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P coda evidence for a layer of anomalous velocity in the upper crust beneath Leduc, Alberta. Somerville, Paul Graham


Previous seismic studies of crustal structure using short-period P coda recorded in the vicinity of Leduc, central Alberta have indicated that serious discrepancies between the experimental observations and those based on a horizontally layered model of the crust exist in both the time and frequency domains. This 'Standard' crustal model is based on well log data for the sedimentary section, and on seismic refraction work in southern Alberta for the lower layers. The principal discrepancy lies in the large amplitude radial motion on the experimental seismograms which lags vertical arrivals by approximately two seconds. It is concluded that this radial motion, which is absent from synthetic seismograms generated using the 'Standard' crustal model, represents the generation of large-amplitude shear waves within the crust. This large radial motion is manifested in the frequency domain by experimental vertical-radial spectral ratios which are considerably lower than those computed using the 'Standard' crustal model. Using vertical-radial spectral ratios-and synthetic seismograms, a modified crustal model has been derived which gives much better agreement between experimental and theoretical results. This model involves the insertion of a layer several kilometers thick having large velocity contrast with respect to the surrounding media at the base of the Precam-brian basement (12 km deep). The new crustal model is discussed in the light of widespread evidence for a low velocity zone in the upper crust in continental regions. Several important discrepancies between experimental and synthetic seismograms remain unresolved: among these are the small onset amplitude and the character of the delayed motion of the radial component.

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