Earthquakes distribution and their focal mechanism in and around Vrancea seismogenic zone

A small portion of the Carpathian Orogen is still seismically active in the Vrancea area. This prominent south-eastern zone of the Carpathian Orogen lies above a nest of strong recent earthquakes (1940, MW = 7.7; 1977, MW 7.5; 1986, MW = 7.2; 1990, MW = 6.9 ) situated between 60 and 200 km depth. This is the only place in the entire Carpathians where folding and thrusting occurred during Pleistocene in the outermost zones between two deep faults in the Moesian Platform. These recent crustal movements in the area are accompanied by moderate crustal seismic activity (between 10 - 50 km depth and Mmax = 5.5), which together with intermediate depth seismicity produce high seismic risk in a densely populated area. In Vrancea area, the original oceanic lithosphere descended to more than 200 km depth, as indicated by the intense intermediate depth seismic activity. Recent works in seismic tomography indicate that anomalies of the seismic velocities appear down to 350 km depth. Available fault plane solutions for the intermediate-depth earthquakes recorded between 1929 and 1997 are analyzed in several depth intervals indicated by the seismic activity. For the same depth intervals, the stress field characteristics, as deduced from P and T axes, are investigated. Predominant cloistering of the principal deformation axes and rupture plane orientation are observed in the Vrancea subcrustal domain in contrast with the earthquakes in the crust, which indicate no clear trend in the stress field. Some prominent orientations of the focal parameters appear. The first and most striking outcome that results from our diagrams is the quasi random distribution of the geometry in the crustal domain in contrast with the subcrustal domain, where significant preferences are pointed out. The lack of near horizontal fault planes ( δ < 30 angle ) is an indicator of a strike-slip component predominance. Another conclusion is that at least two zones are present in the domain of the intermediate earthquakes: the first one, from 60 km to 100 km and the second one from 100-110 km to 200 km in depth. These two zones have clearly different characteristics both in the number of earthquakes occurred and also in the total released energy . They are also different in respect with the orientation angle of the blocks, which is changing at the same depth. In the interval 100 - 140 km depth, it seems that the nodal planes are slightly rotated towards N - S direction. It is interesting to note that this change is also emphasized by seismologic tomography studies, which show a rotation of the lower part of the high-velocity body beneath Carpathians Arc Bend from a NE - SW direction to a N - S direction. (authors)