Fluvial geomorphology of the Yan Shan in North China: Implications for drainage evolution and seismic activity

With an average elevation of similar to 850 m, the east-trending Yan Shan bounds the North China basin to the south and terminates at the Shanxi rift system and Bohai Sea to the west and east. Although the North China basin and the Shanxi rift system host active faults expressed by repeated occurrences of devastating earthquakes in the historical and instrumental records, the Yan Shan region appears to be stable with low seismicity. In this study, we systematically analyze the morphologic parameters of fluvial systems draining across the Yan Shan to explore whether the low seismicity is a transient phenomenon in the past few decades, or it has previously undetected active faults capable of generating devastating earthquakes impactful to regional communities. As shown in this work, the morphometric indices of the fluvial channels across the central Yan Shan display quasi-equilibrium longitudinal profiles, indicating a long-term lack of geomorphological disturbances by active faulting and strong earthquakes. High normalized steepness indices (k(sn)) values and hillslope gradients are concentrated along the southern margin of the Yan Shan. This finding can be explained by the ranging-bounding faults, which are kinematically linked with active northeast-striking right -slip faults to the south in the North China basin. The river longitudinal profile of the Luan River, the trends in watershed migration revealed by Chi(chi) indices, the monadnock phase in the upstream of the Luan River indicated by the low HI value, the present -day drainage pattern, and sedimentary records of the Upper Miocene strata are consistent with an interpretation that the upstream of the Luan River was captured by the proto-Luan River as a result of its headward erosion since the Pliocene.