Seismic Anisotropy and Rock Texture within the Cascadia Megathrust Zone

0545698 Levin两块接骨板的会聚可通过沿接骨板界面连续沿着滑动或通过应力累积和或多或少突然释放的循环来调节。界面上岩石的性质控制着这种行为，决定了板块运动产生的应变是否会积累，如果是这样，板块接触的区域能够积累应变并突然释放应变，这决定了潜在地震的规模。本项目探索了卡斯卡迪亚巨型逆冲断层（下地壳与下地壳之间的界面）的岩石性质，胡安·德富卡大洋板块和北美板块。它试图通过研究岩石的地震波速度来确定岩石的性质，并通过检测和表征地震波速度的方向依赖性（各向异性）来确定岩石内部的变形感。后一种性质通常被认为是由地幔岩石的变形引起的，因此提供了一种测试构造运动情景的手段。该项目的主要目标是确定被称为蛇纹岩的特定类型的岩石是否广泛分布在板块界面区域。这种类型的岩石，由于其不寻常的地震特性，在深度上可以检测到，比地幔中的其他岩石要弱得多。它被认为可以促进板块界面的滑动，并限制大地震中可能滑动的区域范围。 来自遥远地震的地震波记录形成了本研究中使用的数据。它们来自北美西海岸沿着的永久性和便携式地震观测站，包括地球镜项目的美国阵列组成部分部署的统一地震站网格，以及不列颠哥伦比亚省的POLARIS项目。地震属性在深度上的突然变化，就像与板块界面相关的变化一样，会扰动穿过它们的地震波。 这些扰动被隔离，并在各个地震观测站进行检查，然后沿整个太平洋海岸进行沿着比较，从而对选定地点的特定属性及其在整个区域的变化产生约束。

0545698LevinConvergence of two plates may be accommodated by continuous sliding along the plate interface or through cycles of stress accumulation and more or less abrupt release. Properties of rocks lining the interface control this behavior, determining whether strain from plate motion can accumulate at all, and if so, over which part of it. The area of the plate contact capable of accumulating strain and releasing it abruptly determines the size of potential earthquakes.This project explores the nature of rocks lining the Cascadian megathrust - the interface between the down-going oceanic Juan de Fuca plate and the over-riding North American plate. It seeks to determine rock properties through a study of their seismic wave velocities, and the sense of deformation within them through the detection and characterization of the directional dependence (anisotropy) of seismic wave speed. The later property is commonly thought to arise from deformation of mantle rocks, and thus provides a means to test tectonic motion scenarios. The main goal of the project is to determine if specific type of rock called serpentinite is widespread in the plate interface region. This type of rock, detectable at depth because of its unusual seismic properties, is much weaker then the other rocks in the mantle. It is thought to promote sliding on the plate interface and to limit the extent of the area likely to slip in large earthquakes. Records of seismic waves arriving from distant earthquakes form the data used in this study. They come from permanent and portable seismic observatories along the western coast of North America, including the uniform grid of seismic stations deployed by the USArray component of the EarthScope project, and the POLARIS project in British Columbia. Abrupt changes in seismic properties at depth, like the one associated with the plate interface, perturb seismic waves traveling through them. These perturbations are isolated and examined at individual seismic observatories, and then compared along the entire Pacific coast, yielding constraints on specific properties at chosen locations, and on their variation across the region.