Seismicity Variations in Depth and Time in the Vrancea (Romania) Subcrustal Region

Seismicity patterns that characterize the seismic regime of the Vrancea intermediate-depth earthquakes are investigated using an earthquake catalogue extending from 1974 to 1998. The analysis is made separately on two characteristic segments of the subducted plate (active zones) which hosted the major earthquakes of 4 March 1977, 30 August 1986 and 30 May 1990. Precursory anomalies preceding the occurrence of the major shock of 1986 (M<Subscript>w</Subscript>=7.3) in the lower part of the subducted slab are outlined when analyzing the time variation of the γ parameter (defined as the ratio of small to moderate events in a given active zone and a given time interval) and of the fractal dimension of the earthquake space distribution. Nothing similar is noticed in the upper part of the Vrancea slab. The analyzed time interval covering 25 years shows that, in contrast to previous studies, the statistical fluctuations of the γ parameter, computed for a time window of 5 months, appear to be too large to be considered as precursory anomalies. Significant 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. Copyright Kluwer Academic Publishers 1999