Three-dimensional crustal structure beneath the TOR array and effects on teleseismic wavefronts

The temporary seismic station array (TOR) was designed to study the lithosp here-asthenosphere system across the northwestern part of the Trans-Europea n Suture Zone (TESZ) by teleseismic tomography. Teleseismic wavefronts, whe n propagating through complex crustal structure, undergo severe distortion that may result in travel time residual anomalies of significant amplitude. The inversion of teleseismic travel time residuals for deep structures wit hout accounting for such crustal-related anomalies may erroneously map thes e travel time anomalies into features at greater depth. In this study we ap ply a three-dimensional (3-D) technique to estimate effects of a priori kno wn 3-D crustal structure on travel times of teleseismic waves observed at t he TOR seismic array across the TESZ to correct for these effects in future tomographic studies. A uniform 3-D crustal model is developed by use of published two-dimensiona l crustal models from previous active seismic surveys. The parameterization of this 3-D crustal model is designed to adequately represent those crusta l structures that mostly influence the propagation of teleseismic wavefront s. The 3-D model includes lateral variation in velocity structure, Moho top ography, and large and deep sedimentary basins. The teleseismic forward pro blem for this local 3-D model is solved by calculation of travel times to t he base of the model using a standard whole Earth model and by subsequent p ropagation of spherical wavefronts using finite difference methods. Travel time calculations for an event near Japan reveal significant lateral variat ions in the range between -0.3 s and + 0.5 s due to crustal structures. Bei ng able to obtain the full travel time held at the surface of the model has the additional advantage of improving the identification and timing of sei smic phases observed at the TOR seismic array. (C) 1999 Elsevier Science B. V. All rights reserved.