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1976年7月28日唐山7.8级强震前,北京、天津、唐山地区已布设14个形变电阻率观测台站,其中9个台,震前2—3年内记录到形变电阻率长趋势下降异常,这9个台围绕震中形成一个半长轴可达150公里的异常区.位于震中区的两个台还记录到临震前2—3个月内的加速下降异常.根据现场条件下地表浅部岩(土)层受压时,视电阻率下降的实验结果,认为在震中周围较大的区域内,震前浅部岩(土)层被压密,其压缩线应变的数量级约为3×10~(-5).这一认识得到唐山震前地面基线实测资料的支持.
Prior to the MS8.8 Tangshan earthquake on July 28, 1976, 14 stations for deformation resistivity observation were deployed in Beijing, Tianjin and Tangshan. Nine of them were recorded within 2-3 years before the earthquake, indicating a long-term decline in deformation resistivity , These nine stations around the epicenter to form a semi-major axis up to 150 km of anomalous area located in the epicenter area of the two stations also recorded the accelerated decline within 2-3 months before the earthquake anomalies under the conditions of shallow ground conditions When the rock (soil) layer is under compressive stress, the apparent resistivity decreases. The experimental results show that the rock (soil) layer before the epicenter is dense in a large area around the epicenter, and its compressive strain is about 3 × 10 ~ (-5) .This understanding is supported by the measured data of ground surface before Tangshan earthquake.