Strong crustal heterogeneity in the Bolivian Altiplano as suggested by attenuation of Lg waves

Using the seismological data of four temporary networks installed across the Andes of Bolivia and Chile between 1990 and 1995, we study the amplitudes of the Lg phase for 45 regional events located at crustal depths. This analysis reveals that Lg is strongly attenuated for most of the ray paths crossing the Altiplano independent of the azimuth of propagation. To map the anomalies and evaluate their frequency dependence, we conduct a regionalization of the crustal average Q(s) factor using a damped least squares inversion technique in the frequency range 0.6-5 Hz. The resulting maps delineate regions with contrasting values of apparent attenuation. The Altiplano is characterized by strong attenuation with an average Q(s) of 100 at 1 Hz, whereas the Cordilleras are regions of weaker attenuation. The frequency dependence of Q(s) is investigated in an attempt to separate the source of the apparent attenuation into scattering and anelastic absorption. Within the hypothesis of a weak frequency dependence of the intrinsic Q, we show that at 1 Hz, scattering plays a major part in the attenuation of Lg waves whereas at 4 Hz, both mechanisms are of equal importance. These results indicate that the attenuation of Lg waves in the Altiplano is due more to scattering by small-scale heterogeneities than to a high percentage (> 1%) of partial melt involving a large part of the crust.