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

在过去的2亿年里，导致美国大陆地震、火山爆发和地貌演化的主要构造过程是法拉龙板块在北美板块之下的俯冲。了解这种俯冲过程是一个挑战，因为法拉龙板块的大部分现在都在地表以下数百公里处。本研究利用一种分析地震数据的新技术和EarthScope USArray台站收集的丰富数据集，对美国大陆下方俯冲的法拉龙板块进行成像，这一过程类似于医学CT扫描的成像过程。本研究旨在促进对俯冲过程的认识，重点关注以下问题：(1)更古老、更深的法拉龙板块如何与太平洋西北下方正在俯冲的年轻板块连接？(2)法拉龙板块的俯冲作用与黄石火山道的形成有何关系？(3)俯冲的法拉龙板块如何与横跨美洲大陆的深部地幔相互作用？Farallon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提供给更广泛的地球科学界。在本研究的基础上，将开发并在本科教学中实施实践课程项目和实验。在从美国西海岸延伸到美国东部的北美地幔过渡带的广阔区域，用波速模型对板块异常进行了成像。由于数据敏感性和中地幔俯冲过程尚不清楚，在地幔过渡带的波速模型分辨率有限。第二个未解决的问题是深层地幔柱在黄石热点轨道形成中的作用。在本研究中，我们将利用USArray TA台站记录的地震数据，计算在410 km和660 km间断面产生的波的有限频率灵敏度，以成像美国大陆下的高分辨率地幔过渡带结构。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。该研究将为未来地幔过渡带不连续面成像提供坚实的基础，并使科学界能够充分利用频率相关地震数据来解决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.