Surface flow in a 10 000 km 2 expanse of the onset area of Ice Stream D, West Antarctica, was measured by repeat, precise global positioning system surveys over a 1year interval. The pattern of velocity and strain rate shows the development of Ice Stream D, the major flow into which originates south of Byrd station and follows the course of a deep bed channel. Plotting of the driving stress vs the ratio of velocity and ice thickness identifies the onset of streaming flow (roughly 140 km downstream of Byrd station) as a transition between deformation flow and sliding flow. Along the kinematic center line of the developing ice stream, the ice rheology is linear at stresses below 0.6 bar, and appears temperate at the base well before the onset of streaming is reached. The onset corresponds to a maximum driving stress of 0.8 bar. It occurs downstream of a slight increase in longitudinal strain rate where stronger along-flow lineations are apparent in Landsat imagery, and after the ice has passed the center of an overdeepening in the bed channel. No current deviation from equilibrium is detected in this region, but a set of flow stripes misaligned with present flow indicates significant changes in flow have occurred in the past.

Summary A semi-analytical treatment of the seismic response of an arbitrary number of semi-circular viscoelastic alluvial valleys (including an infinite number of periodically distributed ones) in an elastic half-space under obliquely incident plane SH waves is presented. In terms of a radial wave function expansion and a region matching approach, a rigorous solution is derived for general computation for the anti-plane shear wave scattering problem. Upon confirmation of its accuracy with past solutions for single and twin elastic alluvial valleys, the proposed solution is used to present a comprehensive set of numerical examples to illustrate the sensitivity of the ground motion to the size, distribution, modulus, damping, and number of multiple alluvial valleys and frequency and direction of incident waves. It is found that the magnitudes of ground motions of a central alluvial valley are intensified and reduced by the surrounding valleys for vertical and grazing wave incidences, respectively. In addition, the duration of ground motions of the central alluvial valley tends to be prolonged due to the incoming secondary waves generated by surrounding valleys.

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