Groundwater level changes before the 1976 Tangshan Ms 7. 8 earthquake and its relation with the earthquake nucleation process

It is of great scientific and practical significance to explore the field observation evidence of earthquake nucleation process for the study of earthquake nucleation and the practice of earthquake prediction. In this study, observation data of the groundwater level before and after the 1976 Tangshan M(s)7. 8 earthquake in the vicinity of the seismogenic fault were collected, and the changes of the groundwater level before the Tangshan M(s)7. 8 earthquake were retrospectively reviewed, analyzed, and compared with theoretical results of earthquake nucleation. The results show that significant groundwater level changes occurred in mid-term, short-term and impending term before the M(s)7.8 Tangshan earthquake. There is a certain correlation between the slow rate of water level decline in the mid-term stage and the contraction of the weakening-zone core in the late stage of the nucleation, between the accelerated rate of water level decline in the short-term stage and the dilatancy in the weakening-zone core, and between the sharp rise or fluctuation of the groundwater level in the impending term and the rise of stress around the weakening-zone core. The theory of earthquake nucleation can reasonably explain the cause of the change of groundwater level before the Tangshan earthquake and provide practical examples for the field observation of earthquake nucleation to some extent.