Improving depth resolution of teleseismic tomography by simultaneous inversion of teleseismic and global P-wave traveltime data -: application to the Vrancea region in Southeastern Europe

Over the years, teleseismic tomography has developed to be a sophisticated method to study the Earth's upper mantle on a regional scale. Using data from tomographic experiments with temporary station networks, one faces some inherent problems, which include limited resolution at depth and artefacts due to a plane-wave approximation at the bottom of the model volume. Simultaneous inversion of dense regionally recorded teleseismic and global P-wave traveltime data provides an opportunity to overcome these specific problems. The calculation of the entire ray path using a 3-D ray tracing algorithm and a non-linear iterative inversion scheme allow to localize heterogeneities in the Earth's mantle and to improve resolution at depth. Application of a variable parametrization scheme provides not only a regional high-resolution model but additionally allows to include a priori constrained structures such as a crustal model derived from independent studies. We investigated the effect of different inversion strategies for a priori constrained model parameters and found that, for upper-mantle studies, one must allow further perturbation of the known velocity structure during inversion to avoid artefacts down to the mantle transition zone. We apply this approach to the Romanian Vrancea region in Southeastern Europe. The results show a near-vertical, narrow high-velocity body underneath that region extending down to 280 km depth, approximately outlining the narrowly spaced seismogenic volume and a deeper, differently oriented positive anomaly coupled to the shallower segment at the latter's southwestern edge. At north, northwest and west of the Vrancea region, we find an extended region of decreased seismic P-wave velocity down to 200 km depth, being interpreted as a shallow lithosphere-asthenosphere boundary and asthenospheric mantle flow due to lateral migration of the high-velocity body. From synthetic reconstruction tests, we found that inversion of the combined data set of regionally recorded teleseismic and global traveltime data enhances resolution up to depths greater than could be resolved by the teleseismic data alone.