Approximation of surface wave mode conversion at a passive continental margin by a mode-matching technique

Abstract

At a continental margin sharp structural changes cause strong lateral heterogeneity in S-wave velocity, P-wave velocity and density. Therefore, mode conversion of surface wave modes is expected. Passive continental margins may be modelled by vertical discontinuities. Mode conversion is then expressed in terms of reflection and transmission coefficients. Long-period seismograms are calculated by mode summation over incident, reflected and transmitted modes. Reflection and transmission coefficients for surface waves are approximated by a mode-matching technique that is adapted to a spherical earth model. An orthogonality relation for surface wave eigenfunctions on a spherical earth is given that allows one to simplify the linear system of equations for the determination of reflection and transmission coefficients. Transmission of fundamental Love and Rayleigh modes is strongly influenced by upper crustal heterogeneity for frequencies larger than 40 Hz for fundamental Love modes and larger than 60 Hz for fundamental Rayleigh modes. Mode conversion to neighbouring modes can be strong but mode conversion is not necessarily confined to neighbouring modes. Fundamental modes in particular can couple with more distant higher modes. In synthetic seismograms calculated for epicentral distances up to 40°, mode conversion between higher modes and fundamental modes results in shear-coupled Rayleigh waves.