Geometrical constraints for the configuration of the Vrancea (Romania) intermediate-depth seismicity nest

For a seismogenic area like Vrancea (Romania) with well-defined geometrical features of the seismicity production in space and time, the numerical simulation of the earthquake generation process (e.g. cellular automaton) looks highly attractive. The delimitation, as accurately as possible, of the geometrical features of the seismically active system in the Vrancea subcrustal zone is essential to constrain the simulation modeling. As a first approximation, the seismicity pattern is close to a fault plane NE–SW oriented, extended roughly vertically between 60 and 170 km depth. A characteristic median plane is defined by minimizing the distance of hypocenters. The average distance of the hypocenters to the median plane is around 5 km. However, a more detailed investigation of the geometrical configuration of seismicity indicates a fragmentation of the active body located in the upper mantle in two segments. The seismicity pattern is well approximated by a planar distribution in each segment. In the transition zone, between the upper and lower segment, the hypocenter distribution is more dispersed and shows a disruption among the two planar segments, measured by about 9 km apart laterally one relative to the other. The two segments hosted the major Vrancea events recorded in the last two centuries (for which we have available location of acceptable accuracy). The narrow transition zone at about 100 km depth is interpreted as a weaker segment, possibly caused by a dehydration process or by an infiltration of asthenosphere material from the back side of the South-Eastern Carpathian arc system. It is still debatable if fragmentation in two seismically active segments reflects the existence of two neighbouring separate blocks (upper, continental and lower, oceanic block) or a consequence of a breaking process separating a continental block into two parts. The segmentation of the descending lithosphere and the edge effects are apparently stationary, at least for the time interval since 1985 to the present, for which the earthquake catalogue is reliable (homogeneous).