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

The temporary seismic station array (TOR) was designed to study the lithosphere-asthenosphere system across the northwestern part of the Trans-European Suture Zone (TESZ) by teleseismic tomography. Teleseismic wavefronts, when 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 without accounting for such crustal-related anomalies may erroneously map these travel time anomalies into features at greater depth. In this study we apply a three-dimensional (3-D) technique to estimate effects of a priori known 3-D crustal structure on travel times of teleseismic waves observed at the 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-dimensional crustal models from previous active seismic surveys. The parameterization of this 3-D crustal model is designed to adequately represent those crustal structures that mostly influence the propagation of teleseismic wavefronts. The 3-D model includes lateral variation in velocity structure, Moho topography, and large and deep sedimentary basins. The teleseismic forward problem for this local 3-D model is solved by calculation of travel times to the base of the model using a standard whole Earth model and by subsequent propagation of spherical wavefronts using finite difference methods. Travel time calculations for an event near Japan reveal significant lateral variations in the range between -0.3 s and +0.5 s due to crustal structures. Being able to obtain the full travel time field at the surface of the model has the additional advantage of improving the identification and timing of seismic phases observed at the TOR seismic array. (C) 1999 Elsevier Science B.V. All rights reserved.