INTERPRETATION OF SEISMIC SURFACE-WAVES IN REGIONAL NETWORKS - JOINT ESTIMATION OF WAVE-FIELD GEOMETRY AND LOCAL PHASE-VELOCITY - METHOD AND NUMERICAL TESTS

Teleseismic surface waves in general strongly deviate from plane waves as is evident from the analysis of surface-wave data recorded with de nse networks. This causes conventional, ray-based tomographic techniqu es to break down if applied to network surface-wave data. We present a new inversion method based on the acoustic-wave equation and applicab le to vertical-component surface-wave data which successfully deals wi th non-plane wavefield geometries. The basic idea of the method is a j oint estimation of the incoming wavefield and heterogeneous structure within the network region. Crucial to the success of the method is an adequate parametrization of the incoming wavefield which is realized u sing Hermite-Gaussian basis functions. Additionally, we apply a constr aint on the wavefield parameters that expresses the idea that the samp les of the wavefield taken at the station locations should be represen tative for the wavefield in the whole network region. In this way, wav efields that show stronger fluctuations in spectral amplitude than obs erved at the stations are rejected. To represent heterogeneous structu re within the network region we use an expansion into 2-D Hermite-Gaus sian functions. Provided that the density of stations is sufficient, t he proposed method retrieves heterogeneous structure in the network re gion very well. It is not sensitive to noise or non-uniform azimuthal coverage of earthquakes. Moreover, it yields smoothed versions of the true model if the roughness of the latter has been underestimated in t he inversion. Conversely, if the true model is much smoother than anti cipated, inspection of the trade-off between model smoothness and data misfit allows us to find the correct model. The limiting factor for t he inversion is the density of stations, which must allow for a reliab le interpolation of the observed wavefield within the network. Therefo re, in order to perform regional surface-wave studies it is essential to deploy seismic stations in the region of interest itself.