GEOMETRICAL SPREADING AND ATTENUATION OF LG WAVES - A COMPARISON BETWEEN WESTERN ANATOLIA (TURKEY) AND SOUTHERN SPAIN

We compared the attenuation and geometrical spreading coefficient of Lg waves for crustal paths in western Anatolia and southern Spain, at source-receiver distances between 80 and 400 km. We used the coda normalization method which is based on the elimination of the source and site effects from observed spectra of Lg waves. Q for Lg waves in the crust was determined as a function of frequency in the range 1.5-10.0 Hz for western Anatolia and in the range 1.5-12 Hz for southern Spain. We found that Q(Lg) shows a strong frequency dependence in both regions. In western Anatolia, the frequency dependence of Q(Lg) is Q(Lg)(f)= 82.f(1.0) and the geometrical spreading coefficient ranges from 0.6 to 0.9. We found Q(Lg) = 83.f(0.88) and a geometrical spreading coefficient close to 0.5 in southern Spain. Comparing our results with code-Q values obtained in both regions, Q(Lg) and Q(Lg) are similar. Q(Lg) is close to Q(i) in all frequency bands. We interpret the geometrical spreading coefficient higher than 0.5 for western Anatolia as due to a strong layered and heterogeneous crustal structure.