Late-Quaternary strong earthquakes on the seismogenic fault of the 1976 M_s7.8 Tangshan earthquake,Hebei,as revealed by drilling and trenching

Composite borehole profiling combined with trenching is an effective way to acquire evidence of past ruptures of buffed active faults. In this study, three composite borehole profiles and a large-scale trench excavation were carded out across the surface rupture zone of the 1976 Ms7.8 Tangshan earthquake. The following three major conclusions have been reached. （1） The surface rupture zone of the 1976 earthquake extends more than 47 km long to the south of Tangshan city, passing to the west of Sunjialou, to Daodi town in Fengnan County, to Xihe in Fengnan County. （2） The surface rupture zone is divided into south and north branches. The north branch has mainly fight-lateral strike-slip motion, and the vertical displacement of the surface is up on the west and down on the east. On the other hand, the vertical displacement of the south branch is up on the east and down on the west, accompanied by some right-lateral slip. Such a faulting style cannot be explained by the movement of a single normal or reverse fault, but is consistent with the vertical displacement field induced by the fight-lateral strike-slip of the fault belt. The drilling and trenching data from this study verify that such activity continued through the Late Quaternary on the Tangshan Fault. （3） The fault planes exhumed by trenching and the dislocations of strata revealed by the boreholes indicate that multiple faulting events occurred on the Tangshan Fault in the Late Quaternary. The timing of three ruptures prior to the 1976 earthquake was 7.61-8.13, 〉14.57, and 24.21-26.57 ka BP. Counting the earthquake of 1976, the recurrence interval of the four strong events is about 6.7 to 10.8 ka. On one of the three borehole profiles, the Niumaku profile, nine faulting events were detected since 75.18 ka BP with an average interval of 8.4 ka. In addition, this paper also discusses the difference between the Late Quaternary sedimentary environments to the north and south of Tangshan city based on stratum dating.

Composite borehole profiling combined with trenching is an effective way to acquire evidence of past ruptures of buried active faults.In this study,three composite borehole profiles and a large-scale trench excavation were carried out across the surface rupture zone of the 1976 Ms 7.8 Tangshan earthquake.The following three major conclusions have been reached.(1) The surface rupture zone of the 1976 earthquake extends more than 47 km long to the south of Tangshan city,passing to the west of Sunjialou,to Daodi town in Fengnan County,to Xihe in Fengnan County.(2) The surface rupture zone is divided into south and north branches.The north branch has mainly right-lateral strike-slip motion,and the vertical displacement of the surface is up on the west and down on the east.On the other hand,the vertical displacement of the south branch is up on the east and down on the west,accompanied by some right-lateral slip.Such a faulting style cannot be explained by the movement of a single normal or reverse fault,but is consistent with the vertical displacement field induced by the right-lateral strike-slip of the fault belt.The drilling and trenching data from this study verify that such activity continued through the Late Quaternary on the Tangshan Fault.(3) The fault planes exhumed by trenching and the dislocations of strata revealed by the boreholes indicate that multiple faulting events occurred on the Tangshan Fault in the Late Quaternary.The timing of three ruptures prior to the 1976 earthquake was 7.61-8.13,>14.57,and 24.21-26.57 ka BP.Counting the earthquake of 1976,the recurrence interval of the four strong events is about 6.7 to 10.8 ka.On one of the three borehole profiles,the Niumaku profile,nine faulting events were detected since 75.18 ka BP with an average interval of 8.4 ka.In addition,this paper also discusses the difference between the Late Quaternary sedimentary environments to the north and south of Tangshan city based on stratum dating.