Influence of subduction zone structure on coastal and inland attenuation in Mexico

Abstract

Although it has been suggested in the past that the attenuation of seismic waves along the Pacific coast of Mexico and towards the continent differs, the subject has been controversial and, as yet, unresolved. Here we re-examine this issue using more extensive and higher quality data now available. Our analysis of more than 450 local and regional (R < 400 km) records from interplate earthquakes (5.0 <= M-w <= 8.0

10 <= H < 30 km) shows that the Fourier spectral amplitudes at inland sites exceed those at coastal ones, especially in the frequency range of 0.3-2 Hz. We find that site or source effects are not the cause of this difference. The observed disparity of amplitudes is in fact a consequence of the efficient propagation of Lg and SmS phases along the convergence direction, as it is confirmed by numerical modelling. The simulations show that the energy radiated by interplate earthquakes is efficiently guided towards the continent by the wedge-shape geometry and the structure of the subduction zone, thus enhancing the inland wavefield beyond 100-150 km. On the contrary, for coastal paths there is a rapid drop of amplitude due to lateral heterogeneity and low crustal thickness. Corresponding to geometrical spreading terms of [GRAPHICS] and [GRAPHICS] for trajectories along the coast and towards the continent, the Q(f) of the S-wave group is approximately given by 175f(0.52) and 211f(0.46), respectively. Separate attenuation functions for the two types of trajectories improve the model currently used in estimating Fourier amplitude spectrum of ground motion, and thus, may be useful in a more realistic evaluation of the seismic hazard in the Mexican forearc region.