Abstract

The quality factor of shear waves in the crust beneath Central France is evaluated by the combined use of data analysis and theoretical modeling. The measurement is made from Lg wave records of 18 earthquakes at 22 short-period vertical seismic stations of the French seismic network. The geometrical attenuation of Lg waves has been calculated through a numerical simulation in a perfectly elastic model. The station responses (including site effects) have been evaluated from Lg waves and from records of the seismic noise. Q has been calculated for a set of 11 frequencies between 0.5 and 10 Hz and has been found to be in the form

 
Q=290*f0.52

where f is the frequency. The numerical simulation leads to the interpretation of Q of Lg waves as the mean value of Q within the crust. Theoretical and experimental decay of energy with distance are in very good agreement. The Lg waves present a different sensitivity to a low Q zone whether this zone is associated with the sediments or with scattering in the lower crust. The lower crust beneath central France appears from our experimental results to be associated with high values of Q.

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