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LABORATOIRE DE GÉOPHYSIQUE INTERNE ERA CNRS 603 UNIVERSITÉ SCIENTIFIQUE ET MEDICALE DE GRENOBLE, BP 68, 38402 SAINT MARTIN D'HERES CEDEX, France
LABORATOIRES DE DETECTION ET DE GÉOPHYSIQUE COMMISSARIAT A L'ENERGIE ATOMIQUE, BP 136, 92124 MONTOUGE CEDEX, France
Abstract
We present a theoretical study of the generation and geometrical attenuation of regional crustal phases. We do this through the computation of seismograms in the epicentral distance range from 60 to 500 km. The geometrical attenuation of Lg waves with epicentral distance is of the form r–0.83. Pg wave amplitudes display a much stronger decay of the form r–1.5. The spectral density of the crustal transfer function for Pg waves is relatively flat for frequencies between 0.1 and 5 Hz while Lg wave spectra strongly fall off beyond 2 to 3 Hz.
The excitation of Pg wave is insensitive to the depth of the source within the crust while the Lg amplitude is about 50 per cent higher for a source in the upper and middle crust than in the lower crust. The amplitudes of these two phases drastically decrease when the source is below the Moho. These results illustrate the important role of wave guide played by the crust for the propagation of Lg and Pg.
We find that the geometrical attenuation of Pg and Lg waves is independent of source mechanisms. In the case of an explosion, the excitation of Pg is insensitive to the source depth. The Lg wave amplitude is small in comparison to Pg and Rayleigh waves and depends on the closeness of the source to an interface or to the free surface.
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