ATTENUATION DES ONDES SISMIQUES EN BELGIQUE ET DANS LES RÉGIONS LIMITROPHES À PARTIR DE LA CODA DES TREMBLEMENTS DE TERRE LOCAUX.
Observatoire Royal de Belgique, 3, Avenue Circulaire, B-l180 Bruxelles, Belgique
Observatoire Royal de Belgique, 3, Avenue Circulaire, B-l180 Bruxelles, Belgique
Observatoire Royal de Belgique, 3, Avenue Circulaire, B-l180 Bruxelles, Belgique
Abstract
ABSTRACT. Attenuation of seismic waves in Belgium and surrounding regions from coda waves of local earthquakes. The attenuation of shear (S) waves in the crust, expressed by the quality factor Q, in Belgium and surrounding regions has been estimated by the analysis of the decay rate of the coda amplitude of 86 local earthquakes that were recorded by the stations of the Belgian seismic network during the period 1985-1992.
The study of coda waves was limited to a relatively short lapse time (£ 30 s) in order to sample the earth's crust only. Over the broad frequency range 1 - 32 Hz, Q (coda-Q) shows a clear frequency dependency according to a power law, Qc = Qof ' where f is the frequency. For the different regions studied, we obtained the following results :
Hainaut Qc=19.5f1.54
Dour Qc = 26.5 f1.07
Hautes Fagnes Qc =17.0 f1.16
Ardenne Qc. = 21.2 f1.37'
Lower Rhin Graben-Limbourg Qc= 26.9 f1.12'
Plate-Taille Qc= 30.9 f1.19
The values obtained at 1 Hz are very low (17-31). Assuming that the attenuation is entirely due to the scattering loss, the mean free path of S waves is about 30 km in the crust. Considering larger lapse time, the Q, values at 1 Hz increase and reach a mean value of 140 for lapse time betwen 40 and 60 s. This is interpreted as due to attenuation variations of S waves with depth.