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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。
In August 1976, there was a strong earthquake in Songpan, Pingwu, Sichuan, with magnitudes of 7.2, 6.7 and 7.2 respectively. This is the largest earthquake in the middle section of the north-south seismic zone in our country since 1933. A series of moderate-magnitude earthquakes of magnitude 5.0-6.5 occurred during this period from 1933 to 1976. This paper discusses the mid-earthquakes in the epicenter transitions, energy release, the frequency of small earthquakes in epicenter, the epicentral distribution and the aftershocks of moderate earthquakes Intensity attenuation and other aspects of the seismic activity characteristics. It is pointed out that the moderate-strong earthquakes during 1973-1974 should be precursory earthquakes in the 1976 Songpan-Pingdu earthquake, and it is possible to distinguish such moderate-strong earthquakes from the moderate-moderate earthquakes based on the above characteristics. The main characteristics of the activity can be summarized as follows: Since 1972, the anomalous release of seismic energy has been changed from the synchronous change of the original source and peripheral regions to a reversal of the opposite direction. The aftershocks of these moderately strong earthquakes do not spread obviously. On the contrary, It is clear that the time interval between these moderately strong earthquakes and their own greatest strong aftershocks is more than 4 days which is obviously greater than the interval of other moderately strong earthquakes in the vicinity and also greater than the 5.0 At the beginning of 1976, that is, in the first half of the great earthquake, the seismic activity in the source area was unusually calm and lasted until the great earthquake occurred. In addition, the migration characteristics of the historical strong earthquakes in the area may predict the time and location of the Songpan Pingwu earthquake in 1976, and the breeding process seems to be divided into several stages from 1949, 1972 and early 1976.