Mapping Small-scale Mantle Heterogeneities Using USArray

利用USARRAY绘制小尺度地幔不均匀S地球内部岩石部分的深部流被称为地幔对流，这是地球上几乎所有类型地质作用的引擎，不仅产生地震、火山和大陆漂移，而且还调节地磁场甚至大气成分的演化。与此同时，了解这种地幔对流实际上是如何运作的是困难的，而且存在很大的不确定性，因为除了它被称为板块构造的表面表现外，它基本上是隐藏在我们的眼睛之外的。关于地幔对流的基本问题之一是它能以多大的效率混合，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，我们将把最先进的地震成像技术应用于美国阵列数据，以探测北美大陆周围地幔中的小规模非均质性。小尺度非均质性的分布，当在相关地幔剖面的构造历史背景下解释时，有可能阐明地幔对流的长期混合效率。在过去的几年里，PI一直在开发新的数据处理工具来克服与探测小尺度地幔不均匀有关的各种困难，包括一种被称为双自举叠加(DBS)的强大的信号检测技术和基于DBS的远程地震偏移方法。地震偏移是一种成像技术，在这种成像技术中，在地面接收器记录的地震能量被投影回其在地下的原点。由于它直接使用地震数据的整个波形，因此偏移具有获得给定数据所允许的最高空间分辨率的潜力。2013年，美国阵列移动式阵列完成了东迁，利用基于星载卫星的远程地震迁移系统地探索美国阵列数据的海量财富的时机已经成熟。考虑到大量可能的源-接收器组合和要利用的各种散射机制，该项目侧重于以下两个目标作为首要优先事项：(1)基于DBS的迁移的并行化；(2)利用P-to-P散射在北美大陆周围的地幔中寻找小规模的非均质性。基于DBS的迁移是一项非常耗时的处理，但随着计划中的代码并行化，处理大量数据将成为可能。地震记录的P尾波部分以直接P波和PP震相为界，适合于寻找S-P和P-P散射产生的潜在散射相。由于涉及走时，S到P的散射只能照亮地震附近，但P到P的散射可以沿着直接P的射线路径探测。相对于直接P的差异走时的使用限制了震源到接收器的距离小于100°，但太平洋周围的一些好的震源(即，深度超过100公里和兆瓦)将使我们能够成像北太平洋、中美洲，可能还有北大西洋下面的地幔中的散射体。

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.