Late Quaternary Slip Rate of the Zihong Shan Branch and Its Implications for Strain Partitioning Along the Haiyuan Fault, Northeastern Tibetan Plateau

Abstract

Abstract Geometrical complexities such as bends and branches are ubiquitous along strike‐slip faults. Understanding strain partitioning between the different fault strands along such sections is key to assessing kinematics and evolution through time of a fault system and related seismic hazards. The Haiyuan fault, one of the longest strike‐slip faults of the Tibetan Plateau, has developed a multi‐stranded complex fault geometry along the Hasi Shan restraining bend. In this study, we quantified the slip rate of the ∼50‐km‐long Zihong Shan fault, which is the poorly‐known southernmost fault strand of the Hasi Shan restraining bend. We computed high‐resolution DEMs and orthophotos to document the offset landforms along this fault using drone surveys. At selected sites with well‐preserved offset geomorphic markers, we quantified displaced terraces and channels using microtopography analysis. We dated the abandonment age of these terraces using 10 Be cosmogenic depth profiles and OSL dating techniques. It yields a left‐lateral slip rate of 1.9 ± 0.6 mm/yr since ∼13 ka, which is similar to the rate of the main Hasi Shan branch that ruptured during the 1920 Haiyuan earthquake. The minimum total horizontal slip rate system summed over the multiple strands of the Haiyuan fault at the Hasi Shan restraining bend is 4.1 ± 0.6 mm/yr, without considering the vertical deformation rate of these fault strands. The rate is thus slightly smaller than, but comparable to, slip‐rates determined along the rest of the Haiyuan fault, east and west of the Hasi Shan restraining bend.