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The Vs_30 refers to the velocity of transversal seismic waves in the upper earth crust (surface shallow depths) in the 0-30 m interval. This parameter reflects the integral ground properties and is used in almost all seismic hazard assessment software. The determination of Vs_30 is important, especially in the case of urban territories, where in-situ measurements are very difficult and sometimes impossible due to dense urban areas, large anthropogenic noise and the need of expensive boreholes. The paper presents a methodology for extracting necessary data and information from archival sources, mainly geophysical measurements frequently executed for different prospecting purposes. The methodology for assessment of the integral values of Vs_30 is proposed for the definition of seismic hazard maps in Bulgaria. The methodology proposed, based primarily on archive data, represents an effective approach with significant results, especially for the intensive urbanized town territories located in high seismic areas.

Some velocity characteristics of the crust and upper mantle in the central part of the Balkan region are studied on the basis of about 1000 local and teleseismic earthquakes, registered on the 32 seismic stations. An approach for estimation of the large anisotropic structures effects is used together with the general consideration about the connection between the time residuals and velocity inhomogeneities. The main features of the inhomogeneities in the crust and upper mantle are drscussed in relation with some gravity, heat flow and seismotectonic data. The distribution of the crustal inhomogeneities in general corresponds to the configuration of the morphotectonic structures in Bulgaria. The subcrustal inhomogeneities are discordant with the surface structures, but their orientation is in coincidence with the Trans-Balkan seismolineament system. This fact indicates that the crustal seismicity in the region is probably controlled by the upper mantle structures. The high-velocity structures in the deep upper mantle beneath the Rhodope massif probably represent a paleosubduction at a depth more than 300 km.