Before Tangshan earthquake, magnitude 7.8 on July 28, 1976, many stations observing apparent electrical resistivity, ground water level and radon content in well water were set up within an area of radius 200km around the epicentre and the measurements were carried out for a long time at these stations.
About 13 days (mainly within 10 days)before the occurence of Tangshan earthquake, the obvious sudden anomalies of the apparent resistivity, radon content and groundwater level were observed on a normal background at some stations. The stations that observed the anomalies were only a few, but they mainly concentrated near Changdong fault zone, the other stations at which the weaker anomalies were observed were located near Beijing area in the west of the epicenter.
The comparison of these three sorts of anomalise shows that they are at the same temporal and spatial Characters.
According to the fault plane solution of the Tangshan earthquake and investigation of Chang-dong fault and earthquake activities above magnitude 3 before Tangshan earthquake it can be seen that:The spatial distribution of the three immediate anomalies is in agreement with the fault plane solution of the Tangshan earthquake and Chang-dong fault and in agreement with the distribution of the earthquakes. At the same time, the obvious anomalies and weaker anomalies are in agreement with the earthquake zone of M_L≥4 and the earthquake zone of M_L≥3 respectively.
In this paper it is believed that these anomalies may be the reflections of the process of the stress strengthening before the main earthquake.