Seismicity variations in depth and time in the Vrancea (Romania) subcrustal region

Seismicity patterns that characterize the seismic regime of the Vrancea int ermediate-depth earthquakes are investigated using an earthquake catalogue extending from 1974 to 1998. The analysis is made separately on two charact eristic segments of the subducted plate (active zones) which hosted the maj or earthquakes of 4 March 1977, 30 August 1986 and 30 May 1990. Precursory anomalies preceding the occurrence of the major shock of 1986 (M-w = 7.3) i n the lower part of the subducted slab are outlined when analyzing the time variation of the gamma parameter (defined as the ratio of small to moderat e events in a given active zone and a given time interval) and of the fract al dimension of the earthquake space distribution. Nothing similar is notic ed in the upper part of the Vrancea slab. The analyzed time interval coveri ng 25 years shows that, in contrast to previous studies, the statistical fl uctuations of the gamma parameter, computed for a time window of 5 months, appear to be too large to be considered as precursory anomalies. Significan t differences among characteristic depth segments are also outlined in the frequency-magnitude distribution and are possibly related to differences in the physical mechanism of the earthquake generation process.