Investigation of Seismic Velocity Transition Zones and Application to the Moho in the Western United States.
Technical rept. no. 1, 1 May 74-31 May 75,
PURDUE UNIV LAFAYETTE IND DEPT OF GEOSCIENCES
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The ratio of the spectrum of seismic waves reflected at wide angle from a velocity transition zone to the spectrum of waves refracted just below the zone is a sensitive indicator of the velocity structure of the transition zone or the presence of a positive velocity gradient just below the zone. Calculation of the spectral ratio for wide-angle reflected and refracted phases from a single seismogram removes the effects of the source function, instrument and near surface site responses. Theoretical spectral ratios have been determined in the range of 1 to 20 hz from synthetic seismograms calculated for several velocity transition zones representing possible models of the Mohorovicic discontinuity. The ratios are similar at low frequencies but vary widely at higher frequencies and may be used to infer fine structure and the depth extent of the transition zone. Observed seismograms from the Basin and Range province of the western United States, which display prominent refractions Pn and wide angle reflections PmP from the Moho, have been investigated using the spectral ratio method.