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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。
Since late Late Tertiary or Quaternary in the North China Basin, the crustal pull-off rate has been gradually weakened, turning to NE-EH squeeze and up-down motion one after another. Summer pad, Tangshan - Ninghe fault is a deep crustal fault. The gravity change before the earthquake in Tangshan earthquake area was about 200 microg (76 March-76 July). According to the gravity curve of Beijing-Laoting, it is inferred that the density interface that caused the gravity change in Tangshan is about 36 km in depth and about 1 m in height, while the corresponding decrease in summer pad. At the same time, the change of density near the focal point was estimated according to the density interface formula. Tangshan was 2.17 × 10 -4 g / cm -3, and summer pad was -1.46 × 10 -4 g / cm -3.