抄録
Time and spacial variations of the magnitude-frequency relation, on the assumption of the exponential distribution of Gutenberg-Richter, are studied for the microearthquakes which occurred in the Tokai area, central Japan, for the period from January 1981 to December 1985. The smoothness of the estimated value is guaranteed by the introduction of a prior distribution. The parameters of the prior distribution are chosen to minimize the Akaike's Bayesian Information Criterion (ABIC). As a result of the analysis, it is found that the b value in the source region of the Western Nagano earthquake (Sept. 14, 1984, M 6.8) decreased by more than 0.1 about 2.5 years before the earthquake. This change is also detected from the data before the occurrence of this earthquake. Decreases in b value by more than 0.1 for other regions are coincident with the major seismic activities in the respective regions.
