Study on the Focal Mechanism Changing Process in Earthquake Concentrating Area Using Correlation Coefficient of Body-wave Spectral Amplitudes

The ratio of observed spectral amplitudes of direct P and S wave phases is linearly correlated with the ratio of their radiation pattern factors,and the radiation pattern factor depend on geometrical property of focal mechanism,if 2 events occurred in same focal region and recorded by same seismic station.Using low-frequency spectral amplitude values the correlation coefficient can be calculated,and the coefficent gives a normalized measurement for the similarity of focal mechanisms.Applying this method to the Qingdao earthquake sequence in 2003 and the small earthquake concentrating area in conjactive area of Hebei,Shandong and Henan provinces,the events with similar focal mechanism can be grouped,and the temporal average correlation coefficient curve of earthquake sequence can be obtained.The results show: the temporal average correlation coefficient curve of Qingdao earthquake sequence is changed from centralizing to scattering pattern,namely stress strength in focal area was transform from comparatively concentrating to separating;but the coefficient for conjactive area of Hebei,Shandong and Henan was poorer,regional stress concentration is not formed one dominant diraction.