Seismic Imaging of Alaska Using Spectral-Element and Adjoint Methods

本项目将应用谱元和伴随方法建立阿拉斯加地壳和上地幔的三维地震参考模型。从完整的地震波形中提取有限频率的面波和体波，以便为地震成像提供最大的分辨率。主要沉积盆地将利用现有信息，如重力数据和工业地震数据，纳入初始三维模型。一个新的地震矩张量目录将通过反演体波和面波测量地震深度和辐射模式来构建。活动构造环境以地震、大地应变速率、侵蚀和沉积的形式表现出变形。在更长的时间尺度上，这些过程创造了一个高度不均匀的地壳组成。例如，断层可能会抵消两种不同的岩石类型，或者盆地可能会打开并积聚数公里厚的沉积物层。在较短的时间尺度上，这些环境由于地壳内部的应力而经历了大量的地震。利用这些丰富的地震，地震成像和精确的数值方法可以揭示活动构造背景下的极端复杂性。谱元方法已被用于模拟复杂地质环境中的波浪传播，例如那些具有强烈地形变化和沉积盆地的环境。这些波传播方法的精度可以嵌入到伴随逆问题中，以改善地壳的地下图像。主要产品将是阿拉斯加的三维参考模型。该模型将描述阿拉斯加纵波速度、横波速度和密度的空间变化。该模型的质量在地壳中是最高的，它将为整个俯冲和碰撞环境的更大规模层析成像研究提供机会。参考模型将作为即将到来的阿拉斯加地质系统焦点以及预期的地球范围可移动阵列的框架而有用。

This project will apply spectral-element and adjoint methods to develop a three-dimensional seismic reference model for the crust and uppermost mantle of Alaska. Finite-frequency surface waves and body waves will be extracted from full seismic waveforms in order to provide the greatest resolution for seismic imaging. Major sedimentary basins will be incorporated within the initial three-dimensional models by using existing information such as gravity data and industry seismic data. A new catalog of seismic moment tensors will be constructed by inverting measurements of body waves and surface waves for earthquake depth and radiation pattern.Active tectonic settings exhibit deformation in the form of earthquakes, geodetic strain rate, erosion, and sedimentation. On longer time scales, these processes create a highly heterogeneous crustal composition. For example, faults may offset two distinct rock types, or a basin may open and accumulate kilometer-thick layers of sediment. On shorter time scales, these settings experience abundant earthquakes in response to stresses within the crust. By using these abundant earthquakes, seismic imaging with accurate numerical methods can reveal the extreme complexity within active tectonic settings. Spectral-element methods have been used to model wave propagation in complex geological settings, such as those with strong topographic variations and sedimentary basins. The accuracy of these wave propagation methods can be embedded in an adjoint inverse problem to improve subsurface images of Earth's crust. The primary product will be a three-dimensional reference model for Alaska. This model will describe the spatial variations in compressional wave speed, shear wave speed, and density in Alaska. The quality of the model will be highest in the crust, and it will provide opportunities for larger-scale tomographic studies of the entire subduction and collisional setting. The reference model will be useful as a framework for the upcoming GeoPRISMS focus in Alaska as well as the anticipated EarthScope Transportable Array.