The dependence of the maximum magnitude Y of expected tectonic earthquakes upon the velocity X of recent vertical crustal movements in Bulgaria is studied. The two-dimensional random variable (X, Y) is treated as a physical system to which an inner degree of indefiniteness is inherent. With the help of the theory of information, utilizing the possible states A i of the system A and their probabilities p i, the entropy of the partial physical systems X and Y and of the composed system (X, Y) is determined. It is demonstrated that X and Y are not independent and the degree of indefiniteness of X is smaller than that of Y, i.e., that the better known system X could be used as a source of efficient information when studying the state of the less-known (and statistically dependent on it) system Y. This conclusion is further corroborated, by the investigation of the joint sampling distribution of (X, Y) in Bulgaria, made with the help of a new variant suggested by the Chi-Square test of significance aimed at checking a family of statistical hypotheses. A not very close, but statistically significant correlation, of a curvilinear type, between X and Y is established. For the whole country, and especially for its most seismoactive region, the best-fitting regression equations of fourth order, aimed at an express and relatively reliable forecast of Y by means of X, are deducted and experimented. The space structure and the significance of the correlation between X and Y in this region is studied and their regional background surfaces are determined. The results obtained are analysed and a good complex interpretational relation to earlier independent geological, geomorphological and geophysical investigations for this country is established.