1976年7月28日唐山大地震震源区的前兆地震活动平静

Precursory Seismic Quiescence before the Great Tangshan Earthquake of 28. July, 1976 in China

本文以完整性最小震级M_L=1.7的地震目录(1970年—1976年)作为基础资料,研究对比了1976年7月唐山大地震前震源区(余震区)及其周围的四个研究分区中,地震活动速率的时间过程。研究表明:(1)在余震区西半部的分区中,主震前出现了持续时间为38个月的异常地震活动平静,异常在置信水平大于0.99上通过β统计显著性检验。(2)在余震区外围的两个研究分区中,分别记录到16和34个月的活动速率减小的变化,但是,它们仅在置信水平为0.80和0.85上通过β统计显著性检验。考虑到唐山主震、两次最强的余震、与它们相关的地震断层和85％的四级以上的余震都出现在余震区的西半部,作者认为余震区西半部在主震前的地震活动平静是中期前兆现象。前兆地震活动平静区的面积与整个余震区面积之比值为0.7。

The earthquake catalog with the minimum completeness magnitude of ML = 1.7 during the period from 1970 to 1976 has been used as basic data for the research on time variations of seismicity rate in four subregions in source area (aftershock area) and its surroundings before the July 28,1976 great Tangshan earthquake in China. The result shows that (1) there was anomalous seismic quiescence with a duration of 38 months in the subregion located in west part of aftershock area,which is significant statistically at confidence level of 0. 99 and larger by β-test; (2) there were observed decreases of seismicity rate with durations of 16 and 34 months respectively in the annulus subregions around the source area respectively before the Tangshan mainshock, which are significant at confidence level of 0. 80 and 0. 85 by β-test. Because that the Tangshan mainshock,the two strongest aftershocks,and its related earthquake faults and 85 percent of aftershocks ( ML≥4. 0) occurred in the west part of the aftershock area, the authors believe that the seismic quiescence in west part of aftershock area is an intermediate term precursory phenomenon of the great Tangshan earthquake. The ratio of the precursory seismic quiescence area and the aftershock area of the great Tangshan mainshock is about 0. 7.