Seismic Anisotropy and Rock Texture within the Cascadia Megathrust Zone

两个板块的水平收敛可以通过沿着板块界面的连续滑动或通过应力积累和或多或少的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累积，如果可以，会聚集在板块的哪一部分。板块接触能够积累应变并突然释放应变的区域决定了潜在地震的大小。该项目探索了卡斯卡迪亚巨型逆冲--下沉的大洋胡安德富卡板块和俯冲的北美板块之间的界面--两侧岩石的性质。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相关性(各向异性)来确定岩石内部的变形感。后者通常被认为是由于地幔岩石的变形而产生的，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在板块交界处广泛存在。这类岩石由于其不同寻常的地震性质而在深部可探测到，比地幔中的其他岩石弱得多。它被认为促进了板块界面的滑动，并限制了在大地震中可能滑动的区域的范围。来自远距离地震的地震波记录形成了这项研究使用的数据。它们来自北美西海岸的永久性和便携式地震观测站，包括由EarthScope项目的USArray部分和不列颠哥伦比亚省的Polaris项目部署的统一的地震站网格。地震性质在深度上的突然变化，如与板块界面有关的变化，会扰乱穿过它们的地震波。这些扰动在各个地震观测站被隔离和检查，然后沿着整个太平洋海岸进行比较，产生对选定地点的特定属性及其在整个区域的变化的限制。

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.