A reinterpretation of regional seismic surveys in northern Ontario and eastern Manitoba between distances of 127 kilometers and 775 kilometers using seismic model studies of the crust and upper mantle

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Desmarais, Ralph John
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Regional seismic refraction and wide-angle reflection surveys conducted in Manitoba and northern Ontario in recent years have resulted in similar but differing interpretations of structure in the Earth's crust and upper mantle. The present work was an attempt to determine, from a modelling study of the kinematic and amplitude characteristics of compressionial reflected and refracted waves propagating in a spherically stratified earth, those events bottoming in the Earth's crust and upper mantle which would be most prominent on seismic records and the optimum range of observation distances at which the arrivals of these events could be observed. It was found that the existence of positive vertical velocity gradients in a crustal layer would severely reduce the maximum observation distance of primary events bottoming in this layer... The two data sets were found to conform to a single consistent crustal model, 34 km. in thickness, having an intermediate discontinuity at 18 km. with constant velocities of 6.05 km/sec. and 6.99 km./sec. respectively above and below the discontinuity. Both surveys led to the interpretation of a first-order discontinuity in the upper mantle at a depth of 50 km. with a compressional velocity of 8.17 km/sec. above the discontinuity. No unique compressional velocity was obtained for the layer below this interface. This may be attributable to lateral velocity variations over the survey area.