ABSTRACT. The study of the phase of reflections ofP waves off the base of Ice Streams Band C, and Ridge BC, indicates that acoustic impedances of the beds of both ice streams vary laterally. In some places, the impedance is higher than in the ice (a high-impedance bed) and in some places it is less (a low-impedance bed). The estimated impedances in a dilated bed (porosity 0.4) and in a model of the lowermost ice that takes into account the relatively low P-wave speed in ice at or very near the melting point are nearly the same. Whether the impedance in the bed is greater or less than in the ice could depend on minor changes in the nature of the sediments composing the bed, or the physical state of the bed (e.g. porosity) that could occur laterally. Lateral variations of this kind provide a ready explanation for the observations on Ice Stream B. The bed under a substantial part ofIce Stream C that exhibits a low-impedance bed also must be dilated. The evaluation of the state of the bed under the rest ofIce Stream C and on Ridge BC requires further analysis, which is in progress.

ABSTRACT. The study of the phase of reflections ofP waves off the base of Ice Streams Band C, and Ridge BC, indicates that acoustic impedances of the beds of both ice streams vary laterally. In some places, the impedance is higher than in the ice (a high-impedance bed) and in some places it is less (a low-impedance bed). The estimated impedances in a dilated bed (porosity 0.4) and in a model of the lowermost ice that takes into account the relatively low P-wave speed in ice at or very near the melting point are nearly the same. Whether the impedance in the bed is greater or less than in the ice could depend on minor changes in the nature of the sediments composing the bed, or the physical state of the bed (e.g. porosity) that could occur laterally. Lateral variations of this kind provide a ready explanation for the observations on Ice Stream B. The bed under a substantial part ofIce Stream C that exhibits a low-impedance bed also must be dilated. The evaluation of the state of the bed under the rest ofIce Stream C and on Ridge BC requires further analysis, which is in progress.

P and S‐P times from local earthquakes and P times from explosions at seismic stations in the vicinity of the 1989 Loma Prieta earthquake are inverted simultaneously to produce three‐dimensional images of Vp and Vp/Vs structure along the rupture zone. The results confirm the presence of the major heterogeneities identified in previous studies using P waves only, including high Vp along the rupture in areas of significant slip and a low‐velocity wedge along the San Andreas at the SE edge of the rupture. A change in rock properties evidenced by a significant Vp/Vs increase (equal to a 30% Poisson's ratio increase) from the middle crust to the upper crust may have been responsible for the upward termination of the main shock rupture. We speculate that the deep high‐Vp body that lies along the SE portion of the rupture zone consists of two bodies, based on model results indicating that the SW part of the body has relatively high Vp/Vs while the NE part has relatively low Vp/Vs.

First arrival times from P-wave refraction and reflection seismic surveys along Bear Creek Valley on the Oak Ridge Reservation, Tennessee, were inverted to produce refraction tomographic velocity images showing seismic velocity variations within thinly mantled karstic bedrock to a depth of approximately 20 m. Inverted velocities are consistent with two distinct bedrock groups: the Nolichucky Shale (2,730-5,150 m/s) and Maynardville Limestone (3,940-7,575 m/s). Low-velocity zones (2,700-4,000 m/s) in the tomographic images correspond to previously inferred cross-valley strike-slip faults; in places, these faults create permeability barriers that offset or block groundwater flowing along Bear Creek Valley. These faults may also force groundwater contaminants, such as dense non-aqueous phase liquids, to migrate laterally or downward, spreading contamination throughout the groundwater system. Other, previously unmapped cross-valley faults may also be visible in the tomographic images. Borehole logs suggest the low-velocity values are caused by low rigidity fractured and vuggy rock, water zones, cavities and collapse features. Surface streams, including Bear Creek, tend to lie directly above these low-velocity zones, suggesting fault and fracture control of surface drainage, in addition to the subsurface flow system. In some cases, fault zones are also associated with bedrock depressions and thicker accumulations of unconsolidated sediment.

Phases of seismic P-wave reflections from the bed of Ice Stream B ata site on its ice plain have been examined. The survey comprised a 36 km line at ashallow angle (18°) to ice movement and four 3.6 km cross lines. Reversed-phase and unreversed-phase reflections each characterize about half the bed. The corresponding zones can be correlated in stripes quasi-parallel to ice movement. We take this as support for a model previously developed that relates the zones to different types of subglacial sediment dragged along by the ice. There is also evidence for patches of pooled water.