STUDIES OF THE EFFECT OF DEPTH OF FOCUS ON SEISMIC WAVES.
Special technical rept.,
PENNSYLVANIA STATE UNIV UNIVERSITY PARK DEPT OF GEOLOGY AND GEOPHYSICS
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The short-period vertical seismograms of 37 stations for the Peruvian Earthquake of April 13, 1963 were Fourier-analyzed for a time duration of 10 plus or minus 0.5 sec. After proper instrumental corrections, the 37 frequency spectra were found to be quite similar in their overall shape, but very different in their finer structure. The similarity of the envelope appears to be most probably associated with the source while the crustal effect could account for the differences in the finer spectral structure. By using theoretical calculations based on the Thompson-Haskell matrix formulation, the crustal effect of any layered system on an incoming dilatational wave can be obtained. It was found that the distance affects mostly the level of the spectrum, and not its slope and character. The azimuthal effect manifests itself by a change in the slope of the spectral envelope and a shift of the cut-off frequency. The crustal effect appears to be so considerable that the other effects are impossible to measure accurately until the crustal effect has been removed by inverse-filtering. Author