An analysis of the activity of the northwest part of Nankou-Sunhe fault

Abstract：As one of half-buried and buried faults in Quaternary in Zhangjiakou-Bohai seismic belt, Nankou-Sunhe fault is the unique NW-striking active fault in the capital circle. Based on shallow seismic method and borehole engineering, the authors precisely determined the position of the northwest section of Nankou-Sunhe fault. Contrastive analysis was made with drilling core and trench profile, combined with the study of magnetic stratigraphy and 14C dating data. The authors explored the activity of the northwest section of Nankou-Sunhe fault since late Pleistocene. Researches show that the activity of Nankou-Sunhe fault has both stick-slip and creep-slip properties, and the vertical activity rate of the fault was 0.12 mm/a in middle Pleistocene, 0.58 mm/a in late Pleistocene, and 0.31mm/a in Holocnene. The fault's active characteristics show complete regularity, i.e.,s from weak to strong, then to weak. At least two earthquake events can be recognized since the late Pleistocene, with their ages being （9230±45）aBP -（9370±40）aBP and（13620±90）aBP -（13960±60）aBP respectively and the least active displacement fault depth being 1.2m. The authors also explored the research thinking and methods for buried active faults in cities.