Finite difference modelling of elastic wave propagation in the Earth’s uppermost mantle

Most models of the Earth’s upper mantle had previously assumed a homogeneous elastic structure. In contrast many seismological data sets show conclusive evidence for strong scattering. For instance scattering within the uppermost mantle is prominently documented in the so-called high-frequency teleseismic P n phase, generated by mantle velocity fluctuations. This phase, and its correspondence S n , are seen globally in data sets from active and passive seismology. We demonstrate that a wave guide, which is caused by random fluctuations of the mantle’s elastic properties can explain the main features of the teleseismic P n . We focus on the statistical properties of these fluctuations acting as scatterei. To test the hypothesis of an upper mantle scattering wave guide we calculate synthetic seismograms and compare them with observations. To compute realistic seismograms we solved the elastic wave equation numerically. Using a 2D finite difference scheme we calculate synthetic seismograms for a variety of very large models (larger than 1000 wave length). The size of the models employed and the number of time steps computed are unprecedented so far and inconceivable without modern high-performance computing. We developed and optimized an efficient code with High Performance FORTRAN (HPF) for a massive parallel computer system. We discuss the influence of the vertical and horizontal correlation length, RMS velocity fluctuations, and thickness of the heterogeneous layer on the scattering properties of the upper mantle and on the propagation mechanism of the teleseismic P n .