Unveiling seismic and density structure beneath the Vrancea seismogenic zone, Romania

Abstract

The Vrancea seismogenic zone in Romania exhibits an intense intermediate-depth seismicity, confined to a relatively small, roughly cylindrical and elongated region, whose origin is still under debate. Our three-dimensional P and S wave velocity and density images put additional physical constraints on the existing tectonic models to a depth of 200 km. The results appear to substantiate a combination of lithospheric delamination and oceanic subduction. For our analysis, we apply the tomographic inversion method of sequential integrated inversion (SII) to P and S first arrivals from active source data collected during the VRANCEA99 and VRANCEA2001 seismic refraction experiments, local earthquake data collected during the Carpathian Arc Lithosphere X-Tomography (CALIXTO) experiment and recent gravity measurements of the studied area. The reconstructed models, which explain both travel times and gravity data, show a subducting slab which exhibits fast Vp, fast Vs, high density, and a low Vp/Vs ratio consistent with the cold downgoing plate. We associate intermediate-depth seismicity with 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 is particularly evident between 100 and 150 km depth, where the maximum historical seismic energy release is concentrated. Our results indicate the diagnostic power of a combined interpretation of 3-D Vp, Vs, Vp/Vs, and density models.