Mapping Small-scale Mantle Heterogeneities Using USArray

在地球岩石部分使用USArrayDeep电流绘制小尺度地幔非均质性？它是地球上几乎所有地质过程的引擎，不仅产生地震、火山和大陆漂移，而且还调节地磁场的演变，甚至调节大气成分。与此同时，了解地幔对流的实际运作方式是困难的，而且存在很大的不确定性，因为除了板块构造的表面表现外，它大部分都隐藏在我们的眼睛之外。关于地幔对流的一个基本问题是它混合物质的效率有多高，对流混合的效率对地幔对流如何塑造地球的历史及其表面环境有着重要的影响。在这个项目中，我们将对USArray数据应用最先进的地震成像技术，以检测北美大陆周围地幔的小尺度非均质性。在相关地幔剖面的构造历史背景下解释小尺度非均质性的分布，有可能解释地幔对流的长期混合效率。近年来，为了克服小尺度地幔非均质性探测的各种困难，中国地质调查局不断开发新的数据处理工具，包括一种强大的双自激叠加信号检测技术和基于双自激叠加的远震偏移方法。地震偏移是一种成像技术，在地面接收器上记录的地震能量被投影回其在地下的起源。由于它直接使用地震数据的整个波形，因此迁移有可能实现给定数据所允许的最高空间分辨率。USArray的可移动阵列在2013年完成了东移，利用基于db的远震偏移系统挖掘USArray海量数据的时机已经成熟。考虑到大量可能的源-接收器组合和多种散射机制，本项目将重点关注以下两个目标：(1)基于db的偏移并行化；(2)利用P-to-P散射在北美大陆周围地幔中寻找小尺度非均质性。基于数据库的迁移是一个非常耗时的过程，但是通过计划的代码并行化，可以处理大量数据。以P直接到达和PP相位为界的P-尾波部分适合搜索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-to-P散射只能照亮地震附近，但P-to-P散射可以沿着直接P的射线路径探测。使用关于直接P的差分传播时间限制了源-接收器距离小于&amp；#8764；100°，但是太平洋周围的一些良好的震源（即深度超过100公里和毫瓦5.0）将使我们能够成像北太平洋，中美洲，可能还有北大西洋下地幔中的散射体。

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.