唐山地震前后地下水位异常过程的力学解释及与震源演化过程的关系

THE MECHANICAL INTERPRETATION FOR THE PRECURSOR ANOMALIES OF GROUNDWATER LEVEL DURING TANGSHAN EARTHQUAKE AND ITS RELATIONSHIP WITH FOCAL EVOLUTION PROCESS

摘要:

本文探讨了唐山地震前后天津、唐山地区地下水位异常形成的机理、力学过程的解释,以及与震源演化过程的关系,认为:唐山震中区和外围天津地区分别受震源迭加的垂直向上力派生的水平向张应力作用和深部横向水平力拖曳派生的垂直向下牵引力作用,造成地下水位的长期下降。临震时天津地区受震源应力场强化的反馈作用,使深部横向水平力拖曳受阻,引起观测水层产生回压,而震中区则因主破裂前的预滑,引起震源应力场转向,使观测水层的应力状态发生改变,出现地下水位的临震突转回升。又因唐山位于震源体上方,天津紧邻震源体的外围,且观测水层分别为坚硬灰岩和松软土层,具有不同的水文地质力学性质,故地下水位异常发展,显示出不同下降速率的统一孕震力学过程。文末,用唐山震中区地下水位异常资料,讨论了与震源演化过程的关系。

Abstract:

This paper deals with the forming mechanism of groundwater level anomaly in the Tianjin——Tangshan region, the interprectation for the mechanical process as well as its relationship with forcal evolution process. It suggests that in the Tangshan epicentral area there was the horizontal tensile stress derived from vertical-up force in focal process and in the Tianjin area there was the vertical-down drag force in the Earth's depth. This led to the long-term descent of groundwater level. Before shock the feedback effects focal stress field was enhanced in the Tianjin region. Therefore, the horizontal drag force in the Earth's depth was resisted, the observed water——bearing bed was compressed reversely. Meanwhile due to the preseismic slip before the main fracture in the epicentral area the focal stress field turned and the stress condition of observed water-bearing bed changed, as a result the groundwater levels suddenly rose up just before shock. The observed water-bearing bed, which is above earthquake source in Tangshan and near earthquake source in Tianjin, is located in hard limestone and in loose layer, respectively. Their hydrogeological mechanical conditions are different. It shows that the anomalistic process of groundwater level is an unitized mechanical process of brewing earthquake in which there are various descending rates of groundwater level. Finally, the relationship between Tangshan groundwater level anomalies in epicentral area and focal evolution process is discussed.