Surface waves tomography and non-linear inversion in the southeast Carpathians
Introduction
The southeast Carpathians are located between the Afro-Arabian and Eurasian plates. The region consists of a set of tectonic units and it is characterized as a west-directed subduction zone (Nemcok et al., 1998). According to Doglioni et al. (1999) west-directed subduction zones constitute very interesting tectonic features and they present significant differences with respect to classic subduction zones. They form very rapidly, have a very short life, and they are characterized by an age usually less than 50 Ma. The evolution of the highly arcuate Carpathians is driven by the interrelated processes of subduction, slab roll-back, plate boundary retreat into a continental embayment, asthenospheric up-welling, and lateral extension (Tomek, 1996). Moreover, one of the most interesting seismic zones is located in the region: the Vrancea zone, where the epicentres of strong intermediate-depth earthquakes are located within the area of only about 40 km × 80 km (Oncescu et al., 1998).
In this paper we report a surface waves tomography study of the region, based upon regional and global dispersion data, and we discuss the shear-wave velocity structure, to the depth of 250 km, obtained by their non-linear inversion.
Section snippets
Data processing and methods
This study is part of a series of regional tomography investigations made in the Mediterranean area (Panza et al., 2002, Panza et al., 2003, Raykova and Panza, 2002, Guidarelli et al., 2004, Raykova et al., 2004). The regional group velocity measurements are obtained by frequency–time analysis (Levshin et al., 1989 and references therein) of records from seismic stations located in central and southeast Europe. In the present study, a surface-wave tomography method (Yanovskaya, 1997, Yanovskaya
Results and discussion
The tomography maps of the Carpathians domain at different periods show features that are common to other west-directed subduction zones (Gonzalez et al., 2000, Gonzalez et al., 2004, Guidarelli et al., 2004, Pinat, 2005, Vuan et al., 2005a, Vuan et al., 2005b). The group velocity distributions at different periods map the shear-wave velocity distribution in some depth range. The partial derivatives of the group velocity, U(T), calculated for a specific velocity model, show the depth of the
Conclusions
The shear-wave velocity model for the southeast Carpathians is derived by a procedure (frequency–time analysis, surface wave tomography, non-linear inversion and optimized smoothness algorithm) applied to a new set of data: new measurements of group velocity, published regional and global measurements and structural information from different geophysical and geological methods. Independent information deduced from seismicity distribution studies, seismic refraction and reflection investigation,
Acknowledgements
We express our thanks to the reviewers for comments and recommendations that improved the quality of the paper, to Prof. A. Levshin, who provided global group velocity measurements available at CIEI, University of Colorado at Boulder; to Prof. T. Yanovskaya, who provided the 2D tomography software from St. Petersburg State University; Dr. B. Farina who provided the LSO code; to Dr. J. Douglas for language review of the paper. This research was sponsored by the National Science Fund, Bulgarian
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2013, TectonophysicsCitation Excerpt :Down to about 90 km of depth the lateral variation is less marked than at greater depth: the low velocity anomalies come into play first toward NW (Transylvania) around 90 km of depth, than toward SE, below the Moesian Platform at about 130 km of depth and finally toward NE below the Eurasia Platform. The variation of the thickness of the lithosphere in the region from about 90 km to 170 km is evidenced by tomography investigations using body wave (Koulakov et al., 2010; Martin et al., 2006) and surface wave (group velocity) data (Raykova and Panza, 2006). The tendency of isoseismal ellipses to have decreasing minor axis (perpendicularly to the Carpathians Arc) with increasing focal depth, can be thus explained by the uprise of the asthenosphere flow (Doglioni, 1994; Ismail-Zadeh et al., 2012) below the Transylvanian Basin on one side and below the Moesian Platform on the other side.