一个特殊的地震空区

1999 11 0 1,山西省大同市发生了M5 6地震 ,此前 10个月在其震源附近曾发生一群震源定位结果较好的小震。从小震在三维空间的分布情况看 ,在垂直剖面上存在一个NWW向的特殊空区 ,且似乎存在围空地震按顺时针方向旋转发生的现象。采用层状介质点源位错模型 ,由理论地震图拟合观测的垂直向记录 P、 S最大振幅的比值反演小地震震源机制的方法 ,使用大同地震台网和其他地震台网部分台站的记录资料 ,求得小地震的震源机制解。这些震源机制解在 4个象限中呈现出了不同的性质 ,且每两个相对象限中震源机制解的性质相近。5 .6级地震发生在该空区的边缘 ,而余震序列将其填充 ,余震区的长轴方向和空区方向相同。

A Special Seismic Gap before Datong M5.6 Earthquake in Shanxi

A M5.6 earthquake occurred in Datong of Shanxi Province on Nov.1, 1999. There was a group of small earthquakes that was located finely near the hypocenter 10 months before the event. The three-dimensional distribution of these earthquakes showed that there is a special seismic gap with NNW direction in the vertical profile. The occurrence of these gap-surrouding earthquakes showed clockwise order. The focal mechanism solutions of these earthquakes were obtained with point source dislocation model in layered medium. In inversion, the maximum amplitude ratio of and wave in vertical records observed was fitted to that of theoretical seismogram. Data recorded by Datong network and some stations from other networks were used. These focal mechanism solutions showed different property in the 4 quadrants. And the property in opposite quadrant was close. The M5.6 earthquake occurred on the edge of the seismic gap. The seismic gap was filled up by aftershocks. The direction of long axis for the aftershock area is same as the direction of the seismic gap.