对前震和长期地震活动在频度—震级关系式中的b值做了比较。研究了发生在主震前两个非重叠时间间隔内的前震:(1)数小时内;(2)数天内。区域的和全球的地震目录统计分析表明,这些间隔内的b值比长期地震活动的b值下降了一半。这扩展了Molchan和Dmitrieva先前的研究结果(Molchan and Dmitrieva,1990),他们发现在主震前数小时内出现了这样的下降。我们的结果是个统计性的规律,未必在每一单个前震序列中都能出现。 The b value of the relationship between the magnitude and the magnitude of the front earthquakes and the long-term earthquakes is compared. The foreshocks occurring in the two non-overlapping time intervals before the main shock are studied: (1) within a few hours; (2) within a few days. Statistical analysis of regional and global seismic catalogs shows that the b values ​​for these intervals have been reduced by half compared to the b values ​​for long-term seismic activity. This extends the previous findings of Molchan and Dmitrieva (Molchan and Dmitrieva, 1990) who found that such a decline occurred within a few hours of the main shock. Our result is a statistical law that may not appear in every single foreshock sequence.