Acta Geod. Geoph. Hung., Vol. 45(3), pp. 340–355 (2010) DOI: 10.1556/AGeod.45.2010.3.8 POSSIBLE DEEP LITHOSPHERIC ROOTS BENEATH SOUTH-EASTERN CARPATHIANS BACK-ARC REGION C Neagoe, M Popa, M Diaconescu, M Radulian National Institute for Earth Physics, Bucharest-Magurele, Romania, e-mails: cristian.neagoe@infp.ro, mihaela@infp.ro, diac@infp.ro, mircea@infp.ro The Vrancea seismic region represents a unique case of well-defined and intense intermediate-depth earthquake activity as a consequence of specific geodynamic pro- cesses at the continental contact between East-European, Moesian and Intra-Alpine plates. Apart the subcrustal earthquakes generated in the Vrancea slab, the anal- ysis of seismicity puts into evidence two other clusters of subcrustal earthquakes (h> 50 km) toward the back-arc side of the SE Carpathians bend, one to the west (Sinaia), the other to the north-west (Bra¸ sov Depression) relative to the Vrancea seis- mic source. The hypocenters lay down between 50 to 105 km in Sinaia and between 50 and 136 km in Bra¸ sov Depression. The rate of seismic energy release is much lower (Mw ≤ 3.7 in Sinaia and Mw ≤ 3.1 in Bra¸ sov Depression) than for Vrancea activity. The seismicity pattern in the SE Carpathians back-arc region upper man- tle correlates well with the high-velocity structures depicted by seismic tomography investigations suggesting possible remnant deep lithospheric roots apart from the narrow well-defined slab generating Vrancea major earthquakes. The results outline significant lateral heterogeneities in the mantle and provide new data for incorporat- ing seismological, geotectonic and volcanological data in a unified modeling of the complex processes taking place in the study region. Keywords: alkalic magma; lithospheric root; mantle earthquakes; post-collisional mechanics Introduction The Vrancea seismic region represents a unique case of well-defined and intense intermediate-depth earthquake activity as a consequence of specific geodynamic pro- cesses at the continental contact between East-European, Moesian and Intra-Alpine plates. Despite the restraint focal volume, a couple of shocks with magnitude above 7 are generated each century. It is generally supposed that subcrustal earthquakes are focused but in a narrow lithosphere body going down beneath Vrancea and all other earthquakes are generated in the crustal domain (h< 60 km). Recent improvements of the Romania seismic network (Ionescu and Neagoe 2008, Neagoe and Ionescu 2009) and significant progress in quality of earthquake locations bring into evidence the fact that subcrustal events could be encountered over a larger area around Vrancea seismic nest: in the back-arc side of the South-Eastern (SE) 1217-8977/$ 20.00 c 2010 Akad´ emiai Kiad´ o, Budapest