 0.8%) and a depth-interval of increased heterogeneity strength (ϵ= 1%) between 1000 km and 1800 km. The scalelength of heterogeneity is found to be 8 km throughout the mantle. Since mantle heterogeneity of 8 km scale may be linked to subducted oceanic crust, the detection of increased heterogeneity at mid-mantle depths could be associated with stalled slabs due to increases in viscosity, supporting recent observations of mantle viscosity increases due to the iron spin transition at depths of∼ 1000 km.