Characterization of the Oceanic Lithosphere-Asthenosphere Boundary in the Colombian Subduction Zone through Receiver Function Analysis

该奖项全部或部分由《2021年美国救援计划法案》（公法117-2）资助。这个项目的目标是测试两种相互竞争的边界模型，将地球坚硬的外层（岩石圈）与底层较弱的层（软流层）分开。一种模型提出，这个边界是一个狭窄的通道，使岩石圈和软流圈分离。另一种模式提出，这个边界形成了一个宽的变形带，将岩石圈和软流圈耦合在一起。每一个模型都对我们理解板块构造（驱动地球长期地质演化的过程）及其与地球内部的关系具有重要意义。每个模型都预测了岩石圈和软流圈之间的边界。这些预测可以用接收函数来检验。该项目与哥伦比亚国立大学合作，使用位于哥伦比亚俯冲带上方的哥伦比亚国家地震台网地震台站的记录。在这个地区，纳斯卡海洋板块下边界的特征可以在一定深度范围内进行检查，以确定哪种模式最准确。了解哪个模型最准确，将有助于约束作用在构造板块上的力的平衡，特别是在俯冲带附近，在这些力的驱动下，潜在的破坏性地震会发生，从而对社会有益。该项目通过深化美国和哥伦比亚机构之间的国际合作，以及通过为博士后提供地震学培训来促进STEM教育，从而进一步造福社会。对海洋岩石圈-软流圈边界（LAB）的观测支持岩石圈与下垫软流圈耦合的两类相互矛盾的模式，以及相应的对板块构造的矛盾暗示。一类将边界解释为一个狭窄的、富含熔体的通道，使岩石圈板块与软流圈分离。因此，板块运动主要是由板块拉力驱动的。另一类将边界解释为板块与软流层耦合的广阔地带。因此，板块运动受到软流圈流引起的阻力的强烈影响。这些模型预测不同的结构和局部剪切沿LAB。通道模型预测两个紧密间隔，高度剪切的边界，而耦合模型预测一个小剪切的单一边界或一个高度剪切的单一边界。预测特征对俯冲引起的压力增加有不同的响应。水道随着深度的增加而变窄，耦合结构会持续到更大、不同的深度，因此观察板块俯冲的过程为检验这些模型提供了一个理想的自然实验。不同频率内容的地震波与这些结构相互作用，高频能较好地成像通道，低频能较好地成像耦合结构。该项目使用宽带地震台站记录的径向和横向分量的p - s波接收函数分析来检查结构特征和沿边界的剪切局部化。要完整地描述由剪切引起的结构和各向异性，需要从海岸到弧后的地震台站在一系列频带上进行长期观测。该项目利用哥伦比亚国家地震台网26个台站2010年至今的数据，从海岸线100公里深度到弧后400公里深度，对俯冲的纳斯卡海洋板块的LAB进行调查和表征。这些观测将对边界的深度、厚度和各向异性特性（如果存在）提供约束，并使用这些来测试所提出的模型。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

This award is funded in whole or in part under the American Rescue Plan Act of 2021 (Public Law 117-2).The goal of this project is to test two competing models of the boundary separating the Earth’s stiff outer layer (lithosphere) from the underlying, weaker layer (asthenosphere). One model proposes that this boundary is a narrow channel that decouples the lithosphere and asthenosphere. The other model proposes that this boundary forms a broad band of deformation that couples the lithosphere and asthenosphere. Each model has important implications for our understanding of plate tectonics—the process which drives the Earth’s long-term geologic evolution—and its relation to the Earth’s interior. Each model carries predictions about the boundary between the lithosphere and asthenosphere. These predictions can be tested using receiver functions. In collaboration with the Universidad Nacional de Colombia, this project uses recordings from seismic stations of the Colombian National Seismic Network located above the Colombian subduction zone. In this region, the characteristics of Nazca oceanic plate’s lower boundary can be examined over a range of depths to determine which of the models is most accurate. Understanding which model is most accurate will benefit society by helping to constrain the force balance acting on tectonic plates, especially around subduction zones where potentially destructive earthquakes driven by these forces occur. This project further benefits society by deepening international collaboration between institutions in the US and Colombia as well as furthering STEM education by providing training in seismology to a postdoctoral fellow.Observations of the oceanic lithosphere-asthenosphere boundary (LAB) support two categories of contradictory models for coupling between the lithosphere and underlying asthenosphere, and correspondingly contradictory implications for plate tectonics. One category interprets the boundary to be a narrow, melt-rich channel that decouples lithospheric plates from the asthenosphere. Plate motion is thus driven largely by slab pull. The other category interprets the boundary as a broad zone which couples plates to the asthenosphere. Plate motion is thus strongly influenced by asthenospheric flow induced drag. These models predict differing structures and localizations of shear along the LAB. Channel models predict two closely spaced, highly sheared boundaries while coupled models predict either a single boundary with little shear or a single highly sheared boundary. The predicted features respond differently to increasing pressure due to subduction. Channels narrow with depth and coupled structures persist to greater, differing depths, so observing a plate as it subducts offers an ideal natural experiment to test these models. Seismic waves with different frequency contents interact with these structures such that high frequencies can better image channels while low frequencies can better image coupled structures. This project uses P-to-S-wave receiver function analysis of radial and transverse components recorded at broadband seismic stations to examine structural characteristics and shear localization along the boundary. The complete characterization of structure and anisotropy caused by shear requires long-term observations at seismic stations from the coast to the back arc at a range of frequency bands. This project uses data from 2010 to the present from 26 stations in the Colombian National Seismic Network to investigate and characterize the subducting Nazca oceanic plate’s LAB from depths of 100 km at the coastline to 400 km in the back arc. These observations will provide constraints on the depth, thickness, and anisotropic properties (if present) of the boundary and use these to test the proposed models.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.