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

Abstract:

Geometric complexities are ubiquitous along strike-slip faults and often in the form of fault branching into multiple strands of varying kinematics. Strain partitioning between fault strands is important for understanding fault kinematics and the seismic hazard. Near the Yellow river crossing, the Haiyuan fault developed a multi-strand complex geometry at the Hasi Shan. In this study, we focus on the Zihong Shan fault strand, which is poorly known compared with the Hasi Shan branch that broke in the 1920 Haiyuan earthquake. Using small Uncrewed-Aerial-Vehicle technique, we build a series of high-resolution DEM and orthophoto to survey the offset landforms along the Zihong Shan strand. At the Dougou ping site, where offset terrace risers are well preserved, we conduct precise micro-topography geomorphology analysis of displaced terraces and channels, and date the age of abandonment of terrace treads by 10Be depth profile and OSL dating techniques. The left-lateral slip rate is bracketed to be 1.3-2.5 mm/year since ~13 ka along the Zihong Shan fault, which is similar to that on the better-known Hasi Shan branch, 1.9-2.7 mm/yr. The clustering of the smallest geomorphic left-lateral offsets of 3.6 m and 7.6 m along the Zihong Shan fault strand implies that the average coseismic slip may be on the order of ~3.6 m. The horizontal slip rate of the Haiyuan fault at the Hasi Shan thus summed over multiple strands is 3.5-5.7 mm/yr at the minimum, comparable to the slip rate to the east and west sections. This suggests that the Haiyuan fault is through-going at depth despite the surface complexity at the Hasi Shan. The comparable slip rates on the Zihong Shan and Hasi Shan branches suggest that the seismic hazard of the Zihong Shan branch is much underestimated. If the ~43 km-long Zihong Shan strand fails separately by itself, the equivalent magnitude would be Mw7.0, consistent with that average coseismic slip of ~3.6 m. Detailed paleoseismology investigations are therefore needed along the Zihong Shan fault.