Mapping Small-scale Mantle Heterogeneities Using USArray

利用USAray绘制地球岩石部分内的小尺度地幔不均匀性？地幔对流是地球上几乎所有地质过程的发动机，不仅产生地震、火山和大陆漂移，而且还调节地磁场的演化，甚至大气成分。与此同时，了解地幔对流实际上是如何运作的是困难的，并且具有很大的不确定性，因为它主要隐藏在我们的眼睛之外，除了它的表面表现形式称为板块构造。关于地幔对流的一个基本问题是它如何有效地混合事物，对流混合的效率对地幔对流如何塑造地球及其表面环境的历史有许多重要的影响。在这个项目中，我们将应用一个国家的最先进的地震成像技术的USAray数据，以检测小规模的非均质性在地幔周围的北美大陆。小规模的不均匀性的分布，解释的背景下的构造历史的相关地幔部分，有可能阐明长期的地幔对流的混合效率。在过去的几年里，PI一直在开发新的数据处理工具，以克服与小尺度地幔非均质性检测相关的各种困难，包括一种强大的信号检测技术，称为双引导叠加（DBS）和一种基于DBS的微重力迁移方法。地震偏移是一种成像技术，其中在地面接收器处记录的地震能量被投射回其在地下中的起源。由于偏移直接使用地震数据的整个波形，因此有可能实现给定数据所允许的最高空间分辨率。2013年，USAray的可移动阵列完成了向东迁移，通过基于DBS的可移动阵列迁移系统地探索USAray数据的巨大财富的时机已经成熟。考虑到大量可能的源-接收器组合和各种散射机制，该项目重点关注以下两个目标作为首要任务：（1）基于DBS的偏移的并行化，以及（2）使用P-to-P散射在北美大陆周围的地幔中寻找小尺度非均匀性。基于DBS的迁移是一个非常耗时的处理过程，但通过计划中的代码并行化，将有可能处理大量数据。由直接P波到达和PP相位所限定的地震记录的P尾波部分适合于搜索由S-to-P和P-to-P散射产生的潜在散射相位。由于所涉及的传播时间，S到P散射只能照亮地震附近，但是P到P散射可以探测沿直达P的所有沿着射线路径。相对于直达P的差分传播时间的使用将源-接收器距离限制为短于100°，但是许多好的震源（即，深度超过100 km，Mw& #8805; 5.0）将使我们能够对北方太平洋、中美洲和可能的北方大西洋下地幔中的散射体进行成像。

Mapping small-scale mantle heterogeneities using USArrayDeep currents within the rocky portion of the Earth?s interior are called mantle convection, and this is the engine for almost all kinds of geological processes on the Earth, not only generating earthquakes, volcanoes, and continental drift, but also modulating the evolution of the geomagnetic field and even the atmospheric composition. At the same time, understanding how this mantle convection is actually operating is difficult and subject to large uncertainty, because it is mostly hidden from our eyes, except for its surface manifestation known as plate tectonics. One of the fundamental questions regarding mantle convection is how efficiently it can mix things up, and the efficiency of convective mixing has a number of important bearings on how mantle convection has shaped the history of the Earth and its surface environment. In this project, we will apply a state-of-the-art seismic imaging technique to the USArray data, to detect small-scale heterogeneities in the mantle surrounding the North American continent. The distribution of small-scale heterogeneities, when interpreted in the context of the tectonic history of a relevant mantle section, has a potential to elucidate the long-term mixing efficiency of mantle convection. In the last few years, the PI has been developing new data processing tools to overcome various difficulties associated with the detection of small-scale mantle heterogeneities, including a powerful signal detection technique called dual bootstrap stacking (DBS) and a teleseismic migration method based on DBS. Seismic migration is an imaging technique in which seismic energy recorded at surface receivers is projected back to its origin in the subsurface. As it directly uses the entire waveform of seismic data, migration has a potential to achieve the highest spatial resolution allowed by the given data. The Transportable Array of USArray finished its eastward migration in 2013, and the time is ripe to explore the vast wealth of USArray data by DBS-based teleseismic migration in a systematic manner. Given a large number of possible source-receiver combinations and a variety of scattering mechanisms to be exploited, this project focuses on the following two objectives as the top priorities: (1) parallelization of DBS-based migration, and (2) searching for small-scale heterogeneities in the mantle around the North American continent with P-to-P scattering. The DBS-based migration is a very time-consuming processing, but with the planned code parallelization, it will become possible to process a large volume of data. The P-coda part of seismograms, bounded by the direct P arrival and the PP phase, is suitable to be searched for potential scattered phases generated by S-to-P and P-to-P scattering. Because of travel times involved, S-to-P scattering can illuminate only the vicinity of earthquakes, but P-to-P scattering can probe all along the ray path of the direct P. The use of differential travel times with respect to the direct P limits the source-receiver distance to be shorter than ∼100°, but a number of good seismic sources (i.e., deeper than 100 km and Mw ≥5.0) around the Pacific will allow us to image scatterers in the mantle beneath the Northern Pacific, Central America, and possibly the Northern Atlantic.