Focal mechanisms in Romania: statistical features representative for earthquake-prone areas and spatial correlations with tectonic provinces

Abstract

Fault plane solutions for earthquakes recorded in Romania (1929–2012) are analysed on three depth levels: crust (0–50 km), upper (50–110 km) and lower segment (110–201 km). For the Vrancea intermediate-depth source reverse faulting is predominant. However, local-scale variations occur at the upper and lower limits of the active volume: normal faulting (upper side) and strike-slip with normal faulting (lower side). These edge effects are probably caused by the interaction of cold descending lithosphere with hot surrounding asthenosphere acting there. Fault plane solutions of crustal earthquakes reflect complicated patterns associated to local stress sources perturbing the regional field. One important result of our analysis is the delimitation of specific active alignments in North Dobrogea Orogen, Bârlad Depression, Danubian and Banat zones, while seismicity is diffuse and close to random distribution in the other seismogenic zones. The polar diagrams for azimuthal and dip angle distributions and the ternary diagrams for P, T and B axes show prevalence of reverse faulting in Vrancea intermediate-depth source, strike-slip in combination with normal faulting in South Carpathians and Banat region and a deficit of strike-slip faulting south-east of Carpathians. Lack of strike-slip component makes us believe that the deformation field is controlled in the Carpathians Foredeep not by transcurrent deformation along the major faults crossing the region, but rather by subsidence and folding processes as stress release mechanisms in the crust in response to the intense tectonic processes beneath Vrancea region.