Interferometric Imaging of Deep Mantle Reflectors Beneath the Western United States

最近对位于美国西部正下方的上地幔进行的地震学调查已经在很大程度上解决了结构和非均质性问题。这些努力揭示了以前未知的结构，比如大规模的地幔滴水。这些上地幔构造如何与中、下地幔的构造和过程联系起来，目前还没有得到解决。破译上地幔结构与更深地幔层之间相互作用的关键步骤是使用地震不连续的高分辨率制图。特别是位于深度约400 ~ 700 km的不连续层（地幔过渡带）预计将受到其与上地幔结构相互作用的强烈影响，是本研究的重点目标区域。本文发展了一种基于地震干涉测量的新技术来研究上-中地幔区域地震反射体的性质。在过去的十年中，使用干涉测量方法的地震处理已广泛应用于小规模石油勘探工作，但尚未应用于更深层次的目标，因为这种技术需要地震记录仪的密集间距。在USArray项目显著增加地震仪器之前，这种技术是不可能适应的。过去研究地幔不连续结构的努力主要集中在研究地震不连续结构底部的地震波反射。许多这些努力都使用了地震相SS的底面反射（称为SS前兆）。然而，使用SS前体在确定不连续深度方面存在挑战，因为需要了解SS路径的震源端和接收端地震波速度。本研究中使用的技术是利用互相关干涉法，将ss前兆数据与直接到达的s波通过不连续层进行偏移。这就消除了对震源和接收路径上的详细速度结构的需要，并且不依赖于对震源位置的任何先验知识。该技术提供了更好的地幔不连续结构的空间分辨率，并阐明了上、下地幔过程之间的联系。

Recent seismological investigations of the upper-mantle located directly beneath the western United States have resolved a remarkable degree of structure and heterogeneity. These efforts have revealed previously unknown structures, such as large-scale mantle drips. How these upper-mantle structures are related to structure and processes in the mid- and lower-mantle is currently unresolved. A key step in deciphering this interaction between upper-mantle structures with deeper mantle layers is to use high resolution mapping of seismic discontinuities. In particular, discontinuities located approximately 400 to 700 km in depth (the mantle transition zone) are expected to be strongly affected by their interaction with upper mantle structures and are key target areas being investigated in this study.This work has developed a new technique based on seismic interferometry to investigate the nature of seismic reflectors in the upper- to mid-mantle region. Seismic processing using an interferometry approach has been extensively utilized in small-scale oil exploration efforts in the last decade, yet has not been applied to deeper targets as techniques of this kind require dense spacing of seismic recorders. Prior to the significant increase in seismic instruments made possible by the USArray project, this technique would not have been possible to adapt.Past efforts for looking at mantle discontinuity structure have been focused on studying seismic wave reflections off of the underside of seismic discontinuities. Many of these efforts have used underside reflections from the seismic phase SS (termed SS-precursors). However, using SS-precursors presents challenges in determining discontinuity depth, as knowledge of the seismic wave velocity on both the source- and receiver-side of the SS path is needed. The technique used in this study applies cross-correlation interferometry to migrate the SS-precursor data with the direct S-wave arrival passing through the discontinuity. This eliminates the need for detailed velocity structure on both source- and receiver-paths and is not dependent on any prior knowledge of where the seismic source is located. This technique provides better spatial resolution on mantle discontinuity structure and is elucidating the link between upper- and lower-mantle processes.