HR: 1340h
AN: T33A-1341    [Abstracts]
TI: Velocity and Attenuation Structure beneath the Southeastern Carpathians
AU: * Weidle, C
EM: christian.weidle@gpi.uni-karlsruhe.de
AF: Geophysical Institute, Karlsruhe University, Hertzstr. 16 Geb. 6.42, Karlsruhe, 76187 Germany
AB: One of the seismically most active regions in Europe is situated at the southeastern bend of the Carpathian Arc (Vrancea region). Seismologically it is intriguing that the seismicity pattern is dominated by an intermediate-depth, finger-shaped volume with recurring strong earthquakes. From the tectonic evolution of the Pannonian-Carpathian subduction system we know that continental collision (followed by slab detachment) started in the northernmost part of the Carpathians and migrated towards the southeastern bend where the slab seems to be still coupled to the overlying crust. For our studies we used teleseismic P-wave data recorded during the CALIXTO-Experiment. Simultaneous inversion of P-wave traveltimes from the CALIXTO data and a global dataset of ISC- and NEIC-data provides a large-scale, high-resolution image of the P-wave velocity distribution in the upper mantle. The tomographic imaging technique includes implicit correction of the residuals for crustal structure, a variable parameterization, 3-D Ray Tracing and a nonlinear, iterative inversion scheme. Our results show an extended, well-resolved volume of increased P-wave velocity outlining the seismic active volume but extending down to a depth of approximately 280 km beneath the Vrancea region. The hypocenters are distributed within this high-velocity body, which points to stress accumulation induced by gravitational sinking. A low velocity anomaly is mapped northwest and west of the Vrancea region down to a depth of 200 km coinciding with geological constraints of Neogene-Quarternary volcanism. The analysis of the attenuation structure was performed using multiple taper spectral analysis. The slope of linear regression to the fall-off of the spectra is direct proportional to the amplitude absorption (t$^\ast$-Operator). Calculation of t$^\ast$-Operators from teleseismic CALIXTO data indicates that the largest t$^\ast$-values are observed at the bend of the Carpathians, in the mountains as well as in the foreland. It is complicated to interpret the distribution of t$^\ast$ values since observed attenuation parameters include both intrinsic and scattering attenuation effects. Synthetic t$^\ast$-operators can be modeled by calculation of a Q-model based on P-wave velocity structure and thus allow to give a guess on the intrinsic fraction of the observed attenuation. For the Carpathians we find that approximately 75% of the observed attenuation of teleseismic P-waves is induced by scattering, predominantly from complex boundary topographies and heterogeneities in the crust.
DE: 8180 Tomography
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 7218 Lithosphere and upper mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting