Earthquake Characteristics and Earthquake-Explosion Discrimination
Semi-Annual technical rept. no. 6, 1 May-31 Oct 1977
COOPERATIVE INST FOR RESEARCH IN ENVIRONMENTAL SCIENCE BOULDER CO
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Two major advances have been made in the research on synthesis of long-period S waves at distances less than 40 degrees the incorporation of inelastic attenuation and the development of a code to include a representation of the source function. The large effects on the waveforms of rapid variations of velocity in the upper mantle can be calculated and separated from the effects of anelasticity. Examples of synthetic seismograms are included to illustrate the effects of attenuation on the displacement waveforms, as well as the effects of the response of a typical long-period seismograph system. The application of Greens Function techniques to elastodynamics has led to methods for treating a variety of problems in wave propagation and earthquake source representations. The complete theory is given. There is evidence that specific anelastic attenuation, expressed as Q, is frequency dependent. Q models of the Earth based on free oscillations, surface waves and body waves show that in the broad frequency band covered by the input data, Q increases with frequency. The important conclusions is that the P and S attenuation data can only be reconciled by including a bulk loss mechanism, in addition to a shear loss mechanism. Although the results are not unique, this suggests that the bulk loss mechanism is operative in the upper mantle, perhaps within the asthenosphere.