Discovery of a Buried Active Fault to the South of the 1679 M8.0 Sanhe–Pinggu Earthquake in the North China Plain: Evidence from Seismic Reflection Exploration and Drilling Profile

This study focuses on the key structural locations to the south of the 1679 M8.0 Sanhe–Pinggu earthquake. In conjunction with prior deep seismic reflection exploration in the area, we conducted four shallow seismic investigations to the south of Sanhe–Pinggu seismic area to delineate the exact structure of identified faults and to ascertain the precise location, characteristics, and activity levels of active faults within the region. By analyzing the burial depth of the fault’s breakpoint as revealed by high-precision shallow seismic profiles, we postulate that the fault has been active since the middle and late Pleistocene epochs. In addition, we conducted a high-density borehole investigation in tandem with composite drilling profile at the corresponding sites of shallow breakpoints. Using chronological data from neighboring boreholes and accounting for the ages of samples acquired from these boreholes and staggered strata, the fault manifests as a Holocene active fault within the composite borehole–geological section. This study contradicted the previous conception that to the south of 1679 Sanhe–Pinggu seismic area contained no active faults. This new discovery not only has significant application value for evaluating the risk of large earthquakes in the southern part of the capital circle and understanding the earthquake disaster risk in Beijing but also has scientific significance for studying the development and evolution of faults and their deep–shallow coupling characteristics in North China since the late Cenozoic.