Investigation of the crustal and deep structure in Vrancea seismic zone

The Vrancea zone, located at the sharp bend of the Southeast Carpathians, is characterized by an unusually narrow volume of intense seismicity (four shocks with magnitude greater than 7 occurred during the past century) in the depth range of 60 km to 220 km, resulting in significant seismic risk of the densely populated Bucharest area. The oceanic lithosphere slab beneath Vrancea area moved into an almost vertical position when convergence of plates come to a halt and suction force of the subducting plate vanished. In addition, since subduction occurred in an arcuate geometry, the slab is likely to be segmented as suggested by hypocenter distribution. Wortel and Spakman pioneered the idea of slab detachment, based on tomographic images of the upper mantle beneath the Mediterranean. The detachment hypothesis appears to be compatible with the magmatic evolution and the metamorphic patterns in Vrancea collisional orogenic region. Uniform high-precision hypocentre locations of the whole data set are very important in a seismically active area like Vrancea, where the seismic database is a prerequisite for tectonic interpretation and seismic hazard assessment. Well-constrained earthquake data are also extremely useful for studies focused on high-resolution imaging of the complex continental subduction process in Vrancea. To obtain a better image of the structure beneath Vrancea zone it is necessary to have a good velocity model, a well constrained topography model and detailed geological information. To compute a good velocity model we need a data base containing a large number of local, regional and teleseismic well-located events. The recent international tomographic experiment CALIXTO (Carpathian Arc Lithosphere X-Tomography) carried out in 1999, was designed to determine a 3D snapshot of the geodynamic evolution of the Carpathian arc beneath SE-Romania, and in particular, the ongoing slab break-off in the upper mantle beneath Vrancea region. A network consisting of 120 seismic stations (both short-period and broadband instruments) has been temporary deployed. In parallel with the CALIXTO network, events were recorded by Romanian telemetered network (17 vertical-component S-13 seismometers) and three-component digital accelerometers network (30 stations). In the first part of the project we built up a data base for regional (A <300) and teleseismic (A > 300) events. To this purpose we used 340 regional records from 1993-July 2001 and 603 teleseismic waveforms recorded between 1992 and July 2001. Only events with minimum 6 well-recorded P phases were selected. The event locations obtained with the HYPOPLUS program were compared with the location of the International Seismological Centre (ISC-International Seismological Centre, England and USGS-United States Geological Survey, United States of America). In the second part of the project we determined an optimum 1D velocity model that allows consistent and accurate hypocenter locations and that serves as initial reference model for 3D tomography. The minimum 1D model denotes the solution to the coupled hypocenter-velocity model problem and was determined by a trial-and-error process, with internal non-linear (iterative) inversion procedure, using a set of 280 well-locatable earthquakes produced on the Romanian territory. The average velocity model parameters were obtained by minimizing the standard deviations of the travel time residuals. The minimum 1D model leads to a minimum average rms value for the whole data set of earthquakes and reflects more closely the a priori structural information, e.g., by controlled source studies. The test of the minimum 1D velocity model performance shows a significant improvement against the error assessment by routine earthquake location procedures. The station residuals obtained using the new velocity model, correlate regionally and with surface geology. They outline the existence of strong lateral inhomogeneities over the study area. The 3D gravity and geodetic modeling is important in revealing the subducting slab geometry. In a first stage we carried out the topography modeling of the Vrancea area and the distribution of the Bouguer anomaly values. The results outline a minimum anomaly value of large wave lengths in the bending zone of Carpathians. In the second stage, the topography modeling of basement, Conrad and Moho discontinuities is performed. For all this cases, the results show a strong and well constrained sinking beneath the Vrancea area. (authors)