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我们研究了1975年2月4日海城地震(M=7.3)前震的定位及其辐射图象.用六个区域台的到时,相对于一次参考地震,将前震及主震进行了定位.这组前震开始相互很接近,然后随时间及其分布有一定的延伸.最大前震前,前震均位于直径约两公里的小体积内,而在最大前震后,其活动则向北西和南东方向扩展,形成六公里长的在北西方向上伸展的分布带.初动及 P 波 S 波振幅比表明,在前震系列中有两种不同的断裂机制.我们推测这两种辐射图象可能和前震处于分布带的不同部位有关.可能主震震源不处于前震震源所决定的断层上,而是位于这组前震南面6公里、且较这组前震浅几公里处.我们认为,在垂直于主震破裂面及前震分布带走向的方向上,前震和主震相距这么大的距离,可能是由于主震时产生滑动的断层是以雁行排列的.分析了在前震期间断层上滑动所引起的应力变化,认为由前震引起的主震断层上剪应力的增加是很小的.因之由前震直接触发主震的可能性不大.
We studied the location and radiation images of the Haicheng earthquake (M = 7.3) on February 4, 1975. With the arrival of six regional stations, the front and main earthquakes were located relative to a reference earthquake. The foreshocks of this group are close to each other and then extend with time and their distribution. Before the largest foreshock, the foreshocks lie within a small volume of about two kilometers in diameter, while their activities move to the northwest and south east after the largest foreshock And formed a distribution belt extending over a distance of 6 km in the northwesterly direction.The amplitude of initial and P-wave S-waves shows that there are two different fracture mechanisms in the foreshock series.We speculate that the two radiative images may be related to foreshocks Probably in different parts of the distribution belt.Maybe the mainshock source is not located on the fault determined by the pre-earthquake source, but is located 6 km south of this group of foreshocks and a few kilometers shallower than this group of foreshocks.We believe that in the direction perpendicular to the main shock In the direction of the rupture plane and the direction of the pre-earthquake distribution belt, the distance between the foreshock and the main shock may be due to the fact that the faults that generate slip during the mainshock are aligned in the geese. The stress changes caused by the slip during the fore-shock are analyzed ,recognize Cut by the increase on the mainshock fault before the earthquake due to stress is very small. Consequent possibility before the earthquake triggered directly by the main shock is unlikely.