Abstract: The earth crust evolution under the North China basin since Cenozoic Era can be roughly divided into two stages: firstly, this basin was progressively extending since early Tertiary, with the result of wide development of normal faults and a series of depressions, where accumulated voluminous Cenozoic sediments. Then the extension rate of the earth crust was gradually decreasing at the end of Late Tertiary and eventually turned to be ENE-ward horizontal compression and vertical movement.The seismic.sounding shows that the Xiadian and Tangshan-Ninghe faults have deeply cut through the crust and the thickness of the crust is 36km at Tangshan and 38km at Xiadian.The gravity variation of the Tangshan earthquake region from March to July,1976 before mainshock was about 200 microgals. The depth of the density interface (or discontinuity) causing the gravity variation of the Tangshan area as conjectured from the Beijing-Leting gravity variation curve is about 36km. It has uplifted about 1m under Tangshan. The amount of density change at Tangshan was estimated from the density interface formula as 2.17?10^-4g/cm³ and -1.46?10^-4g/cm³ at Xiadian. Tha gravity anomalies at Tangshan and Xiadian must be considered together as two integral sides of the same movement entity. It is supposed that the North China basin has been subjected to ENE-ward horizontal compression, which gave rise to the activation of the Tangshan deep fault and the upwelling of upper mantle hot material to increase the density.

摘要： 华北盆地自晚第三纪末期或第四纪以来,地壳拉张速率逐渐减弱,相继转为北东东向水平挤压和上下运动。夏垫、唐山-宁河断裂为深切地壳的断裂。唐山震区震前重力变化约为正200微伽(76年3月—76年7月)。依据北京—乐亭重力变化曲线,推断引起唐山重力变化的密度界面深度约为36公里,上隆约1米,而夏垫相应下降。同时根据密度界面公式来估算震源附近密度变化量,唐山为2.17×10^-4克/厘米³,夏垫为-1.46×10^-4克/厘米³。