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The effect of focal depth on the spectra of P waves II. Observational studies

DEPARTMENT OF EARTH AND ATMOSPHERIC SCIENCES SAINT LOUIS UNIVERSITY, ST. LOUIS, MISSOURI 63156

Abstract

Using the theoretical development of Part I, properties of transfer functions (radiation patterns) for P waves are investigated for different point sources as functions of depth of burial and orientation of the source in a layered crust. The calculated transfer functions, in general, exhibit two or more minima. These minima shift to longer periods as the source depth increases. The position and relative development of the minima are also functions of depth, of azimuth about the source, and of orientation of the source. In order to investigate whether the predicted effect of focal depth is identifiable in the amplitude spectra of recorded P waves, seismograms of four earthquakes are analyzed. It is found that some of the minima of the observed spectra are due to focal depth, and that for foci in the middle or lower crust, it is possible to determine the depth of focus of an earthquake from the positions of the minima in the spectra.

Footnotes

^* Now with Mathematics Department, Texas Southern University, Houston, Texas 77004.