Laterally heterogeneous scattering explains Lg blockage in the Pyrenees

Abstract : We present an algorithm to simulate the propagation of seismic energy in a model that contains both deterministic structure and statistically described small-scale heterogeneities. With this algorithm, we study the propagation of seismic waves through the Pyrenees. Using a series of seismograms, the variations of the attenuation properties of the crust along the Pyrenean axis are illustrated. The western Pyrenees exhibit strong attenuation of crustal phases, a phenomenon known as Lg blockage. Previous studies demonstrated that the large-scale velocity structure of the western Pyrenees cannot explain the strong lateral variations of attenuation along the Pyrenean range. In this paper we present a model that is able to explain the principal observations. We propose that the Lg blockage is caused by a perturbed region of the crust with scattering and anelastic properties that differ markedly from the surrounding crust. Numerical simulations of the elastic radiative transfer equation in a layered medium show that it is possible to model the observed Lg blockage with increased scattering in the perturbed body. In order to explore the relative importance of scattering and attenuation we performed an inversion of the data using a genetic algorithm. Our study reveals that stronger intrinsic attenuation in the perturbed body improves the fit to the data but that intrinsic attenuation alone cannot explain the observations. Our work puts forward the role of multiple scattering in the attenuation mechanism of crustal phases.