Global mantle structure from multifrequency tomography using P, PP and P-diffracted waves
Global mantle structure from multifrequency tomography using P, PP and P-diffracted waves
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DOI：
10.1093/gji/ggz394
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发表时间：
2020-01-01
影响因子：
2.8
通讯作者：
Igel, Heiner
中科院分区：
文献类型：
--
作者：
Hosseini, Kasra;Sigloch, Karin;Igel, Heiner
在全球尺度的地震层析成像中，超重力P波和PP波主要约束地幔上部三分之二的结构，而岩心衍射波（Pdiff）约束地幔下部三分之一。这项研究是第一个反演Pdiff波的非常大的数据集，达到最高可能的频率。这导致层析成像的分辨率匹配和超过全球S波层析成像，这一直是解释最低mantlestructure.We提出了三个新的全球三维各向同性P波速度的地幔层析成像模型的选择模型。多频互相关走时测量的所有阶段，在通带从30秒的主导期，以产生满意的拟合（约3秒）的最高频率。DETOX-P1模型拟合了约2.5M的地震P波走时。DETOX-P2适合相同的数据，加上Pdiff波近似1.4 M走时的新测量。DETOX-P3拟合与DETOX-P2相同的数据，加上约1.2 M PP走时。合成1秒的主导周期计算全波传播的球对称地球使用谱元素方法AxiSEM。旅行时间与三维速度扰动（dV（P）/V-P）的有限频率Frechet内核，参数化的自适应四面体网格上的近似400 000顶点间隔约80公里的最佳采样区域。为了完成空间覆盖，波形互相关测量增加了大约570万个分析器挑选的、可重复的P到达时间。通过显式矩阵矢量方程的最小二乘解，联合反演了地幔中三维各向同性P速度异常和同相轴校正的P、Pdiff和PP走时，与DETOX-P1相比，包含Pdiff走时（在DETOX-P2、-P3中）提高了最低地幔的空间采样100至1000倍。在大约2400公里深度以下，地震慢异常聚集在南部和赤道纬度，在十几个或更多的直径为600-1400公里的强烈缓慢补丁。这些特征长期以来被划分为两个大的低剪切速度区（LLVP），现在看来是有问题的。相反，补丁的强烈缓慢异常在地幔最低层似乎形成了一个几乎连续的，跨越全球的南半球下的链，根据我们的分辨率增加的LLVP内部细分和新成像的补丁南美洲。我们的层析成像还支持在冰岛、阿森松岛Ascension、阿法尔、凯尔盖朗、加那利、亚速尔、复活节、加拉帕戈斯、夏威夷、法属波利尼西亚和马克萨斯下面存在全地幔柱。可能反映了古-在古生代和中生代早期，俯冲带和弧形的海沟几何形状聚集了东亚和美洲科迪勒拉山脉。中地幔结构主要受Pdiff P波的制约，但Pdiff数据具有稳定作用，例如，在2000 km以上的美洲、欧亚大陆和北方太平洋下，使俯冲板块的几何形状更加清晰。PP走时在上地幔和中地幔中提供了互补的约束，但它们也通过岩石圈结构的向下涂抹在海洋下面引入了低速伪影。我们的三个新的全球P波模型可以通过SubMachine门户网站（http：//submachine.earth.ox.ac.uk/）进行访问和交互式可视化。
In global-scale seismic tomography, teleseismic P and PP waves mainly constrain structures in the upper two thirds of the mantle, whereas core-diffracted waves (Pdiff) constrain the lower third. This study is the first to invert a very large data set of Pdiff waves, up to the highest possible frequencies. This results in tomographic resolution matching and exceeding that of global S-wave tomographies, which have long been the models of choice for interpreting lowermost mantle structure.We present three new global tomography models of 3-D isotropic P-wave velocity in the earth's mantle. Multifrequency cross-correlation traveltimes are measured on all phases in passbands from 30 s dominant period to the highest frequencies that produce satisfactory fits (approximate to 3 s). Model DETOX-P1 fits approximate to 2.5M traveltimes from teleseismic P waves. DETOX-P2 fits the same data, plus novel measurements of approximate to 1.4 M traveltimes of Pdiff waves. DETOX-P3 fits the same data as DETOX-P2, plus approximate to 1.2 M PP traveltimes. Synthetics up to 1 s dominant period are computed by full wave propagation in a spherically symmetric earth using the spectral-element method AxiSEM. Traveltimes are linked to 3-D velocity perturbations (dV(P)/V-P) by finite-frequency Frechet kernels, parametrized on an adaptive tetrahedral grid of approximate to 400 000 vertices spaced by approximate to 80 km in the best-sampled regions. To complete spatial coverage, the waveform cross-correlation measurements are augmented by approximate to 5.7 million analyst-picked, teleseismic P arrival times. P, Pdiff and PP traveltimes are jointly inverted for 3-D isotropic P-velocity anomalies in the mantle and for events corrections, by least squares solution of an explicit matrix-vector equation.Inclusion of Pdiff traveltimes (in DETOX-P2, -P3) improves the spatial sampling of the lowermost mantle 100- to 1000-fold compared to teleseismic P waves (DETOX-P1). Below approximate to 2400 km depth, seismically slow anomalies are clustered at southern and equatorial latitudes, in a dozen or more intensely slow patches of 600-1400 km diameter. These features had long been classed into two large low shear velocity provinces (LLVP), which now appears questionable. Instead, patches of intensely slow anomalies in the lowermost mantle seem to form a nearly continuous, globe-spanning chain beneath the southern hemisphere, according to our increased resolution of LLVP-internal subdivisions and newly imaged patches beneath South America. Our tomography also supports the existence of whole-mantle plumes beneath Iceland, Ascension, Afar, Kerguelen, Canary, Azores, Easter, Galapagos, Hawaii, French Polynesia and the Marquesas.Seismically fast structure in the lowermost mantle is imaged as narrowly elongated belts under Eastern Asia and the Americas, presumably reflecting the palaeo-trench geometries of subduction zones and arcs that assembled Eastern Asia and the American Cordilleras in Palaeozoic and early Mesozoic times. Mid-mantle structure is primarily constrained by teleseismic P waves, but Pdiff data have a stabilizing effect, for example, sharpening the geometries of subducted slabs under the Americas, Eurasia and the Northern Pacific in the upper 2000 km. PP traveltimes contribute complementary constraints in the upper and mid mantle, but they also introduce low-velocity artefacts beneath the oceans, through downward smearing of lithospheric structure.Our three new global P-wave models can be accessed and interactively visualized through the SubMachine web portal (http://submachine.earth.ox.ac.uk/).