1989年大同—阳高地震及其前兆特征

Datong-Yanggao Earthquake of 1989 and Its Precursor Characteristics

１９８９年１０月１８～２３日在大同—阳高地区发生了一组中强地震，最大震级为Ｍｓ６．１。这次地震是在华北地区１９６６～１９７６年强震活动高潮之后，在连续１０ａ无６级以上地震活动的平静背景上发生的。由于本区观测台网较为密集，因而在震前观测到较为丰富的前兆现象。通过对这些前兆特点及其发展过程的研究，发现在大同—阳原盆地加速下降后（震前２～３ａ），在前兆中长期—中期阶段，前兆异常由震源区向远区扩展（１３５→３３０ｋｍ），在前兆异常的短期阶段，大部分异常从远场逐渐向孕震区收缩。有限元模拟计算结果表明，大同—阳高地震的孕育是一个长期的过程，由于大同—阳高孕震区地壳内存在断层面的障碍体或硬包体，它们具有较强的抵抗变形的能力，从而使得该区的应力随时间不断积累，在此过程中，周围地震的发生对其应力场具有不同程度的影响，有的使其增加，如渤海地震，有的使其减小，如邢台地震，到１９８５年前后。

A group of moderately strong earthquakes occurred in Datong Yanggao region of North China on Oct.18～23,1989,the maximum magnitude was Ms6 1.The group of earthquakes occurred for the quiet background without M≥6 0 earthquake for many years after the active period(1966～1976)of strong earthquake in North China.Because there are concentrated seismic networks in the region,the more rich precursor phenomena were observed before the earthquakes.Through the analysis for the characteristics and the cavalcade of the precursors,it is discovered that after Datong-Yanggao basin descending quickened(2～3 years before the earthquakes),the precursor anomalies spread from the focal region to the far area(135330km)in the medium long term and the medium term of the precursor cavalcade,and greater part of the anomalies converged from the far field to the seismogenic region in the short term of the precursors.The results of the finit element simulation calculation showed that the preparation of the Datong-Yanggao earthquake was a long term process,because there are the barrier bodys or the inclusions of the fault plane in the seismogenic zone of the Datong-Yanggao,and they have stronger deformation resistant ability,the stress of the zone successively accumulated with time,in the process of the stress accumulation,the earthqueake in its neighboring area affected its stress field at different levels,some increased its stress(such as the Bohai earthquake),some decreased it(such as the Xingtai earthquake),before and after 1985,the preparation of the Datong-Yanggao earthquake was essentially complete.