abstract

Azimuthal radiation patterns of short-period (0.5-2.0 cps) seismic energy obtained from integrals of the seismograms from two underground nuclear explosions and two earthquakes are used to study the propagation and source characteristics of the Pg and Lg phases in the United States. In addition, the energy spectrum is divided into two bands, greater than and less than 1.4 cps, and the ratio of higher-to-lower-frequency energy is mapped to study the nature of propagation as a function of frequency. Both the total energy and the ratio show large fluctuations with azimuth and distance. However, a general correlation is found between the energy and ratio contours and the major tectonic provinces of the United States. This correlation is attributed to focusing, resulting from lateral variations in velocity and to regional differences in attenuation of the seismic energy. The range in the Q values across the United States, based on the assumption of symmetrical surface wave propagation, is from 200 to 1000, about a factor of 5. The transverse (T) component shows about the same total energy and ratio contour patterns as the vertical (Z) and longitudinal (L) components. Also, energy contour maps are similar to maps obtained using the maximum amplitudes of the Pg and Lg phases. For the events examined, it seems that the nature and distribution of tectonic features along the propagation path are more important in detemining the resultant radiation patterns than the initial conditions at the source. The particle motion at most stations does not give direct proof for the surface wave nature of the Pg and Lg phases, except that Pg tends to be longitudinal or mixed and Lg transverse or mixed.

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