High-resolution Imaging of the Mantle Transition Zone using EarthScope USArray

在过去2亿年里，导致美国大陆地震、火山和地貌演变的主要构造过程是法拉隆板块俯冲到北美板块之下。理解这种俯冲过程一直是一个挑战，因为法拉隆板块的大部分现在都在地表以下数百公里处。这项研究利用了一种分析地震数据的新技术和EarthScope US阵列站收集的丰富数据集来对美国大陆下方俯冲的法拉隆板块进行成像，这一过程类似于医学CT扫描中的成像过程。这项研究旨在加深对俯冲过程的理解，重点研究以下问题：(1)较老的、更深的俯冲法拉隆板块如何与现在俯冲在太平洋西北下方的年轻板块相连？(2)法拉隆板块的俯冲与黄石火山带的形成有什么关系？(3)俯冲的法拉隆板块如何与美国大陆更深的地幔相互作用？法拉隆板块的高分辨率图像、地震测量以及基于新地震理论的计算机代码将向更广泛的地球科学界提供。基于这项研究的实践课程项目和实验室将在本科教学中开发和实施。在北美地幔过渡区从西海岸延伸到美国东部的大范围区域，板条异常已在波速模式中成像。由于数据的敏感性，波速模型的分辨率在地幔过渡带受到限制，中地幔俯冲过程尚不清楚。第二个悬而未决的问题是深部地幔热柱在黄石热点轨道形成中的作用。在这项研究中，我们将计算在410公里和660公里不连续面上产生的波的有限频率敏感度，以使用美国阵列TA站记录的地震数据来成像美国大陆以下的高分辨率地幔过渡带结构。这些高分辨率模型将为俯冲动力学提供新的见解，特别是美国西部和东部俯冲物质之间的关系，以及板块破碎在黄石热点轨道形成中的作用。这项研究将为未来地幔过渡带不连续面的成像提供坚实的基础，并使社区能够充分利用依赖于频率的地震数据来解决关于地幔对流动力学的科学问题。

The dominant tectonic process responsible for earthquakes, volcanoes and landform evolution in the continental United States in the past 200 million years has been the subduction of the Farallon plate beneath the North American Plate. Understanding this subduction process has been a challenge as most of the Farallon plate is now several hundred kilometers below the surface. This research takes advantage of a new technique to analyze seismic data and a rich data set collected by EarthScope USArray stations to image the subducted Farallon slab beneath the continental United States, a process similar to the imaging in medical CT Scan. This research aims to advance understanding of the subduction process with focuses on the following questions: (1) How does the older and deeper subducted Farallon plate connect with the younger plate now subducting beneath the Pacific Northwest? (2) What is the relation between the subduction of Farallon plate and the formation of the Yellowstone volcanic track? (3) How does the subducted Farallon plate interact with the deeper mantle across the US continent? High-resolution images of the Farallon slab, seismic measurements as well as computer codes based on the new seismic theory will be made available to the broader Geosciences community. Hands-on course projects and labs based on this research will be developed and implemented in undergraduate teaching. Slab anomalies have been imaged in wavespeed models in a broad region in North America mantle transition zone, extending from the west coast to the eastern US. The resolutions of wavespeed models are limited in the mantle transition zone due to data sensitivity and the subduction process in the mid mantle remains unclear. The second unresolved question is the role of a deep mantle plume in the formation of the Yellowstone hotspot track. In this research, we will compute finite-frequency sensitivities for waves generated at the 410-km and 660-km discontinuities to image high-resolution mantle transition zone structure beneath the continental US using seismic data recorded at USArray TA stations. The high-resolution models will provide new insights into the dynamics of subduction, especially the relation between subducted materials in the western and eastern US and the role of slab fragmentation in the formation of the Yellowstone hot-spot track. This research will provide a solid foundation for future imaging of mantle transition zone discontinuities and allow the community to take full advantage of frequency-dependent seismic data to tackle scientific questions on mantle convection dynamics.