Wavefield distortion imaging of Earth's deep mantle
Wavefield distortion imaging of Earth's deep mantle
复制标题
地幔深处的波场畸变成像
DOI：
10.1016/j.epsl.2023.118011
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发表时间：
2023
影响因子：
5.3
通讯作者：
Rost S
中科院分区：
文献类型：
--
作者：
Rost S
地表记录的地震波场携带着地震震源和地震路径沿线地球结构的信息，这对于了解地球内部至关重要。40年来，地震层析成像研究利用地震传播时和波形越来越详细地解决了三维地震速度结构。这些研究推动了我们对地球动力学和演化的理解，但由于层析反演的限制，它们的空间分辨率仅限于100秒到1000公里的成像尺度。对地震波形的详细研究可以解决更精细的尺度结构，但通常依赖于偶然的源接收器组合，并且提供了非常不均匀的地球覆盖范围。因此，我们经常缺乏对地球精细尺度结构的理解，而这对于理解地幔柱或板块循环等结构和过程非常重要。在这里，我们展示的证据表明，我们可以利用地震波场的慢度矢量偏差，将我们对地球结构的了解扩展到使用大数据集的更小尺度。通过对地震阵列数据的分析，我们发现远震pandp差分波场在地球深部取样时具有很强的可测量的聚焦和散焦效应。我们将p波结果与附加的sandsdiffdata数据进行比较，发现两种波类型之间有很好的一致性。我们可以将波场偏差与强烈的速度变化联系起来，假设沿深地幔路径采样了明显的边界。该数据集对太平洋和墨西哥湾的油井进行了采样，并显示了太平洋（向西高达14°）和墨西哥湾下方（向东和向西高达5至8°）强烈的水平入射（反向）偏差。相对于±9 km/s的速度变化，反方位角偏差也反映在±0.8 s/°的慢度偏差上。使用3D光线追踪，我们能够对检测到的pandpdiffdataset的反向变化进行前向建模。纵波的高频率、射线路径的密度以及正演计算的低计算成本，使我们能够建立一个比以前的方法更高分辨率和更详细的夏威夷海底速度异常模型。最适合太平洋的速度模型包含位于夏威夷皇帝链顶端N25°/W155°和N25°/W165°的两个低速区。太平洋异常直径(D)为6°和2°，速度降低（dVP）为8%和4%，高度(H)分别高于CMB 70 km和至少200 km。我们还在北太平洋发现了一个速度增加3%的快速区域，在CMB上方至少300公里处，直径为12°，N60°/W175°。在墨西哥湾下面，我们发现了模糊的结果，要么是一个慢区（N25°/W85°，H= 200 km,dVP=-3%，D= 2°），要么是一个快区（N15°/W75°，H= 200 km,dVP= 3%，D= 4°）能够解释这些数据。因此，我们表明，地震波场的指向性信息（很大程度上未被开发）可以用于高精度地解决地球内部的精细尺度速度结构，并可以提供对地球深部动力学的额外见解。
The seismic wavefield, as recorded at the surface, carries information about the seismic source and Earth's structure along the seismic path, essential for the understanding of the interior of our planet. For 40 years seismic tomography studies have resolved the 3D seismic velocity structure in growing detail using seismic traveltimes and waveforms. These studies have been driving our understanding of the dynamics and evolution of the planet, but are limited in their spatial resolution to imaging scales of a few 100 s to 1000 km due to the constraints of the tomographic inversion. Detailed studies of seismic waveforms can resolve finer scale structure but are often reliant on serendipitous source-receiver combinations and provide very uneven coverage of the planet. Therefore, we often lack an understanding of the fine scale structure of the Earth that is important to understand structures and processes such as mantle plumes or details of slab recycling. Here we show evidence that we can exploit slowness vector deviations of the seismic wavefield to extend our knowledge of Earth structure to smaller scales using large datasets. Analysing seismic array data, we show strong and measurable focussing and defocussing effects of the teleseismicPandPdiffwavefield sampling the deep Earth. We compare theP-wave results to additionalSandSdiffdata and find good agreement between both wavetypes. We can link the wavefield deviations to strong velocity variations assuming sharp boundaries are sampled along the path in the deep mantle. The dataset samples the Pacific and Gulf of Mexico well and shows strong horizontal incidence (backazimuth) deviations in the Pacific (up to 14° westwards) and beneath the Gulf of Mexico (up to 5 to 8° east- and west-ward). The backazimuth deviations are also reflected in slowness deviations in the range of ± 0.8 s/° relating to velocity variations in the range of ± 9 km/s. Using 3D raytracing we are able to forward model the detected backazimuth variations of thePandPdiffdataset. The high frequencies of theP-waves, density of the ray-paths, and low computational cost of our forward calculation allow us to construct a higher resolution and more detailed model of velocity anomalies under Hawaii than was possible with previous methods. The best-fitting velocity model for the Pacific contains two low-velocity regions located at N25°/W155° and N25°/W165° beneath the tip of the Hawaii Emperor chain. The Pacific anomalies have diameters (D) of 6° and 2° with velocity reductions (dVP) of 8% and 4% with heights (H) above the CMB of 70 km and at least 200 km, respectively. We also detect a fast region of 3% velocity increase in the North Pacific rising at least 300 km above the CMB with a diameter of 12° at N60°/W175°. Beneath the Gulf of Mexico we find ambiguous results with either a slow region (N25°/W85°,H= 200 km,dVP=-3%,D= 2°) or a fast region (N15°/W75°,H= 200 km,dVP= 3%,D= 4°) able to explain the data. We thus show that the directivity information of the seismic wavefield - largely underexploited - can be used to resolve the fine scale velocity structure of the Earth's interior with great accuracy and can deliver additional insight into deep Earth dynamics.
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