FREQUENCY-DEPENDENCE AND SPATIAL-DISTRIBUTION OF SEISMIC ATTENUATION IN FRANCE - EXPERIMENTAL RESULTS AND POSSIBLE INTERPRETATIONS

We processed digital records from 431 earthquakes at 25 stations of the Laboratoire de Detection Geophysique network in France. We computed spectral amplitudes of Pg and Lg in different group velocity windows. This large data set allowed us to compute a map of mean crustal attenuation through simultaneous consideration of source amplitude, site response and apparent Q. Our inversion procedure combines an iterative reconstruction technique with an adjustment of source amplitudes and site responses at each step. The stability of the result is tested with different subsets of data and the resolution is evaluated. Maps of apparent attenuation at frequencies between 1.5 and 10 Hz were computed from Pg, Lg and early coda of Lg. These results are in good agreement with the predictions of the single-scattering model both for frequency dependence and for Q(s):Q(p) ratios. The comparison between the attenuation anomaly found in the Variscan belt and results from a deep reflection seismic profile confirms the prominent part played by scattering in the apparent attenuation of seismic waves in the crust.