营口—海城地区震群的震源机制一致性特征

采用P波初动方法计算1999年以来营口—海城地区5次震群序列的中小地震震源机制解,并定义震源机制一致性参数θ为中小地震震源机制解3个正交的应力主轴与华北地区构造应力场3个应力主轴在三维空间的夹角之和。计算结果显示:营口—海城地区震群中较大地震发生前10天~2个月,θ都有一个持续低值的过程;1999年岫岩前震序列的θ变化幅度在20°~30°之间,且大部分小于65°。相比用震源机制P轴取向趋于一致来判定震源区应力状态,θ更有其优越性。

Focal Mechanism Consistency Features of Earthquake Swarms in Yingkou-Haicheng Area

We solved the five focal mechanisms of moderate and small swarm earthquakes occurring since 1999 in Yingkou and Haicheng using the P-wave first-motion method. We defined the sum of the inclinations of three orthogonal and principal stress axes of the focal mechanisms for the moderate and small earthquakes and that of the tectonic stress field in corresponding areas in the 3D coordinate system as parameter of the consistency of the mechanisms. It is a quantitative index used to measure the release stress field of an earthquake source and the conformance degree of the regional tectonic stress field. The conformance degree of the regional stress field and the focal mechanisms increase as the value decreases. The conformance degree of the regional stress field and the focal mechanisms of moderate and small earthquakes are higher because of the enhanced tendency of the regional tectonic stress field and higher possibility of occurrence of a stronger earthquake in the future. The result shows that the parameter of the consistency of the focal mechanisms was reduced to a minimum value of approximately 10~60 days before the occurrence of larger earthquakes for the earthquake swarms in Yingkou and Haicheng. Time and fluctuation range of the low value may be related to the characteristics of the earthquake swarm. The consistency parameter fluctuated significantly in common swarms such as the 2008 Haicheng earthquake swarm and the 2012 and 2013 Qingshiling earthquake swarms in Gaizhou City. All the mean values of the consistency parameter were greater than 125°. The value was low before the occurrence of the two stronger earthquakes (M5.4 on November 29, 1999 and M5.1 on January 12, 2000) in the foreshock sequence in Xiuyan. The value varied between 20° and 30°, and the minimum value was less than 65°. The consistency parameter did not vary significantly, and the mean value is not high (73°) for the 2014 earthquake swarms of Xihaiyu and Gaizhou, thus indicating some strengthening tendency of the stress field. However, because the consistency parameter is not consistently low, further research must be conducted in the future. Nevertheless, the parameter of consistency of focal mechanism is more useful for judging the stress condition in the focal region than using the P-axis trend of focal mechanism.