Investigation on Zhaotan site in NW-trending ground fissure zone of 1976 M_S7.8 Tangshan earthquake in Laoting-Luannan region

Field investigation after the 1976 M_S7.8 Tangshan earthquake reveals that, in Laoting and Luannan region exists a NW-trending ground fissure zone, which is from Shenying village, Luannan county and extended to Wanggezhuang village, Wangtan town, Laoting county, with more than 30 km in length and mainly in dextral strike slip. Laoting-Luannan ground fissures are formed by the 1976 Tangshan M_S7.8 earthquake, but its position closes to the extending line of the NW-trending surface rupture zone of Luanxian M_S7.1 earthquake. In order to further study the seismogenic structure of the Tangshan M_S7.8 earthquake, based on previous data, we investigated the Zhaotan site on the NW-trending Laoting-Luannan ground fissure zone, implemented the trench excavation and drilling exploration. The trench reveals small sand veins and sand vein approximately 1.0—1.2 m wide, which are formed by the 1976 M_S7.8 Tang-shan earthquake and another strong earthquake before the quake, respectively. The borehole profile shows that there is no clearly vertical displacement between the two sides of the main sand vein, which coincides with the pheno-mena that the ground fissures of the 1976 M_S7.8 Tangshan earthquake in this position is of dextral strike slip and the vertical displacement is not obvious that is revealed by the Zhaotan trench.