Seismic Anisotropy and Rock Texture within the Cascadia Megathrust Zone

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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.