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ON A SOURCE OF SEISMIC AMPLITUDE ANOMALIES DUE TO DILATIONAL WAVES.
GEOTECH GARLAND TX
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An idea of elastically restrained boundary is introduced for partial explanation of the amplitude anomalies customarily observed near the low-velocity, weathered layer of the earths surface. Nondimensional parameters are used as measures of restraint for particle motion within this weathered layer overlying an isotropic, homogeneous, semi-infinite solid earth. It is found that the degree of surface restraint has considerable effect on the amplitude anomaly when the nondimensional parameter for the restraint of particle motion becomes greater than unity. The value of this nondimensional parameter ranges from zero to infinity. The usual free-boundary condition corresponds to the case where the parameter becomes zero. The solution given in this paper is quite general with an arbitrary choice of Poissons ratio, angle of incidence, degree of surface restraint, and depth of instrument burial. As might be expected, the extent of the amplitude anomaly due to change in Poissons ratio is not significant for the usual teleseismic events under ordinary earth materials. The instrument burial depth has a considerable influence on signal character. Special attention is given to those cases where the average shearing stress within this weathered layer is very small. A set of P-values indicating relative properties of an earth surface is successfully assigned to two subarrays near the Tonto Forest Observatory. Author
APPROVED FOR PUBLIC RELEASE