Computations of SV waves in realistic Earth models
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Abstract
The reflectivity method for the calculation of theoretical body-wave seismograms is extended to include a double-couple point source. Theoretical seismograms of SV waves from this type of source are presented for models of the Earth's crust and the crust-mantle boundary, and for models of the whole Earth. In the models of the crust-mantle boundary, there are up to four SV head waves, depending on the sharpness of the transition. The most remarkable one is slightly slower and later than Sn and has unusually low frequencies. Theoretical SV-wave seismograms for models of the whole Earth for periods from 15 s to 60 s and in the epicentral distance range from 10° to 160° show as prominent phases S, ScS, SKS and SKKS. SKS, and SKKS are different in wave form, in agreement with observations from long-period WWNSS stations. A diffracted wave SPdiffKS + SKPdiffS is found in the theoretical seismograms whose travel-time curve is tangential to that of SKS at a distance of 107 °. The resulting interference causes the wave form of SKS to change markedly around 120°. Since the theoretical seismograms are the complete response of the Earth models from the crust-mantle boundary down to the inner core, they also include many multiple and converted phases. Besides those associated with the crust-mantle boundary there are phases related to the transition zones in the upper mantle. Their amplitudes depend strongly on the sharpness of these zones.
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