GRAVITY ANOMALY, M-DISCONTINUITY DEFORMATION AND SEISMOGENETIC PROCESS OF THE TANGSHAN EARTHQUAKE
Zhu Yueqing (Center for Analysis and Prediction, State Seismological Bureau)
The comprehensive analyses are made on the subsurface tectonic environments of the 1976 Tangshan earthquake region, the pre- and post-earthquake gravity changes and the pattern of short wavelength mantle convection in the Beijing-Tianjin-Tangshan-Bohai region. It is suggested that the mantle convection starts from the Gulf of Bohai Sea and is obstructed near the Tangshan area, then forced to upwell through the channels formed by great deep fractures. This might be a dynamic source which produces the preseismic inherited deformation of uplifting M-discontinuity and inherited activation of great deep fractures, so that the strong NW principal tensile stress is generated within the crust of the Tangshan area finally leading to the occurrence of Tangshan earthquake.