Since the Xingtai M_S6.8 earthquake in 1966, many means of earthquake precursor observation and research have been carried out in China. Vertical deformation observation represented by levelling is one of the important means. The Xingtai earthquake occurred in North China, which hosts Beijing, the nation’s political, cultural and populus core. Strong earthquakes have occurred in this region in history, such as the 1679 Sanhe-Pinggu M8 earthquake, so extensive work has been carried out on vertical deformation monitoring. The 1975 Haicheng M_S7.3 earthquake and 1976 Tangshan M_S7.8 earthquake occurred in North China, which provide favorable conditions for researches on strong earthquake monitoring and prediction by crustal deformation observation. Notably, strong earthquakes with abundant deformation observation data are extremely rare in this region. With far more comprehensive theoretical understanding available today, we re-analyze the decades-old archived data from an updated perspective. For this purpose, we reviewed historical files preserved in the Science and Technology Archive Section and Science and Technology Resource Section of the First Monitoring and Application Center of China Earthquake Administration. And we retrived observation data, maps and text reports of deformation anomalies in North China and its surrounding areas before the Tangshan earthquake. By analyzing the viewpoints and conclusions recorded in these archived materials, we identified several issues worthy of further discussion. This paper summarized and discussed representative deformation anomaly patterns, put forward new insights for interpreting the anomalies and their relationship with strong earthquakes.

In the study, we found several drawings in the daily work reports compiled by grass-roots technical researchers. These plots, whose data were calculated by abacus and manually drawn, were very original, but also very real and worthy of attention. Based on a thorough review of these materials, several unresolved scientific issues emerge. Integrating these archived records with updated research findings, this paper puts forward new understandings that can provide a reference for advancing strong earthquake monitoring and prediction in future work. We present one of prediction maps for the Haicheng earthquake and Tangshan earthquake, and the prediction opinions were not officially published in publications at that time. Another dynamic profile of vertical deformation from Tangshan to Han’gu is also shown. There exists a notably spatial correspondence between abnormal uplift deformation characteristics and Tangshan M_S7.8 main shock and its Ninghe M6.9 aftershock, which merits in-depth investigation. And correspondence between the Fengnan M_S5.2 earthquake occurred on May 25, 1970 and the uplift anomaly also requires thorogh analysis. Subsequently, the leveling data before Tangshan earthquake and Haicheng earthquake have been further processed, analyzed and studied, including expanding the temporal and saptial coverage of datasets, strengthening data screening, optimizing calculation methods, etc. Good results have also been achieved in the prediction of medium-long term strong earthquake danger zone, but no prediction results like those found in the archives of this paper have been published. In view of this, this paper makes an in-depth analysis of this phenomenon and its possible causes, and obtains a series of new views and understandings. These findings reveals that the understanding such complex issuess needs a continuous, iterative and deepening cognitive process.

The records and maps of the groundwater before the Tangshan earthquake were also found in the archives, which showed that there was a relatively large decline of groundwater in the late 1960s, and the decline speed increased obviously in the early 1970s. So the conclusion at that time was that the decline of groundwater caused a large and wide range of land subsidence. It seriously interfered with the analysis and research using vertical deformation anomaly to predict earthquake at that time. Now review and think about a new understanding.

The decline of groundwater level and the land subsidence caused by it are usually regarded as the interference to the abnormal information of earthquake deformation. Based on the comprehensive analysis of various research results, this paper concluded that the industry in this area was not developed enough before the Tangshan earthquake, and the change in industrial water consumption was limited, which was mainly caused by the sharp increase of agricultural water consumption. But the increase in agricultural water use is closely related to successive years of severe droughts. At this time, it is believed that the ground subsidence caused by pumping should not be simply regarded as interference, which obviously contains the causal relationship between the major earthquake preparation and the major drought. If the ground uplift caused by the intensification of crust stress and land subsidence resulting from severe drought-induced groundwater depletion are respectively regarded as anomalous information before the major earthquake, the two types of signals can complement each other, so that the deformation anomaly information before the large earthquake can be relatively reasonable highlighted.

Finally, the research and thinking of this paper are summarized as follows: ① the large area deformation map given by the computer shows the spatiotemporal low-frequency deformation information; the small range deformation map given by hand can highlight the spatiotemporal high-frequency deformation information, and the two have complementary effects; ② The early archived scientific and technological literature will show the information that could not be recognized at that time, which is worth reviewing, and these more primitive, more basic and rare materials have non-negligible research value; ③ Land subsidence is usually considered as an interference to seismic deformation analysis, but if drought is the main factor, and drought may be the mid-term precursor before a major earthquake, when drought occurs in the risk area of a major earthquake, the relative uplift in the subsidence area is more worthy of vigilance and attention, and the weight of the anomalous information of relative deformation uplift in the prediction of strong earthquakes should be increased; ④ More comprehensive research should be done on the analysis and judgment of interference information. While considering the adequacy and authenticity of the anomaly caused by interference, it is also necessary to realize that interference may be a trigger factor of earthquakes, and more importantly, the indirect correlation and complexity between interference factors and strong earthquakes.