CHANGES IN THE MAGNITUDE FREQUENCY B-VALUE PRIOR TO LARGE (M-GREATER-THAN-OR-EQUAL-TO-6.0) EARTHQUAKES IN JAPAN

On the basis of Akaike's Bayesian information criterion (ABIC), a new method of estimating temporal and spatial variations in the magnitude-frequency relation (b-value) is developed. The space-time volume studied is divided into a number of segments having equal volumes, for which b-values are estimated. The smoothness of the estimated b-value is guaranteed by the introduction of a prior distribution. The parameters of the prior distribution are chosen in such a way as to minimize the ABIC. This method is employed to study the temporal and spatial variations of b-values for microearthquake activity in the Kanto, Tokai and Tottori areas, where seven earthquakes of M greater-than-or-equal-to 6.0 have recently occurred. The temporal variation of b-values for the source region of the western Nagano earthquake (Sep. 1984, M 6.8) in the Tokai area shows a decrease in b-values about 2 years before the occurrence of the main shock. Similar findings before the southwestern Ibaraki (Feb. 1983, M 6.0), the southern Ibaraki (Oct. 1985, M 6.0) and the Chiba (Dec. 1987, M 6.7) earthquakes in the Kanto area, and the Tottori earthquake (Oct. 1984, M 6.2) in the Tottori area were obtained. These findings are confirmed by a precursor analysis (PA), in which effects of aftershocks are eliminated. Using a quantitative definition, temporal variations of b-values are classified into three groups: increase, decrease and no change. The space-time volume of decrease accounts for approximately 10% of the total volume. For a period before the occurrence of earthquakes, the decrease appears more frequently than might be expected from random chance. The results suggest that the decrease in b-values is a promising candidate to act as a precursory phenomena.