S-wave attenuation characteristics beneath the Vrancea region in Romania: New insights from the inversion of ground-motion spectra

The S-wave attenuation characteristics beneath the Vrancea region in Romania are analyzed from the spectra (frequency range 0.5-20 Hz) of more than 850 recordings at 43 accelerometric stations of 55 intermediate-depth earthquakes (M 4.0-7.1) that occurred in the Vrancea seismogenic zone. The method commonly chosen for this type of investigation in the case of crustal earthquakes is the generalized inversion technique (GIT) (e. g., Andrews, 1986; Castro et al., 1990). Yet the Vrancea dataset is entirely different from common crustal datasets. Because of the strong clustering of the hypocenters within a very small focal volume, there are only few crossing ray paths from sources to receivers. As a consequence, inhomogeneities in the attenuation properties are not averaged out, which leads to unphysical results if the standard GIT approach is adopted. The problem is discussed qualitatively by performing tests with synthetic data and solved quantitatively by adapting the GIT technique in view of these peculiarities. With the optimally adapted inversion scheme, it is possible to unravel differences in the attenuation characteristics between two (or more) sets of stations. The results show that the attenuation of seismic waves is roughly comparable in the low frequency range (< 4-5 Hz) but stronger by up to an order of magnitude at higher frequencies within the Carpathian mountain arc as compared with the foreland area. Modeling this strongly frequency-dependent lateral variation of seismic attenuation by a significantly lower Q beneath Vrancea (1) provides a very good fit of observed strong-motion characteristics, (2) sheds new light on the distribution of intensities of the previous strong earthquakes, (3) will have strong implications for future hazard assessment, and (4) is fully compatible with structural models from deep seismic sounding, tomography, and teleseismic attenuation.