STUDY ON TANGSHAN-HEJIAN-CIXIAN EARTHQUAKE FAULT ZONE BY SHALLOW SEISMIC EXPLORATION METHOD

The location of the buried faults, the fault broken layers and the depth of breakpoints in the Tangshan-Hejian-Cixian seismotectonic zone are not clear. We implemented 4 shallow seismic exploration profiles on the Daming Fault, Cangxi Fault, and Dachengdong Fault. Line DZ1 is located on the Daming Fault in the southeast of Daming County. Five breakpoints were dectectd, which are all normal faults, with depths of 95~125m and displacements about 6~12m, offsetting late Pleistocene but not the Holocene. Line DZ2 is located in the east of Xianxian County to dectect the Cangxi Fault. Three breakpoints were detected, all are normal faults, with depths of 170~190m and displacements about 7~10m. The upper breakpoints of the three faults cut the middle Pleistocene. The lines DZ3 and DZ4 are located in the west of Litan Town, Dacheng County. Four breakpoints were detected, with the upper breakpoint depth of 120~130m and displacements about 5~15m. They are all normal faults, and the upper breakpoints of the faults cut the Pleistocene strata.
The result of the exploration of Cixian-Daming Fault is not consistent with the buried depth 1 200m proposed by XU Hua-ming. It is proved that the activity of the fault is also consistent with the overall activity of the Cixian-Daming Fault, which is an active fault since late Pleistocene.
The Dachengdong Fault and Cangxi Fault offset the middle Pleistocene strata. Although the late Pleistocene active faults are generally defined as active faults in the practice of active tectonics research in China, strong earthquakes in eastern China have shorter recurrence period, and earthquakes of magnitude 6 or so may also occur in some middle Pleistocene active faults.
During the compilation of GB18306-2015 “Seismic ground motion parameter zonation map of China”, there were no late Pleistocene active faults in the M6~6.5 potential source areas in eastern China. Therefore, we believe that the Dachengdong and Cangxi faults still have the ability to generate earthquake of magnitude 6 or so, and the faults have some similarities with the seismogenic structures of Xingtai earthquake swarm. Under the action of the latest tectonic stress field, the “deep faults” tearing ruptured successively and expanded upwards, resulting in stress migration and loading between two neighbouring en-echolon concealed faults, so, the Dachengdong and Cangxi faults are the product of this three-dimensional rupture process. The Dachengdong Fault is a “newly-generated” fault resulting from the tearing rupturing and upward expanding of the pre-existing concealed “deept faults” in the middle and lower curst.