Seismic source properties: indications of lithosphere irregular structure on depth beneath the Vrancea region

The previous research focused on seismic regime properties in the Vrancea region (seismicity, tomography, seismic source scaling) revealed significant variation along the subducting slab. It has been assumed that this irregularity should be in connection with multiple parameter and multi-scale fields, and should reflect differences in the physical, geochemical and tectonic processes at different scale lengths. The recent development of the seismic network on the Romanian territory through the installation of digital accelerometers within the Collaborative Research Centre 461 Programme and the tomography experiment CALIXTO'99 has provided a substantial new amount of high-quality data in addition to the routine data coming from the Romanian seismic network (18 short-period, one-component instruments), dedicated firstly to seismicity monitoring. The main purpose of the present study is to incorporate this new data in the analysis of the depth dependence of the seismic source scaling in the Vrancea subducting slab. Spectral analysis and spectral ratios methods are applied for a set of 100 earthquakes with magnitudes 2.9 w between 1996 and 2001. The two methods are employed independently, first to individual events, the second to pairs of collocated events and similar focal mechanism, in order to inspect the scaling properties over the seismic active depth domain (60-220 km). The results are discussed in connection with previous studies based on fractal statistics analysis. The clustering properties of time, space and size distributions as well as the scaling variations in the subducting lithosphere are ascribed to differences in specific source mechanisms or/and structural hierarchical inhomogeneity properties. Key words: seismic source, lithosphere, fractal analysis, Vrancea region. (authors)