Forschungs- & Entwicklungsinformationsdienst der Gemeinschaft - CORDIS

Final Activity Report Summary - SIILEG (3D Modelling ... of Pand S Travel Times from local Earthquakes and gravity data: the crustal structure of the Eastern Carpathians in Romania)

Within this project, three-dimensional velocities of primary (P-wave) and secondary waves (S-waves), i.e. Vp, Vs, Vp/Vs and density parameters characterising the Vrancea seismogenic Zone were defined for a 200 km thick volume covering an area of 230 times 230 km.

With our results:
1. we overcame two major limitations of the previous work, namely the lack of a high resolution three-dimensional S-wave and density model down to 200 km; and
2. we demonstrated the diagnostic power of a combined interpretation of three-dimensional Vp, Vs, Vp/Vs and density earth models.

For our goal, we successfully applied the tomographic inversion method of sequential integrated inversion proposed by Tondi and de Franco (2006) to shot data collected during the Vranchea99 (Hauser et al., 2001) and Vranchea2001 (Landes et al., 2004) seismic refraction experiments to local earthquake data collected during the Calixto (EOS, 1998) experiment and to recent gravity measurements of the studied area (Besutiu et al., 2004).

We viewed the measured data as a realisation of a stochastic process generated by the physical parameters to be sought and the solution was found through the minimisation of a probability density function which included the information from gravity measurements, the information from picked travel times and the information on the physical correlation among density and P-wave or S-wave velocity. The reliability of the reconstructed models, which explained equally well both travel times and gravity data, was quantified through a restoring test and the estimation of travel times and gravity residuals.

The analysis of the results, while putting constraints on the material properties within the slab structure allowed us to reduce ambiguity of the interpretation of the nature of the convergence process. Hence:
1. we substantiated the subduction model and eliminated the continental lithospheric delamination model;
2. we associated intermediate depth seismicity of the eastern Carpathians to the observed sharp lateral Vp/Vs variations presumably generated by contact between the dense and cold slab and the lithospheric mantle in the shallower part or the asthenosphere in the deeper part.
This contrast was particularly evident between 100 and 150 km of depth, where the maximum historical and analysed seismic energy release was concentrated.

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