尾波持续时间比τ_H/τ_V 在强震短临预报中的应用

APPLICATION OF LASTING PERIOD RATIO OF CODA IN SHORT—TERM AND IMPENDING PREDICTION OF STRONG EARTHQUAKE

本文根据Aki等人提出的尾波理论,导出了地方震尾波水平分量与垂直分量的持续时间比的具体表达式: τ_H/τ_V=I_H/I_V(Q_H/Q_V)^(1/4)·B_H/B_V。该式表明,τ_H/τ_V的变化主要反映了震源体一定范围内,由于介质的各向异性而引起的尾波在不同方向上的激发及衰减能力的差异。本文还讨论了地震前τ_H/τ_V短临异常的物理机制,认为异常的产生与孕震期间介质内裂隙的出现和闭合有关。1986年门源6.4级地震和1975年海城7.3级地震前,τ_H/τ_V都有不同程度的短期低值异常及临震高值突跳。门源地震前后门源台记录的直达S波的最大振幅比A_(mH)/A_(mv)也有与尾波持续时间比类似的异常。最后通过对一些震例的分析,初步得到震级与异常时间的关系为M=0.657lnT+3.44。

In the light of the coda theory advanced by Aki et al.,this paper deduces a concrete formula for the lasting period ratio τ_H/τ_V of the horizontal and the vertical component of the local earthquake coda. After the approximate treatment, we get:τ_H/τ_V=I_H/I_V(Q_H/Q_V)^(1/4)·B_H/B_VThe formula shows that the changes of τ_H/τ_V mainly reflect the focal body's difference in excitation and attenuation force to the coda wave from different directi6ns in certain range caused by the anisotropy of the medium. The paper also discusses the physical mechanism of the τ_H/τ_V short and imminent anomalies, and rends to think that the occurrences of the anomalies are related to the appearance and closure of the cracks in the medium in earthquake preparatory stage.The results obtained after processing the Mcnyuan earthquake Ms6. 4 of 1986 and the Haicheng earthquake Ms7.3 of 1975 show that there appeared the short—period low value anomalies and imminent high value jump in various, degrees.In comparison with the maximum amplitudes, of the direct S wave recorded by Menyuan station before and after the 1986 Menyuan earthquake, it is found that A_(mH)/A_(mV) has the similar case with the anomalies of the coda τ_H/τ_V.Through the analysis to the data of some earthquake cases, the paper finally gives the relation of the magnitude with the anomalous period asM=0.657lnt+3.44~ ., , ,~