Lower Mantle Seismic structures investigated with unusual seismic phases

了解地球下地幔的结构提供了有关地幔内矿物学和动力学的信息。尤其是最下部的地幔已被证明在几个长度尺度上包含许多不同的结构，包括不同深度的反射体和大范围的减速区域(LLSVP)。虽然有可能测试地球上的许多区域，但更好的覆盖范围往往受到以下因素的限制：1)可用的震源-接收器组合；2)研究特定地震波所需的距离范围，从而研究地幔中的特定区域。在这项研究中，我们的目标是利用波场中的异常地震波，了解和验证它们的来源，并最终利用它们来研究北太平洋、西太平洋和大西洋的深部地幔，以扩大我们对目标区下地幔结构的覆盖。因此，这一提议在一定程度上是探索性的，因为我们想测试我们在以前的数据集中观察到的新的地震到达，但我们还不知道这些到达的来源，尽管初步迹象表明它们起源于下地幔。这两个目标区位于LLSVP边缘附近，了解这些区域的构造将有助于进一步了解地幔动力学和矿物学，以及下地幔反射体与LLSVP之间的联系。我们还将使用标准地震波来绘制下地台反射层的地图，并提取它们的属性，以验证异常地震波的来源。一个重要的步骤是比较两个目标区域，即北太平洋和西太平洋与大西洋的结果，因为这将有助于理解：i)成像的结构是源于全球还是更多的局部现象，以及ii)非洲和太平洋LLSVP之间是否存在差异。只有通过整合所有的波信息，包括振幅、极性、波形、到达时间和频率相关性以及随后的模拟结果，我们才能实现对地震结构的详细了解。以前的研究通常只使用一种方法或一种类型的地震波来破译LLSVP的起源，但在这里，我们的目标是通过结合标准和不寻常的地震震相观测来更全面地了解该结构。

Understanding structures in the Earth lower mantle provides information about mineralogy and dynamics within the mantle. Especially the lowermost mantle has been shown to contain a number of different structures on several length scales including reflectors at different depth and large regions of reduced velocity (LLSVPs). While it is possible to test many regions in the Earth, a better coverage is often limited by i) the available source-receiver combinations and ii) distance ranges needed to study a particular seismic wave and hence a particular region in the Earth's mantle. In this study, we aim to use unusual seismic arrivals in the wavefield, understand and validate their origin, and eventually use them to study the deep mantle in the Northern and Western Pacific and Atlantic in order to enhance our coverage of the lower mantle structures in the target areas. This proposal is therefore partly exploratory, since we want to test new seismic arrivals that we have observed in previous datasets but we do not understand the origin of these arrivals yet, although first indications suggest that they originate in the lower mantle. The two target regions are places near the edges of the LLSVPs and understanding structures in these areas will provide further knowledge on mantle dynamics and mineralogy as well as connections between lower mantle reflectors and LLSVPs. We will also use standard seismic waves to map reflectors in the lower mantel and extract their properties also for validation of the origin of the unusual seismic waves. An important step is the comparison of the results in the two target regions, i.e. the Northern and Western Pacific with the Atlantic, since this will help to understand i) whether the imaged structures are of global origin or more local phenomena and ii) whether there are differences between the African and the Pacific LLSVP. Only through an integration of all wave information, including amplitudes, polarities, waveforms, arrival times and frequency dependence and subsequent modelling results can we achieve to gain detailed knowledge on seismic structures. Previous studies often use only one type of method or one type of seismic wave to decipher the origin of LLSVPs but here we aim for a much more complete view of the structure by using combinations of standard and unusual seismic phase observations.