By using two kinds of principle extraction methods, I.e. Uniform and non-uniform, we extract earthquakes from the complete earthquake catalogues of the Northern China and the Southwestern China, and then we obtain the empirical formula of the relation between directory missing rate and change of b-valued. Our results show that missing rate by non-uniform extraction is proportional to the reduction of b-value. With uniform extraction calculated b-value from the least square method and the maximum likelihood method is relatively complex, in which b-value could increase or decrease with the change of directory missing rate. With non-uniform extraction, the effect of directory missing rate on b value is greater from the maximum likelihood method than from the least square method.%本文通过对华北地区和西南地区完整地震目录按照均匀抽取和非均匀抽取2种原则进行抽取地震,得到了地震目录遗漏率和b值变化量间的经验关系式.结果表明,在进行非均匀抽取地震时,得到的地震目录遗漏率和b值的减小量间呈正比关系；在进行均匀抽取地震时,利用最小二乘法和极大似然法计算得到的b值变化较复杂,b值随地震目录遗漏率的增加,或为增加,或为减少.同时,在进行非均匀抽取地震时,地震目录遗漏率对极大似然法计算的b值影响大于最小二乘法计算的b值影响.
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