唐山地震前后的应力变化

在距唐山地震震源约百公里的大港油田,以水压致裂法,在深度1000——4000米之间,测定了69次地应力值,对比了1973——1980年大地震前后八年内的应力值变化.在主震前,构造应力场出现明显变化:(1)水平应力梯度降低,即在最大与最小水平主应力方向呈现应力松弛;(2)水平应力与深度的相关系数降低,即构造应力场受到扰动;(3)断层滑动系数增高,断层出现无震蠕动.此外,结合地质构造与应力测量结果,对唐山地震发生过程中的一些异常现象,如无震蠕动、地下水位变化、浅层和外围地区的应力松弛,以及应力沿断层面向深层和闭锁区的转移等,进行了分析和讨论.

THE VARIATION OF REGIONAL STRESS BEFORE AND AFTER THE 1976 TANGSHAN EARTHQUAKE

In the Dagang oil field about 100 km from the source region of the 1976 Tangshan earthquake, 69 in-situ stress measurements have been made by the hydrofracturing method to show variations of stress at depths of 1000-4000 meters within 8 years from 1973-1980 before and after this main shock.The state of tectonic field prior to the major earthquake was changed as follows:1. The gradient of horizontal stresses decreased, that is, the state of stress relaxed in the directions of maximum and minimum principal horizontal stresses.2. The correlation coefficient between horizontal stresses and depth decreased, that is, the in-situ tectonic stress field was disturbed.3. The fault slip coefficient increased, and aseismic creep appeared on the fault. Besides, in combining geological structure and the result of stress measurements.some anomalous phenomena of the seismogenetic process of the Tangshan earthquake, such as the aseismic creep, variation in ground water level, the stress relaxation in shallower layers and around the source region, the stress transfer along the fault plane to deeper layers and closed regions etc. are also analysed and discussed.