Frequency Dependence and Spatial Distribution of Seismic Attenuation in France: Experimental Results and Possible Interpretations
Rept. for 15 Jul 1988-14 Jul 1989
RADIOMANA PARIS (FRANCE)
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We processed digital records from 431 earthquakes at 25 stations of LDG network in France. We computed spectral amplitudes of Pg and Lg in different group velocity windows. We used this large data set to compute a map of mean crustal attenuation by evaluating simultaneously source amplitude, site response and apparent Q. We used a procedure combining an iterative reconstruction technique with an adjustment of source amplitudes and site responses at each step of the inversion. The stability of the results is tested with different subsets of data and the resolution is evaluated. We obtained maps of apparent attenuation at frequencies between 1.5 and 10 Hz, computed from Pg, Lg and early coda of Lg. The results obtained are in a good agreement with the predictions of the single scattering model both for frequency dependence and Qs-Qp ratio. The comparison between the attenuation anomaly found in the Variscan belt and a deep reflexion seismic profile confirms the prominent part played by scattering for the apparent attenuation of seismic waves in the crust. Keywords Seismic datareflection.