High-resolution attenuation structure from the ambient seismic field

要更好地了解美国大陆岩石圈的结构和构造历史，在很大程度上依赖于从地表到数百公里的高分辨率图像。 传统上，这些图像是从地震激发的地震波记录中获得的。在过去的几年里，科学家们已经开始使用环境地震场（ASF）进行高分辨率层析成像。与地震源相反，ASF是一种几乎无法察觉的、无处不在的信号，被世界各地的地震台站记录，因此为断层成像提供了理想的源。该建议将利用ASF在美国大陆进行地震层析成像，但除了典型的速度结构外，还将估计衰减结构。通过结合这两个观测量（速度和衰减），可以获得对岩石圈热结构和化学结构的关键约束。我们建议使用虚拟地震方法（VEA）的速度和衰减层析成像。该方法已被证明对加州南部和日本未来大地震的盆地放大分析和地面运动预测是非常有用的。VEA将一个远距离地震台站视为虚拟震源，通过计算感兴趣区域内密集地震台阵的脉冲响应函数（IRF），可以估计基面波的到达时间和相对振幅。通过这种方法得到的波形类似于传统的基于地震的方法，其优点是不受地震数量和位置的限制。我们的速度和衰减图像的分辨率将取决于台站密度而不是地震位置。VEA方法的选择提供了额外的优势，包括更完整的理论框架来表征振幅，缺乏方位平均，以及由于ASF源分布和多相（体波和更高模式表面波）而减少的潜在偏差。考虑弹性聚焦和散焦与基于地震的方法所需的相似。拟议的研究将利用ASF提供的大量信息，并利用地震台站沿着美国大陆的分布来限制岩石圈的速度和衰减结构，例如比较美国西部和东部的衰减，或比较构造活跃的加州和新马德里地震带之间的衰减。

A better understanding of the structure and tectonic history of the lithosphere in the continental US relies heavily in high-resolution images from the surface down to hundreds of kilometers. Traditionally, these images are obtained from records of seismic waves excited by earthquakes. In the last few years, scientists have started using the ambient seismic field (ASF) for high-resolution tomographic imaging. Contrary to earthquake sources, the ASF is an almost imperceptible, ubiquitous signal that is recorded by seismic stations all around the world, and thus provides an ideal source for tomographic imaging. This proposal will take advantage of the ASF to perform seismic tomography in the continental US, but in addition to the typical velocity structure, the attenuation structure will also be estimated. Key constraints on the thermal and chemical structure of the lithosphere can be obtained by combining these two observables (velocity and attenuation). We propose to use a Virtual Earthquake Approach (VEA) for velocity and attenuation tomography. This method has proved very useful for basin amplification analysis and ground motion prediction of future large earthquakes in southern California and Japan. The VEA treats one distant seismic station as a virtual source, and both the arrival times and relative amplitudes of fundamental surface waves can be estimated by computing the impulse response functions (IRF) with respect to a dense seismic array in a region of interest. The resulting waveforms obtained thru this approach are similar to traditional earthquake-based methods with the advantage of not being limited by the number and location of earthquakes. The resolution of our velocity and attenuation images will depend on station density rather than earthquake location. The choice of the VEA approach provides additional advantages, including a more complete theoretical framework to characterize the amplitudes, the lack of azimuthal averaging and a reduction of the potential bias due to ASF source distribution and multiple phases (body waves and higher mode surface waves). Accounting for elastic focusing and defocusing is similar to what is required for earthquake-based methods. The proposed research will exploit the vast information available from the ASF and take advantage of the distribution of seismic stations along the continental US to constrain the velocity and attenuation structure of the lithosphere, for example comparing attenuation in the Western and Eastern US, or between tectonically active California and the New Madrid Seismic Zone.