Engineering geological conditions in the Sofia Kettle and consequences from the earthquakes

The historical review of the seismic activity shows that strong earthquakes with destructive effects took place in the Sofia kettle. There are data for the display of sufficiently powerful seismic events in the Sofia zone that had been reflected in historical documents as early as in the XV century. Four strong earthquakes occurred in this region during the period from the beginning of the XIX century till now that caused significant damages, the most powerful one being in 1858 (intensity of IX degree according to the MSK-64 scale and magnitude of about 6.5-7.0).
   According to the seismic zoning of Bulgaria for a period longer than 1000 years and average engineering geological conditions, the kettle falls entirely within a region with expected intensity of IX degree according to the MSK-64 scale with a seismic coefficient K = 0.27 (Boncev et aI., 1982; Norms for ..., 1987).
   The engineering geological characteristics of the Sofia kettle are the basis for determining the influence of the soil conditions on the effects of the seismic activity. Complex geologicalgeophysical investigations were carried out in the Sofia kettle during the period 1967-1970 in connection with the seismic microzoning of the territory of the Sofia City and the Sofia basin. On the basis of the engineering geological studies and the micro-seismic investigations (Petkov, Christoskov, 1965; Demirev et aI., 1971; Petrov, Iliev, 1971), the first attempts for preliminary seismic micro-zoning of the Sofia City and its surroundings were made, which were applied in practice too. The coefficient of variation of the seismic intensity is calculated according to the formula of Medvedev, the parameters of the monzonites and syenites of the Vitosha pluton being considered as reference ones. The values of this coefficient reach up to 3, and for the most unfavorable ground conditions - even to 3.5.