地震活动性图象
SEISMICITY PATTERN
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摘要: 本文叙述了在中国,特别是在华北地区观测到的一些地震活动性图象。高震级(M≥7.5)地震具有特定的间隔距离。华北近年7级地震分布成条块。强震前的临震地震活动具有相似性。特定地区的地震具有明显的相关性,分析其特征,表明这种相关现象不是一种传播过程;不能用“相关地震间具有直接的断层联系”来解释;而可能是大范围地层同时受力的结果。观测到大震前总有中小地震为其作前奏,并且大小地震间遵循一定的比例关系。以上这些活动性图象,可用地层受力屈曲的机理给以解释。根据这种观点,可推算出一般地震的b值将取0.75,火山地震、水库地震以及某些震群的b值将取1.5,而沿断层分布的地震的b值将为0.5.这些数值与实际情况比较符合,因而本文叙述的现象与分析结果,可能具有普遍意义。Abstract: The seismicity pattern of China, especially that seen in North China is. described in this paper. Strong earthquakes with magnitudes of M≥7.5 are found to occur at certain definite intervals of distance. In North China, recent earthquakes with magnitudes M≥7.0 occurred within a belt in the form of a stripe. The seismicity immediately before the strong earthquakes show a similar distribution pattern. The strong earthquakes of given separate regions are thus obviously correlatable. Such a correlation can neither be shown to be due to a process of propagation nor be explained by the assumption that correlatable earthquakes are linked by faults. It is thought to be possibly the result of the action of an ambient stress field over a broad region. Medium and small shocks, whose numbers maintain certain constant ratios, are usually observed as preludes to the strong earthquakes.All of these facts may be considered to be the results of buckling of crustal layers under an ambient stress field. According to this view-point, we are able to calculate the values of b: equalling 0.75 for average earthquakes; 1.5 for volcanic earthquakes, reservoir earthquakes and some earthquake swarms; and 0.5 for earthquakes on the same fault system. These figures agree rather well with actual observations, so the discussion presented here might have some general significance.
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