Off-great-circle propagation of intermediate-period surface waves observed on a dense array in the French Alps

Array analysis is performed on surface waves recorded in the French Alps using a small-aperture (25 km) temporary array of six broad-band stations. The analysis shows that both Rayleigh and Love waves deviate relative to the great-circle path. The deviations are particularly strong, up to 30 degrees, between 20 and 40 s period. To interpret these observations, we first study the effect of large-scale structures using ray tracing in a smooth, laterally heterogeneous model of the Earth. Second, we evaluate the local effect by considering a model for the French Alps including strong lateral heterogeneities around the array that were not taken into account in the ray tracing. By combining these two possible causes of the observed deviations, we propose an explanation for the general trend in the observed deviations. Finally, we show that by taking into account azimuthal deviations, phase velocities measured at a regional scale can be significantly improved.