张北地区震前、震时和震后的震源机制

对于张北地震之前出现在小震条带上的地震，我们收集了区域地震台网的P波初动和垂直向记录的直达P、S的最大振幅，由理论地震图反演出15次地震的震源机制；同样的方法反演得到145次余震的震源机制；连同刘瑞丰给出的6．2级主震、哈佛大学提供的5．6级强余震的矩张量解一起分析。发现，震前小震震源机制优势取向的平均解、震后余震震源机制优势取向的平均解和主震以及强余震的矩张量解，基本一致。震前、震时和震后的震源机制的优势取向，没有发生可以觉察的显著变化。表明震区始终受到构造应力场的控制，呈现出一定的稳定性。

THE FOCAL MECHANISMS OF EARTHQUAKES BEFORE AND AFTER ZHANGBEI MAIN EVENT IN THE AREA

for the earthquakes in a belt formed before Zhangbei event,their first motion of P wave and maximum amplitude of P and S wave in vertical records that recorded by local seismic network are collected,and 15 focal mechanisms are inversed with synthetic seismogram.145 focal mechanisms of aftershocks are determined with the same method;Combined with the focal mechanism of the M6.2main event given by Liu Ruifeng and the M5.6 strong aftershock from Harvard,these mechanisms are analyzed.The results show that the average dominant orientations of the focal mechanisms from the small earthquakes before and after the main shock are consistent with the moment tensor solutions of the main shock and its strong aftershock.There are no remarkable change in the dominant orientation of the focal mechanisms before and after the event.It indicates that the seismic region controlled by the tectonic stress field is stable.