Depth-dependence properties of the Vrancea seismic source

Recent results on seismic source in the Vrancea region showed how important is to relate source properties with information coming from other fields, such as geology, tectonics, rheology, geochemistry, volcanism. In particular, the variation in space of source properties indicates significant inhomogeneities in the structural and geophysical parameters and should reflect differences in the physical, geochemical and tectonic processes at different scalelengths. Seismicity and tomography results (Trifu and Radulian, 1991; Martin et al., 2002) suggest the existence of two seismic active segments in Vrancea, one located in the upper part (60 ≤ h ≤ 110 km), the other in the lower part (110 < h ≤ 220 km) of the subducting slab. They show different characteristics both in seismicity rate, clustering and total seismic energy release. They also differ in respect with the orientation angle of the blocks, which is changing around 90 - 110 km depth from NE-SW orientation (in the upper part) to N-S orientation (in the lower part). According to Oncescu and Trifu (1987), the focal mechanism is also changing in the transition zone (around 100 km depth), relative to the predominant solution which characterizes the entire seismic volume. We assume that the physic-chemical conditions there are in favor of a dehydration reaction in the slab. The source parameters and their scaling relations reflect depth variations over the considered depth domain (60-220 km) as well. Thus, the b slope of the frequency-magnitude distribution is decreasing in the lower slab in correlation with the increasing of the stress drop. At the same time, the slope of the seismic moment - magnitude relation is larger in the lower part. Fractal statistics (variation coefficient, integral correlation methods) is applied to detect possible clustering features in the Vrancea seismic regime. Our analysis outlines differences in earthquake process between the upper and lower segments. More abundant moderate (asperity-like) events than small (crack-like) events are generated in the lower zone in comparison with the upper zone (reflected in the behavior of the Cv and γ parameters). (authors)