Upper Mantle Earth Structure in Africa From Full-Wave Ambient Noise Tomography
Upper Mantle Earth Structure in Africa From Full-Wave Ambient Noise Tomography
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DOI：
10.1029/2018gc007804
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发表时间：
2019-01-01
影响因子：
3.5
通讯作者：
Bao, X.
中科院分区：
文献类型：
--
作者：
Emry, E. L.;Shen, Y.;Bao, X.
我们对非洲大陆的构造发展及其地质区之间的相互作用的理解受到分布不均匀的地震仪器的阻碍。为了更好地了解非洲大陆，我们使用了长周期环境噪声全波形层析成像技术，对从整个非洲和周边地区的186个宽带地震台站收集的数据进行了分析，以更好地对上地幔结构进行成像。利用频率-时间归一化方法从背景地震噪声中提取经验绿色函数，并提取了周期为7-340 s的相干信号。我们模拟波传播通过一个非均匀的地球，使用球面有限差分方法获得合成波形，测量数据和合成之间的相位延迟的失配，计算数值灵敏度内核使用散射积分的方法，并迭代反演结构。由此产生的各向同性，剪切波速度为大陆的图像显示分段，低速上地幔下的高度岩浆北方和东部部分的东非裂谷系统（EARS）。在南部和西部部分，高速上地幔占主导地位，和独特的，低速异常仅限于目前的火山活动地区。在更深的深度，南部和西部的EARS过渡到低速度。除EARS外，几个低速异常还散布在安哥拉和北非的浅层上地幔中，其中一些低速异常可能与更深的特征有关。不同的上地幔高速异常的成像在整个大陆，并建议在刚果地区和撒哈拉Metacraton内可能的超声波根多个超声波根。
Our understanding of the tectonic development of the African continent and the interplay between its geological provinces is hindered by unevenly distributed seismic instrumentation. In order to better understand the continent, we used long-period ambient noise full-waveform tomography on data collected from 186 broadband seismic stations throughout Africa and surrounding regions to better image the upper mantle structure. We extracted empirical Green's functions from ambient seismic noise using a frequency-time normalization method and retrieved coherent signal at periods of 7-340 s. We simulated wave propagation through a heterogeneous Earth using a spherical finite-difference approach to obtain synthetic waveforms, measured the misfit as phase delay between the data and synthetics, calculated numerical sensitivity kernels using the scattering integral approach, and iteratively inverted for structure. The resulting images of isotropic, shear wave speed for the continent reveal segmented, low-velocity upper mantle beneath the highly magmatic northern and eastern sections of the East African Rift System (EARS). In the southern and western sections, high-velocity upper mantle dominates, and distinct, low-velocity anomalies are restricted to regions of current volcanism. At deeper depths, the southern and western EARS transition to low velocities. In addition to the EARS, several low-velocity anomalies are scattered through the shallow upper mantle beneath Angola and North Africa, and some of these low-velocity anomalies may be connected to a deeper feature. Distinct upper mantle high-velocity anomalies are imaged throughout the continent and suggest multiple cratonic roots within the Congo region and possible cratonic roots within the Sahara Metacraton.