1976年松潘大震前中强震的活动特征

1976年8月四川松潘,平武发生了强烈地震,震级分别为7.2,6.7和7.2,这是我国南北地震带中段自1933年以来的最大地震。1933—1976年间该段上发生了一系列5.0—6.5级中强地震,本文讨论了这些中强震在震中跳迁,能量释放,大震源区小震频度,震中分布以及中强震余震的强度衰减等方面的地震活动特征。指出1973—1974年间的中强震活动,应当是1976年松潘平武地震的前兆地震活动,並且依据上述特征有可能将这类中强震从一般的中强震中区分出来。主要的活动特征可以归结为:自1972年开始,地震能量释放一反常态由原来源区和外围区的同步成比例变化改为截然反向的变化,这些中强震余震的扩散不明显,相反,主震却很清楚;这几个中强震和他们自已的最大强余震之间的时间间隔都大于4天,这明显大于附近地区其他中强震的这一间隔,也大于国内近年来大多数5.0—6.0级中强震的这一时间隔;1976年初,即大震前半年,震源区的地震活动异常平静,一直持续到大震发生。此外结合该区历史强震的迁移特点有可能预示1976年松潘平武地震的时间和地点,而其孕育过程似乎可由1949,1972及1976年初分为几个阶段。

THE MODERATE EARTHQUAKE BEFORE LARGE SONGPAN EARTHQUAKE IN 1976

In August 1976, the strong earthquakes with Ms=7.2,6. 7 and 7.2 respectively occurred in Songpan-Pingwu, Sichuan Province, which is the largest event in the middle section of the north-south earthquake belt in China since 1933. A series of moderate earthquakes of magnisitudes 5.0-6.5 occurred here during 1933-1976. This Paper deals with the characteristics of the moderate earthquakes, such as: shock in epicenter, the release of seismic energy, the change of small shock frequency in focus region, epicenter distribution as well as magnitude attenuation of the aftershoks. It suggests that moderate seismicity during 1973-1974 should be the precursor of songpan-Pingwu earthquake in 1976. we can separate them from commom ones based on their features mentioned above. The significant characteristics can be regarded as: from 1972, the release of seismic energy had assumed a striking contrast between focus region and its surrounding region, a reversal past proportional change relation: The epicenter diffusion of aftershocks of these moderate events couldn''t be seen; on the contrary, the main shocks were very clear. The durations between moderate earthquakes and their own largest aftershocks lasted over 4 days, not only in near regions but also in many other areas. In early 1976, i. e. half a year before largeearthquakes, an abnormal seismic quiet had been kept in the region till their occurence.Besides, combinating with the migrating feature of historical large earthquakes in these regions, it is possible to prereveal the location and time of Songpan-Pingwu earthquakes in 1976. The preparing process of the shocks seemed to be divided into stages by 1949, 1972 and early 1976 respective 17.