采用体波谱振幅相关系数方法研究地震序列的震源机制变化过程

Study on the changing process of earthquake sequence′s focal mechanisms using correlation coefficient of body-wave spectral amplitudes

发生在同一震源区的两个地震,当地震之间的距离远小于地震到台站之间的距离时,同一个台站的直达P波、S波零频观测谱值之比与其辐射图型因子之比线性相关,而辐射图型因子取决于震源机制解的几何特性。利用同一震源区两次事件的相同台站的波形记录计算谱振幅相关系数,该系数可认为是震源机制异同的归一化的数值描述;据此对地震序列事件进行聚类分组以及计算序列事件的滑动平均相关系数时序曲线。对2001年雅江和2003年大姚强震序列,以及2002年漾濞和2003年洱源小震序列进行分析的研究结果表明:①强震序列与小震序列的滑动平均相关系数时序曲线形态存在明显差异;②某些强震序列的初期震源机制有从散乱到一致的过程。分析认为,孕震应力场的应力强度对孕震区内地震的破裂方向有明显的约束作用。

When 2 events occur within a same focal region and the distance between 2 events is far shorter than the distance between events and the station, the ratio of observed spectral amplitudes of the direct P and S phases is linearly correlated with the ratio of their radiation pattern factors, and radiation pattern factors depend on geometrical property focal mechanisms. If the 2 closely located events are recorded by the same station, we calculate correlation coefficients using low-frequency spectral amplitude values, and the correlation coefficients gives a normalized measurement of the similar focal mechanism. Applying this method to earthquake sequence, the events with similar focal mechanism can be grouped, and temporal average correlation coefficient curve of earthquake sequences can be obtained. Based on above principle, we calculated and analyzed 2 strong earthquake sequences of 2001 Yajiang M6.0 Sichuan, 2003 Dayao M6.2, Yunnan, and 2 small earthquake sequences of 2002 Yangbi, Yunnan, 2003 Eryuan, Yunnan. The results show： First, the features of temporal average correlation coefficient curves are different between strong and small earthquake sequences. Second, some strong earthquake sequences＇ focal mechanisms are changed from scattering to centralized pattern at the beginning period. This suggests that focal stress intensity has restraining effect on earthquake＇s rupture direction.