WAVE-THEORETICAL INVERSION OF TELESEISMIC SURFACE-WAVES IN A REGIONALNETWORK - PHASE-VELOCITY MAPS AND A 3-DIMENSIONAL UPPER-MANTLE SHEAR-WAVE-VELOCITY MODEL FOR SOUTHERN GERMANY

Using teleseimic surface-wave data from 110 selected earthquakes recor ded at 10 broad-band stations in southern Germany we construct phase-v elocity maps of Rayleigh waves for southern Germany. In a further step these maps are inverted for a three-dimensional model of the SV veloc ity of the upper 200 km of the mantle. We attempt to lake into account the effect of heterogeneous structure outside the study region by joi ntly inverting the data for heterogeneous phase velocity inside the st udy region and the distortion of the incoming wavefields by the surrou nding structure. The total wavefield in the study region is computed w ith a scattering formalism which includes multiple forward scattering and single backscattering. Since, in principle, the data can be perfec tly fit by the incoming wavefields without any heterogeneous structure inside the study area, we impose additional constraints on the incomi ng wavefields to reduce the non-uniqueness. The most important constra int is an energy criterion which states that the energy of the modelle d wavefield in the study area, averaged over many events, should be eq ual to the energy sampled by the stations. We demonstrate that enforci ng this criterion generates phase-velocity maps with heterogeneous str ucture. Nevertheless, we are able to satisfy the energy criterion with out any heterogeneous structure at the price of an only slightly incre ased data misfit. Hence, it must be concluded that a seismic network o f size and station density such as the one used in this study is still insufficient to demonstrate convincingly the existence of heterogenei ties in the network area using teleseismic surface waves. Any reasonab le structure combined with the appropriate incoming wavefields would a llow an acceptable fit of our data. This frustrating conclusion, of co urse, applies to all other comparable studies which use teleseismic su rface waves. Although we cannot convincingly show that any phase-veloc ity map we find should be preferred over others, we are able to obtain good reconstructions of test structures from realistic synthetic data with the same station and event distribution as the real data. Moreov er, we find that the geometric pattern of the phase-velocity maps obta ined from real data depends only weakly on the constraints applied in the inversion, while the amplitude of the phase-velocity perturbations is almost completely determined by the constraints. For all periods c onsidered the fit to the data is extremely good. The reduction of the quadratic misfit relative to the case of plane incoming waves and no s tructure is dramatic for the shorter periods. The 3-D model of vertica l shear-wave velocity down to a depth of 200 km exhibits a basic divis ion into four quadrants separated by a vertical plane intersecting the surface along a nearly west-east line and a horizontal plane at about 130 km depth. The northern quadrants show high velocities in the top 120 km and low velocities below 140 km. The opposite is the case for t he southern quadrants. An exception to this general feature is a prono unced low-velocity zone in the northwestern corner of the region.