Mantle Structures Beneath USArray

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One of the grand challenges in seismology is to develop a three-dimensional seismic velocity field of the upper-mantle beneath the various plates and at their boundaries. It is at these depths where the mechanically strong outer lithosphere layer transitions into the relatively soft asthenosphere which deforms and flows to produce plate motions. The continental lithosphere beneath old cratons is cold and appears to be depleted in heavy minerals such as iron. Thus, they remain relatively stable. The objective of this study is to sharpen existing tomographic images to provide a basis for high-resolution dynamic modeling of the plate motions. With the advent of USArray, there is an unprecedented opportunity for investigating the transition from the Pacific Basin to the North American Craton. This data set displays a great deal of P and S body waveform complexity depending on direction of arrivals both along the Coastal Ranges and the Rocky Mountain Front. These features can be explained by multi-pathing where there are two paths to reach the station, one traveling along a fast path relative to a slow path. A Multi-Path Detector (MPD) processing technique has been developed to map out the sharpness of structures such as down-going slabs and plume-like features. Many of these structures can be seen in existing tomographic images and only require some sharpening to produce synthetic seismograms matching these complex waveforms. The North American Craton in contrast appears relatively one-dimensional in nature but with complicated vertical layering, at least for the upper-mantle transition zone. These modeling results of the upper-mantle fine structure will be followed by a second stage effort of constructing a detailed Trans-Pacific-Craton corridor for the entire mantle.