Abstract
Based on Akaike's Bayesian Information Criterion (ABIC), a new method is proposed for detecting changes in the magnitude-frequency relation. Earthquakes are classified into a few magnitude classes and the ratio between the numbers of shocks in these classes is estimated by using ABIC. The time variation of the magnitude-frequency relation is investigated by this method for the source areas of three events with magnitude 6.0 and above in central Japan; the Southern Ibaraki Earthquake of 1983, the Eastern Yamanashi Earthquake of 1983 and the Western Nagano Earthquake of 1983. In.the southern Ibaraki and the Eastern Yamanashi regions, the frequency of larger shocks in comparison with smaller shocks increased about 1 year preceding the main shocks. In the Western Nagano region, the number of large shocks may have increased 2 years before the main shock. The time variations of the former cases are statistically significant.
