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In this paper the shortages of explanations on the cause of Tangshan earthquakes in 1976 are pointed out. Earth-quake phenomena around Tangshan earthquakes are analyzed synthetically, it is noticed that the most prominent seismic phenomenon are seismic denseness of ML=4, but ML=3 and ML=2 is not active in the same tempo-ral-spatial interval, which occurred from 1973 to 1975. We think that the phenomenon should correspond to rela-tive integrity of the crust medium under higher regional stress. Assuming that the seismicity in circumjacent region could reflect the jostling extent of surrounding plates toward the Chinese mainland, it is inferred that there are multi-dynamical processes in North China region in 1970s, which supply the basic dynamical source to Tangshan earthquakes. A model of multi-dynamical processes and local weakening of the crust is proposed to explain the cause of Tangshan earthquakes. This model could unpuzzle many seismic phenomena related to Tangshan earth-quakes.
In this paper the shortages of explanations on the cause of Tangshan earthquakes in 1976 are pointed out. Earth-quake phenomena around Tangshan earthquakes are analyzed synthetically, it is noticed that the most prominent seismic phenomena are seismic denseness of ML = 4, but ML = 3 and ML = 2 is not active in the same tempo-ral-spatial interval, which occurred from 1973 to 1975. We think that the phenomenon should correspond to rela-tive integrity of the crust medium under higher regional stress. Assuming that the seismicity in circumjacent region could reflect the jostling extent of surrounding plates toward the Chinese mainland, it is inferred that there are multi-dynamical processes in North China region in 1970s, which supply the basic dynamical source to Tangshan earthquakes. A model of multi-dynamical processes and local weakening of the crust is proposed to to explain the cause of Tangshan earthquakes. This model could unpuzzle many seismic phenomena related to Tangshan earth-quakes.