INVESTIGATION ON THE THERMAL FORCE SOURCES OF 1976 TANGSHAN EARTHQUAKE

In this paper, the possible force sources of the 1976 Tangshan earthquake are discussed from the view point of stress field. The Tangshan area was affected obviously by the neighbouring geothermal sources besides the action of the Pacific Ocean and Indian Plates before the earthquake. The rising temperature zone of upper mantle uplift of Xialiaohe—Bohai—Huanghua centred on the Bohai Bay center and the horizontal temperature gradient zone of Cangdong Fault made the Tangshan area concentrated with geothermal stresses. Based on the observation data, three mechanical models, simplified the temperature variation area as a circle, a long narrow rectangle and an ellipse respectively, are presented in order to evaluate the magnitude of geothermal stresses. The analytical solutions of the geothermal stress field of above the models are calculated. The results show that the normal stresses are all compressive stresses and shear stress is small within the temperature rising area; the radial stress is compressive stress but the hoop stress is tensile stress. All the stresses attenuate outside the temperature rising area. The hoop stress and horizontal shear stress are maximum on the edge of the temperature rising area and their value may reach several handreds bars. Therefore, the geothermal stresses resulting from the thermal sources of the Xialiaohe—Bohai—Huanghua uplift and Cangdoug Fault zones may be an important force sources for the Tangshan earthquake.Besides the Tangshan earthquake, several present strong earthquakes such as the Xingtai, Hejian, Bohai, Haicheng, and Heze earthquakes occurred in north China were also located on the high value zones of the geothermal stresses. Their occurrences may all be connected with the above geothermal force sources.